1
Rhodium and Iridium Complexes of Bulky Tertiary
Phosphine Ligands. Searching for Isolable Cationic M(III)
Alkylidenes
Jesús Campos,† ‡ Ernesto Carmona†*
†Instituto de Investigaciones Químicas-Departamento de Química Inorgánica.
Universidad de Sevilla-Consejo Superior de Investigaciones Científicas. Avda. Américo
Vespucio 49, 41092 Sevilla (Spain).
‡Present address: Inorganic Chemistry Laboratory, Department of Chemistry,
University of Oxford, South Parks Road, Oxford OX1 3QR, UK.
TOC Graphic
2
Abstract
Cyclometalated chloride complexes of rhodium and iridium based on (5-
C5Me5)M(III) fragments that result from the metalation of the xylyl substituent of a
coordinated PR2(Xyl) phosphine (Xyl=2,6-Me2C6H3) have been prepared by reaction of
the appropriate metal precursor with the corresponding phosphine. For iridium, four
complexes derived from the phosphines PiPr2(Xyl), 1a; PCy2(Xyl), 1b; PMe2(Xyl), 1c
and PPh2(Xyl), 1d, have been prepared, whereas for rhodium only the complexes
derived from PiPr2(Xyl), 2a and PMe2(Xyl), 2d have been studied. Chloride abstraction
from compounds 1 and 2 by NaBArF (BArF= B(3,5-C6H3(CF3)2)4) leads to either
cationic dichloromethane adducts or to cationic hydride-alkylidene structures resulting
from -H elimination. The rhodium complexes investigated yield only dichloromethane
adducts. However, in the iridium system the less sterically demanding phosphines
PMe2(Xyl) and PPh2(Xyl) provide also dichloromethane adducts as the only observable
products, whereas for the bulkier PiPr2(Xyl) and PCy2(Xyl) ligands the hydride-
alkylidene formulation is the prevailing one. Nonetheless, variable temperature NMR
studies reveal that in solution each of these two structures exists in equilibrium with
undetectable concentrations of the other by means of facile reversible -H elimination
and migratory insertion reactions. Reactivity studies on the cationic hydride-alkylidene
complexes of iridium are reported too.
3
Introduction
Transition metal alkyl and carbene complexes are renowned families of
organometallic complexes that have been intensively investigated in the past decades.
Metal alkyls1,2 are at the heart of organometallic chemistry, as their reactive M-C
bonds participate in many useful reactions. In turn, the chemistry of metal carbenes has
become one the most successful areas of research in organometallic catalysis due to the
rich reactivity of the M=C bond.3,4,5 Even though effort has focused on carbocyclic6 and
heteroatom-stabilized carbenes,7,8 encompassing N-heterocyclic carbenes,4a,9 metal
alkylidenes, that is M=CR2 complexes where R= hydrogen or hydrocarbyl fragment,
continue to attract widespread attention.
In general, alkylidene M=CR2 complexes of the late transition elements contain
the metal in a low oxidation state.10 With reference to iridium, many Ir(I) alkylidenes
have been reported11 comprising examples of the Ir=CH2 parent unit, which
interestingly, may exhibit either electrophilic or nucleophilic carbene reactivity.11b-11d In
contrast, their Ir(III) counterparts are somewhat elusive species that are often proposed
as reactive intermediates for many relevant transformations.12 Recently, the oxidative
addition of CH3F to a pincer-ligated Ir(I) complex has been proposed to involve the
intermediacy of an Ir(III)=CH2 species that results from C-H activation of the
fluorocarbon followed by -fluorine migration.13
Not unexpectedly, cationic Ir(III) alkylidenes are rather fleeting species,
although examples have been known for many years. A transient hydride-ethylidene
complex (A in Figure 1) was generated by our group at -100ºC by protonation of an Ir-
CH=CH2 unit at the carbon atom, but at -50ºC it rearranged irreversibly to the
thermodynamically more stable hydride-ethylene isomer.14a,b Incorporation of the Ir=C
functionality to a metalacyclic structure hindered -H elimination and allowed for the
isolation of a stable hydride-alkylidene and observation of reversible carbene migratory
insertion and -H elimination.14c However, analogous 1,2-C shifts are usually
irreversible processes. We have reported recently that the cationic bis(iridacycle) B
(Figure 1), that contains Ir-CH2R and Ir=CHR termini, participates actively in C-H
activation and C-C bond forming reactions that ultimately lead to unusual hydride-
phosphepine structures.15 Intermediate B was in fact generated in situ by -hydride
4
M-P-CH2
elimination from the neutral dialkyl C (Figure 1) which derived formally from double
metalation of a bis(xylyl) phosphine ligand, PMe(Xyl)2, for Xyl = 2,6-Me2C6H3.15
Figure 1. Previous examples of cationic Ir(III) alkylidenes [IrIII=C(H)(R)]+ structures
(A and B) and the bimetalacycle C precursor for B.14a,15
In view of these results we planned the synthesis of isolable cationic alkylidene
complexes of Rh(III) and Ir(III) from metalacyclic structures akin to those
represented for iridium in Figure 1. To avoid the irreversible C-C coupling reaction that
prevented the isolation of B, mono(xylyl) phosphines PR2(Xyl), rather than bis(xylyl)
phosphines, PR(Xyl)2,15 were chosen for this work. We envisaged that cationic
cyclometalated16 rhodium and iridium solvento complexes of type D in Scheme 1 might
experience reversible -H elimination leading to the desired hydride-alkylidene
complexes (E, in Scheme 1).17 As the pioneer work of Schrock on the somewhat related
-H abstraction reaction, that converts a M(CH2R)2 fragment into M=C(H)R with
elimination of RCH3, demonstrated the importance of increased steric pressure at the
metal coordination sphere,18 four tertiary phosphines PR2(Xyl) of different steric
properties were assayed. As indicated also in Scheme 1, the R substituents range from
the small methyl group to the very bulky cyclohexyl, with phenyl and i-propyl having
intermediate, increasing size. In the following sections we provide details of this work
that has led to stable complexes of the two kinds depicted in Scheme 1, i.e. cationic M-
CH2Cl2 adducts (D) and hydride-alkylidene structures, E.
