Spectroscopic and Reactivity Studies on

Graphite-Conjugated Salen Complexes

by

Jeffrey N. Rosenberg

B.S. in ChemistryCalifornia Institute of Technology

Submitted to the Department of Chemistryin partial fulfillment of the requirements for the Degree of

MASTER OF SCIENCE IN CHEMISTRY

AT THE

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

February 2020

© 2020 Massachusetts Institute of Technology. All rights reserved.

Signature of Author: Signatureredacted

Certified by:

Accepted by:

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OF TECSLOY

MAR 0 3 020

LIBRARIES

Department of ChemistryJanuary 17 th, 2020

Yogesh SurendranathPaul M Cook Career Development Assistant Professor

Signature redacted

C-b

Robert W. FieldHaslam and Dewey Professor of Chemistry

Chair, Departmental Committee on Graduate Students

1

Spectroscopic and Reactivity Studies onGraphite-Conjugated Salen Complexes

by

Jeffrey N. Rosenberg

Submitted to the Department of Chemistryon January 17, 2020 in partial fulfillment of the requirements for

the Degree of Master of Science in Chemistry

ABSTRACT

Metal oxo intermediates are evoked and observed in a wide range of natural and man-madecatalytic system. These varied systems demonstrate divergent reactivity modes dependent onperturbations to the electronic structure and reaction conditions. It is necessary to uncoverelectronic structure characterization of reactive metal oxo intermediates to provide an improvedmeans of understanding how perturbations to the electronic structure will direct towards thesecompeting pathways.

Herein, we have developed a platform for the study of site isolated metal oxo intermediates with aunique electronic structure derived from a conjugated linkage between graphitic carbon edge sitesand a chelated metal center. This linkage results in a high degree of electronic coupling betweenthe isolated metal site and the band structure of the graphitic carbon material, referred to asgraphite-conjugated catalysts (GCCs). Specifically, we have synthesized a pyrazine-linkedconjugated salen-type ligand platform which has been metallated with Mn2+ and used tofunctionalize graphitic carbon electrodes and powder. XPS surface characterization on the N andMn chemical environments and relative abundance indicate the desired surface condensation toform a pyrazinic linkage has proceeded as desired. Electrochemical characterization offunctionalized electrodes by cyclic voltammogram show three distinct features. Two reversibleredox features consistent with proton-coupled electron transfers at the pyrizinic linkages and athird feature, putatively assigned to the metal site.

Future work on this project will involve 1) the expansion of the scope of metallation to additionalfirst row transition metals, 2) thermal reactivity studies, 3) in-situ XAS studies, and 4)electrochemical oxygen-reduction reactivity studies.

Thesis Supervisor: Yogesh SurendranathTitle: Paul M Cook Career Development Assistant Professor

2

Table of Contents:

Abstract 2

1 Introduction 4

1.1 Metal Oxo Reactivity 4

1.2 Redox Non-innocence and GCCs 6

1.3 Qualitative Predictions ofElectronic Structure 7

1.4 Project Aims 10

2 Synthesis, Functionalization, and Characterization 10

2.1 Ligand Synthesis 10

2.2 Metallation Studies 11

2.3 Functionalization ofElectrodes 13

2.4 XPS Characterization 14

2.5 Electrochemical Characterization 15

2.6 Functionalization ofPowder Samples 16

3 Future Directions 18

3.1 Expand Thermal Reactivity Studies 18

3.2 In situ XAS Studies 18

3.3 ORR Reactivity Studies 19

4 Experimental Section 20

5 Acknowledgements 30

6 References 31

3

1 Introduction

1.1 Metal Oxo Reactivity

Metal oxo intermediates are central to a diverse range of catalytic reactions, ranging from

biological C-H activation in CyP4501 2, to epoxidations at Mn-Salens 3-5 , to electrochemical

oxygen evolution in metal oxide materials. 6 In these examples and others, oxo intermediates are

active for hydroxylation, halogenation, sulfoxidation, epoxidation, as well as additional oxidation

reactions. 7 The stability and reactivity of metal oxo species is extremely sensitive to local

electronic structure, leading to divergent reactivity modes. For example, the electronic structure of

the oxo determines whether it is nucleophilic vs. electrophilic, prone to radical H-abstraction or

oxygen atom transfer (OAT). Clearly, control over metal oxo electronic structure is critical to

designing next generation catalysts for selective oxidation reactions.

Metal oxo systems have been characterized by a wide range of methods, notably including

X-ray crystallography, X-ray absorption spectroscopy (XAS), and thermodynamic reactivity

studies. Crystallography and XAS have allowed for characterization of the coordination

environment, bond metrics, and electronic structure of stable metal oxo intermediates. 7-9 This has

been especially useful in the assignment of oxidation state of the metal center with implications

for the redox activity of the ligand environment. Additionally, bond metrics can provide insights

into the approximate bond order of the metal oxo and hence the electronic structure and predicted

reactivity of the metal oxo moiety. Finally, the strength of the pre-edge feature in XAS has been

used to indicate the degree of orbital hybridization and the related bond order of metal oxo

intermediates.

