Synthesis, Characterization, and Hydrogenation Activity of Group 10 Metal Complexes Featuring Bulky Phosphine

Ligands

by

Erin Amanda Gwynne

A thesis submitted in conformity with the requirements for the degree of Master of Science

Department of Chemistry University of Toronto

© Copyright by Erin Amanda Gwynne 2010

ii

Synthesis, Characterization, and Hydrogenation Activity of

Group 10 Metal Complexes Featuring Bulky Phosphine Ligands

Erin Amanda Gwynne

Master of Science

Department of Chemistry University of Toronto

2010

Abstract

Bulky, electron-rich phosphine ligands facilitate unique reactivity in various chemical systems

and can stabilize metal species in unusual oxidation states or environments. Routes to bulky

bis(phosphine) chelating ligands that mimic the sterics of the exceptionally bulky tri-tert-

buylphosphine are explored with the ultimate goal of preparing novel catalyst systems of group

10 metals capable of hydrogenation. Attempts to target bulky phosphines from phosphinimine

precursors highlight some interesting phosphinimine reactivity, however attempts to reduce the

phosphinimine bond revealed limitations. Bis(aminophosphine) ligands present an alternate route

to bulky bis(phosphines) and allow for tunability of the environment around phosphorus. The

coordination of these ligands with palladium and nickel exhibit a novel bonding mode in which

C-H or N-H activation of the ligand occurs to form strained metallacycles. Prepared compounds

showed some activity as catalysts under hydrogen and isomerized 1-hexene to 2-hexene, offering

support for their potential use as hydrogenation catalysts.

iii

Acknowledgments

There are many people I would like to thank for their contributions to this process.

My greatest acknowledgement is to my supervisor, Dr. Doug Stephan, for all his mentoring and

guidance. His excitement for chemistry is contagious. I am extremely grateful for the opportunity

to have worked with his excellent research group under his strong leadership.

To all the members of the Stephan group who have helped me in one way or another: I am

grateful for all the helpful discussions, ideas, and friendship. Each person in the group has taught

me something, and I learned at least one new thing every single day (which is clearly an amazing

thing). Dr. Ian Blackmore really helped me to ‘get my feet wet’ in the lab – I would have been

nowhere without the basic introductions to glove boxes, Schlenk lines, etc., and I am grateful for

this foundation. A thank you to Dr. Preston Chase, who helped to expand my ideas and calm my

fears in stress-filled moments. I owe a special thanks to Meghan Dureen, who literally tackled

every single question I had. She always knew precisely how I needed to have an idea explained;

this is an invaluable resource. Thank you for the help with ideas, edits, and X-ray. I would also

like to acknowledge Chris Brown, Steve Geier, and Mike Sgro for X-ray crystallography.

My family was a dynamic support network for me throughout my degree. Thank you for always

encouraging me.

I would also like to thank all of the professors who have expanded my knowledge of chemistry

through their questions, conversations and academic curiosity. A particular ‘thank you’ to Dr.

Datong Song for reading my thesis. The chemistry community here is excellent; the Department

and the University of Toronto have been fantastic places to spend my time.

These individuals and this institution have allowed me to explore a corner of chemistry, and this

experience has been truly rewarding.

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Table of Contents Abstract ........................................................................................................................................... ii

Acknowledgments .......................................................................................................................... iii

Table of Contents ........................................................................................................................... iv

List of Tables ................................................................................................................................. vi

List of Schemes ............................................................................................................................. vii

List of Figures .............................................................................................................................. viii

List of Abbreviations and Symbols .............................................................................................. viii

1 Introduction ................................................................................................................................ 1

1.1 Overview ............................................................................................................................. 1

1.2 Hydrogenation ..................................................................................................................... 1

1.2.1 History of Metal-Catalyzed Hydrogenation ............................................................ 1

1.3 Ligand Design for Hydrogenation Catalysts ....................................................................... 4

1.4 Bis(phosphine) Systems ...................................................................................................... 6

1.5 Bis(aminophosphine) systems ............................................................................................ 6

1.6 Research Objectives ........................................................... Error! Bookmark not defined.

2 Phosphinimines in the Synthesis of Tertiary Phosphines .......................................................... 8

2.1 Introduction ......................................................................................................................... 8

2.1.1 Synthetic Strategies ................................................................................................. 9

2.1.2 Reduction of Phosphine-Chalcogen Bonds .......................................................... 10

2.1.3 Conversion of Phosphinimines to Phosphine Oxides or Phosphine Sulfides ....... 12

2.2 Results and Discussion ..................................................................................................... 13

2.2.1 Precursor Ligand ................................................................................................... 13

2.2.2 Phosphinimine Reduction Attempts ..................................................................... 14

2.2.3 Interaction with Reducing Agents ........................................................................ 15

v

2.2.4 Other Routes to the Linked Phosphine ................................................................. 17

2.3 Conclusions ....................................................................................................................... 19

2.4 Experimental Section ........................................................................................................ 19

2.4.1 General Considerations ......................................................................................... 19

2.4.2 Starting Materials and Reagents ........................................................................... 20

2.4.3 Crystallography ..................................................................................................... 20

2.4.4 Synthesis and Characterization ............................................................................. 22

3 Complexes of Aminophosphine Ligands ................................................................................. 25

3.1 Introduction ....................................................................................................................... 25

3.1.1 Metal Complexes of Bis(aminophosphine) Ligands ............................................. 25

3.1.2 Aminophosphine Pincer-Type Complexes (PNCNP Complexes) ........................ 27

3.2 Results ............................................................................................................................... 28

3.2.1 Synthesis of aminophosphine ligands ................................................................... 28

3.2.2 Preparation of PCP-pincer complexes .................................................................. 31

3.2.3 Preparation of PNP-Type Pincer Complexes ........................................................ 36

3.2.4 Bidentate Chelate Complexes ............................................................................... 37

3.2.5 Other PCP-Type Pincer Complexes ...................................................................... 39

3.2.6 Hydrogenation Activity ........................................................................................ 45

3.4 Experimental Section ........................................................................................................ 46

3.4.1 General Considerations ......................................................................................... 46

3.4.2 Starting Materials and Reagents ........................................................................... 47

3.4.3 Crystallography ..................................................................................................... 47

3.4.4 Synthesis and Characterization ............................................................................. 52

References ..................................................................................................................................... 60

vi

List of Tables

Table 2.1 31P NMR Chemical Shifts of Tertiary Phosphinimines Treated with LiAlH4

Table 2.2 Crystallographic Parameters for Complexes 1, 2, and 3

Table 3.1 Selected bond distances (Å) and angles (°) for compounds 7, 8, and 9

Table 3.2 Selected bond distances (Å) and angles (°) for compounds 10, 11, and 12

Table 3.3 Selected bond distances (Å) and angles (°) for compounds 13, 14, and 15

Table 3.4 Selected bond distances (Å) and angles (°) for compounds 16 and 17

Table 3.5 Activity of Palladium Systems on 1-Hexene

Table 3.6 Activity of Nickel Systems on 1-Hexene

Table 3.7 Crystallographic Parameters for Compounds 7, 8, and 9

Table 3.8 Crystallographic Parameters for Compounds 10, 11, and 12

Table 3.9 Crystallographic parameters for compounds 13, 14, and 15

Table 3.10 Crystallographic Parameters for Compounds 3, 16, and 17

vii

List of Schemes

Scheme 1.1. Catalytic cycle for a metal-based hydrogenation catalyst.

Scheme 1.2. Catalytic cycle of Wilkinson’s catalyst.

Scheme 2.1. Staudinger reaction for the synthesis of phosphinimines.

Scheme 2.2. Preparation of the bulky phosphinimine CH2=C[CH2C(Me2)P(t-Bu2)NSiMe3]2.

Scheme 2.3. Possible mechanism of phosphimine reduction by HSiCl3 in the presence of Ph3P.

Scheme 2.4. Phosphine oxide reduction by LiAlH4in the presence of a methylation reagent.

Scheme 2.5. Preparation of phosphine oxides and phosphine sulfides from phosphinimines.

Scheme 2.6. Complexation of 1 with LiAlH4.

Scheme 2.7. Other routes to the lithiated phosphine species.

Scheme 3.1. Bidentate binding of a bis(aminophosphine) ligand.

Scheme 3.2. Oxidative addition of ammonia by an aliphatic iridium pincer complex.

Scheme 3.3. General procedure for preparation of bis(aminophosphines).

Scheme 3.4. Synthesis of aminophosphine ligands.

Scheme 3.5. Preparation of PCP-PdI pincer complexes.

Scheme 3.6. Preparation of PNP- and PCP-NiCl complexes.

Scheme 3.7. Preparation of PNP complexes.

Scheme 3.8. Preparation of asymmetric PCP-pincer complexes.

Scheme 3.9. Routes to the asymmetric PCP-pincer complex.

Scheme 3.10. Activity of 1-hexene under hydrogen in the presence of 2 mol% of catalyst.

viii

List of Figures

Figure 1.1. Steric effects of bulky phosphine ligands on Ni0.

Figure 1.2. (a) Tri-tert-butylphosphine (b) Tri-tert-butylphosphine-type end groups on a bis(phosphine).

Figure 2.1. Resonance forms of (a) phosphinimine and (b) phosphine oxide.

Figure 2.2. CH2=C[CH2C(Me2)P(t-Bu2)NSiMe3]2 as a synthetic precursor to a bulky bis(phosphine).

Figure 2.3. ORTEP representation of 1.

Figure 2.4. ORTEP representation of 2.

Figure 2.5. POV-Ray depiction of 3.

Figure 3.1. Bis(aminophosphine) ligands with ethylene (A) and propylene (B) spacers in the literature.

Figure 3.2. ORTEP representation of 3.

Figure 3.3. ORTEP representation of 6 and 7.

Figure 3.4. ORTEP representations of 8 and 9.

Figure 3.5. ORTEP representations of 10 and 11.

Figure 3.6. ORTEP representation of 12.

Figure 3.7. POV-Ray depictions of complexes 13-15.

Figure 3.8. ORTEP representation of 16.

Figure 3.9. Paramagnetic nickel salts formed in the presence of HCl.

Figure 3.10. POV-Ray representation of 17.

ix

List of Abbreviations and Symbols

° degrees

Å Ångstrom, 10-10 m

δ chemical shift

Δ change

σ standard deviation

6311-G a type of basis set

Anal Calcd calculated (elemental) analysis

Ar aryl

B3LYP a type of DFT exchange-correlational functional

br broad

C Celsius

ca. circa

cat. catalytic

Cy cyclohexyl

d doublet

ddd doublet of doublets of doublets

dt doublet of triplets

DCM dichloromethane

DFT density functional theory

DME 1,2-dimethoxyethane

equiv. equivalents

e.s.d estimated standard deviation

Et ethyl

FT Fourier transform

x

g gram

h hour

HSQC heteronuclear single quantum correlation

Hz Hertz, s-1

iBu isobutyl, CH2CH(CH3)2

iPr isopropyl, CH(CH3)2

J symbol for coupling constant

kcal kilocalorie

kJ kilojoule

m multiplet

M molarity

Me methyl

mg milligram

MHz megahertz, 106 s-1

min minute

mL milliliter(s), 10-3 L

mmol millimole(s), 10-3 mol

mol mole(s)

MW molecular weight

nBu n-butyl, C4H8

NMR nuclear magnetic resonance

Np naphthalene, C10H8

ORTEP Oak Ridge thermal ellipsoid plot

Ph phenyl, C6H5

xi

ppm parts per million

q quartet

s singlet

t triplet

tBu tert-butyl, C(CH3)3

THF tetrahydrofuran

1

1 Introduction

1.1 Overview

The work presented in this thesis is part of an ongoing research project in collaboration with the

chemical company Lanxess concerning the chemistry of bis(phosphine) ligands and their

coordination to late transition metals. As part of this project, this thesis will discuss the routes to

preparing catalysts for hydrogenation of nitrile butadiene rubber, specifically targeting the olefin

functionalities. In this chapter, a short background of catalytic hydrogenation will be given, and

an introduction to the targeted ligand systems will be provided.

1.2 Hydrogenation

1.2.1 History of Metal-Catalyzed Hydrogenation

The potential use of hydrogen in future energy storage and delivery systems based on renewable

energy sources has stimulated interest in many aspects of hydrogen production, storage, and

usage. Hydrogenation is among the most important of organic transformations utilized in both

academic and industrial fine chemical synthesis. For example, the upconversion of crude oil into

gasoline and other useful products is a heterogeneously catalyzed hydrogenation process.1 As

such, activation of molecular hydrogen and chemical hydrogen sources are under constant

investigation, and there is ongoing interest in finding new ways to mediate the interaction

between hydrogen and various chemical species.

The complexes from which hydrogenation catalysts are derived may or may not initially contain

metal hydride bonds, however they are almost always capable of generating an intermediate of

2

the type MH2. Once a [MH2(alkene)] complex is formed, migratory insertion of a hydride to the

adjacent coordinated alkene, followed by reductive elimination of the resulting alkyl species,

yields the saturated alkane (Scheme 1.1).

Scheme 1.1. Catalytic cycle for a metal-based hydrogenation catalyst.

The discovery of catalytic hydrogenation of olefins with metallic nickel by Sabatier in 1897 was

a landmark in heterogeneous catalysis and paved the way for the incorporation of homogeneous

hydrogenation into modern industrial processes. Indeed, he was awarded half the Nobel Prize in

1912 for this work. In “The Method of Direct Hydrogenation by Catalysis” Sabatier described

the ability of colloidal nickel to affect the hydrogenation of organic materials “without itself

being visibly modified,3” that is, catalytically. Following the communication of this

heterogeneous hydrogenation system, developments towards homogenous catalysts for

hydrogenation began to progress. One benefit of the homogeneous system over a heterogeneous

process is that it is much easier to characterize reaction intermediates and thus rationally design

improved or novel catalysts; another is the higher activity per metal atom. The first documented

example of homogeneous hydrogenation carried out by a discrete, mononuclear metal complex

was the copper mediated hydrogenation of para-benzoquinone by Calvin in 1938. The following

year the first report of a rhodium-based homogeneous catalyst was described.4 Soon thereafter

the syntheses of rhodium phosphine complexes represented the most significant advances in

homogeneous hydrogenation. Reports of the complex [RhH(CO)(PPh3)3], first prepared by Bath

and Vaska5, and the detailed studies of its catalytic activity ultimately opened up an entirely new

perspective on catalytic hydrogenation. This was led by the discovery of catalytic hydrogenation

3

with the rhodium-based phosphine-ligated system [ClRh(PPh3)3],6 commonly known as

Wilkinson’s catalyst (Scheme 1.2) which is active even under mild conditions.