5
Scheme 1. Presumed reversible 1,2-shifts in the cationic Rh(III) and Ir(III) complexes
investigated in this work.
Results and Discussion
Synthesis of Iridium and Rhodium Precursors
As stated implicitly, the four phoshine ligands selected are prone to
cyclometalation.16 They were prepared by conventional procedures (see Supporting
Information, SI, for details). The corresponding iridium complexes (1a - 1d, Scheme 2)
resulted from the straightforward reaction of the Ir(III) dimer [{(5-C5Me5)IrCl2}2] with
the appropriate phosphine in the presence of the non-coordinating base 2,2,6,6-
tetramethylpiperidine (TMPP in Scheme 2). Under the specified conditions, complexes
1a and 1b were obtained as the exclusive reaction products in yields around 90%.
Nevertheless, compounds 1c and 1d formed together with the non-metalated dichlorides
(5-C5Me5)Ir(Cl)2PR2(Xyl) in ca. 1:5 (PMe2(Xyl)) and 2:1 (PPh2(Xyl)) ratios. Mild
heating of dichloromethane solutions of the non-metalated compounds (45ºC) in a
sealed flask in the presence of TMPP resulted in their quantitative conversion to the
desired 1c and 1d complexes.
6
Scheme 2. Synthesis of cyclometalated iridium chloride complexes 1a-1d. Lower-case letters a-
d will be used throughout this paper to denote complexes of the above phosphines as specified
in this scheme.
For rhodium, only the PMe2(Xyl) and PiPr2(Xyl) derivatives were investigated.
In contrast to the iridium analogs, complexes 2a and 2c could be obtained in yields
<50% only after heating in refluxing toluene for four days, and were accompanied by
other unidentified compounds. Accordingly, the synthetic route of Scheme 3, that takes
advantage of the capacity of Zn(C5Me5)2 to act as a mild Cp* transfer reagent,19 was
utilized. Once more, the PiPr2(Xyl) complex 2a was the exclusive product of this
transformation while for the PMe2(Xyl) reaction, 2c was the minor product as it was
accompanied by (5-C5Me5)Rh(Cl)2PMe2(Xyl) in ca. double molar quantities.
Conversion of the former into the latter required treatment with 1 equiv of LinBu at -
20ºC, and gave 2c in moderate yields (ca. 60%).
Scheme 3. Synthesis of rhodium chloride complexes 2a and 2c.
7
The new complexes 1 and 2 feature characteristic 31P{1H} resonances which for
the rhodium complexes appear as the expected doublet with 1JPRh values of ca. 160 Hz.
In the 1H NMR spectra, the C5Me5 ring gives a doublet (4JHP 1.5 - 2.5 Hz) at roughly
1.7 ppm while the diastereotopic M-CH2 protons resonate between 3 and 4 ppm and
exhibit a two-bond 1H-1H coupling constant of around 12-14 Hz. In the iridium
compounds, only one of these two proton signals exhibit coupling to the phosphorous
nucleus (3JHP of ca. 4-5 Hz). Corresponding 13C{1H} resonances appear at around 17
(Ir) and 30 (Rh) ppm (2JCP ≈ 3 (Ir), 8 (Rh) and 1JCRh ≈ 23 Hz). Slow diffusion of pentane
into concentrated dichloromethane solutions of compounds 1a, 1c and 2a provided
suitable crystals for X-ray studies (Figure 2 and Figure S5), that further confirmed the
cyclometalation of the xylyl-substituted phosphine. In the three structures, the M–C,
M–P and M–Cl bond distances are similar to those found for related compounds
previously reported by our group.15,20,21 Bond angles for the three-legged piano stool,
are close to the ideal 90º value.
Figure 2. ORTEP diagram for compound 1a and 2a. Thermal ellipsoids are drawn at the 50 % probability
and most hydrogen atoms have been omitted for clarity.
Chloride Abstraction from Rh and Ir Chloride Complexes.
As anticipated in the Introduction section, we foresaw that chloride abstraction from
complexes of types 1 and 2 would generate cationic cyclometalated dichloromethane
adducts that could further rearrange by -H elimination and formation of the desired
hydride-alkylidene derivatives. As summarized in Scheme 4, treatment of
dichloromethane solutions of complexes 1a and 1b of the bulky PiPr2(Xyl) and
PCy2(Xyl) ligands with NaBArF (BArF= B(3,5-C6H3(CF3)2)4) led to the targeted
8
alkylidenes 3a+ and 3b+ that formed as the only observable reaction products. Instead,
the analogous chloride complexes of the less sterically demanding phosphines
PMe2(Xyl) and PPh2(Xyl) originated the corresponding cationic adducts, 4c·CH2Cl2+
and 4d·CH2Cl2+, in quantitative yields (by NMR). In a similar manner, the rhodium
chloride precursors 2a and 2c led also to the dichloromethane adducts 5a·CH2Cl2+ and
5c·CH2Cl2+, once more as the only observable products. These complexes are not
represented in Scheme 4 but possess structures alike that of the iridium analogs
4·CH2Cl2+. It is rather intriguing that NMR investigations on the new compounds
provided no indication for the existence of a structure in which the cyclometalated
R2P(Xyl) unit binds to the metal center in a 4-P,C,C',C" fashion, similarly to our
previous findings for iridium15 and rhodium21 compounds constructed around the
PMe(Xyl)2 ligand. It seems clear that two Xyl units are required to stabilize such an
unusual 4 binding mode.
Scheme 4. Generation of cationic hydride-alkylidene and CH2Cl2 adducts of iridium, 3+ and 4·CH2Cl2+,
respectively. For R = iPr and Cy the 3+ ↔ 4+ equilibria favor 3 (i.e. 3a+ or 3b+), while for R = Me, Ph
only compounds of type 4·CH2Cl2+ have been isolated ([4·CH2Cl2]BArF). The rhodium analogs of
compounds 1 (viz. 2a and 2c, Scheme 3) form exclusively dichloromethane adducts (5a·CH2Cl2+ and
5c·CH2Cl2+, respectively).