4

O O1I e- (E") %j iMX M-

H+ (pKa) H• H+ (pKa)

+ H

Figure 1 - Square scheme for PCET on a generic molecular metal oxo intermediate, brokendown into proton transfer and electron transfer.

Thermodynamic reactivity studies of metal oxo intermediates have been used to

characterize proton-coupled electron-transfer (PCET) reactions and OAT reactions.8 9 The

thermodynamics of PCET reactivity, often applied for H-atom abstraction by metal oxo

intermediates to form a metal hydroxo intermediate, can be broken down into a square-scheme of

proton transfer and electron transfer (Figure 1). Proton transfer is characterized thermodynamically

by pKa while electron transfer is characterized thermodynamically by E, allowing for

determination of the bond dissociate free energy (BDFE) of the O-H bond formed in PCET

reactions with metal oxo intermediates. This understanding has been applied to molecular metal

oxo intermediates through reactivity testing to place a set of bounding thermodynamic values

characterizing the reactive intermediate and allowing for comparison with other known metal oxo

intermediates.8

5

I

1.2 Redox Non-innocence and GCCs

Our group has developed a new class of catalysts that incorporate molecularly well-

defined, highly-tunable active sites into heterogeneous graphite surfaces."" These graphite-

conjugated catalysts (GCCs) feature a unique conjugated linkage between a discrete molecular

active site and the delocalized states of graphitic carbons. These materials are prepared by

exploiting the site-selective irreversible condensation with substituted phenylenediamines to form

robust, aromatic pyrazine linkages. By synthetically altering the diamine precursor, the active sites

on the surface of the GCC can be tuned systematically, affording a molecular-level precision that

heterogeneous catalysts usually lack.' 0

Electrochemical and spectroscopic studies have established that conjugation of redox

active moieties in GCCs leads to strong electronic coupling." This strong electronic coupling

results in several noteworthy effects: the redox state of the conjugated site is pinned relative to the

Fermi level of graphite, electron transfer is only observed if the interfacial reaction is ion coupled,

and when electron transfer is observed there is no concomitant change in the redox state of the

metal center. Combined, these observations indicate that GCCs behave as metallic surface sites

rather than isolated molecules, bridging the traditionally disparate worlds of molecular and

heterogeneous catalysis. This can alternatively be understood as an isolated metal site interacting

with a massive redox non-innocent ligand in the extreme.

Ligand redox non-innocence is known to play a critical role in metal oxo reactivity.

However, in almost all cases, the ligand environment can only host one additional redox

equivalent.'" We envisioned that new reactivity patterns could be uncovered if the redox non-

innocent capacity of the ligand is expanded and appropriately tuned. Here I aimed to do this by

investigating the oxo reactivity of metal oxo units conjugated to the metallic band structure of

6

graphitic carbon materials. In addition to metal oxo intermediates, this study could be extended to

the study of the isoelectronic metal imido intermediates as well, as they will be amenable to similar

spectroscopic and reactivity studies. 16'?

Our work on graphic conjugated systems has also been applied to PCET reactivity. 4 This

has led to a better understanding of the thermochemistry of reactions at the isolated sites on graphic

conjugated molecules, acting as a model site for the study of interfacial PCET more generally. Our

recent work on graphite conjugated acids has shown that we can apply the approach of square

schemes for breaking down the separate thermodynamic components of PCET reactions. This

showed that the thermochemistry of interfacial PCET reactions on a graphic conjugated system

can be predicted based on the pKa of the molecular analogue and the potential of zero free charge

of the electrode. We propose the application of this method for studying the thermochemistry of

PCET reactions on various graphite conjugated metal oxo intermediates. This understanding will

allow for a more nuanced comparison with the molecular analogues and thus the impact of

conjugation to a band structure on the electronic structure and reactivity of metal oxo

intermediates.

1.3 Qualitative Predictions ofElectronic Structure

In addition to studying the impact of electronic coupling to a band structure on the stability

and reactivity of metal oxo intermediates, this system will potentially provide a more nuanced

understanding of the impact of alterations at a metal site on local electronic structure in these

systems. While we have engaged extensively in electrochemical studies of these system, we aimed

in this work to engage with new avenues of thermal catalysis. Our group has begun additional

investigations on thermal catalysis for our porphyrin based GCC systems with similar oxidative

7

reactivity in mind. Rather than shifting the metal based orbitals in tandem with the band states

through application of an external potential, these studies alter the local electronic structure

through a change in the local coordination environment around the isolated metal sites. This

perturbation to the local electronic environment should induce an overall electronic rearrangement

in the system. Specifically, by doing this with oxo ligands, we will be able to employ the wide

range of spectroscopic handles and reactivity studies employed in molecular metal oxo literature

to develop a detailed picture of the local electronic environment of graphite conjugated metal oxo

intermediates.

tBu- I

tBu

N N N O N O N[Graphite] | [ [M]

N N N O N 0 NtBu

U N,

t' 2 2'

tBu AO A

Figure 2 - [M]-salophen-GCC (1) active site on graphite and two potential intermediates

accessible through OAT; a metal oxo (2) and an OAT adduct (2').