Scheme 1.2. Catalytic cycle of Wilkinson’s catalyst.

Phosphines play a fundamental role in modern coordination and organometallic chemistry, with

their transition metal complexes of primary importance as homogeneous catalysts for many

processes. Furthermore, it is widely acknowledged that modification of the substituents on

phosphorus ligands can significantly alter the properties and therefore behaviour of the ligands

and their transition metal complexes.7 Rhodium complexes of bulky phosphines have proven to

yield extremely active catalyst systems.8-11 Shaw and coworkers demonstrated the importance of

bulky phosphine substituents in the generation of active catalysts; they found that the complex

trans-[RhH2Cl(PtBu3)2], formed from the treatment of RhCl3 with tBu3P, is highly active for the

hydrogenation of olefins.10 Over the years, the coordination chemistry of diphosphine ligands has

been widely studied with a variety of transition metals. Diphosphine complexes of palladium, for

example, have been employed widely in catalytic transformations such as hydroformylation,

hydrogenation, and C-C coupling.12 Furthermore, bulky, electron-rich phosphine ligands often

facilitate unique reactivity in various chemical systems.8 The distinctive properties of tri-tert-

butylphosphine, have allowed for its use and the use of similar bulky phosphines to stabilize

metal species in unusual oxidation states or novel metal-ligand environments (Figure 1.1). The

4

[(dtbpe)Ni] fragment, for example, displays interesting features due to the combination of an

exceedingly bulky chelating ligand coupled with the small nickel metal centre, which helps

promote stabilization of lower coordination modes. As a result, (dtbpe)Ni0-alkene complexes

have remarkably low kinetic reactivities in comparison to the displacement of alkene ligands

from other Ni0 complexes.13

Figure 1.1. Steric effects of bulky phosphine ligands on Ni0.

Wilkinson’s catalyst is still commonly employed industrially, however the high cost, low

abundance, and toxicity of rhodium makes it desirable to seek out an alternate, non-precious

metal catalyst system where metal recovery and recycling would not be necessary. In efforts to

contribute to the development of a more cost-efficient alternative, strategies for the development

of a highly active, non-rhodium based hydrogenation catalyst are explored.

1.3 Ligand Design for Hydrogenation Catalysts

Central to inorganic and organometallic chemistry is the design of a well-tailored ligand system

to exercise control over the properties of a metal centre. The ligands coordinated to a transition

metal greatly influence its properties and thus variations in the ligands result in modification of

catalyst stability, reactivity, and selectivity. Ligands can be carefully tuned along several axes of

variability such as size, spatial conformation, and electronic properties.

Steric Considerations

One of the important roles occupied by ligands in metal catalysts is to offer steric protection to

the catalytically active site. Bulky substituents can protect the metal centre by kinetically

5

hampering side reactions and decomposition. Sterically demanding ligands stabilize ligand-to-

metal bonding by increasing the σ-donor strength relative to those bearing smaller alkyl

substituents.14

Structural Considerations

Chelation, that is, the binding of a ligand to a metal via two or more bonded atoms, is a versatile

method to impose greater control over the properties of a metal complex. Chelating ligands are

more robust ancillary ligands than their monodenate congeners by virtue of the chelate effect.

The utility of chelating bis(phosphine) ligands in organometallic chemistry and in homogeneous

catalysis is well documented.15

Electronic Considerations

The use of strong σ-donor ligands such as trialkylphosphines16 to generate electron-rich metal

complexes has been shown to facilitate in promoting reactions which were not possible using

metal complexes of arylphosphines.17 Generation of a more electron-rich metal centre results in

greater stabilization of the higher oxidation state and hence makes oxidative addition reactions

more facile. This effect is most apparent and useful in catalytic cycles where oxidative addition

is the rate-determining step.

Metal Centre

An important consideration is the selectivity of a catalyst for a targeted functional group. Late

metals tend to show preference for binding olefins over other unsaturated substrates due to

synergistic interactions, and thus nickel- and palladium- based catalysts will be targeted.

These considerations are not mutually exclusive; steric effects can, for example, have marked

electronic consequences, and a delicate balance between these competing factors must be struck.

The bulkiness of tBu3P results in a decreased binding ability relative to smaller tertiary

phosphines such as PMe3, described in terms of ligand cone angle.7 Taking advantage of the

combination of strong σ-donor ability and high congestion, sterically hindered alkylphosphine

ligands might provide the ideal combination of electron rich contribution to the metal centre, and

sterically favoured dissociation.

6

1.4 Bis(phosphine) Systems

While the area of bulky bidentate phosphine ligands continues to grow, there does not yet exist a

bidentate phosphine that provides the same steric environment as tri-tert-butylphosphine at each

donor site. In order to achieve this sort of steric encumberance, it would be necessary to

incorporate geminal-dimethyl groups into the backbone of a bis(tri-tert-butylphosphine) chelate

(Figure 1.2). Preparation of bis(phosphine) ligands is typically achieved by reaction of secondary

phosphines of the type LiPR2 or HPR2 with a primary dihaloalkane.18 This method does not

provide opportunity for incorporation of any substitution on the linking fragment. Thus, in the

search for bis(phosphines) which contain geminal-substitution on the backbone, alternate

synthetic strategies must be targeted. This thesis will explore potential pathways to

bis(phosphines) of this type.

(a) (b)

Figure 1.2. (a) Tri-tert-butylphosphine (b) Tri-tert-butylphosphine-type end groups on a bis(phosphine).

1.5 Bis(aminophosphine) systems

Aminophosphine ligands present another viable route to functionalized bulky phosphine ligands.

Bis(aminophosphine) ligands of the type [R2PN(R’)(CH2)nN(R’)PR2] provide the opportunity

for more accessible ligand modification; the modular nature both of the backbone and the

phosphine synthons allows for access to a library of related ligands. Changing the atom in the

alpha position to one which is inherently more functionalizable expands the synthetic

7

possibilities. Though bis(phosphines) with a carbon backbone of varying lengths have been well

studied, those containing heteroatom spacers are far less explored. Heteroatom-containing

ligands in which there exist both hard and soft donor centres potentially allow for the formation

of weak metal-heteroatom bonds along with strong coordination of the phosphorus atoms to the

metal centre. These systems are well suited for homogeneous catalysis, as the more labile donor

centre can readily dissociate from the metal to open a vacant coordination site which can produce

an active intermediate in a catalytic process.19

The largest foray into this area has been with POCOP-type ligands,20 which contain ethoxy

spacers in the ligand backbone. Although there now exists a huge body of work on the chemistry

of both phosphines and phosphites, ligands containing P-N instead of P-C or P-O bonds have

been relatively neglected. Incorporating heteroatoms such as nitrogen that are further

functionalizable opens up an even wider array of potential reactivity and tunability. Despite this

lack of development, there are indications of the potential utility of transition metal complexes of

aminophosphines. For example, rhodium(I)21 and platinum(II)22 complexes of chiral

aminophosphines have proven to be efficient catalysts for asymmetric hydrogenation and

hydroformylation reactions respectively, and nickel complexes have been employed in the

cyclodimerization of buta-1,3-diene.23

1.6 Research Objectives

The lack of a documented route to bis(phosphines) with the same steric bulk as tri-tert-

butylphosphine represents a significant gap in the literature. The development of such a ligand

set and its coordination to metals could lead to new and potentially very interesting reactivities.

Furthermore, a bulky phosphine chelating ligand should possess several desirable characteristics

for a good late metal hydrogenation catalyst. The following chapters will present routes towards

ligands of this type and describe some preliminary studies on the resulting complexes as

hydrogenation catalysts.

8

2 Phosphinimines in the Synthesis of Tertiary Phosphines

2.1 Introduction

Phosphinimines (R3P=N-R’) were first described in 1919 from the reaction between tertiary

phosphines (R3P) and organic azides (R’N3),24 and have since then found many applications in

organic synthesis and as polymer building blocks.

Scheme 2.1. Staudinger reaction for the synthesis of phosphinimines.

Reactivity of phosphinimide complexes is a result of the polarity of the P-N bond and the steric

demands of the substituents on phosphorus. Like their phosphine oxide counterparts, resonance

forms of the phosphinimine bond can be drawn which illustrate the representation as either a

phosphorus(V) double-bonded species or as a zwitterions (Figure 2.1). The unique nature of

phosphoranimine, or phosphinimine, ligand complexes has been demonstrated for a wide variety

of elements through the work of several groups, the largest contribution of which came from the

groups of Dehnicke and Stephan.25,26

Figure 2.1. Resonance forms of (a) phosphinimine and (b) phosphine oxide.

9

2.1.1 Synthetic Strategies

The reaction of the phosphinimine RtBu2PNSiMe3 (R = Me, iPr)27,28 with an alkyllithium reagent

has been previously shown to generate the species (LiCR’2)tBu2PNSiMe3 (R’ = H, Me)27,28.

Alhomaidan et al. showed that two of these lithiated species can be linked into a

bis(phosphinimine) via reaction with 3-chloro-2-chloromethyl-1-propene in the appropriate

stoichiometry to afford the bidentate phosphinimine CH2=C[CH2C(Me2)P(t-Bu2)NSiMe3]2

(Scheme 2.2).

Scheme 2.2. Preparation of the bulky phosphinimine CH2=C[CH2C(Me2)P(t-Bu2)NSiMe3]2.

Since the bis(phosphinimine) CH2=C[CH2C(Me2)P(t-Bu2)NSiMe3]2 contains two bulky

phosphinimines structurally similar to t-Bu3PNSiMe3, this has been targeted as a potential

synthetic precursor to the bulky bis(phosphine) CH2=C[CH2C(Me2)P(t-Bu2)]2 that consists of

two arms with phosphines structurally similar to t-Bu3P (Figure 2.2).

Figure 2.2. CH2=C[CH2C(Me2)P(t-Bu2)NSiMe3]2 as a synthetic precursor to a bulky bis(phosphine).

In order to carry out this organic transformation from the P(V) to the P(III) species, reduction of

the phosphine-nitrogen double bond would have to be accomplished. While there is no precedent

for reduction of a phosphinimine bond, strategies from the reductions of phosphine oxides and

phosphine sulfides to their phosphine analogs were investigated with phosphinimines.

10

2.1.2 Reduction of Phosphine-Chalcogen Bonds

The reduction of phosphine oxides is often a component of phosphine ligand synthesis due to the

incompatibility of tertiary phosphines with common synthetic organic techniques. A number of

different methods for the reduction of the phosphine oxide moiety have been documented. Metal

hydride or silylhydride reagents such as SiHCl3,29 SiHCl3/PPh3, PhSiH3,30 MeOTf/LiAlH4,31

DIBAL-H,32 BH3,33 and AlH334,35 have been quite extensively explored. Titanium (IV) catalyzed

reduction of phosphine oxides by Ti(OiPr)4/(EtO)3SiH or Ti(OiPr)4/PMHS provide an efficient

method for the practical synthesis of phosphines and phosphonium salts with retention of

stereochemistry at phosphorus.36 Alternate approaches target somewhat milder reagents, and take

advantage of the strategy of using an “oxygen acceptor” such as hexachlorosilane.37

In particular, the use of an oxygen acceptor can be especially useful in cases involving sterically

hindered or electron-deficient phosphine oxides. The effectiveness of silylhydride reagents to

reduce phosphine oxides is believed to lie in an oxygen transfer taking place between the

phosphine oxide and a sacrificial phosphine which is mediated by the silane.38 Treatment of a

phosphinimine with HSiCl3 in the presence of an oxygen-accepting phosphine such as Ph3P

might follow a similar pathway (Scheme 2.3).

Scheme 2.3. Possible mechanism of phosphimine reduction by HSiCl3 in the presence of Ph3P.

11

The stereospecific reduction of phosphine oxides to phosphines by LiAlH4 has been well

explored, and use of a methylation agent as an activator has been demonstrated to promote this

reaction.31 The reduction of triphenylphosphine oxide was accomplished in > 94% yields in the

presence of different methylation agents; in the absence of a methylation reagent, the yield was

only 4%. The phosphine oxide is believed to first be methylated by methyl triflate or methyl

iodide, and the phosphonium salt that results then undergoes nucleophilic attack by a hydride,

thereby releasing H2 and liberating the free phosphine (Scheme 2.4).

Scheme 2.4. Phosphine oxide reduction by LiAlH4in the presence of a methylation reagent.31

Aqueous workup is essential in methods involving metal hydrides, as in the case of reductions

with HSiCl3, Si2Cl6, and LiAlH4 however complications sometimes arise as a small amount of

reoxidation always seems to accompany an aqueous workup. Reduction of

alkyldiphenylphosphine oxides by alane, on the other hand, does not require an aqueous workup.

Reactions can be quenched with anhydrous methanol and filtered through Celite to give the

corresponding phosphine in > 95% yield.34 Furthermore, alane demonstrates more

chemoselectivity than does LiAlH4, particularly with respect to sulfides, sulfones, alkylbromides,

and nitrobenzene.35

Transformation of some phosphine oxides to the corresponding phosphine-boranes can be

accomplished by treatment of the phosphine oxide with SMe2BH3.39 Imamoto et al. were able to

lithiate the methyl group of the phosphine borane Ph2(CH3)P·BH3 with sec-butyllithium.40

Removal of the borane functionality from the phosphine borane (MeOPh)(Ph)(CH3)P·BH3 was

accomplished through reaction with an amine such as diethylamine to afford the corresponding

phosphine with retention of configuration.