Variable temperature NMR studies for the cationic iridium complexes of
Scheme 4 evince their interesting solution dynamic behavior. Thus, the room-
temperature 1H NMR spectra of hydride-alkylidenes 3+ show equivalent iPr (3a+) and
9
Cy (3b+) phosphine substituents and no signals attributable to their key Ir–H and
Ir=CH functionalities. However, upon cooling to -60 ºC, 1H NMR resonances at ca.
15.5 and -15.2 ppm due to Ir=CH and Ir–H protons of 3a+ (Figure S1) and 3b+ become
discernible. The low-frequency peaks correspond to the hydride ligands and exhibit a
two-bond coupling to the phosphorus nucleus of around 25 Hz, whereas signals due to
the carbenic protons exhibit no resolvable nuclear coupling. At these low temperatures
(below -60 ºC), the phosphine iPr substituents of 3a+ become inequivalent and give rise
to four somewhat broad, albeit clearly distinct, resonances. In almost all probability this
solution dynamic behavior results from a rapid and reversible 1,2-H shift between Ir and
the alkylidene carbon (vide infra). We analyzed the exchange of these protons by 1D-
EXSY studies. Rate constants of 14.7 and 45.5 s-1 were measured at -80 ºC for
compounds 3a+ and 3b+, respectively. An Eyring analysis (see SI, Figure S3) in the
temperature interval from –70 to –90 ºC yielded values of the activation parameters ΔH‡
= 7.3 ± 0.6 kcal mol-1 and ΔS‡ = -12 ± 1 cal mol-1K-1, with ΔG‡298K
= 11 ± 1 kcal mol-1
for compound 3a+ (and similar values for compound 3b+: ΔH‡ = 9 ± 2 kcal mol-1 and
ΔS‡ = -6 ± 1 cal mol-1K-1, with ΔG‡298K
= 11 ± 2 kcal mol-1).
We thought it of interest to demonstrate further the existence of an alkylidene unit in
3a+ by converting its cis hydride ligand to bromide by action of N-bromosuccinimide
(Scheme 5). The new alkylidene, 6a+, features dark green color and carbenic NMR
resonances observable at room temperature at δ 16.1 (1H) and 262.2 (13C) ppm, the
latter with 1JCH = 152 Hz. As expected, complex 6a+ displays no dynamic behavior in
solution.
Scheme 5. Reaction of 3a+ with N-bromosuccinimide
10
The molecular structures of the cationic alkylidenes 3a+ and 6a+ were confirmed by
X-ray diffraction studies (Figure 3). Characteristic Ir=C bond lengths of 1.896(5) (3a+)
and 1.907(2) (6a+) Å, which are appreciably shorter than the Ir–CH2 bond distances of
ca. 2.10 Å found in other related complexes discussed in this paper or elsewhere,15,20 are
in accordance with the proposed alkylidene formulation. Other geometrical parameters
are comparable to those in related structures already considered and need no further
discussion.
Figure 3. ORTEP diagrams for compounds 3a+ and 6a+. Thermal ellipsoids are drawn at the 50 %
probability and hydrogen atoms and counterion have been omitted for clarity.
Despite the lability of the coordinated molecule of CH2Cl2 in the cationic solvento
complexes of iridium 4c·CH2Cl2+ and 4d·CH2Cl2
+ (Scheme 4), α-H elimination was
not observed in any case. Evidently, for these complexes derived from the less bulky
PMe2(Xyl) and PPh2(Xyl) ligands, the dichloromethane-solvated Ir-CH2 metalacyclic
structure is preferred relative to the non-solvated hydride-alkylidene formulation (D and
E, respectively, in Scheme 1). However, the dynamic behavior exhibited by these
complexes in solution may be suggestive of a fast equilibration of 4c·CH2Cl2+ and
4d·CH2Cl2+ with undetectable concentrations of the corresponding hydride-alkylidene
structures. Thus, the two cations present broad 31P{1H} NMR signals centered at 8.3
(4c·CH2Cl2+) and 35.5 ppm (4d·CH2Cl2
+). Furthermore, the room temperature 1H
NMR spectrum of 4d·CH2Cl2+ contains signals similar to those of the neutral chloride
precursor 1d, except for resonances due to the Ir-CH2 protons which are missing.
Cooling at -20ºC results in the appearance of two broad doublets centered at 3.82 and
3.42 ppm, with a 2JHH coupling of ca. 16 Hz. For cation 4c·CH2Cl2+ with the least
11
bulky phosphine substituents, cooling at -90ºC is needed for the observation of two
broad signals with 3.52 and 3.20 ppm. At 20ºC, they convert into a broad peak shifted
to ca. 2.90 ppm. These observations may be indicative of the attainment in solution of
the equilibria shown in Scheme 6, implying fast reversible CH2Cl2 dissociation
accompanied by also reversible -H elimination.22
Scheme 6. Proposed dynamic behavior for 4c·CH2Cl2+ and 4d·CH2Cl2
+
As summarized in Scheme 7, adducts 4c·py+ and 4d·NCMe+ were instantly
generated upon addition of a slight excess of the Lewis base to solutions of the
appropriate 4·CH2Cl2+ cationic species. They display sharp 31P{1H} NMR resonances
at -0.5 (4c·py+) and 2.84 ppm (4d·NCMe+) with chemical shifts close to those of the
neutral chloride precursors (3.3 and 28.6 ppm for 1c and 1d, respectively). In addition,
the superior binding properties of C5H5N and NCMe in comparison with CH2Cl2, result
in sharp IrCH2 resonances. The molecular structures of the two complexes were
ascertained by X-ray crystallography (see SI, Figure S7).
Scheme 7. Reaction of cationic adducts 4c·CH2Cl2+ and 4d·CH2Cl2
+ with Lewis Bases.
At variance with the iridium cations 4·CH2Cl2+, the analogous rhodium
complexes, 5a·CH2Cl2+ and 5c·CH2Cl2
+, exhibited no dynamic behavior in solution, in
agreement with their failure to undergo α-H elimination reactions. For instance, for
12
cation 5a·CH2Cl2+, the 31P{1H} NMR spectrum consists of a doublet with 77.3 ppm
and 1JRhP= 158 Hz. The rhodium benzylic linkage, Rh-CH2-, gives rise to a proton
multiplet centered at 3.49 ppm and to a 13C{1H} doublet with 33.9 ppm and one-bond
13C -103Rh coupling of 23 Hz. It is worth remarking that all Rh and Ir dichlorometane
adducts studied in this work feature good thermal stability at room temperature and
remain unaltered after prolonged periods of time when kept under an inert atmosphere.