We proposed a model in which the relative energy level of the Fermi level of the material

compared to that of the metal-oxygen bonding and anti-bonding orbitals would determine the

stability and reactivity of a GCC metal oxo (Figure 3). We would predict based on this that if the

bonding orbitals are below the Fermi level, becoming fully occupied, and the anti-bonding orbitals

are above, becoming fully unoccupied, that we would have a stable metal-oxygen triple bond,

similar to the vanadyl oxo molecular system (Figure 3a). On the other extreme, if all of the anti-

bonding orbitals sit below the Fermi level, we would expect them to be occupied, thus failing to

8

form a stable observable system (Figure 3c). Most of the first row transition metal series commonly

studied in metal oxo system would likely fall in this intermediate range, making predictions of the

stability and reactivity of such GCC metal oxo intermediates difficult to predict a priori and

resulting in a range of predicted bond order and reactivity (Figure 3b).

dx2-y2

dz2

d d

dxz dyz dx2-y2

ddz2

E____

F z1 4 r__dx2-y2Sxy

I I ~~dzu1

I xz yz

o 0 0

M M

(a) (b) (c)

Figure 3 - Qualitative d-orbital splitting diagram for a metal oxo intermediate with orbital

occupancy determined by relative energy level compared to Fermi level, EF, of solid.

Qualitatively, we predict that the pattern of stability and reactivity for this intermediate

range of d-orbital occupancy should reflect that of the molecular counterparts. This would mean

that the increased occupancy of the anti-bonding orbitals will result in a more reactive oxidant and

thus more difficult to capture and characterize spectroscopically. However, the site isolated nature

of the system could allow us to observe reactive species which would otherwise be impossible to

9

observe in a comparable molecular system. We would predict that spectroscopic characterization

by XAS should yield information on the coordination environment, bond-metrics, and electronic

structure for these intermediates.

1.4 Project Aims

This project aimed to investigate the electronic structure-reactivity relationship of metal

oxos conjugated to graphite. Specifically, we sought to advance the following aims: (1) Synthesize

graphite conjugated salophen complexes; (2) Prepare and characterize conjugated metal oxo and

imido intermediates; (3) Investigate OAT/nitrene transfer (NT) and PCET reactivity on conjugated

metal oxo and imido intermediates; (4) Apply salophen-GCCs to electrochemical oxygen

reduction reaction (ORR).

2 Synthesis, Functionalization, and Characterization

2.1 Ligand Synthesis

To enable these studies, I synthesized a conjugated salophen ligand architecture 1 (Figure

2). This features 2,9-bisphenoxide substitution on 1,10-phenanthroline which is conjugated to the

surface by a phenazine linkage, referred to here as salophen-GCC. The diamine precursor to

salophen-GCC is synthesized in 8 steps. As an overview, oxidation of 1,10-phenanthroline to 5,6-

dione-1,10-phenanthroline is followed by reduction and alkylation to install protecting ether

chains. Subsequent stepwise nucleophilic arylation with 2,4-di-t-butyl-6-magnesium-bromide-

anisole and oxidative rearomatization furnished the alky protected ligand. Ether protecting groups

are cleaved with BBr3 to furnish the final salen coordination environment and 5,6-diol. After

oxidation of 5,6-diol functionality to 5,6-dione, salophen-diamine is generated by condensing

10

1,2,4,5-benzene tetraamine to form a phenazine linkage and the desired o-diamine group to be

condensed on to graphite. Despite the large number of steps, the ligand can be prepared in 28%

overall yield and at gram scale. The detailed sequence can be found below in section 4.

tBU 'Bu

NaS204

N TBABTBAB CP tBu CP tBuO N BCH C1 H210 N T CH210 N OMe THE C1 0H210 -N OMeBrlH1r.t., 24h 5000, 48h

o - NH,/TH N 2. MnO 2 2. MnO 2 CH2 0 -OH20 HF C1 0H21 0 210CM C2MH 21 20 N DCM 10 N OMe

'Bu

'Bu

BBr3DCE

tBu tBu 0°C -> 700C tBu

B H2N NH2 tBu N tBu'Bu 0 4HCI t Ag20O

H2N N N OH H2N NH2 0 -N OH DCM HO -N OH

H2N N N OH K2CO 3 0 /N OH HO / N OHN EtOH/H 20, Ar I | t~u

Bu 110°C, 2h tBU Bu

tBu tBu tBu

Scheme 1 - Ligand synthesis of Salophen-diamine.

This ligand synthesis procedure can be altered in the final steps to produce a molecular

analogue useful for comparative studies with the target GCC species. This can be done by taking

the penultimate salophen-dione product and condensing alternative aryl diamine species.