Reduction of phosphine sulfides has been demonstrated with several of the same reducing agents

as for phosphine oxides, as well as a few others. Gilbertson and Wang have also shown that the

reduction of tertiary phosphine sulfides can be accomplished via hydrogenation over Raney

nickel.41 Phosphine sulfides and phosphine selenides can be reduced by (Me3Si)3SiH, owing to

12

the affinity of the silyl radical for sulfur and selenium atoms.42 Reductions of the Ph3P=S and

Ph3P=Se to Ph3P can be accomplished in high yields, as can those of nBu3P=S and nBu3P=Se

however these require longer reaction times and higher concentration of the reducing agent.

However, attempts to reduce the analogous phosphine oxides by this method showed no success

even at elevated temperatures.

2.1.3 Conversion of Phosphinimines to Phosphine Oxides or Phosphine Sulfides

Staudinger showed that triphenylphosphinimines added alkyl halides to produce

dialkylaminotriphenylphosphonium halides, which could be hydrolyzed to give

triphenylphosphine oxide and azalkylamines.24 Similarly, the imine functionality can be

exchanged for an oxide or sulfide via reaction with metal carbonyl complexes and metal oxides

to produce isocyanide and nitrene functions by elimination of phosphine oxide or phosphine

sulfide, respectively43 (Scheme 5).

Scheme 2.5. Preparation of phosphine oxides and phosphine sulfides from phosphinimines.

13

2.2 Results and Discussion

2.2.1 Precursor Ligand

The preparation of the NH-phosphinimine CH2=C[CH2C(Me2)P(t-Bu2)NH]2 was described by

Alhomaidan et al. from heating of the silylphosphinimine in methanol.44 In our efforts to isolate

this ligand for use as a potential precursor to the bis(phosphine), colourless crystals were

obtained from a solution in pentane. X-ray diffraction studies revealed the solid-state structure to

be consistent with the previously reported spectral data and furthermore provided some useful

information about bond distance and angles (Figure 2.3). The P-N bond lengths are ca. 1.57 Å,

consistent with a typical phosphinimine bond.25

Figure 2.3. ORTEP representation of 1. 50% probability ellipsoids. Most hydrogen atoms are omitted for clarity. Selected bond distances (Å): P(1)-N(1) 1.5763(16), P(2)-N(2) 1.5719(16), C(1)-C(2) 1.3250(25).

14

2.2.2 Phosphinimine Reduction Attempts

The phosphinimine tBu3P=NH was used to model the behaviour of the phosphinimine chelate 1

in the presence of reducing agents. Reduction attempts were monitored by 31P NMR. Treating

the phosphinimine tBu3P=NSiMe3 with 10 equivalents of LiAlH4 at both ambient and elevated

temperatures over a period of three days did not show any evidence of conversion to the free

phosphine. The same procedure was attempted with LiHBEt3, AlMe3, AliBu3, DIBAL-H and

Me2S·BH3; all reactions were monitored by 31P NMR, and no conversion to the free phosphine

was observed in any case.

In order to obtain a slightly less sterically hindered phosphinimine, tBu3P=NSiMe3 was heated at

reflux in methanol to hydrolyze the trimethylsilyl group, affording the phosphinimine tBu3P=NH

as a white solid. Conversion to the NH-phosphinimine is evidenced by a downfield shift of the

resonance in the 31P NMR spectrum from 34.4 ppm to 68.6 ppm. The NH proton can also be

observed as a broad singlet at 2.94 ppm in the 1H NMR spectrum, accompanied by a

disappearance of the resonance at 0.18 ppm for the trimethylsilylmethyl protons. Attempts to

reduce this phosphinimine with the same reducing agents applied in the reduction attempts of tBu3P=NSiMe3 had the same result: no reduction products were observed.

Though reduction of phosphine oxides with borane has been demonstrated, this proves slightly

more complicated for application to phosphinimines, since phosphinimines have been shown to

form adducts with boranes.45 In the preparation of catecholborane-phosphinimides from

chlorocatecholborane and silylphosphinimines, steric factors determine whether the product

formed is monomeric or dimeric as the reaction between the phosphinimine and the

pinacolborane proceeds via two different pathways, depending on the steric demands of the

phosphinimine. Sterically demanding phosphinimines result in pinacolborane-phosphinimide

derivatives, while those with less demanding phosphorus substituents undergo reduction to the

free phosphine accompanied by formation of borylamines.

To probe whether the nature of the phosphorus substituents affect the reactivity of the

phosphinimine bond – particularly, whether the sterics are influencing the accessibility of the P-

N bond – the less sterically hindered tertiary phosphinimines of the form R3P=NH (R = Et, Ph,

15

iPr, Cy) were reacted with LiAlH4 to try and establish the limitations of phosphinimine

reactivity. All phosphinimines were formed from the precursor phosphinimine R3P=NSiMe3 by

heating at reflux in methanol. In the case of the least sterically encumbered phosphinimines (R =

Et, Ph, iPr), some phosphine oxide product was also formed in this reaction (chemical shifts are

reported in Table 2.1). When treated with LiAlH4, in all cases the species were unchanged, with

the exception of the ethyl derivative, in which conversion to free triethylphosphine was

accomplished. This result indicates that reduction of the phosphinimine P-N bond is chemically

possible by LiAlH4, and further suggests that sterics is the primary limiting factor in trying to

reduce R3P=NH type phosphinimines. It is also possible that the greater electronic contribution

from the more strongly σ-donating R groups contributes to the nature of the reactivity of the P=N

bond.

Table 2.1. 31P NMR Chemical Shifts of Tertiary Phosphinimines Treated with LiAlH4

R

31P δ (ppm) [R3PNSiMe3]

31P δ (ppm) [R3PNH]

31P δ (ppm) [R3P=O]

31P δ (ppm) [PR3]

R1 = R2 = R3 = Et 16.0 70.3 52.4 -18.1 R1 = R2 = R3 = Ph -0.5 25.8 24.4 [-4.8]b

R1 = R2 = R3 = iPr 25.6 54.4 47.2 [18.5]b R1 = R2 = R3 = Cy 17.0 39.0 49.0a [10.5]b

R1 = iPr, R2 = R3 = tBu 30.1 55.3 n/a [47.5]b R1 = R2 = R3 = tBu 32.4 68.2 40.1a [66.1]b

a literature values; b [ ] indicates theoretical chemical shift (product not observed)

2.2.3 Interaction with Reducing Agents

Reaction of the bis(phosphinimine) 1 with lithium aluminum hydride did not yield any of the

reduced phosphine product, however some reaction between the phosphinimine and reducing

agent was observed (Scheme 2.6). Extraction of the waxy white solid from the reaction mixture

into benzene led to the formation of a very large colourless crystal. X-ray diffraction studies

16

revealed its solid-state structure to be a large complex consisting of two equivalents of the

phosphinimine ligand and one equivalent of LiAlH (Figure 2.4). The structure is based around a

central four-membered planar Li-Al-H-N ring, with one phosphinimide ligand chelating to the

aluminum atom in a bidentate fashion, while the other ligand coordinates to the same aluminum

centre in a monodentate fashion, with the other end of the ligand associating with the lithium

cation of the central ring.

Scheme 2.6. Complexation of 1 with LiAlH4.

Figure 2.4. ORTEP representation of 2 with 50% probablility ellipsoids. Solvent molecules (C6D6) and most hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (o): Al(1)-Li(1) 2.5576(26), Al(1)-N(1) 1.8022(12), Li(1)-N(2) Al(1)-N(2) 1.8058(13), Al(1)-N(3) 1.8682(12), N(1)-P(1) 1.5327(12), N(2)-P(2) 1.5352(12), N(3)-P(3) 1.5562(12), N(4)-P(4) 1.5839(13), Al(1)-N(2)-Li(1) 84.26(9).

17

When 1 was treated with DIBAL-H at ambient temperatures, no reduction to the bis(phosphine)

was observed. On heating the mixture at 90°C to facilitate reduction, once again an unexpected

result was obtained. NMR analysis of the isolated crystalline solid revealed that complexation

had taken place, with resonances in the 1H NMR spectrum corresponding to four equivalents of

AliBu2 and one equivalent of phosphinimine. A colourless crystal of this complex was obtained

and the solid-state structure was determined by X-ray diffraction studies to be the

tetrakis(diisobutylaluminum) phosphinimide complex 3 (Figure 2.5), in agreement with the

NMR evidence. The AliBu2 groups are complexed via the phosphinimide nitrogen centres.

Figure 2.5. POV-Ray depiction of 3 with hydrogen atoms omitted for clarity. Colour scheme: Al, beige; P, orange; N, blue; C, black. Selected bond distances (Å): Al-N, 1.884(14); P-N, 1.596(12).

2.2.4 Other Routes to the Linked Phosphine

The tendency towards phosphinimide reactivity with the metal centres of the metal-hydride

based reducing agents motivated investigation into other meant of ‘protecting’ the phosphine. In

order to deprotonate the isopropyl group of iPrtBu2P to allow for incorporation of a carbon linker

to make the bidentate chelate, the phosphorus centre must be oxidized to a phosphorus (V)

18

species to render the phosphorous centre more electropositive and make the methine proton

sufficiently acidic for deprotonation. As an alternate approach to targeting bis(phosphinimines)

as a synthetic precursor, other phosphorus (V) species were probed for their capacity to undergo

the lithiation and subsequent salt metathesis reactions to afford a new category of oxidized

bis(phosphines). Given the literature precedent for reduction of tertiary phosphine sulfides, this

seemed a logical starting target. Stirring iPrtBu2P in the presence of S8 afforded a white

crystalline solid (Scheme 2.7). A downfield shift of the 31P resonance to 83.0 ppm from 47.5

ppm is consistent with oxidation of the phosphine to iPr(tBu)2P=S, 4. Treating the phosphine

sulfide species with tBuLi affords the lithiated product 5, characterized by the disappearance of

the septet in the 1H NMR spectrum representing the isopropyl proton, and by a shift in the 31P

NMR resonance to 74.6 ppm.

Scheme 2.7. Other routes to the lithiated phosphine species.

Similarly, treating iPrtBu2P with BH3SMe2 resulted in a white solid. A downfield shift of the 31P

signal to 51.3 ppm and a quartet signal characteristic of P-B coupling indicated adduct formation

to produce the phosphine-borane 6. Lithiation of this species is accomplished by treatment with tBuLi to afford 7, as evidenced by the disappearance of the septet resonance for the isopropyl

proton in the 1H NMR spectrum and by a phosphorus resonance shift to 33.4 ppm. Future work

will investigate whether these lithiated species will undergo reaction with 3-chloro-2-

chloromethyl-1-propene to yield the linked species.

19

2.3 Conclusions

Attempts to reduce the bulky bis(phosphinimine) 1 resulted in unexpected complexation with the

metal hydride reducing agents to form large phosphinimide complexes. Investigating the

susceptibility of the P=N bond to reduction by LiAlH4 revealed that the phosphinimine bond can

be reduced, however there is likely a steric factor preventing access to the bond. Since sterics

preclude the reduction of bulky phosphinimines to their phosphine analogs, other routes will

have to be explored in targeting bulky phosphine ligands.

2.4 Experimental Section

2.4.1 General Considerations

All manipulations of air- and/or water-sensitive compounds were carried out under an

atmosphere of dry oxygen-free nitrogen using standard Schlenk techniques or an MBraun inert

atmosphere glovebox. 1H, 13C{1H}, and 31P{1H} NMR spectra were acquired on a Bruker

Avance 400 MHz spectrometer, a Varian Mercury 300 MHz spectrometer, or a Varian Mercury

400 MHz spectrometer. 1H resonances were referenced internally to the residual protonated

solvent resonances, 13C resonances were referenced internally to the deuterated solvent

resonances, and 31P resonances were referenced externally to 85% H3PO4. 1H-13C HSQC

experiments were carried out using conventional pulse sequences to aid in the assignment of

peaks in the 13C{1H} NMR. Coupling constants (J) are reported as absolute values.

20

2.4.2 Starting Materials and Reagents

Anhydrous solvents including toluene, pentane, hexanes, ether, tetrahydrofuran, and

dichloromethane were purchased from Aldrich and purified using Grubbs’ column systems

manufactured by Innovative Technology. C6D6 and toluene-d8 were purchased from Cambridge

Isotopes Laboratories, vacuum distilled from Na/benzophenone, and freeze-pump-thaw degassed

(x3). Hyflo Super Cel® (Celite) was purchased from Aldrich and dried for at least 12 h in a

vacuum oven or on a Schlenk line prior to use. Molecular sieves (4 Å) were purchased from

Aldrich and dried at 100 ºC under vacuum using a Schlenk line. Phosphines were prepared

according to literature procedures or purchased from Strem. Phosphinimines were prepared via

the Staudinger reaction according to literature procedures. All reducing agents were purchased

from Aldrich and used without further purification.

2.4.3 Crystallography

X-Ray Data Collection and Reduction. Crystals were coated in Paratone-N oil in the glovebox,

mounted on a MiTegen Micromount, and placed under a N2 stream, thus maintaining a dry, O2-

free environment for each crystal. Crystal data were collected on a Bruker Apex II diffractometer

at 150((2) K for all crystals. Data frames were integrated with the Bruker SAINT software

package using a narrow-frame algorithm. Data were corrected for absorption effects using the

empirical multiscan method (SADABS). Table 2.2 provides crystallographic data for 1-3.

Structure Solution and Refinement. Non-hydrogen atomic scattering factors were taken from the

literature tabulations. The heavy atom positions were determined using direct methods

employing the SHELXTL direct methods routine. The remaining non-hydrogen atoms were

located from successive difference Fourier map calculations. The refinements were carried out

by using full-matrix least-squares techniques on F, minimizing the function ω (Fo - Fc)2, where

the weight ω is defined as 4Fo2/2σ (Fo

2) and Fo and Fc are the observed and calculated structure

factor amplitudes, respectively. In the final cycles of each refinement, all non-hydrogen atoms

21

were assigned anisotropic temperature factors in the absence of disorder or insufficient data. In

the latter cases, atoms were treated isotropically. C-H atom positions were calculated and

allowed to ride on the carbon to which they were bonded, assuming a C-H bond length of 0.95Å.