This is in contrast with reports on related complexes.15,23 We therefore performed X-ray
studies on [5a·CH2Cl2] BArF, with the results shown in Figure 4. Only a few rhodium
complexes with coordinated CH2Cl2 have been authenticated by X-ray
crystallography,23b,c,24 of which two are closely related to 5a·CH2Cl2+, namely [(η5-
C5Me5)Rh(PMe3)Me(CH2Cl2)]+ and [(η5-C5Me5)Rh (PMe3)Ph(CH2Cl2)]
+. The two are
air sensitive, thermally unstable solids that readily decompose at room temperature even
under inert atmosphere. In contrast, 5a-CH2Cl2+ is stable in solution for many days,
when stored under argon, and remained unaltered after several months in the solid state.
We propose that the existence of an intramolecular CH–π interaction between the
coordinated molecule of the dichloromethane and the metalated xylyl ring (Figure 4)
might account for this remarkable stability. This weak interaction is characterized by
CH(25b)···C(12) and CH(25b)···C(13) bond distances of ca. 2.64 and 2.73 Å, whereas
the C–H(25b)···Centr. angle has a value of ca. 157.6º (Centr. = metalated xylyl ring
centroid).
Figure 4. (a) ORTEP diagram for the complex 5a·CH2Cl2+. Thermal ellipsoids are drawn at the 50 %
probability and hydrogen atoms and counterion have been omitted for clarity. (b) Intramolecular CH–π
contacts in 5a·CH2Cl2+ (Centr. = xylyl ring centroid).
13
Reactivity Studies of Hydride-Alkylidene Complex 3a+
The solution dynamic behavior of hydride-alkylidene complexes 3+ (Scheme 4) that
was ascribed to very facile migratory insertion of the hydride ligand into the cationic
Ir=CH- functionality, was a clear sign of the otherwise expected electrophilicity of these
complexes. To gain additional insight into this chemical functioning, we performed
supplementary reactivity studies employing 3a+ as a representative complex. As shown
in Scheme 8, its reaction with PMe3 resulted in an immediate color change, due to the
generation of the phosphonium ylid 7a+ as the major reaction product (85 %). The
cationic adduct 4a·PMe3+ was formed too, albeit in low proportions (ca. 15 %).
Compound 4a·PMe3+ exhibits a 31P{1H} NMR signal at 48.1 ppm (very close to
corresponding signals due to 1a, at 49.6 ppm, and 4a·C2H4+, at 42.7 ppm; vide infra),
which appears as a doublet (2Jpp= 21 Hz) due to coupling to the PMe3 phosphorus
nucleus. The latter also features a doublet at -47.3 ppm. In contrast, the 31P{1H} NMR
signal due to the metalated phosphine of compound 7a+ is found at 59.3 ppm, while its
phosphonium PMe3 group yields a resonance at 33.1 ppm, highly deshielded with
respect to the corresponding resonance in compound 4a·PMe3+ (-47.3 ppm), and
without observable coupling to the phosphorus center of the cyclometalated phosphine.
A characteristic doublet due to the Ir–H unit appears in the 1H NMR spectrum at -17.7
ppm (dd, 2JHP = 33.3, 3JHP = 10.6 Hz). In the 13C{1H} NMR spectrum, the signal due to
the IrCHPMe3 is observed at 6.3 ppm (d, 1JCP = 32 Hz), widely shifted with respect to
the resonance due to the carbenic carbon of 3a+ (263.8 ppm). Compound 4a·PMe3+ was
further characterized by X-ray diffraction studies, whose details can be found in the
Supporting Information (Figure S6).
14
Scheme 8. Reaction of 3a+ with PMe3.
Once more, formation of the phosphonium ylide 7a+ in the above reaction was an
obvious sign of the electrophilicity of the carbene linkage of complex 3a+. However,
7a+ was the kinetic product of the reaction as heating a mixture of 4a·PMe3+ and 7a+ at
40 ºC for 16 hours caused quantitative conversion of the ylide species into the
phosphine adduct (Scheme 8). Monitoring the reaction by 31P{1H} NMR spectroscopy
provided a first-order kinetic rate constant of 3.4·10-5 s-1, which corresponds to a ∆Gǂ of
24.8 ± 0.3 kcal·mol-1 at 40 ºC (see SI, Figure S4). The latter compound is thermally
stable and does not revert to 7a+, even after heating its solution in ClCH2CH2Cl over 80
ºC for several hours. It seems probable that whereas the ylid complex 7a+ could result
from direct nucleophilic attack by PMe3 at the carbene carbon, adduct 4a·PMe3+ must
have been formed by PMe3 coordination to an unsaturated intermediate alike F in
Scheme 6.7d
Treatment of a dichloromethane solution of 3a+ with C2H4 caused an immediate
color change, from the characteristic intense red of the alkylidene to pale yellow, due to
formation of the cationic ethylene adduct 4a·C2H4+ (Scheme 9). The reaction with
LiMe led to the formation of the neutral methyl complex 9a as the major product (ca. 70
% yield), along with other minor unidentified species. Use of the milder alkylating
reagent ZnMe2, led cleanly and quantitatively to 9a. The molecular structure of this
complex was confirmed by X-ray diffraction studies (see Figure S6 for details). We also
studied the reaction of 3a+ with H2, which resulted in quantitative formation of the
cationic alkyl-dihydride species 8a+. This reaction might proceed either by direct
addition of H2 to the Ir=CH unit, or more likely by dihydrogen-induced migratory
insertion chemistry, that would involve coordination of H2 to a species analogous to F
in Scheme 6. Interestingly, reaction of 3a+ with D2 led to the formation of [D5]-8a-D2+,
in which all benzylic positions of the xylyl ring became deuterated. We recently
15
reported an analogous C-H/D scrambling for the parent compound bearing the
PMe(Xyl)2 ligand.15
Scheme 9. Additional reactivity studies performed with complex 3a+
Compounds 4a-C2H4+ and 9a feature characteristic 1H NMR signals due to
diastereotopic Ir–CH2 protons at 3.16 (d, 2JHH = 14.2 Hz) and 2.05 (dd, 2JHH = 14.2, 3JHP
= 3.4 Hz) ppm (9a), and 3.67 (d, 2JHH = 12.5 Hz) and 3.39 (dd, 2JHH = 12.5, 3JHP = 3.3
Hz) ppm (4a-C2H4+). The Ir-Me unit of 9a leads to a shielded doublet at -0.20 ppm,
with a three-bond scalar coupling to phosphorus of 3.4 Hz. The ethylene ligand of 4a-
C2H4+ provides two broad resonances (2H each) at 2.77 and 2.29 ppm, with
corresponding 13C{1H} NMR resonances (also broad) at 46.8 and 42.5 ppm. The
cationic dihydride 8a+ exhibits a remarkable dynamic behavior in solution (Figure 5),
consisting in the exchange of the two hydride ligands and the two methylene protons.