Additional substituents on the aryl condensation could provide solvating groups, either bulky

organic groups to increase solubility in organic solvents or polar groups to increase solubility in

aqueous or other polar solvents used in electrochemical comparisons.

2.2 Metallation Studies

With this versatile ligand platform in hand, I set out a general metallation strategy

combining the pro-ligand withMCl2 (M=Cr, Mn, Fe, Co, and Ni) and triethylamine. This strategy

11

has successfully produced Cr- and Mn-salophen-diamine species, reproducibly. The paramagnetic

Cr and Mn species were evaluated by MALDI-TOF mass spectrometry, providing a clear mass hit

for each with a clear match for the isotopic envelop for each species.

A number of additional metallation strategies were attempted with inconsistent results. One

strategy involved the deprotonation of the phenolic protons by various bases. Attempts with

heterogenous deprotonation, such as NaH, resulted in deprotonation of the amines in addition to

the phenolic protons. Deprotonation with homogenous bases resulted either in decomposition or

incomplete reactions. Further, these incomplete deprotonations failed to produce the clear

metallation observed with the combination of excess triethylamine as an extrinsic base andMCl 2

salts. Additional attempts at metallations with other first row transition metal salts incorporating

intrinsic or extrinsic bases failed to reproduce clean reactions and were not pursued further.

Ongoing investigation of the metallation procedure with additional first row transition metals is

needed to expand upon this work.

The majority of the remaining work has focused on carrying forward Mn-salophen

chemistry. Mn offers comparisons with an extremely rich literature precedence surrounding

molecular Mn oxo chemistry, including Jacobsen's catalyst and Mn porphyrinoid complexes. 3,4

Additionally, Mn oxo offers a desirable balance of stability and reactivity compared to that of other

first row transition metal oxos.7 To ensure high purity, Mn-salophen-diamine (3) was additionally

recrystallized from THF by layering diethyl ether. 3 was also characterized by Evans method

resulting inpeff= 5.98, in close agreement with ps= 5.92 expected for an S = 5/2 Mn(II) high spin

system. MALDI-TOF shows a single mass envelope with a splitting pattern consistent with 3.

12

2.3 Functionalization ofElectrodes

With salophen-diamine pro-ligand and 3 in hand, I aimed to functionalize glassy carbon

electrodes, analogously to our previous work on GCCs, by double imine condensation of the o-

diamine group onto native o-quinones on the pre-anodized glassy carbon button electrodes. This

was done by dissolving 5 mg of salophen-diamine in 2 mL dry degassed DMF then heating

electrodes in the solution to 110 °C for 18 hours. The electrodes were then washed with water and

ethanol to remove physisorbed material and mono imine linkages. Functionalization with 3,

resulting in Mn-salophen-GCC (4) was done with the addition of NaOAc. Without NaOAc,

functionalization resulted in over incorporation of free Mn2+ by X-ray photoelectron spectroscopy

(XPS) integration by a factor of 15.

There are a few hypotheses which could explain how the addition of NaOAc prevents over

incorporation of Mn into the carbon. Over incorporation of Mn2+ would imply either an additional

source of Mn2+ as an impurity in 3 or lability of Mn2+ in the salen coordination environment of 3.

Regardless of the source of free Mn2+ on the surface, the addition of Na* may block O-containing

sites on the electrodes, preventing a slight impurity of MnCl2present from binding to these sites.

Alternatively, the addition of OAc- may coordinate Mn2+ and crash it out of solution, preventing

it from binding to sites on the carbon. This will require further study to confirm and will be of

great value to understanding for additional first row lability issues on similar GCC systems in the

future.

13

2.4 XPS Characterization

The electrodes are then characterized by XPS, indicating the loading and chemical

environment of N and Mn on the surface, as well as any other element of interest (Figure 4a). This

has revealed N content on the surface of-4.5 atom% for and -0.75 atom% Mn, consistent with a

6:1 N:Mn ratio expected for 4. Additionally, peak fitting on high resolution XPS of Nis reveals a

chemical environment consistent with four pyridinic and two pyrazinic N (Figure 4b).18 High

resolution XPS on Mn2pof 4 also indicates a single environment, again consistent with that of 3

(Figure 4c). Based on comparison to literature for various Mn oxidation states, both 3 and 4 are

consistent with Mn(II) based on the splitting of Mn2ppeak, peak shape, and presence of the Mn2p

shakeup peak between the Mn2pi/2and Mn2p3/2peaks. 9

14

a) b)

Survey

=i

0500binding energy I eV

4-

Mn2p

I iI Ii

667 657 647 637binding energy IeV

b)

407 39402 397binding energy I eV

Figure 4 - XPS of 4, including survey (a),high resolution Nis (b), and high resolution

Mn2p(c). Raw data in grey point, baseline andcombined in solid lines, and peak fitting ofthe Nis and Mn2pin respective dashed lines.