H-atom temperature factors were fixed at 1.10 times the isotropic temperature factor of the C

atom to which they were bonded. The H-atom contributions were calculated, but not refined. The

locations of the largest peaks in the final difference Fourier map calculation as well as the

magnitude of the residual electron densities in each case were of no chemical significance.

Table 2.2 Crystallographic Parameters for Complexes 1, 2, and 3

1 2 3 Formula C26H56N2P2 C52H110AlLiN4P4 C51H90Al4N2P2 Formula weight 580.77 1104.45 695.15 Crystal system monoclinic triclinic monoclinic Space group P21 P-1 P21/c a (Å) 8.6233(5) 12.7148(14) 21.329(4) b (Å) 10.6931(7) 15.5934(16) 12.342(2) c (Å) 14.9997(9) 17.9564(19) 26.608(4) α (deg) 90.00 106.340(3) 90.00 β (deg) 91.660(3) 93.058(4) 108.940(5) γ (deg) 90.00 94.916(3) 90.00 V (Å3) 1382.54(15) 3392.6(6) 6625.2(18) z 2 2 8 dcalc (g cm-3) 1.102 1.081 2.048 Abs coeff, µ (cm-1) 0.173 0.163 0.304 Data collected 12434 57028 41435 Rint 0.0279 0.0270 0.0642 Data Fo

2 > 3σ(Fo2) 6139 15543 11635

No. of parameters 293 671 595 R1

0.0340 0.0382 0.1947 wR2 0.0806 0.1040 0.5419 Goodness of fit 0.944 1.027 2.983

R1 = ∑||Fo|-|Fc||/∑|Fo|; wR2 = [∑w(Fo2 – Fc

2)2/∑w(Fo2)2]1/2

22

2.4.4 Synthesis and Characterization

Preparation of 1: Based on procedure previously reported by Alhomaidan et al.44; all NMR

spectra resonances are in agreement with those previously reported. The complex was further

characterized by X-ray crystallography. An ORTEP representation is given in Figure 2.3 and

crystallographic parameters are given in Table 2.2.

Preparation of 2: A solution of 1 (500 mg, 1.09 mmol) in THF (15 mL) was stirred in the

presence of LiAlH4 (350 mg, 10.9 mmol). The mixture was heated at reflux overnight and then

filtered through Celite. Removal of volatiles under vacuum and subsequent extraction into

benzene yielded large colourless crystals, identified by X-ray diffraction. 1H NMR gave broad,

indistinguishable signals, and no phosphorus signal could be found by 31P NMR, thus the

complex was characterized crystallographically.

Preparation of 3: To a solution of 1 (120 mg, 0.262 mmol) in toluene was added a solution of

DIBAL-H (4 equiv., 187 mg, 1.04 mmol). The mixture was heated at 90°C for 12h and then

filtered through Celite. Volatiles were removed under vacuum and large colourless crystals were

grown from pentane. 1H NMR (CD2Cl2, 400 MHz) δ: 5.17 (s, 2H, H2C=C), 3.93 (br d, 3JH-P =

32.8 Hz, 2H, AlH), 2.70 (d, 3JH-P = 5.2 Hz , 4H, H2C), 1.89 (sept, 3JH-H = 6.8 Hz, 8H, HC(Me2)),

1.49 (d, 3JH-P = 12.8 Hz, 36H, C(CH3)3), 1.43 (d, 3JH-P = 14.0 Hz, 12H, PC(CH3)2), 0.96 (d, 3JH-H

= 6.8 Hz, 48H, CH(CH3)2), 0.19 (d, 3JH-H = 6.8 Hz, 16H, AlCH2); 31P{1H} NMR (C6D6, 121

MHz) δ: 62.3.

Synthesis of 4: tBu2PiPr (1.95 g, 10.4 mmol) was dissolved in toluene and stirred at room

temperature for 5h in the presence of an excess of S8. The mixture was filtered to remove

unreacted sulfur and toluene was removed under vacuum to give a white crystalline solid (2.15 g,

94%). A downfield shift of the 31P signal confirmed oxidation of the phosphine. 1H NMR (C6D6,

23

400 MHz) δ: 1.97 (sept d, 3JH-H = 7.3 Hz, 2JH-P = 1.8 Hz, HC(CH3)2, 1H), 1.16 (d, 3JH-P = 14.0

Hz, 6H, C(CH3)2), 1.16 (d, 3JH=P = 14.0 Hz, 18H, PC(CH3)3); 31P{1H} NMR (C6D6, 121 MHz) δ:

83.0; 13C{1H} NMR (C6D6, 100 MHz) δ: 44.9 (d, 1JC-P = 40.6 Hz, PC(CH3)2), 38.8 (d, 2JC-P =

37.7 Hz, PC(CH3)2), 30.3 (d, 1JC-P = 38.7 Hz, PC(CH3)3), 28.4 (s, PC(CH3)3).

Synthesis of 5: A solution of tBuLi in pentane (1.7 M, 0.53mL) was added to a solution of 4 (200

mg, 0.912 mmol) in toluene (5 mL) at room temperature. After 5 h stirring, volatiles were

removed under vacuum and the resulting waxy white solid was recrystallized from pentane to

give a white crystalline solid (71%). 1H NMR (C6D6, 400 MHz) δ: 1.90 (d, 3JH-P = 10.0 Hz, 6H,

C(CH3)2), 1.25 (d, 3JH=P = 7.3 Hz, 18H, PC(CH3)3); 31P{1H} NMR (C6D6, 121 MHz) δ: 74.6; 13C{1H} NMR (C6D6, 100 MHz) δ: 38.8 (d, 2JC-P = 37.9 Hz, PC(CH3)2), 37.7 (d, 1JC-P = 41.8 Hz,

PC(CH3)2), 30.3 (d, 1JC-P = 38.6 Hz, PC(CH3)3), 28.2 (d, 2JC-P = 47.0 Hz, PC(CH3)3).

Synthesis of 6: A solution of H3BSMe2 (2.0 M, 3 mL) was added by syringe to a solution of tBu2PiPr (1.0 g, 5.32 mmol) in toluene. After stirring overnight, the solution turned slightly

cloudy. Volatiles were removed under vacuum to give a white solid (1.06 g, 99%). 1H NMR

(CD2Cl2, 300 MHz) δ: 2.18 (sept d, 3JH-H = 7.3 Hz, 2JH-P = 1.6 Hz, 1H, CH(CH3)2), 1.44 (d, 3JH-P

= 12.5 Hz, 6H, CH(CH3)2), 1.33 (d, 3JH-P = 12.0 Hz, 18H, PC(CH3)3), 0.36 (qd, 1JH-B = 95.0 Hz, 2JH-P = 12.2 Hz, 3H, BH3); 31P NMR (CD2Cl2, 121 MHz) δ: 50.3 (q, 1JP-B = 63.7 Hz).

Synthesis of 7: To a solution of 6 (514 mg, 2.56 mmol) in hexanes was added tBuLi (1.7M, 1.5

mL). The pale yellow solution was stirred overnight and then. Leaving at -40°C precipitated

most of the product as a white crystalline solid which was then washed with cold hexanes (62%).

1H NMR (CD2Cl2, 300 MHz) δ: 2.08 (d, 3JH-P = 17.9 Hz, 6H, C(CH3)2), 1.34 (d, 3JH-P = 11.0 Hz,

36H, PC(CH3)3); 31P NMR (121 MHz) δ: 40.8 (q, 1JP-B = 118 Hz).

24

General procedure for phosphinimine reduction attempts: R3P=NR’ (100 mg) was dissolved

in toluene or THF to which was added 10 equiv. of reducing agent with stirring. Reactions were

stirred at room temperature or at reflux temperatures for 24h. Conversion to the free phosphine

was monitored by 31P NMR, and in all cases no conversion to the reduced phosphine was

observed.

General procedure for conversion from R3P=NSiMe3 to R3P=NH: R3P=NSiMe3 (500 mg)

was stirred in MeOH (20 mL) and heated at reflux for 3h. Solvent was removed under vacuum to

yield the R3P=NH product as a white crystalline solid.

General procedure for attempted reduction with LiAlH4: R3P=NH (200 mg) was dissolved in

THF (4 mL) and this solution was added dropwise to a suspension of LiAlH4 (10 equiv.) in THF

(8 mL). After stirring overnight, unreacted LiAlH4 was removed by filtration through Celite and

a 31P NMR spectrum was obtained.

25

3 Complexes of Aminophosphine Ligands

3.1 Introduction

3.1.1 Metal Complexes of Bis(aminophosphine) Ligands

The versatile coordination chemistry of aminophosphines and their potential utility in catalytic

applications makes them an interesting target for new ligand design. The presence of two

different potential donor atoms can confer very different electronic and steric properties on the

ligand and its resulting metal complexes. Although they possess two possible donor atoms, their

coordination compounds form almost exclusively through formation of a metal-phosphorus

bond. The lack of coordination through nitrogen is attributed to the lowered basicity of the amine

nitrogen resulting from the P-N π-interaction.46

Little coordination chemistry of these ligands has been explored to date, with most focusing on

aryl-substituted aminophosphine ligands. Some aminophosphine ligands featuring ethylene and

propylene linkers have previously been reported,47-49 however the literature is quite limited. With

the exception of the bis(aminophosphine) ligand iPr2PN(CH2Ph)(CH2)2N(CH2Ph)PiPr2 reported

by Wollins and coworkers,50 all examples feature amine and phosphorus substitution that is not

particularly electron-rich (Figure 3.1). Bis(aminodiphenylphosphine) ligands based on an

ethylene backbone featuring unsubstituted amines of the type 1A,51,52 amino-substituted ligands

(R = benzyl, phenyl) of the type 2A,47,48 and substituted amines featuring chiral substituents (R =

Ph, Np, Cy) of the type 3A.53 Bis(aminophosphine) ligands based on a propylene linker are even

more scarce, with the only two examples reported by the groups of Gusev49 and Vogt,53 with

unsubstituted amines (1B) and chiral amine substituents (R = Ph) (2B). Umbrella-type

complexes featuring two ethylene or one ethylene and one propylene linker have also been

prepared.54

26

Figure 3.1. Bis(aminophosphine) ligands with ethylene (A) and propylene (B) spacers in the

literature.

The coordination chemistry of these ligands has also been described to some degree.47-50,55

Treatment of a metal halide with any of the ligands seen in Figure 3.1 (A) affords the

corresponding chelate complex (Scheme 3.1).

Scheme 3.1. Bidentate binding of a bis(aminophosphine) ligand.

Preparation of bis(aminophosphine) ligands that are strongly electron-rich in nature and their

complexation to metals represents a significant gap in the study of these ligand systems.

Furthermore, none of these aminophosphine-based complexes have been explored for their

reactivity in the presence of hydrogen. In this vein, the research presented in this chapter will

focus on the generation of new alkyl-substituted bis(aminophosphine) ligands with strong σ-

donor characteristics and their complexation with nickel and palladium.

27

3.1.2 Aminophosphine Pincer-Type Complexes (PNCNP Complexes)

Though many cis-chelating diphosphines have proven instrumental as catalysts, a growing

number of trans-chelating diphosphines – so-called “pincer” ligands – are increasingly appearing

in the literature.56-58 These ligands offer a strong chelating nature and impose a rigid geometry on

the metal centre resulting in high thermal and oxidative stability. The major attraction of PCP

ligands is their ability to give rise to exceptionally robust metal complexes. Their outstanding

stability permits their use as catalysts under forcing conditions. These complexes have found

enormous utility in Heck coupling, alkane dehydrogenation, and transfer hydrogenation, for

example.59

Pincer ligands and their complexes have been well studied since they were first reported in the

1970s.60,61 Shaw and coworkers first described62 the synthesis of several late transition metal

complexes with the general formula MX[CH(C2H4PtBu2)2], in which the central carbon atom

metallates, thereby forming two fused five-membered rings. Seligson and Trogler demonstrated

the utility of alkyl-pincer Pd-Cl, Pd-Me, and Pd-H complexes in catalytic transformations,

including the utility of the methyl derivative as a long-lived olefin amination catalyst.61

Variations in the typical PCP-pincer framework result in altered electronic and steric properties

of their metal complexes. Modifying the metallated carbon from an sp2 to sp3 hybridized centre,

incorporating heteroatoms into the ligand skeleton, and varying the substituents on phosphorus

can improve the catalytic efficiency (eg. of PCP-Pd complexes in the Heck reaction).55,63 The

most extensively investigated complexes feature ligands based on a meta-substituted arene

skeleton (PCsp2P); however, ligands based on an aliphatic skeleton featuring an sp3-metallated

carbon atom (PCsp3P) are rising in popularity given that they possess some enhanced or unique

reactivities. For example Hartwig and coworkers demonstrated that the aliphatic iridium-pincer

complex (Scheme 3.2) can activate the N-H bond of ammonia and undergo oxidative addition to

generate a stable amido hydride.64

28

Scheme 3.2. Oxidative addition of ammonia by an aliphatic iridium pincer complex.

While several PCP systems have found wide application in catalysis and in bond activation,64-66

complexes of this type are overwhelmingly dominated by those based on bis(phosphine) ligands

with only carbon atoms in the backbone. More recently, pincer ligands featuring nitrogen donor

atoms have been developed,67 and proven useful in catalytic applications. Monoanionic PNP-

type pincer ligands have shown utility in N2 activation chemistry68 and in the preparation of low-

coordinate 14e- species,69 however most of these are based on inclusion of a pyridine fragment in

the ligand backbone; few aliphatic skeletons are heteroatom containing. Largely absent from the

literature are PCP ligands in which the phosphine donors are connected to the carbon skeleton

via NH or NR linkers. To date, there has been only nominal investigation of the coordination

chemistry of pincer complexes based on (bis)aminophosphine ligands. These complexes are

typically of the type MCl[CH(CH2N(R)PPh2)2] (R = H, Ph), and their coordination chemistry

with ruthenium and palladium has been described,49,53,70,71 however the library of this class of

heteroatom-substituted pincer complexes is rather sparse.

The remainder of this chapter will detail the preparation and coordination chemistry of several

bis(aminophosphine) ligands and explore new modes of pincer-type binding with palladium and

nickel based on these ligand sets.