This exchange probably proceeds by coupling of one of the hydrides with the Ir–CH2
bond and formation of a monohydride agostic intermediate.15a,25 At room temperature,
the 1H NMR spectrum does not contain any observable resonances due to these four
protons but at -20 ºC two broad signals become hardly visible at 2.45 and 2.93 ppm (Ir–
CH2), along with a broad resonance at -13.62 due to the hydride ligands. Coalescence is
attained at ca. -10 ºC and further cooling to -40 ºC allowed identifying a two-bond
coupling constant of 13.0 Hz between the diastereotopic methylene protons. At -50 ºC
16
the signal for the two hydride ligands splits into two different peaks, one of them
displaying coupling to the phosphorous center (2JHP = 16.1 Hz). Heating the solution at
45 ºC resulted in the appearance of a broad signal attributable to four protons at -4.98
ppm (not shown in Figure 5). Lineshape analysis of the corresponding resonances at
various temperatures in conjunction with Eyring analysis of the observed rate constants
yields the following activation parameters for the overall process: ΔH≠ = 20.3 ± 0.5
kcal·mol-1, ΔS≠ = 29 ± 2 cal·mol-1·K-1 and ΔG≠300K = 12 ± 2 kcal·mol-1 (more details in
the SI).
Figure 5. Solution dynamic behavior of complex 8a+ by 1H NMR
Concluding Remarks
Cationic hydride alkylidene complexes of the (5-C5Me5)Ir(III) fragment in which
the alkylidene functionality is part of a five-membered iridacycle that contains also a
phosphine terminus (complexes 3a+ and 3b+) have been prepared and characterized
using bulky PR2(Xyl) phosphines which are prone to cyclometalation. Their pivotal Ir-
H and Ir=CH- units derive from a metalated benzylic Ir-CH2- linkage by reversible -H
elimination. In accordance with previous findings, increased steric pressure at the metal
coordination sphere favors -H elimination, thereby stabilizing the hydride-alkylidene
8a+
17
structure. Indeed, this structure has only been observed for the two bulkiest phosphines
employed, namely PiPr2(Xyl) and PCy2(Xyl). At variance with these observations, the
cyclometalated Ir-CH2- binding motif of complexes of the less sterically demanding
PMe2(Xyl) and PPh2(Xyl) ligands is favored over the Ir(H)(=CH-) structure, leading to
stable, cationic dichloromethane adducts (complexes 4c·CH2Cl2+ and 4d·CH2Cl2
+,
respectively). Only CH2Cl2 adducts analogous to the latter compounds can be isolated
for the similar (5-C5Me5)Rh(III)- PR2(Xyl) reaction system.
Experimental Section
General Synthesis of Iridium Chloride Compounds 1a-1d. [Cp*IrCl2]2 (0.30 g,
ca. 0.38 mmol) dissolved in dry CH2Cl2 (5 mL) and cooled at 0ºC was reacted with a
dichloromethane solution of the phosphine (0.76 mmol), in the presence of 2,2,6,6-
tetramethyl piperidine (TMPP, 130 µL, 0.76 mmol). The reaction mixture was allowed
to warm to room temperature and additionally stirred for 2 h (16 hours at 45 ºC in the
case of PMe2(Xyl) and 4 hours at this temperature when the phosphine was PPh2(Xyl)).
The solvent was removed under vacuum and the product extracted with toluene. The
solution was evaporated to dryness providing a bright yellow powder, which was
washed with pentane to yield the desired chloride complexes in yields ca. 90 %.
Variable amounts of the non-metalated (5-C5Me5)IrCl2(PR2(Xyl)) compounds were
identified when performing the reactions with PMe2(Xyl) and PPh2(Xyl) at room
temperature. Compound 1a. 1H NMR (500 MHz, CD2Cl2, 25 ºC) : 7.22 (d, 1 H, Ha),
7.06 (td, 1 H, 5JHP = 1.6 Hz, Hb), 6.85 (d, 1 H, Hc), 3,67 (d, 1 H, 2JHH = 13.9 Hz,
IrCHH), 3.24 (m, 1 H, CH(iPr)), 3.07 (dd, 1 H, 2JHH = 13.9, 3JHP = 4.4 Hz, IrCHH), 2,50
(s, 3 H, Meα), 2.27 (m, 1 H, CH(iPr)), 1.71 (d, 15 H, 4JHP = 1.3 Hz, C5Me5), 1.30 (dd, 3
H, 3JHP = 18.6, 3JHH = 6.8 Hz, Me(iPr)}), 1.24 (dd, 3 H, 3JHP = 12.8, 3JHH = 7.2 Hz,
Me(iPr)), 1.18 (dd, 3 H, 3JHP = 14.6, 3JHH = 7.1 Hz, Me(iPr)), 0.73 (dd, 3 H, 3JHP = 15.7,
3JHH = 7.1 Hz, Me(iPr)). All aromatic couplings are of ca. 7.5 Hz. 13C{1H} NMR (125
MHz, CD2Cl2, 25 ºC) : 162.9 (d, 2JCP = 26 Hz, C1), 140.5 (C3), 133.6 (d, 1JCP = 48 Hz,
C2), 129.9 (CHb), 128.0 (d, 3JCP = 7 Hz, CHc), 127.5 (d, 3JCP = 12 Hz, CHa), 91.9
(C5Me5), 30.9 (d, 1JCP = 30 Hz, CH(iPr)), 27.4 (d, 1JCP = 29 Hz, CH(iPr)), 22.8 (Meα),
20.9 (d, 2JCP = 7 Hz, Me(iPr)), 20.2 Me(iPr)), 19.2 (d, 2JCP = 5 Hz, Me(iPr)), 18.5
(Me(iPr)), 16.9 (d, 2JCP = 3 Hz, IrCH2), 9.0 (C5Me5).31P{1H} NMR (202 MHz, CD2Cl2,
18
25 ºC) δ: 49.6. Anal. Calcd. for C24H37ClIrP: C, 49.34; H, 6.38. Found: C, 49.3; H, 6.6.