2.5 Electrochemical Characterization

Electrochemical characterization of 4 by cyclic voltammetry (Figure 5a) in 0.1 M KOH

aqueous solution reveals three distinct reversible redox couples. The features occur at -40, 100,

and 720 mV vs. RHE reference electrode. These waves integrate to an equivalent amount of

change. This is consistent with the features at -40 and 100 mV assigned as PCET redox events

15

1000

c)

a)

M

Njs

F0P, a

Ql)

2

10

previously observed on phenazine linked GCCs (Figure 5b). After treatment of the electrode with

0.1 M HC104aqueous solution provides a CV in which the pyrazine assigned redox couples are

persistent while the couple at 720 mV disappears. XPS confirms the loss of Mn from acid

treatment. This indicates that the couple at 720 mV is a metal-based wave. This could either be a

Mn-aquo/Mn-oxo couple or a Mn-bisaquo/Mn-bishydroxo couple (Figure 5c). Further

investigation of the pH response of these redox couples may provide greater insight into the nature

of this unassigned couple.

a) 300

200 -

100 - b) N +2e-,+2H' N

E 0 N NH

-100 OH 2 -2e,-2H* 9Mn- 'Mn

-200 c)300 OH2 -2e-,-2H. OH

Mn ' Mn-400 OH2 OH

-0.4 0.1 0.6 1.1EI V vs RHE

Figure 5 - Cyclic voltammogram of 4 (blue) and 4 post exposure to 0.1M H2SO4for 1 hour(orange), data recorded in 0.1M KOH aqueous electrolyte at 10 mV/s (a). Proposed redox events

corresponding to couples at -40 and 100 mV (b) and 720 mV (c).

2.6 Functionalization ofPowder Samples

In addition to glassy carbon electrodes, I have functionalized a high surface area graphitic

carbon powder, Monarch, with 4. The difference in procedure from GC electrodes involved the

thorough degassing of Monarch in DMF through three freeze-pump-thaw cycles and higher

concentration, ranging from 0.1-0.2 mM in 3, to account for the high surface area of Monarch. The

16

functionalized Monarch is then washed via Soxhlet extraction with ethanol followed by toluene

for one day each. 4 on Monarch shows weaker signal than seen on the button functionalization,

but with identical Nis and Mn2ppeak positions. The loading decreases to -1.2 atom% N and ~0.2

atom% Mn, consistent with previous results seen for other GCC work in our lab.13 This support

was used in thermal catalysis studies, including steady-state gas flow catalysis.

A

D GC

Figure 6 - Diagram of thermal catalysis plugged flow setup, including heating mantle (A), glass

wool plugs (B), catalyst plug (C), gas flow (D), and outflow to GC.

For thermal catalytic studies, we set up a plugged flow experimental setup in which the

catalyst was packed to create a bed to flow the gaseous reagents through, collecting the outflow to

monitor in situ through an in-line GC. Specifically, this was attempted with N20 and ethylene

controlled by mass flow controllers and heated by an external heating mantle. The desired

reactivity sequence was OAT from N20 to the Mn site, forming a metal oxo, or otherwise

formation of a reactive N20 adduct. Next, the ethylene would accept the O-atom in an epoxidation

reaction, forming ethylene oxide. Flowing a combination of 0.5% N20 and 4% ethylene in argon

carrier gas and heating to 250°C for one hour resulted in detection of ethylene oxide by GC.

However, similar reactivity was observed with the control carbon as well, making this study

inconclusive. Further thermal catalytic studies on plugged flow catalyst beds should serve as an

effective means of exploring this reactivity further. A similar setup can also be applied to in-situ

XAS measurements of a packed capillary under exposure to different gas sources. This may allow

us to characterize the ensemble of reactive intermediated produced in such systems.

17

3 Future Directions

3.1 Expand Thermal Reactivity Studies

The proposed family of metallo-salophen-GCCs will provide the basis for detailed

investigation of graphite-conjugated metal oxo and imido structure (aim 2). We can attempt to

form GCC metal oxos by treating 2 with various OAT transfer reagents, including N20, peroxides,

N-oxides, and iodosyl arenes. Imido species could be prepared from treatment of 2 with aryl azides.

These species will be characterized in house by air-free XPS measurements. High resolution Mn2p

XPS could potentially indicate any oxidation state change occurring on the Mn center. 19

Additionally, high resolution Ols and Nls XPS could indicate the presence of metal oxo and

imido, respectively.'8 Aryl imido species offer the possibility of incorporation of further

spectroscopic handles, such as fluorine tags on the arene.

3.2 In situ XAS Studies

Ex situ XAS studies on 4 could allow for assignment of oxidation state through X-ray

Absorption Near-Edge Structure (XANES) and coordination environment through Extended X-

ray Absorption Fine Structure (EXAFS). Most importantly, these measurements, combined with

comparisons to standards, could provide the necessary confirmation of the uniformity of Mn sites

on the surface, allowing for in situ experiments (vide infra) and structure-function relationships to

be drawn based on the molecularly well-defined active site.