3.2 Results

3.2.1 Synthesis of aminophosphine ligands

N,N’-Substituted diamine derivatives react with two equivalents of chlorodi-tert-butylphosphine

to afford the corresponding bis(aminophosphine) compounds in good yield (Scheme 3.3).

29

Compounds 1 and 2 were prepared in this manner from ethylenediamine and 1,3-

diaminopropane and fully characterized by 1H, 13C, and 31P spectroscopy and elemental analysis.

The 31P NMR spectrum of 1 reveals one sharp singlet at 78.7 ppm and the phosphorous

resonance for 2 is at 78.3 ppm.

Scheme 3.3. General procedure for preparation of bis(aminophosphines).

Attempts to prepare bis(aminophosphines) from secondary diamines, revealed that for those

with electron rich substitution on the amino groups (eg. where R = Me, iPr, tBu), simple

deprotonation in the presence of an organic base was not possible, owing to the reduced acidity

of the amine proton. Instead, an alkyllithium reagent had to be used to deprotonate the diamine

prior to reaction with the chlorophosphine (Scheme 3.4). Thus, 3 was prepared in this manner

and isolated as a crystalline solid in 76% yield. The 31P NMR spectrum reveals that this ligand

exhibits some fluxional behavior on the NMR time scale, likely due to inversion of the bulky

substituents at the pyramidalized nitrogen atoms. This inversion process gives rise to three broad

resonances in the 31P NMR spectrum which can be resolved as sharp singlets at -30°C (δ =

108.0, 104.8, and 104.0 ppm); on heating the sample to 50°C, these signals coalesce to give one

broad resonance at 50 ppm. The ligand was further characterized by X-ray crystallography

(Figure 3.2). The P-N bond lengths, at 1.47(1) Å and 1.64(1) Å are noticeably shorter than the

typical single P-N bond length of 1.77 Å.6 Furthermore, this reveals a slightly different solid-

state environment about the nitrogen centres, with the P-N bond lengths differing by ca. 0.1 Å.

Both atoms are slightly pyramidalized, however not to the same degree: the sum of the angles

30

around N(1) is 358.76°, corresponding to the shorter P-N bond and supportive of some π-

bonding, and for N(2) is 355.78°, in accordance with the slightly longer P-N bond.

Figure 3.2. ORTEP representation of 3. Ellipsoids drawn at 30% probability. Hydrogens are omitted for clarity.

Efforts to prepare the aminophosphine from substituted diamines with bulkier substituents such

as iPr and tBu proved less trivial. Treatment of N,N’-di-iso-propylethylenediamine and N,N’-di-

tert-butylethylenediamine with two equivalents of tBuLi followed by addition of two equivalents

of chlorodi-tert-butylphosphine afforded only the respective monosubstituted products 4 and 5

(Scheme 3.4).

Scheme 3.4. Synthesis of aminophosphine ligands.

31

3.2.2 Preparation of PCP-pincer complexes

The bis(phosphine) ligand tBu2P(CH2)5PtBu2 reacts with PdI2 to afford the C-H activated pincer

complex 6 (Scheme 3.5a). The 31P NMR spectrum contains a sharp singlet resonance at 82 ppm

that confirms the equivalence of the phosphorus nuclei in accordance with a trans geometry. The

appearance in the 1H spectrum of the characteristic pattern of virtual triplets for the tBu

hydrogens is consistent with strong coupling of these nuclei to mutually trans phosphorus nuclei.

Also, present in the 13C{1H} NMR spectrum are low-field triplets attributed to the metallated

carbon atom, giving support to the formation of the pincer species. To determine whether the

presence of amino groups in the ligand backbone would cause any deviation from this expected

reactivity, the bis(aminophosphine) ligand 2 was reacted with PdI2 (Scheme 3.5b). Stirring the

two species in THF at room temperature afforded the symmetrical PCP-pincer complex 7,

characterized by the appearance of a single resonance in the 31P NMR spectrum at 130 ppm and

the same observed pattern of virtual triplets for the tBu groups in the 1H and 13C{1H} NMR

spectra as seen in 6. The phosphorus chemical shift of 130 ppm is significantly downfield from

that for the bis(phosphine) complex 6, in accordance with the more electron-rich environment

provided by the amino groups.

32

Scheme 3.5. Preparation of PCP-PdI pincer complexes.

The solid-state structure of 7 was obtained by X-ray crystallography (Figure 3.3) and confirms

this PCP-pincer structure with the central carbon of the ligand backbone coordinating to the

palladium centre [Pd-C(2) = 2.062(3) Å]. Selected parameters are given in Table 3.1.

Figure 3.3. ORTEP representation of 6 and 7. Ellipsoids drawn at 50% probability. Hydrogen atoms and tBu groups are omitted for clarity.

33

There is no evidence of any interaction between the metal and either of the backbone amino

groups in the solid state. The structure confirms this PCP-pincer bonding with the P-N bond

lengths ca. 1.67 Å; considerably shorter than a standard P-N single bond, and slightly shorter

than the P-N bond lengths of the free ligand. The geometry around the metal centre is square

planar, however the P(1)-Pd-P(2) angle is 164.41(2)°, deviating from the expected 180° for a

square planar complex due to the rigid pincer structure. The C(2)-Pd-I angle of 173.9(2)° is bent

slightly out of the plane defined by P(1)-Pd(1)-I(1).

Curiously, when 2 is treated with NiCl2(dme), the pincer complex is not obtained. In this case,

rather, NH-activation is observed and the bright purple PNP-pincer type complex 8 is formed.

The 31P NMR spectrum contains two doublets at 106 and -29 ppm, with 2JP-P = 261 Hz,

consistent with the phosphines arranged trans to one another. This arrangement of the phosphine

nuclei on nickel suggests formation of a pincer-type structure, rather than a bidentate chelate in

which the ligand arms would likely coordinate in a cis fashion. Furthermore, the 1H NMR

reveals only two doublets for the tBu groups on the different phosphorus atoms; no virtual

triplets are observed. The equivalence of the tBu groups above and below the coordination plane

suggests that the backbone is somewhat flexible and therefore likely not centrally anchored as it

is with the PCP-pincer complexes.

Scheme 3.6. Preparation of PNP- and PCP-NiCl complexes.

34

The PNP complex 8 can be converted to the yellow PCP complex 9 in the presence of

tris(pentafluorophenyl)boron (Scheme 3.6), characterized by a single phosphorus resonance at

119 ppm in the 31P NMR spectrum. Conversion to 9 is also consistent with the emergence of

virtual triplets in the 1H and 13C{1H} NMR spectra for the tBu groups, as well as a triplet for the

metallated carbon atom in the 13C{1H} NMR spectrum (1JP-C = 12 Hz). The solid-state structures

of both nickel complexes were obtained by X-ray crystallography and confirm the conversion

from the PNP to PCP complex (Figure 3.4).

Figure 3.4. ORTEP representations of 8 and 9. Ellipsoids drawn at 50% probability. Hydrogen atoms are omitted for clarity.

Both complexes are characterized by a square planar geometry with respect to the nickel centre.

The angles in 8 deviate quite significantly from 90° due to the highly strained 3-membered P-N-

Ni ring, while the angles around the nickel centre in 9 all fall between ca. 84° - 96°. This 3-

membered M-P-N metallacycle interaction with the nitrogen atoms of the aminophosphine

backbone is virtually unprecedented, with the exception of two examples involving ruthenium72

and iridium73 complexes of aminophosphine ligands. There are no previous reports of this

35

coordination behaviour occurring spontaneously upon interaction with the ligand. The

asymmetric metallation in 8 has a large effect on the overall geometry of the molecule,

particularly with respect to the pocket around nickel. This is most noticeable in the P-Ni-P angle

which for 8 is 145.95(5)°, and for 9 is 168.10(4)°. Conversion to the PCP-pincer is also

accompanied by a change in the P-N bond lengths. In the PNP-species, the two P-N bond lengths

differ quite significantly (P(1)-N(1) = 2.161(1) Å; P(2)-N(2) = 2.222(1) Å) based on their

distinctly different environments; the PCP species 9 features P-N bond lengths that are a perfect

average of these ca. 2.19 Å. DFT studies were performed at the B3LYP/6311-G(d,p) level to

assess the electronic structure of 8 and 9. The crystal structure of each served as an initial

prediction for the optimized structure. These calculations revealed only a small energy

difference; the C-H activated complex 9 is slightly lower in energy than the N-H activated

complex 8 (ΔG = 9.96 kcal mol-1). Examination by 11B NMR suggests the formation of H-

B(C6F5)3, evidenced by a peak at -25.1 in the boron NMR spectrum. Though the precise

mechanism of this rearrangement is not clear, the reaction appears to be mediated by formation

of a borohydride, which promotes a change to the slightly more stable product.

Table 3.1 Selected bond distances (Å) and angles (°) for compounds 7, 8, and 9.

7 (M = Pd, X = I)

8 (M = Ni, X = Cl)

9 (M = Ni, X = Cl)

M-X 2.7227(3) 2.1990(12) 2.2338(10) M-C(2) 2.0621(27) -- 1.9761(40) M-N(1) -- 1.8911(35) -- M-P(1) 2.3090(6) 2.1614(12) 2.1907(10) M-P(2) 2.3190(6) 2.2218(12) 2.1943(10) P(1)-N(1) 1.6721(20) 1.6045(35) 1.6782(32) P(2)-N(2) 1.6769(21) 1.6784(36) 1.6723(34) P(1)-M-P(2) 164.41(2) 145.95(5) 168.10(4) P(1)-M-C(2) 81.98(8) 46.05(11) 83.82(13) P(2)-M-C(2) 82.44(8) 99.89(11) 84.72(13) C(2)-M-X 173.91(15) 157.14(11) 171.74(17)

36

3.2.3 Preparation of PNP-Type Pincer Complexes

Treating the shorter chain bis(aminophosphine) 1 with PdI2 and with NiCl2(dme) in both cases

led to the NH-activated PNP-type pincer complexes 10 and 11, consisting of 3- and 6-membered

rings (Scheme 3.7). There is no CH-activated product observed in either reaction.

Scheme 3.7. Preparation of PNP complexes.

Both species are characterized by the presence of two coupled phosphorus species in the 31P

NMR spectra. The chemical shifts of these doublets in the spectrum for the nickel complex are

considerably more separated at 85 and -32 ppm (Δ = 117 ppm) with a 2JP-P of 262 Hz, versus the

palladium complex in which the phosphorus chemical shifts are at 87 and 18 ppm (Δ = 69 ppm)

and JP-P of 372 Hz. The 1H NMR spectrum also reveals only two different tBu environments,

again suggesting some degree of symmetry with respect to the coordination plane, as seen in 8.

The backbone methylene protons are represented by two multiplets in the 1H NMR spectrum.

These peaks integrate for four protons, consistent with metallation occuring through one of the

amines rather than via one of the carbon spacers. These protons show complex coupling patterns

as they are coupled to the adjacent inequivalent methylene protons, as well as to both

inequivalent phosphorus centres. The solid-state structures of 10 and 11 were determined via X-

ray crystallography (Figure 3.5), confirming the formation of a PNP-type pincer complex, as

37

seen with the longer chain ligand and nickel. Comparison of these structures reveals only subtle

differences between the palladium and nickel analogs, namely in the P-M-P angles, where the

palladium complex features a slightly smaller angle [139.50(5)°] as compared to the nickel

complex [141.52(6)°].Unsurprisingly, the nickel complex also contains shorter bond lengths by

ca. 0.1 Å for atoms bonded to the metal centre.

Figure 3.5. ORTEP representations of 10 and 11. Ellipsoids drawn at 50% probability. Hydrogen atoms are omitted for clarity.

3.2.4 Bidentate Chelate Complexes

When 1 was treated with the Pd(0) source Pd2(dba)3, the bidentate chelate complex

[tBu2PN(H)(CH2)2N(H)PtBu2]Pd(dba) 12 was formed. The 1H NMR spectrum of complex 12

displays very broad peaks, which is typical for Pd(dba) complexesput reference here, while the

31P NMR spectrum shows a singlet at δ 108 ppm. Large orange crystals suitable for X-ray

diffraction were isolated from toluene at -38°C (Figure 3.6).The phosphine nuclei chelate in a cis

fashion to the palladium centre, with a P-Pd-P angle of 105.20(2)°. The double bond of one dba

38

molecule remains coordinated to the metal in an η2 fashion. The coordinated double bond is

elongated [1.3562 Å] compared to the uncoordinated double bond of dba [1.2907 Å]. The

palladium-phosphine bonds are lengthened compared to those seen in the PNP-Pd complex of

the same ligand, as well as the nitrogen-phosphorus bonds.

Figure 3.6. ORTEP representation of 12. Ellipsoids drawn at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond distances and angles are given in Table 3.2.

Table 3.2 Selected bond distances (Å) and angles (°) for compounds 10, 11, and 12.

10 M = Pd, X = I

11 M = Ni, X = Cl

12 M = Pd, X = dba

M(1)-X(1) 2.626(1) 2.219(3) 2.1430(18), 2.1538(18) M(1)-N(1) 2.077(8) 1.828(2) -- M(1)-P(1) 2.246(2) 2.185(2) 2.3766(5) M(1)-P(2) 2.312(3) 2.185(2) 2.3690(5) P(1)-N(1) 1.613(9) 1.584(4) 1.677(2) P(2)-N(2) 1.668(9) 1.655(4) 1.703(2) P(1)-M(1)-P(2) 139.50(5) 141.47(4) 105.20(2) P(1)-M(1)-N(1) 43.60(24) 45.41(13) -- P(2)-M(1)-N(1) 96.27(24) 96.06(13) -- N(1)-M(1)-X(1) 157.26(24) 156.17(12) --

39

3.2.5 Other PCP-Type Pincer Complexes

To probe the effect of substitution at nitrogen, complexation of 3 with palladium and nickel was

investigated. When 3 is treated with PdI2 the result was surprising; the 31P NMR spectrum

contained two doublets (2JP-P = 388 Hz), indicative of two coupled trans phosphorus

environments. 1H NMR spectrum revealed four distinct tBu environments, giving further support

to the existence of two separate phosphorus environments, and also suggesting that some degree

of rigidity exists in the ligand backbone, thereby rendering the tBu groups above and below the

plane of the ring inequivalent. This is consistent with the formation of a PCP-type pincer

complex in which CH-activation of the ligand backbone occurs to yield the asymmetric pincer

species 13, which features a 4- and 5-membered ring (Scheme 3.8). This pincer-type binding is

in contrast to what has been observed for the activity of similar substituted bis(aminophosphine)

ligands on palladium(II),47,48 where ligands bind in a bidentate fashion to give 7-membered

metallacycles. The PCP-PdBr analog 14 and PCP-PdCl analog 15 were also prepared.