Corresponding data for the remaining compounds of this type can be found in the SI.
Figure 6. Labeling scheme used for 1H and 13C{1H} NMR assignments.
Synthesis of Rhodium Chloride Compounds 2a, 2c. A solution of the phosphine
(P(iPr)2(Xyl): 111 mg, 0.5 mmol; PMe2(Xyl): 83 mg, 0.5 mmol) in 2 mL of THF was
added, at -40 ºC, to a solution of [RhCl(C2H4)2]2 (100 mg, 0.25 mmol) in 3 mL of THF.
The reaction mixture was stirred for 3 h at this temperature. Then, a solution of ZnCp2*
(84 mg, 0.25 mmol) in 1 mL of THF was added and the mixture stirred for 5 h, allowing
to warm slowly to -25 ºC. The solvent was removed under vacuum, the residue was
extracted with diethyl ether and it was then evaporated to dryness. The solid was
dissolved in 5 mL of CHCl3 and stirred for 3 h at room temperature. The solvent was
removed under vacuum and the crude product washed with pentane to yield chloride
complexes 2a and 2c as orange solids, which were purified by column chromatography
from Et2O/pentane (2a, 175 mg, 70 %; 2c, 110 mg, 83 %). Reaction with PMe2(Xyl)
gave a mixture of metalated (2c) and (5-C5Me5)RhCl2(PMe3(Xyl) complexes in a ca.
ratio of 30:70. Both compounds were separated by column chromatography from
Et2O/pentane to give pure samples of 2c (42 mg, 19 %) and the non-metalated
dichloride (130 mg, 55 %) as crystalline orange solids. In order to increase the amount
of 2c, a THF solution of the latter product (50 mg, 0.105 mmol) was reacted with a
solution of LinBu (3 M in hexanes, 45 µL) at -20 ºC. After 30 min of stirring at this
temperature, the reaction was quenched with MeOH (10 µL) and the volatiles removed
19
under vacuum. The product was extracted with Et2O, the solvent evaporated to dryness
and the orange solid washed with pentane to give complex 2c in 62 % yield (29 mg).
Compound 2a. 1H NMR (500 MHz, CDCl3, 25 ºC) : 7.14 (d, 1 H, Ha), 7.01 (td, 1 H,
5JHP = 2.2 Hz, Hb), 6.81 (dd, 1 H, 4JHP = 2.4 Hz, Hc), 3.42 (br. d, 1 H, 2JHH = 12.1 Hz,
RhCHH), 3.24 (br. d, 1 H, 2JHH = 12.1 Hz, RhCHH), 2.95 (septet, 2 H, 3JHH = 7.0 Hz,
CH(iPr)), 2.42 (s, 3 H, Meα), 2.29 (m, 2 H, CH(iPr)), 1.63 (d, 15 H, 4JHP = 2.4 Hz,
C5Me5), 1.36 (dd, 6 H, 3JHP = 19.1, 3JHH = 6.8, Me(iPr)), 1.24 (dd, 6 H, 3JHP = 12.3, 3JHH
= 7.1, Me(iPr)), 1.19 (dd, 6 H, 3JHP = 14.5, 3JHH = 7.1, Me(iPr)), 0.80 (dd, 6 H, 3JHP =
15.5, 3JHH = 7.1, Me(iPr)). All aromatic couplings are of ca. 7.5 Hz. 13C{1H} NMR (160
MHz, CDCl3, 25 ºC) : 160.7 (d, 2JCP = 28 Hz, C1), 139.5 (C3), 131.2 (d, 1JCP = 42 Hz,
C2), 129.6 (d, 4JCP = 2 Hz, CHb), 127.7 (d, 4JCP = 6 Hz, CHc), 126.6 (d, 4JCP = 15 Hz,
CHa), 98.1 (t, 1JCRh = 2JCP = 4 Hz, C5Me5), 30.5 (dd, 1JCRh = 23, 2JCP = 8 Hz, RhCH2),
30.1 (d, 1JCP = 21 Hz, CH(iPr)), 27.5 (d, 1JCP = 22 Hz, CH(iPr)), 22.8 (Meα), 20.7 (d,
2JCP = 8 Hz, Me(iPr)), 20.2 (Me(iPr)), 19.4 (d, 2JCP = 5 Hz, Me(iPr)), 19.1 (Me(iPr)), 9.6
(C5Me5).31P{1H} NMR (200 MHz, CDCl3, 25 ºC) δ: 83.0 (d, 1JRhP = 159 Hz). Anal.
Calcd. for C24H37ClPRh: C, 58.25; H, 7.54. Found: C, 58.4; H, 7.4. Corresponding data
for the remaining compounds of this type can be found in the SI.
Synthesis of cationic hydride-alkylidenes 3a+ and 3b+. To a solid mixture of 1a or
1b (0.08 mmol) and NaBArF (72 mg, 0.08 mmol) placed in a Schlenk flask was added 5
mL of CH2Cl2. The resulting solution with intense red color was stirred for 15 min at
room temperature, then filtered and the solvent removed under vacuum. The red solid
was washed with pentane to give the BArF- salts of alkylidenes 3a+ or 3b+ in ca. 95 %
yield. These complexes can be recrystallized from a 1:2 mixture of CH2Cl2:pentane.