XAS studies on metal oxo intermediates could provide M-O bond metrics, coordination

environment, and metal oxidation state. Molecular terminal metal oxo intermediates are known to

undergo bimolecular decomposition pathways as well as formation of t-oxo dimers, a common

thermodynamic sink for metal oxo intermediates. The site isolated nature of GCCs on a solid

18

support could allow for the generation of and characterization of highly active metal oxos. In situ

studies could allow for a direct comparison of the resting state, metal oxo and imido, and, upon

exposure to acceptor substrates, the complete catalytic cycle. While late transition metals may

prove recalcitrant to oxo or imido formation, XAS could provide a direct probe whether an adduct

forms with an OAT/NT reagent and whether elevated temperatures can be used to drive further

reactivity. As the nature of the metal oxo and imido species are revealed, this could allow for more

in depth study of various reactivity modes, including OAT/NT and C-H bond activation. We could

begin by studying thermal heterogeneous catalysis in batch reactions with various phosphines,

thioether, and olefins as acceptor molecules for OAT/NT.

3.3 ORR Reactivity Studies

Our understanding of metal oxo stability and reactivity could also be applied to

electrochemical ORR on salophen-GCCs. Recent studies from the Machan group have

demonstrated that Mn-salen species are competent for ORR catalysis." Selectivity of ORR is split

between the two-electron two-proton pathway to hydrogen peroxide vs. the four-electron four-

proton pathway to water. The four-electron pathway passes through a metal oxo intermediate such

that the selectivity of ORR is highly dependent on the stability of a metal oxo intermediate. We

could run standard rotating ring disc electrode experiments to quantify the faradaic efficiency and

selectivity for each ORR pathway across the series of salophen-GCCs. To gain a more detailed

mechanistic picture of ORR on salophen-GCCs, we could perform in situ XAS studies with

polarization under Ar and02.With a robust comparison to surface bound molecular species, we

could discern the impact of the redox non-innocence of the carbon towards the stabilization of

metal oxo intermediates and selectivity of ORR.

19

4 Experimental Details

1,10-Phenanthroline-5,6-dione (> 98 %) was purchased from TCI. NMR solvents were

obtained from both Cambridge Isotope Laboratories and Sigma-Aldrich. All syntheses were

performed with solvents of ACS grade purity or better and reagents were used as received

without further purification. Argon (ultra high purity) were purchased from Airgas. All aqueous

solutions were prepared with reagent grade water (Millipore Type 1, 18 M-cm resistivity).

Concentrated hydrochloric acid (ACS reagent grade), concentrated nitric acid (68-70%, ACS

grade), and concentrated sulfuric acid (ACS grade) were purchased from EMD Millipore.

All electrochemical measurements were performed at ambient temperature (21 1 'C)

using a Biologic VSP 16-channel potentiostat and a three-electrode electrochemical cell with a

porous glass frit separating the working and auxiliary compartments. Prior to use,

electrochemical cells were soaked in concentrated nitric acid overnight, rinsed with Millipore

water, and dried in an oven at 120 °C for a minimum of 1 h. All measurements on functionalized

glassy carbons were conducted using a glassy carbon RDE tip working electrode (5 mm

diameter, 0.196 cm2 , Pine Research Instrumentation). Prior to each experiment, glassy carbon

(GC) electrodes were successively polished in an aqueous 1, 0.3, 0.05 m alumina slurry against

a Buehler MasterTex polishing pad. Unless otherwise stated, current densities were normalized

to the geometric surface area of the working electrode. In all cases, the auxiliary electrode was a

high surface area platinum mesh (Alfa Aesar, 99.997%). All electrochemical measurements in

non-aqueous electrolyte were performed in an N2-filled Purelab HE 4GB 2500 Glovebox.

Alkylation of 5,6-dione 1,]0-phenanthroline

20

NaS204TBAB

0 I KOHo - N BrC1OH 2 C1 0H210 N

o /~ N H 20/THF C10H210 / N48h

Combined 5,6-dione 1,10-phenanthroline (8.4 g, 40 mmol, 1 eq), tetrabutyl ammonium bromide

(9.0 g, 28 mmol, 0.7 eq), and sodium dithionite (42.0 g, 240 mmol, 6 eq) in a round bottom flask

with 200 mL water and 200 mL tetrahydrofurane. Added bromo decane (33 mL, 160 mmol, 4

eq). Sparged solution with Ar. Dissolved potassium hydroxide (33.6 g, 600 mmol, 15 eq) in 200

mL water and added to reaction flask. Sealed under Ar and heated to 40°C for 48 hours. Let cool

then sep funneled into ethyl acetate and water, collected organics, dried, filtered, and pumped

down. Column chromatography on silica to purify, began with 10% ethyl acetate in hexanes to

remove bromo decane then increased gradient to 100% ethyl acetate to collect the desired

product. 'H NMR in CDC 3 : 8 = 9.11 (dd, 2 H), 8.56 (dd, 2 H), 7.63 (dd, 2 H), 4.24 (t, 4 H), 1.89

(p, 4 H), 1.54 (p, 4 H), 1.26-1.43 (m, 24 H), 0.89 (m, 6 H). Yield of 75%.