Scheme 3.8. Preparation of asymmetric PCP-pincer complexes.

40

Figure 3.7. POV-Ray depictions of complexes 13-15. Hydrogen atoms are omitted for clarity. Colour scheme: Pd, brown; Cl, green; Br, red; I, magenta; P, orange; N, blue; C, black.

41

X-ray diffraction studies helped establish the solid-state structures of 13, 14, and 15. The ORTEP

representations of these complexes are shown in Figure 3.7; crystal data and collection

parameters are listed in Table 3.7 and selected bond distances and angles are given in Table 3.4.

The palladium centre in all three structures adopts a distorted square planar geometry defined by

the chelating phosphorus atoms, the metallated carbon atom, and a halogen atom. The P-Pd-P

bond angle deviates significantly (P-Pd-P ca. 149°) from the expected 180° imposed by the

square planar environment, owing to the buckling of the ligand backbone upon metallation of the

skeletal carbon atom.

Table 3.3 Selected bond distances (Å) and angles (°) for compounds 13, 14, and 15.

13 (M = Pd, X = I)

14 (M = Pd, X = Br)

15 (M = Pd, X = Cl)

Pd(1)-X(1) 2.6806(4) 2.5204(32) 2.3973(8) Pd(1)-C(1) 2.0491(75) 1.7669(167) 2.0198(47) Pd(1)-P(1) 2.2981(11) 2.2998(61) 2.2867(7) Pd(1)-P(2) 2.3030(11) 2.3269(58) 2.2996(7) P(1)-N(1) 1.6838(55) 1.6320(207) 1.6698(28) P(2)-N(2) 1.6621(57) 1.5588(227) 1.6720(29) P(1)-Pd(1)-P(2) 149.33(5) 149.1(26) 149.56(3) P(1)-Pd(1)-C(1) 70.41(49) 72.84(61) 70.05(22) P(2)-Pd(1)-C(1) 79.59(49) 76.39(61) 80.36(22) C(1)-Pd(1)-X(1) 170.37(31) 174.92(61) 173.78(22)

In an attempt to prepare the nickel analog of 15, 3 was reacted with NiCl2(dme), yielding a bright

turquoise product. NMR spectroscopy revealed that this was not the expected pincer complex nor

the chelate compound, but rather a paramagnetic species that would be consistent with

tetrahedral species of the type [LnNiX4-n]n-2. Two broad singlets could be observed in the 31P

NMR spectrum at 50.5 and 53.5 ppm, indicative of two different phosphorus environments.

Recrystallizations of this turquoise solid afforded light green crystals which were identified as

42

the zwitterionic nickel-phosphonium complex [{(tBu2PH)(NMe)(CH2)2(NMe)(tBu2P)}NiCl3], 16

which features monodentate phosphine binding to a NiCl3- moiety (Figure 3.8). The geometry

with respect to the nickel centre is tetrahedral, with P-Ni-Cl angles of 111.66°, 112.71°, and

106.50°. This latter angle is slightly smaller due to a small hydrogen bonding interaction

between that chlorine atom and the phosphonium proton (distance of 3.053 Å).

Figure 3.8. ORTEP representation of 16. Ellipsoids drawn at 30% probability. Hydrogen atoms are omitted for clarity.

The formation of 16 may result from reaction with HCl liberated in situ upon generation of the

desired pincer complex, either with the pincer complex itself, or with the aminophosphine ligand

prior to its interaction with NiCl2. Reaction in the presence of triethylamine did not preclude the

formation of the zwitterion, giving strength to the hypothesis that that the phosphine is

protonated preferentially to the added base. A similar zwitterionic byproduct (Figure 3.9a) was

observed by Zargarian and coworkers in their efforts to isolate the [PCP]NiCl pincer complex

from tBu2P(CH2)5PtBu2 and NiCl2.5 They further observed that protonation of the expected

pincer complex with HCl leads to the doubly protonated ionic species

[{tBu2PH)(CH2)2}2CH2][NiCl4] (Figure 3.9b).

43

Figure 3.9. Paramagnetic nickel salts formed in the presence of HCl.

Abstraction of 1 equiv. of HCl should afford the target pincer complex, however efforts to

encourage this by abstraction of the phosphonium proton using triethylamine, tri-tert-

butylphosphine, and proton sponge all proved elusive. Abstraction of a chloride by the Lewis

acid tris(pentafluorophenyl)boron afforded the target pincer species 17 (Scheme 3.7), evidenced

by the appearance of the characteristic doublets in the 31P NMR spectrum at 120.7 and 31.9 ppm

and the large phosphorus coupling (2JP-P = 289 Hz), consistent with a trans arrangement of the

phosphine atoms. The 11B NMR spectrum contains a singlet resonance at -6.7 ppm, consistent

with the formation of Cl-B(C6F5)3.

44

Scheme 3.9. Routes to the asymmetric PCP-pincer complex.

The solid-state structure of 17 was verified by X-ray crystallography (Figure 3.10) and shows

very similar metrical parameters to those for 13-15. The nickel analog shows a slightly larger P-

M-P angle than its palladium counterparts [154.39(7)°], due to the smaller size of nickel and the

overall more compact binding.

Figure 3.10. POV-Ray representation of 17 with hydrogen atoms omitted for clarity. Colour scheme: Ni, yellow; Cl, green; P, orange; N, blue; C, black.

Table 3.4 Selected bond distances (Å) and angles (°) for compounds 16 and 17.

16 17 Ni(1)-Cl(1) ca.2.26 2.2383(43) Ni(1)-C(1) -- 1.6947(312) Ni(1)-P(1) 2.3723(10) 2.1868(39) Ni(1)-P(2) -- 2.1757(40) P(1)-N(1) 1.6720(35) 1.6913(148) P(2)-N(2) 1.6244(36) 1.6981(131) P(1)-Ni(1)-P(2) -- 154.39(17) P(1)-Ni(1)-C(1) -- 70.46 (1.10) P(2)-Ni(1)-C(1) -- 84.40(1.10) C(1)-Ni(1)-X(1) -- 172.11(1.15)

45

3.2.6 Hydrogenation Activity

It was one of the initial goals of this research to develop metal catalysts systems that are active

towards the hydrogenation of olefins. To model the activity of the prepared complexes on

olefins, reactivity under an atmosphere of hydrogen was examined on 1-hexene as a substrate. 1-

hexene is good for this purpose as it is a terminal olefin, which makes the double bond inherently

more accessible and easier to hydrogenate. Furthermore, the olefinic signals are easy to identify

and follow by 1H and 13C NMR spectroscopy, and any isomerization behaviour can be easily

detected by this method.

Scheme 3.10. Activity of 1-hexene under hydrogen in the presence of 2 mol% of catalyst.

Hydrogenation attempts were carried out in the presence of 2 mol % of catalyst, under 4 atm

(Scheme 3.10). of H2. The reactions were monitored by 1H NMR at regular intervals based on

the integration of the olefinic 1-hexene peaks with respect to hexamethylbenzene (internal

standard), and 31P NMR was used to monitor any catalyst decomposition. All hydrogenations

were first tried at room temperature, and then at 80°C.

For all catalysts, isomeriation of 1-hexene to 2-hexene was observed, rather than hydrogenation

to hexane. The palladium systems (Table 3.5) generally performed better than their nickel

counterparts (Table 3.6), with complex 7 showing full conversion to 2-hexene at room

temperature. All other systems performed relatively poorly at room temperature.

Table 3.5 Activity of Palladium Systems on 1-Hexene

Catalyst Time Temperature 1-hexene hexane 2-hexene

13 72h 80°C 100% 0% 0%

10 24h 80°C 0% 0% 100%

7 48h rt 0% 0% 100%

12 72h 80°C 0% 0% 100%

46

Table 3.6 Activity of Nickel Systems on 1-Hexene

Catalyst Time Temperature 1-hexene hexane 2-hexene

11 72h 80°C 90% 0% 10%

8 6h 80°C 0% 0% 100%

Though there was no evidence of hydrogenation to hexane, the isomerization behaviour does

support generation of a metal hydride species, which is essential for the hydrogenation to occur.

Activity was not tested at pressures higher than 4 atm., however these preliminary results do

offer promise of the possibility of successful conversion to 1-hexene at higher pressures.

3.3 Experimental Section

3.3.1 General Considerations

All manipulations of air- and/or water-sensitive compounds were carried out under an

atmosphere of dry oxygen-free nitrogen using standard Schlenk techniques or an MBraun inert

atmosphere glovebox.

1H, 13C{1H}, and 31P{1H} NMR spectra were acquired on a Bruker Avance 400 MHz

spectrometer, a Varian Mercury 300 MHz spectrometer, or a Varian Mercury 400 MHz

spectrometer. 1H resonances were referenced internally to the residual protonated solvent

resonances, 13C resonances were referenced internally to the deuterated solvent resonances, and 31P resonances were referenced externally to 85% H3PO4. 1H-13C HSQC experiments were

carried out using conventional pulse sequences to aid in the assignment of peaks in the 13C{1H}

NMR. Coupling constants (J) are reported as absolute values.

47

Calculations were performed with the Gaussian03 program using density functional theory

(DFT). The geometry of compounds 7 and 8 were optimized starting from the X-ray structures of

these complexes using B3LYP exchange-correlational functional with the 6-311-G(d,p) basis set.

Optimizations were performed without (symmetry) constraints, and the resulting structures were

confirmed to be minima on the potential energy surface by frequency calculations (the number of

imaginary frequencies is zero). Visualization of the computed structures and molecular orbitals

was achieved using the program WebMO.

3.3.2 Starting Materials and Reagents

Anhydrous solvents including toluene, pentane, hexanes, ether, tetrahydrofuran, and

dichloromethane were purchased from Aldrich and purified using Grubbs’ column systems

manufactured by Innovative Technology. Deuterated solvents were purchased from Cambridge

Isotopes Laboratories. THF-d8 was purchased in 1 g ampoules and used over activated Seives.

C6D6 and toluene-d8 were vacuum distilled from Na/benzophenone, and freeze-pump-thaw

degassed (x3). Hyflo Super Cel® (Celite) was purchased from Aldrich and dried for at least 12 h

in a vacuum oven or on a Schlenk line prior to use. Molecular sieves (4 Å) were purchased from

Aldrich and dried at 100 ºC under vacuum using a Schlenk line. All diamines were purchased

from Aldrich and degassed prior to use. The phosphines tBu2P(CH2)5tBu2 and tBu2PCl were

purchased from Strem. Unless otherwise noted, all metal starting materials were purchased from

Strem and used as received.

3.3.3 Crystallography

X-Ray Data Collection and Reduction. Crystals were coated in Paratone-N oil in the glovebox,

mounted on a MiTegen Micromount, and placed under a N2 stream, thus maintaining a dry, O2-

free environment for each crystal. Crystal data were collected on a Bruker Apex II diffractometer

at 150((2) K for all crystals. Data frames were integrated with the Bruker SAINT software

48

package using a narrow-frame algorithm. Data were corrected for absorption effects using the

empirical multiscan method (SADABS). Table 3.7 provides crystallographic data for 7-9; Table

3.8 provides crystallographic data for 10-12; Table 3.9 provides crystallographic data for 13-15;

Table 3.10 provides crystallographic data for 3, 16, 17.

Structure Solution and Refinement. Non-hydrogen atomic scattering factors were taken from the

literature tabulations. The heavy atom positions were determined using direct methods

employing the SHELXTL direct methods routine. The remaining non-hydrogen atoms were

located from successive difference Fourier map calculations. The refinements were carried out

by using full-matrix least-squares techniques on F, minimizing the function ω (Fo - Fc)2, where

the weight ω is defined as 4Fo2/2σ (Fo

2) and Fo and Fc are the observed and calculated structure

factor amplitudes, respectively. In the final cycles of each refinement, all non-hydrogen atoms

were assigned anisotropic temperature factors in the absence of disorder or insufficient data. In

the latter cases, atoms were treated isotropically. C-H atom positions were calculated and

allowed to ride on the carbon to which they were bonded, assuming a C-H bond length of 0.95Å.

H-atom temperature factors were fixed at 1.10 times the isotropic temperature factor of the C

atom to which they were bonded. The H-atom contributions were calculated, but not refined. The

locations of the largest peaks in the final difference Fourier map calculation as well as the

magnitude of the residual electron densities in each case were of no chemical significance.