Compound 3a+. 1H NMR (500 MHz, CD2Cl2, 25 ºC) : 7.69 (d, 1 H, Ha), 7.62 (dd, 4JHP
= 2.6 Hz, Hc), 7.34 (td, 5JHP = 2.2 Hz, 1 H, Hb), 2.73 (m, 2 H, 2 CH(iPr)), 2,69 (s, 3 H,
Me), 2.17 (s, 15 H, C5Me5), 1.06, (dd, 6 H, 3JHP = 17.2, 3JHH = 6.9 Hz, Me(iPr)), 0.89
(dd, 3 H, 3JHP = 18.5, 3JHH = 6.9 Hz, Me(iPr)). All aromatic couplings are of ca. 7.5 Hz;
1H NMR (500 MHz, CD2Cl2, -80 ºC) δ: 15.51 (s, 1 H, IrCH), -15.21 (d, 1 H, 2JHP = 24.7
Hz, Ir−H). Hydride and carbene signals are only detectable at temperatures below -
50ºC. 13C{1H} NMR (125 MHz, CD2Cl2, 25 ºC) : 263.8 (Ir=CH), 166.4 (d, 2JCP = 27
Hz, C1), 144.2 (C3) 137.2 (d, 3JCP = 7 Hz, CHc), 135.2 (C2, overlapped with BArF),
134.1 (CHb), 128.7 (d, 3JCP = 12 Hz, CHa), 104.5 (C5Me5), 25.5 (d, 1JCP = 32 Hz,
CH(iPr)), 22.0 (Me), 18.7, 18.3 (Me(iPr)), 10.3 (C5Me5). 31P{1H} NMR (202 MHz,
20
CD2Cl2, 25 ºC) δ: 73.1. IR (Nujol): υ(IrH) 2165 cm-1. Anal. Calcd. for C56H48BF24IrP: C,
47.67 ; H, 3.43. Found: C, 47.4; H, 3.5. See SI for corresponding data for 3b+.
Synthesis of cationic bromide-alkylidene 6a+. Chloride complex 1a (100 mg, 0.171
mmol) and NaBArF (152 mg, 0.171 mmol) were placed in a Schlenk and dissolved in
CH2Cl2 (5 mL) under argon. After stirring at room temperature for 15 minutes the red
solution of the resulting of complex 3a+ was filtered over a Schlenk flask containing N-
bromosuccinimide (30 mg, 0.171 mmol), with a change in color to dark green. The
reaction mixture was stirred for 15 minutes and then the succinimide removed by
extraction with deoxygenated water. Compound [6a]BArF was obtained as dark green
crystals (215 mg, 84 %) by slow diffusion of pentane into a dichloromethane solution of
the alkylidene.1H NMR (500 MHz, CD2Cl2, 25 ºC) : 16.1 (s, 1 H, Ir=CH), 8.10 (d, 1 H,
Ha), 6.98 (dd, 1 H, 5JHP = 2.9 Hz, Hc), 7.56 (Hb, overlapped with NaBArF), 3.74
(dseptet, 1 H, 2JHP = 11.9, 3JHH = 6.8 Hz), 2.79 (s, 3 H, Meα), 1.98 (m, 1 H, CH(iPr)),
1.94 (t, 15 H, 4JHP = 1.1 Hz, C5Me5), 1.66 (dd, 3 H, 3JHP = 15.8, 3JHH = 6.9 Hz, Me(iPr)),
1.30 (m, 6 H, 2 Me(iPr)), 0.24 (dd, 3 H, 3JHP = 17.3, 3JHH = 7.0 Hz, Me(iPr)). All
aromatic couplings are of ca. 7.5 Hz. 13C{1H} NMR (125 MHz, CD2Cl2, 25 ºC) : 262.2
(1JCH = 152 Hz, Ir=CH), 167.2 (d, 2JCP = 25 Hz, C1), 145.5 (C3), 139.9 (d, 3JCP = 8 Hz,
CHa), 139.1 (d, 1JCP = 47 Hz, C2), 134.2 (CHb), 131.1 (d, 3JCP = 12 Hz, CHc), 107.6
(C5Me5), 29.7 (Meα), 28.1 (d, 1JCP = 28 Hz, CH(iPr)), 27.2 (d, 1JCP = 31 Hz, CH(iPr),
22.4 (Me(iPr)), 19.5 (d, 2JCP = 4 Hz, Me(iPr)), 18.6 (d, 2JCP = 6 Hz, Me(iPr)), 18.5
(Me(iPr)), 9.4 (C5Me5). 31P{1H} NMR (200 MHz, CD2Cl2, 25 ºC) δ: 60.3. Anal. Calcd.
for C56H47BBrF24IrP: C, 45.15; H, 3.18. Found: C, 45.1; H, 3.2.
Reaction of 3a+ with PMe3. A solution of PMe3 in toluene (33 µL, 1.1 M, 0.033
mmol) was added under argon over a CH2Cl2 (1 mL) solution of alkylidene 3a+ (BArF-
salts 40 mg, 0.028 mmol) placed in a Schlenk flask. The solution rapidly cleared up and
then it was heated at 40 ºC for 16 hours. The volatiles were removed under vacuum and
the residue washed with pentane to give complex 4a·PMe3+ as a pale orange powder
(35 mg, 84 %). Crystals suitable for X-ray analysis were obtained by slow diffusion
from CH2Cl2/pentane. In turn, characterization of the related ylide 7a+ was achieved by
the following procedure. A screw-capped NMR tube was charged with 3a+ (BArF- salt;
30 mg, 0.021 mmol) and CD2Cl2 (0.6 mL). The tube was shaken, placed at -40 ºC and
PMe3 (2.5 µL, 0.026 mmol) was added at this temperature. 31P{1H} NMR monitoring of
the reaction showed immediate conversion of the alkylidene to a mixture of the ylide
21
7a+ and the cationic adduct 4a·PMe3+ in a ca. ratio of 97:3. Spectroscopic data were
obtained at -40 ºC without further purification in order to avoid isomerization to
4a·PMe3+, which is slow at this temperature. Compound 4a·PMe3
+. 1H NMR (400
MHz, CD2Cl2, 25 ºC) : 7.27 (d, 1 H, Ha), 7.20 (td, 1 H, 5JHP = 2.7 Hz, Hb), 6.98 (dd, 1
H, 5JHP = 3.1 Hz, Hc), 3.43 (m, 1 H, IrCHH), 3.29 (dd, 1 H, 2JHH = 7.6, 3JHP = 3.1 Hz,
IrCHH), 3.28 (dseptet, 1 H, 2JHP = 10.9, 3JHH = 7.1 Hz, CH(iPr)), 2.51 (s, 3 H, Meα),
2.33 (m, 1 H, CH(iPr)), 1.77 (t, 15 H, 4JHP = 1.9 Hz, C5Me5), 1.28 (dd, 3 H, 3JHP = 13.2,
3JHH = 7.2 Hz, Me(iPr)), 1.24 (d, 9 H, 2JHP = 10.3 Hz, PMe3), 1.15 (dd, 3 H, 3JHP = 18.9,
3JHH = 7.1 Hz, Me(iPr)), 1.03 (dd, 3 H, 3JHP = 16.5, 3JHH = 6.7 Hz, Me(iPr)), 0.59 (dd, 3
H, 3JHP = 16.3, 3JHH = 7.1 Hz, Me(iPr)). All aromatic couplings are of ca. 7.5 Hz.