21

.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5fl (ppm)

Stepwise arylation/oxidation of 5,6-didecyl ether 1,10-phenanthroline

OMe OMeBr tBu Mg BrMg ' tBu

THF

tBu 600C, 1-4h tBu

CP

Heated Mg shavings (1.2 g, 50 mmol, 10 eq) in a Schlenk tube to under vacuum. Added 10 mL

dry THF to the Mg then added 2-bromo-4,6-di-t-butyl anisole (3 g, 10 mmol, 2 eq) in a degassed

dry 10 mL THF solution to the Mg. Sealed under Ar and heat to 60°C. Progress could be seen

visually by a suspension of Mg in solution making the flask appear black. Checked for

completion of Grignard by quenching a sample of reaction in methanol and analyzing by NMR

22

in CDCl3: 8 = 7.32 (d,1 H), 7.19 (dd, 1 H), 6.81 (d,1 H), 3.82 (s, 3 H), 1.39 (s, 9 H), 1.31 (s, 9

H). Allowed to cool to room temperature once completed. Yield 100%.

Iii IA AL

I . . . . . . . I |. .............|1 1 .....|| |. ...-I I I I I i I I

7.6 7.4 7.2 7.0 6.8 6.6 6.4 6.2 6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.44.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.fl (ppm)

'Bu

CP tBu

C 10H 2 10 N TH C1 0H 210 IN OMe210 T I q r.t., 24h

N 2. MnO2 CC10H210N DCM C10H210 N

Combined 5,6-didecyl ether 1,10-phenanthroline (2.5 g, 5 mmol, 1 eq) and 10 mL THF in a

Schlenk tube. Added Grignard to the reaction via syringe or cannula, sealed, and stired at room

temperature for 24 hours to ensure it ran to completion. Checked progress by NMR prior to

quench. Once the reaction was complete, quenched with methanol. Pumped down then took up

23

kI

in dichloromethane. Added activated manganese dioxide (2.5 g, 29 mmol, 6 eq) and stired for a

few hours. Once completed, filtered through celite to remove the excess MnO2and washed

through with more DCM. Filtered through silica plug. Dried with magnesium sulfate, filtered,

and pumped down. Checked by NMR in CDC 3: 8= 9.12 (d, 1 H), 8.58 (m, 2 H), 8.09 (d, 1 H),

7.71 (d, 1 H), 7.62 (dd, 1 H), 7.43 (d, 1 H), 4.24-4.32 (m, 4 H), 3.33 (s, 3 H), 1.88-1.98 (m, 4 H),

1.54-1.63 (m, 4 H), 1.26-1.43 (m, 42 H), 0.87-0.93 (m, 6 H). NMR below also includes residual

quenched coupling partner, which is reported above. No isolated yield obtained, approximately

90% based on NMR integrations.

tl I"ii I. L

24

I ' . . . . . . . . . . I ' I I ' I ' ' ' .' ... ..5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

fi1(ppm)

J1 LL

tBu tBu

'Bu CP tBu

ClH210 N OMe THE C10H210 N OMe5000,48h q

01H1 0 -N 2. MnO2 -C1H210' N.CnM C 10H210 N OMe

Bu

tBu

For the second arylation/oxidation sequence, combined first arylation product in a Schlenk with

10 mL dry THF. Added Grignard (2 eq) to the reaction via syringe or cannula, sealed, and stired

at 500C for two days. Checked progress by NMR. Once the reaction was complete, quenched

with methanol. Pumped down then took up in dichloromethane. Added activated manganese

dioxide (2.5 g, 29 mmol, 6 eq) and stired for a few hours. Once completed, filtered through

celite to remove the excess manganese and washed through with more DCM. Dried with

magnesium sulfate, filtered, and pumped down. Checked for completion by NMR. Purified

product by chromatography starting with hexanes then increased gradient to 10% ethyl acetate in

hexanes. NMR in CDCl3: 8 = 8.58 (d, 2 H), 8.18 (d, 2 H), 7.85 (d, 2 H), 7.41 (d, 2 H), 4.26 (t, 4

H), 3.36 (s, 6 H), 1.93 (in, 4 H), 1.57 (in, 4 H), 1.26-1.43 (m, 60 H), 0.89 (t, 6 H). Yield 80%.