49

Table 3.7 Crystallographic Parameters for Compounds 7, 8, and 9 7 8 9 Formula C19H43N2P2PdI C19H43N2P2NiCl C19H43N2P2NiCl Formula weight 594.79 455.65 455.65 Crystal system monoclinic orthorhombic monoclinic Space group P21/n P212121 P21/n a (Å) 15.454(2) 11.6462(10) 11.8428(8) b (Å) 10.441(1) 14.1879(13) 14.4447(9) c (Å) 16.245(2) 15.0410(13) 14.7145(11) α (°) 90.00 90.00 90.00 β (°) 112.477(4) 90.00 103.688(4) γ (°) 90.00 90.00 90.00 V (Å3) 2491.6(6) 2485.3(4) 2445.7(3) z 4 4 4 dcalc (g cm-3) 1.581 1.218 1.238 Abs coeff, µ (cm-1) 2.188 1.022 1.039 Data collected 19258 40754 49993 Rint 0.0288 0.1284 0.0872 Data Fo

2 > 3σ(Fo2) 5716 5706 5626

No. of parameters 217 226 226 R1

0.0221 0.0483 0.0532 wR2 0.0552 0.1016 0.1403 Goodness of fit 0.942 0.906 0.998

R1 = ∑||Fo|-|Fc||/∑|Fo|; wR2 = [∑w(Fo2 – Fc

2)2/∑w(Fo2)2]1/2

50

Table 3.8 Crystallographic Parameters for Compounds 10, 11, and 12 10 11 12 Formula C18H41N2P2PdI C18H41N2P2NiCl C35H56N2OP2Pd Formula weight 580.77 441.63 695.15 Crystal system triclinic monoclinic orthorhombic Space group P-1 P21/m Pbca a (Å) 8.8808(8) 8.0241(11) 17.5311(18) b (Å) 10.1886(8) 14.249(2) 22.158(2) c (Å) 13.7003(13) 10.4438(14) 22.345(2) α (deg) 89.746(5) 90.00 90.00 β (deg) 88.662(5) 99.858(9) 90.00 γ (deg) 79.960(5) 90.00 90.00 V (Å3) 1220.33(19) 1376.4(3) 8680.2(15) z 2 2 8 dcalc (g cm-3) 1.581 1.247 1.260 Abs coeff, µ (cm-1) 2.160 1.078 0.536 Data collected 19258 16131 198723 Rint 0.0528 0.0576 0.0682 Data Fo

2 > 3σ(Fo2) 5574 3870 14089

No. of parameters 217 123 478 R1

0.0836 0.0726 0.0331 wR2 0.2091 0.2382 0.0761 Goodness of fit 0.814 1.053 1.001

R1 = ∑||Fo|-|Fc||/∑|Fo|; wR2 = [∑w(Fo2 – Fc

2)2/∑w(Fo2)2]1/2

51

Table 3.9 Crystallographic parameters for compounds 13, 14, and 15 15 14 13 Formula C20H45N2P2PdCl C20H45N2P2PdBr C20H45N2P2PdI Formula weight 517.37 561.83 608.82 Crystal system monoclinic monoclinic monoclinic Space group P21/c P21/c P21 a (Å) 8.7937(3) 9.0634(6) 9.4299(3) b (Å) 11.7012(4) 11.6618(8) 11.7919(4) c (Å) 24.3196(9) 24.2011(18) 11.7917(4) α (deg) 90.00 90.00 90.00 β (deg) 90.332(2) 90.154(4) 90.2030(10) γ (deg) 90.00 90.00 90.00 V (Å3) 2502.37(15) 2557.9(3) 1311.19(8) z 4 4 2 dcalc (g cm-3) 1.373 1.459 1.542 Abs coeff, µ (cm-1) 0.984 2.420 2.014 Data collected 42913 24118 47499 Rint 0.0356 0.0546 0.0369 Data Fo

2 > 3σ(Fo2) 9802 6362 12108

No. of parameters 235 236 235 R1

0.0513 0.2528 0.0581 wR2 0.1702 0.5623 0.1639 Goodness of fit 1.283 4.299 0.683

R1 = ∑||Fo|-|Fc||/∑|Fo|; wR2 = [∑w(Fo2 – Fc

2)2/∑w(Fo2)2]1/2

52

Table 3.10 Crystallographic Parameters for Compounds 3, 16, and 17 3 16 17 Formula C20H46N2P2 C20H46N2P2NiCl3 C20H45N2P2NiCl Formula weight 376.53 712.45 378.19 Crystal system triclinic orthorhombic monoclinic Space group P1 Pna21 P21/c a (Å) 6.2240(6) 10.8771(6) 8.637(4) b (Å) 8.0779(8) 20.2402(12) 11.727(5) c (Å) 12.4180(12) 15.8448(11) 24.616(8) α (deg) 94.467(3) 90.00 90.00 β (deg) 99.62 90.00 90.215(19) γ (deg) 104.88 90.00 90.00 V (Å3) 590.12(10) 3488.3(4) 2493.3(17) z 1 4 4 dcalc (g cm-3) 1.060 1.357 1.251 Abs coeff, µ (cm-1) 0.189 1.199 1.021 Data collected 7072 39600 19001 Rint 0.0435 0.0452 0.1797 Data Fo

2 > 3σ(Fo2) 4125 or 3074 10343 or 7932 5723 or 2865

No. of parameters 217 307 230 R1

0.1430 0.0556 0.1690 wR2 0.3930 0.1493 0.4049 Goodness of fit 1.008 1.033 1.023

R1 = ∑||Fo|-|Fc||/∑|Fo|; wR2 = [∑w(Fo2 – Fc

2)2/∑w(Fo2)2]1/2

3.3.4 Synthesis and Characterization

Synthesis of 1: A solution of PtBu2Cl (2.00 g, 11.1 mmol) in THF (15 mL)

was added with stirring to a solution of ethylenediamine (500 mg, 8.32

mmol) and triethylamine (2.0 g, 19.8 mmol) in THF (5 mL). After stirring overnight, the white

salt precipitate was allowed to settle and the clear supernatant was filtered through Celite. THF

and unreacted diamine were removed under vacuum and the resulting oily solid was

recrystallized from hexanes at -40°C to give a white crystalline solid (3.24 g, 84%). 1H NMR

(C6D6, 400 MHz) δ: 3.04 (m, 4H, CH2), 1.24 (br d, 2JP-H = 11.0 Hz, 2H, NH), 1.09 (d, 3JP-H =

53

11.1 Hz, 36H, C(CH3)3); 31P{1H} NMR (C6D6, 162 MHz) δ: 78.3; 13C{1H} NMR (C6D6, 100

MHz) δ: 53.83 (dd, 2JC-P = 27.4 Hz, 3JC-P = 7.0 Hz, 2C, CH2), 34.46 (d, 1JC-P = 21.4 Hz, 4C,

C(CH3)3), 28.91 (d, 2JC-P = 15.3 Hz, C(CH3)3).

Synthesis of 2: A solution of 1,3-diaminopropane (695 mg, 9.38 mmol) and

triethylamine (2.0 g, 19.8 mmol) was stirred in THF (5 mL). To this was added

a solution of PtBu2Cl (1.4 equiv., 2.43 g, 13.4 mmol) in THF (15 mL). After stirring overnight,

the white salt precipitate was allowed to settle and the clear supernatant was filtered through

Celite. THF and unreacted diamine were removed under vacuum to give a colourless liquid (2.36

g, 97%). 1H NMR (C6D6, 400 MHz) δ: 3.00 (m, 4H, CH2), 1.56 (dt, 2JH-P = 6.8 Hz, 3JH-H = 6.8

Hz, 2H, NH), 1.06 (d, 3JH-P = 11.1 Hz, 36H, C(CH3)3); 31P{1H} NMR (C6D6, 162 MHz) δ: 78.8; 13C{1H} NMR (C6D6, 100 MHz) δ: 48.53 (d, 2JC-P = 29.3 Hz, 2C, NCH2CH2), 37.67 (t, 3JC-P =

6.5 Hz, 1C, CCH2C), 34.48 (d, 1JC-P = 21.8 Hz, 4C, C(CH3)3), 28.98 (d, 2JC-P = 15.3 Hz,

C(CH3)3).

Synthesis of 3: n-BuLi (22.66 mmol, 1.7M in THF, 13.3 mL) was added

dropwise to a solution of N,N’-dimethylethylenediamine (1.0 g, 11.34 mmol)

in THF (20 mL). After 3h stirring, a solution of PtBu2Cl (2.40 g, 22.7 mmol) in THF (15mL) was

added slowly to the lithiated diamine and stirred at room temperature for 6h. The mixture was

filtered to remove LiCl and volatiles were removed under vacuum. The resulting white waxy

solid was recrystallized from pentane to give a colourless crystalline solid (3.46 g, 81%) which

was identified as tBu2PN(CH3)CH2CH2N(CH3)PtBu2. 1H NMR (C6D6, 400 MHz) δ: 3.34 (m,

CH2), 2.71 (6H, d, 3JH-P = 6.0 Hz, NCH3), 1.23 (d, 3JH-P = 11.5 Hz, 36H, C(CH3)3); 31P{1H}

NMR (C6D6, 162 MHz) δ: 108.0, 104.8, 104.0; 13C{1H} NMR (C6D6, 100 MHz) δ: 60.7 (d, 2JC-

P = 41.3 Hz), 39.1 (br s), 36.1 (d, 1JC-P = 28.2 Hz, PC(CH3)3), 30.1 (d, 2JC-P = 16.5 Hz,

PC(CH3)3). Anal. Calcd for C20H46N2P2: C, 63.78; H, 12.34; N, 7.44. Found: C, 64.07; H, 12.12;

N, 7.43.

54

Synthesis of 6: A solution of PdI2 (100 mg, 0.278 mmol) in THF (6 mL) was added

dropwise with stirring to a solution of tBu2P(CH2)5PtBu2 (100 mg, 0.277 mmol) in

THF (5 mL). Stirring overnight led to formation of a red solution, which was filtered

through Celite. THF was removed under vacuum to give 98 mg (62 %) of a pale red solid. 1H

NMR (toluene-d8, 400 MHz) δ: 2.66 (m, 1H, HCPd), 2.13 (br m, 4H, P(CH2) 2C), 1.32 (vt, JH-P =

6.6 Hz, 18H, PC(CH3)3), 1.27 (d, 3JH-P = 6.5 Hz, 18H, PC(CH3)3); 31P{1H} NMR (C6D6, 162

MHz) δ: 82.1

Synthesis of 7: A suspension of PdI2 (100 mg, 0.277 mmol) in THF (6 mL) was

added with stirring to a solution of 2 (100mg, 0.276 mmol) in THF (4 mL). The

resulting red-brown solution was stirred overnight and then filtered through Celite.

Volatiles were removed under vacuum and the dark orange solid was recrystallized

from pentane (121 mg, 72% yield). 1H NMR (C6D6, 400 MHz) δ: 3.37 (m, 1H, HCPd), 2.84 (m,

4H, CH2N), 1.46 (br s, 2H, NH), 1.37 (vt, JH-P = 7.0 Hz, PC(CH3)3), 1.31 (vt, JH-P = 6.7 Hz,

PC(CH3)3); 31P{1H} NMR (C6D6, 162 MHz) δ: 130. 13C{1H} NMR (C6D6, 100 MHz) δ: 61.2 (t, 2JC-P = 5.1 Hz, HCPd), 55.3 (t, 2JC-P = 6.6 Hz, NCH2), 40.2 (t, 1JC-P = 5.8 Hz, PC(CH3)3), 37.6 (t, 1JC-P = 11.8 Hz, PC(CH3)3), 29.1 (t, 2JC-P = 3.4 Hz, PC(CH3)3), 28.5 (t, 2JC-P = 3.2 Hz, PC(CH3)3).

Anal. Calcd for C19H43N2P2PdI: C, 38.36; H, 7.30; N, 4.71. Found: C, 37.88; H, 7.17; N, 4.61.

Synthesis of 8: A suspension of NiCl2(dme) (108 mg, 0.554 mmol) in THF (6 mL)

was added with stirring to a solution of 2 (200mg, 0.552 mmol) in THF (4 mL) in

the presence of triethylamine. The resulting green-brown solution was stirred

overnight to give a purple mixture which was then filtered through Celite to give a bright fuschia

solution. Volatiles were removed under vacuum and the purple solid was recrystallized from

pentane. Yield: 197 mg, 78%. 1H NMR (toluene-d8, 400 MHz, 100°C) δ: 3.05 (m, 2H, CCH2C),

2.54 (m, 2H, CH2N), 2.09 (m, 2H, CH2N), 1.44 (d, 3JH-P = 12.4 Hz, 18H, PC(CH3)3), 1.37 (d, 3JH-

P = 14.9 Hz, 18H, PC(CH3)3; 31P{1H} NMR (C6D6, 162 MHz) δ: 106, -29 (d, 2JP-P = 261 Hz). 13C{1H} NMR (toluene-d8, 100 MHz) δ: 41.7 (d, 2JC-P = 16.4 Hz, CH2N), 40.7 (d, 2JC-P = 4.6,

CCH2C), 38.1 (dd, 1JC-P = 10.3 Hz, 3JC-P = 3.5 Hz, C(CH3)3), 34.4 (dd, 1JC-P = 6.1 Hz, 3JC-P = 1.7

55

Hz, C(CH3)3), 29.0 (dd, 2JC-P = 4.4 Hz, 4JC-P = 1.0 Hz), 28.8 (d, 2JC-P = 5.4 Hz), (27.46 (s)). Anal.

Calcd for C19H43ClN2NiP2: C, 50.07; H, 9.53; N, 6.15. Found: C, 49.70; H, 9.87; N, 5.75.

Synthesis of 9: To a purple solution of 8 (20 mg, 0.044 mmol) in toluene (2 mL) was

added a colourless solution of B(C6F5)3 (22 mg, 0.044 mmol) in toluene (4 mL).

After 10 min stirring at room temperature, the solution turned yellow and a small

amount of green oil had formed. The yellow solution was decanted off the green oil

and toluene was removed under vacuum to afford a yellow crystalline solid (isolated yield:

48%). Yellow crystals suitable for X-ray diffraction were grown from hexanes. 1H NMR (C6D6,

400 MHz) δ: 3.10 (m), 3.01 (m), 1.45 (d, J = 6.6 Hz), 1.44 (d, J = 6.6 Hz), 1.38 (d, J = 6.2 Hz),

1.37 (d, J = J = 6.2 Hz); 31P{1H} NMR (C6D6, 162 MHz) δ: 119.0. 13C{1H} NMR (C6D6, 100

MHz) 55.0 (vt, JC-P = 10.3 Hz, 2C, CH2NP), 42.3 (vt, JC-P = 11.6 Hz, 1C, HCNi), 39.0 (vt, JC-P =

5.1 Hz, 2C, C(CH3)3), 38.0 (vt, JC-P = 11.5 Hz, 2C, C(CH3)3), 28.4 (vt, JC-P = 2.9 Hz, 6C,

C(CH3)3), 27.9 (vt, JC-P = 2.6 Hz, 6C, C(CH3)3). 11B{1H} NMR (THF-d8, 128 MHz) -4.4 (br), -

25.4 (s).