13C{1H} NMR (100 MHz, CD2Cl2, 25 ºC) : 158.5 (d, 2JCP = 24 Hz, C1), 140.1 (C3),
132.4 (d, 1JCP = 47 Hz, C2), 131.9 (d, 4JCP = 3 Hz, CHb), 130.4 (d, 3JCP = 7 Hz, CHc),
126.9 (d, 3JCP = 12 Hz, CHa), 98.5 (C5Me5), 31.2 (dd, 1JCP = 29, 3JCP = 2 Hz, CH(iPr)),
26.3 (d, 1JCP = 30 Hz, CH(iPr), 22.6 (Meα), 20.7 (Me(iPr)), 20.4 (d, 2JCP = 5 Hz,
Me(iPr)), 20.0 (d, 2JCP = 6 Hz, Me(iPr)), 18.6 (Me(iPr)), 18.4 (d, 1JCP = 39 Hz, PMe3),
9.8 (C5Me5), 6.4 (dd, 2JCP = 8, 2JCP = 3 Hz, IrCH2). 31P{1H} NMR (160 MHz, CD2Cl2,
25 ºC) δ: 48.1 (d, 2JPP = 21 Hz, P(iPr)2Xyl), -47.3 (d, 2JPP = 21 Hz, PMe3). Anal. Calcd.
for C59H58BF24IrP2: C, 47.62; H, 3.93. Found: C, 47.9; H, 3.8. Compound 7a+. 1H
NMR (500 MHz, CD2Cl2, 25 ºC) : 7.19 (m, 2 H, Ha, Hb), 7.10 (m, 1 H, Hc), 3.60 (d, 1
H, 2JHP = 11.0 Hz, IrCHPMe3), 2.94 (m, 1 H, CH(iPr)), 2.56 (s, 3 H, Meα), 2.31 (m, 1 H,
CH(iPr)), 1.90 (t, 15 H, 4JHP = 1.8 Hz, C5Me5), 1.33 (d, 9 H, 2JHP = 11.9 Hz, PMe3), 1.23
(dd, 3 H, 3JHP = 12.8, 3JHH = 7.4 Hz, Me(iPr)), 1.13 (dd, 3 H, 3JHP = 18.3, 3JHH = 6.8 Hz,
Me(iPr)), 1.03 (dd, 3 H, 3JHP = 12.0, 3JHH = 7.2 Hz, Me(iPr)), 0.16 (dd, 3 H, 3JHP = 16.0,
3JHH = 7.1 Hz, Me(iPr)), -17.7 (dd, 1 H, 2JHP = 33.3, 3JHP = 10.6 Hz, IrH). All aromatic
couplings are of ca. 7.5 Hz. 13C{1H} NMR (125 MHz, CD2Cl2, -40 ºC) : 150.6 (d, 2JCP
= 27 Hz, C1), 141.8 (C3), 133.4 (C2, overlapped with BArF), 130.2 (CHb), 129.3 (CHc),
125.4 (CHa), 92.1 (C5Me5), 27.6 (d, 1JCP = 28 Hz, CH(iPr)), 24.7 (d, 1JCP = 36 Hz,
CH(iPr), 22.0 (Meα), 18.5 (Me(iPr)), 18.3 (Me(iPr)), 17.8 (Me(iPr)), 16.5 (Me(iPr)), 10.6
(d, 1JCP = 56 Hz, PMe3), 9.8 (C5Me5), 6.3 (dd, 1JCP = 32 Hz, IrCHPMe3). 31P{1H} NMR
(200 MHz, CD2Cl2, 25 ºC) δ: 59.3 (P(iPr)2Xyl), 33.1 (PMe3).
Associated Content
22
Supporting Information: general experimental details, synthesis and characterization
of phosphine ligands and rhodium and iridium complexes, solution dynamic 1H NMR
spectroscopy studies, kinetic studies on the conversion of 7a+ into 4a·PMe3+ and X-ray
crystallographic studies. This material is available free of charge via the Internet at
http://pubs.acs.org.
Author Information
Corresponding Author: *E-mail: [email protected]
Notes: The authors declare no competing financial interest.
Acknowledgments
Financial support from the Spanish Ministry of Science and Innovation (Projects
CTQ2010-17476 and Consolider-Ingenio 2010 CSD2007-00006) and the Junta de
Andalucía (Projects FQM-119 and P09-FQM-4832) is gratefully acknowledged. J. C.
thanks the Spanish Ministry of Education for a research grant (AP-20080256).
Dedication
This paper is dedicated to the memory of Professor M. F. Lappert, in recognition of
an outstanding career and of his most valuable contributions to the development of
Inorganic and Organometallic Chemistry.
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22 It is worth mentioning in this regard that the color of solutions of both cationic dichloromethane
adducts is temperature dependent. At low temperatures (below -20 ºC) the solutions are yellow, as found
for 1a-d and other cationic adducts described in this work. However, warming above 10 ºC results in
darkening of the mixture to the characteristic intense orange color typical of hydride-alkylidenes 3+.
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