25

8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

Deprotection/Oxidation of Salophen-dione

tBu tBU tBu

I BItBU tB

C10H210 -N OMe DOE HO H-N OH Ag20 0 -N OH

C 10H 210 / N OMe OC >70°C HO / N OH DCM 0 / N OH

tBu tBu tB

tBu tBu tBu

Starting material (2.8 g, 3 mmol, 1 eq) was dissolved in 70 mL dry dichloroethane and degassed

with Ar. After cooling to 0°C in an ice bath, boron tribromide (2.85 mL, 30 mmol, 10 eq) was

added dropwise to via syringe. The reaction was then kept under Ar and heated to 70°C for at

least 6 hours. After cooling to 0°C in an ice bath, the reaction was quenched by adding saturated

26

U

U

potassium carbonate dropwise until no more gas evolves. The mixture was sep funneled into

DCM and water, collecting the organics, dry, filter, and pump down. The organic mixture was

then taken up in DCM and combined with silver oxide, stirred for 30 minutes. The reaction was

then filtered through celite and washed through with more DCM, then pumped down. If product

was sufficiently pure, could recrystallize from methanol and DCM. Chromatography to purify

the product was done by starting with hexanes then increased to 10% EtOAc in hex to collect

product as dark red band. However, if the mono arylated product was also present, it was

necessary to elute with 5% EtOAC/45% tol/50% hex to achieve separation of the mono and

diarylated products. NMR in CDCl3: 6= 13.46 (s, 2 H), 8.60 (d, 2 H), 8.15 (d, 2 H), 7.72 (d, 2

H), 7.57 (d, 2 H), 1.52 (s, 18 H), 1.40 (s, 18 H). Yield 70%.

hI 1 I- I

27

13.5 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5fl (ppm)

I

k

Condensation of benzene tetraamine on salophen-dione

tBu tBu

H 2N NH2'Bu C 4HCI tBu

O .-N OH H2N NH2 H2N N N OH

0 N OH K2 CO 3 H2N N N OHEIOH/H2O, ArtBu 110°C, 2h tBu

tBu tBu

Charged Schlenk tube with 32 mL ethanol and 4 mL water and sparged for 10 min with Ar.

Added K2C03 (0.67 g, 4.8 mmol, 2.4 eq), benzene tetraamine tetrahydrochloride (0.68 g, 2.4

mmol, 1.2 eq), and salophen-dione (1.25 g, 2 mmol, 1 eq) to Schlenk. Sealed under Ar and

heated to 110°C for two hours, checked for completion by running TLC in 50/50 EtOAc/Hex.

There should only be the one yellow spot at Rf=0.2 and no presence of the red spot for starting

material at Rf=0.7. Once completed, filtered reaction on frit and washed with water, ethanol, and

hexanes. Took up the product in THF and added in 1 M aqueous HC while stirring, crashing out

the HCl salt as a purple solid. Collected the solid by filtration onto a celite plug, washing through

with additional THF. Took the solids into a flask once more, adding THF and saturated aqueous

NaHCO3. This deprotonated the ligand and dissolved it into the THF. After stirring until the

purple solids are no longer present, preformed a sep funnel work up to collect the product in the

organic phase. Checked by NMR in d6-DMSO for purity. 1H NMR in d6-DMSO: 6= 14.54 (s, 2

H), 9.68 (d, 2 H), 8.71 (d, 2 H), 8.01 (d, 2 H), 7.44 (d, 2 H), 7.20 (s, 2 H), 6.41 (s, 4 H), 1.51 (s,

18 H), 1.41 (s, 18 H). Yield 80%.

28

I I 1 1l

14.5 13.5 12.5 11.5 10.5 9.5 8.5 7.5 6.5 5.5 4.5 3.5 2.5 1.5fi (ppm)

Metallation ofsalophen-diamine with MnCl 2

'Bu 'Bu

tBu MnCI 2 NBu

H 2N N N OH NEt3 H2N N N 0Mn

THFH2N a N N OH H 2N N - N O

tBu tBu

tBu tBu

Dissolved salophen-diamine (100 mg, 139 pmol, 1 eq) in THF (4 mL) under an N2 atmosphere

of a glovebox. Added in excess of anhydrous MnCl2 (200 mg, 1.60 mmol, 11.5 eq), this

primarily remained undissolved. Finally, added in excess NEt3 (0.2 mL, 1.43 mmol, 10.3 eq) and

29

I I I j

stired, there was an immediate color change from bright yellow of the dissolved ligand to dark

red. Allowed to stir at room temperature for six hours. Passed the solution through a celite plug

to remove excess of MnCl2, then pumped down to dryness under vacuum. Dissolved residue in

THF and passed through a silica plug to remove additional excess MnCl2.Pumped down again.

Yield was near quantitative 3. Mass by MALDI-TOF, calculated: m/z = 773.34, observed: m/z=

773.32.

5 Acknowledgements

I would like to thank Professor Yogi Surendranath for his mentorship and sharing his passion forscientific investigation with all of his students. The Surendranath lab provided me with a fertileenvironment to develop as a scientist and for that I am grateful to all of its members. Specifically,I would like to thank my postdoc mentors, Professor Alex Murray, Dr. Ryan Bisbey, Dr. MikePegis, and Dr. Patrick Smith for their advice and guidance at the whiteboard and the bench. Mythesis chair, Professor Dan Suess, provided additional insights on the direction of my project alongthe way. I would like to thank the NDSEG for providing the funding that made my work possible.The staff of the DCIF and the Chem Education Office are crucial to the operation of ourdepartment, and were always a joy to interact with. Finally, I would like to thank my friends andfamily for supporting me through this program, especially my cat for his tireless love.

30

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