Synthesis of 10: A solution of 1 (100mg, 0.287 mmol) in THF (4mL) was added

dropwise to a solution of PdI2 (103mg, 0.287mmol) in THF (5mL). The dark

solution was stirred overnight and then filtered through Celite to remove unreacted

metal. Removal of solvent under vacuum and washing the resulting solid with Et20 (3 x 3mL)

afforded a pale yellow crystalline solid. Crystals suitable for x-ray diffraction were obtained

from toluene at -40°C. 1H NMR (C6D6, 400 MHz) δ: 2.71 (m, 2H, CH2), 2.63 (m, 2H, CH2),

1.17 (d, 3JP-H = 13.8 Hz, 18H, PC(CH3)3), 1.05 (d, 3JP-H = 15.8 Hz, 18H, PC(CH3)3); 31P{1H}

NMR (C6D6, 162 MHz) δ: 86.5, 17.7 (d, 2JP-P = 372 Hz). 13C{1H} NMR (C6D6, 100 MHz) δ:

52.3 (t, 2JC-P = 9.1 Hz, NCH2), 47.4 (dd, 2JC-P = , NCH2), 37.8 (dd, 3JC-P = 10.1 Hz, 1JC-P = 5.9

Hz, PC(CH3)3), 37.2 (dd, 3JC-P = 9.8 Hz, 1JC-P = 3.8 Hz, PC(CH3)3), 29.0 (d, 2JC-P = 3.0 Hz,

PC(CH3)3), 28.3 (d, 2JC-P = 6.7 Hz, PC(CH3)3). Anal. Calcd for C18H41N2P2PdI: C, 37.22; H,

7.13; N, 4.82. Found: C, 35.86; H, 7.18; N, 5.17.

56

Synthesis of 11: On addition of orange solution of (dme)NiCl2 (60 mg, 0.287

mmol) in THF (6mL) to clear solution of 1 (100 mg, 0.287 mmol) in THF (5mL)

with stirring, the reaction mixture turned dark red. After 15 min, all (dme)NiCl2 had

dissolved and the solution was dark fuschia. The solution was stirred for 4 h and

then filtered through a plug of Celite to remove a forest green precipitate, affording a bright

fuschia solution. The solvent was removed under vacuum to give a purple crystalline solid (65

mg, 54 %). 1H NMR (C6D6, 400 MHz) δ: 2.56 (br m, 2H, NCH2), 2.18 (br m, 2H, NCH2), 1.45

(d, 3JP-H = 13.2 Hz, 18H, PC(CH3)3), 1.28 (d, 3JP-H = 14.8 Hz, 18H, PC(CH3)3); 31P{1H} NMR

(C6D6, 162 MHz) δ: 98, -19 (d, 2JP-P = 262 Hz) 13C{1H} NMR (C6D6, 100 MHz) δ: 48.7 (dd, 3JC-

P = 9.2 Hz, 2JP-C = 4.4 Hz, NCH2CH2), 43.7 (dd, 3JC-P = 16.4 Hz, 2JC-P = 5.1 Hz, NCH2CH2), 38.0

(dd, 1JC-P = 13.5 Hz, 3JC-P = 3.8 Hz, PC(CH3)3), 35.2 (dd, 1JC-P = 5.7 Hz, 3JC-P = 3.7 Hz,

PC(CH3)3), 29.4 (dd, 2JC-P = 5.5 Hz, 4JC-P = 1.5 Hz, C(CH3)3), 28.7 (dd, 2JC-P = 5.8 Hz, 4JC-P = 0.8

Hz, C(CH3)3). Anal. Calcd for C18H41ClN2NiP2: C, 49.21; H, 9.38; N, 6.49. Found: C, 49.21; H,

9.89; N, 6.49.

Synthesis of 12: To a solution of 2 (100mg, 0.287 mmol) in toluene (4mL) was

added a purple solution of Pd2(dba)3 (131 mg, 0.143 mmol) in toluene (6mL).

The solution was stirred overnight to give a red-orange solution, which was

filtered through Celite and then put under vacuum to remove toluene. Recrystallization from

toluene at -40°C afforded large orange crystals suitable for X-ray diffraction. 1H NMR (toluene-

d8, 400 MHz) Ligand peaks are buried in br signal for dba ca. 1.2 ppm. 31P{1H} NMR (C6D6,

MHz) δ: 107.9.

Synthesis of 13: A solution of PdI2 (285 mg, 0.792 mmol) in THF (6 mL) was

added dropwise with stirring to a solution of 3 (300 mg, 0.796 mmol) in THF

(6mL) in the presence of Et3N. The dark solution was stirred overnight and then

filtered through Celite. Volatiles were removed under vacuum and the resulting

golden-brown solid was extracted into toluene and filtered again through Celite. Removal of the

toluene under vacuum gave a yellow solid (417 mg, 86% yield). 1H NMR (C6D6, 400 MHz) δ:

57

2.98 (m, 1H, NC(H)H), 2.84 (m, H, HCPd), 2.74 (m, 1H, NC(H)H), 2.52 (d, 3JH-P = 3.8 Hz, 3H,

NCH3), 2.30 (d, 3JH-P = 8.9 Hz, 3H, NCH3), 1.49 (d, 3JH-P = 13.8 Hz, 9H, PC(CH3)3), 1.35 (d, 3JH-

P = 8.8 Hz, 9H, PC(CH3)3), 1.33 (d, 3JH-P = 14.0 Hz, 9H, PC(CH3)3), 1.31 (d, 3JH-P = 8.1 Hz, 9H,

PC(CH3)3). 31P{1H} NMR (C6D6, 162 MHz) δ: 132.6, 15.2 (d, 2JP-P = 388 Hz). 13C{1H} NMR

(C6D6, 100 MHz) δ: 67.4 (dd, 2JC-P = 18.8 Hz, 2JC-P = 8.6 Hz, HCPd), 46.4 (dd, 2JC-P = 9.5 Hz, 3JC-P = 6.6 Hz, H2C), 39.6 (d, 2JC-P = 3.6 Hz, CH2N(CH3)P), 38.7 (d, 2JC-P = 7.1 Hz,

CHN(CH3)P), 30.5 (dd, 2JC-P = 7.4, JC-P = 1.4 Hz, PC(CH3)3), 30.3 (dd, 2JC-P = 5.9 Hz, JC-P = 1.4

Hz, PC(CH3)3), 29.8 (dd, 2JC-P = 5.8 Hz, JC-P = 1.3 Hz, PC(CH3)3), 28.7 (d, 2JC-P = 5.1 Hz,

PC(CH3)3). Anal. Calcd for C20H45N2P2PdI: C, 39.45; H, 7.46; N, 4.60. Found: C, 38.98; H,

7.09; N, 4.47.

Synthesis of 14: 3 (80mg, 0.212mmol) was dissolved in THF (4 mL) to which a

solution of PdBr2 (57 mg, 0.214 mmol) in THF (5 mL) was added dropwise with

stirring. The reaction mixture was stirred overnight and then filtered through Celite

to give a yellow solution. The solvent was removed under vacuum to give a light

yellow solid (26 mg, 22 %). 1H NMR (C6D6, 400 MHz) δ: 2.98 (td, 3JH-P = 9.6 Hz, 3JH-P = 2.2

Hz, 1H, PdCH), 2.76 (m, 1H, CHH), 2.68 (m, 1H, CHH), 2.51 (d, 3JH-P = 4.06 Hz, 3H, NCH3),

2.30 (d, 3JH-P = 8.8 Hz, 3H, NCH3), 1.49 (d, 3JH-P = 14.0 Hz, 9H, PC(CH3)3), 1.35 (d, 3JH-P =

14.45 Hz, 9H, PC(CH3)3), 1.32 (d, 3JH-P = 14.0 Hz, 9H, PC(CH3)3), 1.32 (d, 3JH-P = 14.0 Hz, 9H,

PC(CH3)3); 31P{1H} NMR (C6D6, 162 MHz) δ: 129.3, 19.0 (d, 2JP-P = 396.6 Hz).

Synthesis of 15: 3 (100 mg, 0.265 mmol) was dissolved in THF (4 mL) to which a

solution of PdCl2(CH3CN)2 (95 mg, 0.265 mmol) in THF (5 mL) was added

dropwise with stirring. The reaction mixture was stirred overnight and then filtered

through Celite to give a yellow solution and an insoluble yellow precipitate. The

precipitate was removed by filtration and washed with THF (3 x 3mL). The filtrate and washes

were combined and solvent was removed under vacuum, leaving a pale yellow crystalline solid.

Recrystallization from dichloromethane and hexanes (1:1) afforded light yellow crystals suitable

for X-ray diffraction (29 mg, 21%).

58

Alternative preparation: A solution of PdCl2(MeCN)2 (0.5 equiv.) and Pd2(dba)3 (0.25 equiv) in

THF (8 mL) was added dropwise to a clear solution of 3 in THF (5 mL). The mixture was stirred

overnight to produce a dark yellow-green mixture. Removal of an olive green precipitate via

filtration left a golden solution which upon removal of volatiles afforded 15 as the exclusive

product. 1H NMR (CD2Cl2, 300 MHz) δ: 3.05 (m, 1H, PdCH), 2.97 (m, 1H, PdC(H)CHH), 2.77

(m, 1H, PdC(H)CHH), 2.82 (d, 3JH-P = 3.9 Hz, 3H, NCH3), 2.62 (d, 3JH-P = 9.0 Hz, 3H, NCH3),

1.39 (d, 3JH-P = 13.8 Hz, 9H, PC(CH3)3), 1.33 (d, 3JH-P = 14.1 Hz, 9H, PC(CH3)3), 1.32 (d, 3JH-P =

14.7 Hz, 9H, PC(CH3)3), 1.28 (d, 3JH-P = 14.1 Hz, 9H, PC(CH3)3); 31P{1H} NMR (C6D6, 162

MHz) δ:128.9, 22.9 (d, 2JP-P = 403.1 Hz). 13C{1H} NMR (C6D6, 100 MHz) δ: 67.3 (dd, 2JC-P =

19.3 Hz, 2JC-P = 8.8 Hz, HCPd), 43.0 (dd, 2JC-P = 10.7 Hz, 3JC-P = 5.6 Hz, H2C), 39.9 (d, 2JC-P =

6.8 Hz, CH2N(CH3)P), 38.6 (d, 2JC-P = 4.4 Hz, CHN(CH3)P), 29.7 (dd, 2JC-P = 7.7, JC-P = 1.2 Hz,

PC(CH3)3), 29.4 (dd, 2JC-P = 6.4 Hz, JC-P = 1.2 Hz, PC(CH3)3), 29.3 (dd, 2JC-P = 5.9 Hz, JC-P = 1.1

Hz, PC(CH3)3), 28.7 (d, 2JC-P = 5.9 Hz, PC(CH3)3).

Synthesis of 16: A solution of 3 (100 mg, 0.265 mmol) in THF (4 mL)

was added to a suspension of (dme)NiCl2 in THF (6mL) and stirred at rt

for 4h to give a green solution. Filtration of the mixture through Celite

and evaporation of the solvent under vacuum afforded a blue-green

crystalline solid. The paramagnetic species was determined to be 16 by X-ray crystallography. 31P{1H} NMR (CD2Cl2, 121 MHz) δ: 55.3, 51.1. Anal. Calcd for C20H47N2P2NiCl3: C, 44.26; H,

8.75; N, 5.16. Found: C, 44.70; H, 8.35; N, 5.56.

Synthesis of 17: Addition of B(C6F5)3 to a suspension of 16 in C6D6 immediately

produced a green oil and yellow solution. Decanting the solution to separate the oil

gave a bright yellow solution which was revealed by NMR to contain exclusively

the PCP pincer complex. 1H NMR (C6D6, 400 MHz) δ: 2.49 (d, 3JH-P = 3.9 Hz, 3H,

NCH3), 2.33 (m, 1H, HCNi), 2.31 (m, 2H, H2CC), 2.23 (d, 3JH-P = 8.6 Hz, 3H, NCH3), 1.56 (d, 3JH-P = 12.9 Hz, 9H, PC(CH3)3), 1.46 (d, 3JH-P = 13.7 Hz, 9H, PC(CH3)3), 1.44 (d, 3JH-P = 12.9

Hz, 9H, PC(CH3)3), 1.41 (d, 3JH-P = 13.3 Hz, 9H, PC(CH3)3); 31P{1H} NMR (C6D6, 162 MHz) δ:

59

120.7, 31.9 (d, 2JP-P = 289 Hz). 13C{1H} NMR (C6D6, 100 MHz) δ: δ: 58.6 (dd, 2JC-P = 19.2 Hz, 2JC-P = 8.8 Hz, HCNi), 41.1 (dd, 2JC-P = 10.0 Hz, 3JC-P = 5.2 Hz, H2C), 39.1 (d, 2JC-P = 7.0 Hz,

CH2N(CH3)P), 38.2 (d, 2JC-P = 4.4 Hz, CHN(CH3)P), 28.8 (dd, 2JC-P = 7.5, JC-P = 1.3 Hz,

PC(CH3)3), 28.7 (dd, 2JC-P = 6.4 Hz, JC-P = 1.2 Hz, PC(CH3)3), 28.5 (dd, 2JC-P = 5.5 Hz, JC-P = 1.2

Hz, PC(CH3)3), 28.4 (d, 2JC-P = 5.8 Hz, PC(CH3)3). 11B (128 MHz): -7.2. 19F (377 MHz): -131.66

(d, 3JF-F = 17.2, 6F, o-F), -163.41 (t, 3JF-F = 20.2 Hz, 3F, p-F), -167.71 (t, 3JF-F = 18.1 Hz, 6F, m-

F).

General Procedure for Hydrogenation Attempts: 1-hexene (20 mg, 0.238 mmol) was

dissolved in toluene-d8 in a J. Young tube in the presence of 2 mol% of catalyst and

hexamethylbenzene (2 mg) as an internal standard. Tubes were pressurized under 4 atm. H2,

shaken gently, and left to stand at room temperature or heated to 80°C in an oil bath. Reactions

were monitored regularly by 1H and 31P NMR.

60

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