Mechanistic considerations for C-C bond reductive coupling at a cobalt(III) center

Article abstract of DOI:10.1021/ja2072548

The diamagnetic cobalt(III) dimethyl complex, cis,mer-(PMe ₃)₃Co(CH₃)₂I, was found to promote selective C-C bond formation, affording ethane and triplet (PMe ₃)₃CoI. The mechanism of reductive elimination has been investigated by a series of kinetic and isotopic-labeling experiments. Ethane formation proceeds with a rate constant of 3.1(5) × 10^-5 s ^-1 (50 °C) and activation parameters of ΔH ^a = 31.4(8) kcal/mol and ΔS ^a = 17(3) eu. Addition of free trimethylphosphine or coordinating solvent strongly inhibits reductive elimination, indicating reversible phosphine dissociation prior to C-C bond-coupling. EXSY NMR analysis established a rate constant of 9(2) s^-1 for phosphine loss from cis,mer-(PMe₃)₃Co(CH₃)₂I. Radical trapping, crossover, and isotope effect experiments were consistent with a proposed mechanism for ethane extrusion where formation of an unobserved five-coordinate intermediate is followed by concerted C-C bond formation. An unusual intermolecular exchange of cobalt-methyl ligands was also observed by isotopic labeling.

Full text of DOI:10.1021/ja2072548

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pubs.acs.org/JACS
Mechanistic Considerations for CÀC Bond Reductive Coupling at a
Cobalt(III) Center
Hongwei Xu and Wesley H. Bernskoetter*
Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
S Supporting Information
b
study.5 The relative weakness of first-row transition metalÀcarbon
ABSTRACT: The diamagnetic cobalt(III) dimethyl com-
plex, cis,mer-(PMe3)3Co(CH3)2I, was found to promote
selective CÀC bond formation, affording ethane and triplet
(PMe3)3CoI. The mechanism of reductive elimination has
been investigated by a series of kinetic and isotopic-labeling
experiments. Ethane formation proceeds with a rate con-
stant of 3.1(5) Â 10À5 sÀ1 (50 °C) and activation para-
meters of ΔHq = 31.4(8) kcal/mol and ΔSq = 17(3) eu.
Addition of free trimethylphosphine or coordinating solvent
strongly inhibits reductive elimination, indicating reversible
phosphine dissociation prior to CÀC bond-coupling. EXSY
NMR analysis established a rate constant of 9(2) sÀ1 for
phosphine loss from cis,mer-(PMe3)3Co(CH3)2I. Radical
trapping, crossover, and isotope effect experiments were
consistent with a proposed mechanism for ethane extrusion
where formation of an unobserved five-coordinate inter-
mediate is followed by concerted CÀC bond formation. An
unusual intermolecular exchange of cobaltÀmethyl ligands
was also observed by isotopic labeling.
bonds, the frequent presence of alternative β-hydride elimination
routes, and the exceptional thermal and air sensitivity of many base-
metal hydrocarbyl complexes are just some origins of this dearth of
examples. These factors have been of particular hindrance to the study
of difficult Csp3ÀCsp3 bond formations. However, in the 1970s,
Yamamoto and Ikariya reported a series of investigations into the
thermal decomposition of dialkylcobalt(III) complexes supported by
phosphine and acetylacetonato (acac) ligands.6 Among the reactions
studied, a rare example of selective ethane reductive elimination from
cobalt(III) was described for cis-(PhPMe2)2(acac)Co(CH3)2.6c Un-
fortunately, the alkane elimination occurred with degradation of the
presumed cobalt(I) product, obviating further mechanistic study.6c
Inspired by these observations, as well as the recent advances in the
use of cobalt as a CÀC bond cross-coupling catalyst,7 our laboratory
began a study into the mechanism for metal-centered, two-electron
CÀC bond elimination at strong field cobalt complexes.
Given the significant challenges associated with deciphering
between the multiple possible redox pathways available to base
metals in CÀC bond reductive elimination reactions and the
rarity of selective hydrocarbyl eliminations from cobalt centers,
our initial investigations were directed toward cobalt species
likely to undergo solely metal-based redox changes.2,5 The con-
veniently prepared dialkyl cobalt complex, cis,mer-(PMe3)3-
Co(CH3)2I (1-Me2),8 has proven a useful platform for detailed
study of this class of transformation.
ransition metal-mediated carbonÀcarbon bond-forming re-
T
actions have been at the forefront of synthetic method
development for more than five decades.1 The broad impact of
this methodology has spurred efforts to expand the scope of
transition metal catalysts for CÀC bond cross-coupling and
other transformations beyond the successful but expensive
platinum group metals.2 Accordingly, economical and less toxic
first-row transition metal sources have become targets of growing
interest.2 Many exciting examples of base-metal-promoted carbonÀ
carbon coupling reactions have been reported and suggest that
the key bond-forming step(s) may occur via a range of redox
pathways including ligand-centered, combined metalÀligand,
and metal-centered net two-electron reductive elimination reac-
tions.3 In addition, these transformations at base metals may
easily traverse between open- and closed-shell spin systems.2 By
contrast, the carbonÀcarbon bond-forming reductive elimina-
tion process in precious metal cross-coupling reactions is be-
lieved to proceed primarily through metal-centered, two-electron
redox events which have been implicated in many elegant
mechanistic studies.4
The strong-field, redox-innocent phosphine and methyl ligands
afford a diamagnetic complex which is reasonably stable in
hydrocarbon or ethereal solvents. Although 1-Me2 was first
characterized by Klein and Karsch in 1975,9 little has been
reported regarding its reactivity. Our laboratory observed that
1-Me2 completes a smooth reductive elimination of ethane at
50 °C in benzene or heptane solutions over the course of hours
(eq 1). Significantly, the reaction afforded a stable cobalt(I)
triplet species, (PMe3)3CoI (1),10 and no detectable quantity of
methane by 1H NMR spectroscopy. The selective ethane elim-
ination suggests that 1-Me2 circumvents deleterious radical
extrusion pathways in preference for a net two-electron CÀC
bond coupling. This attribute, along with the modest pace of
elimination and relative stability of the cobalt product, provided
The mechanistic models for CÀC bond elimination which
have guided catalysis at precious metals have received consider-
ably more investigation than their lighter first-row metal
congeners.4 The limited range of isolable base-metal complexes
which undergo selective, stoichiometric carbonÀcarbon bond re-
ductive elimination may account for the modest scope of mechanistic
Received: August 2, 2011
Published: September 07, 2011
r
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Figure 1. Reversible PMe3 dissociation prior to ethane loss.
an unusual opportunity to gain insight into the kinetics and
mechanism of CÀC bond formation at strong-field cobalt(III)
centers.
Figure 2. Possible paths for ethane elimination from 1-Me2.
Reductive ethane elimination from octahedral complexes has
substantial precedent in late transition metals, although most
examples occur at second- and third-row metals.4,11 There has
been significant interest in determining whether these heavier
metal complexes extrude alkanes via a direct reductive elimina-
tion from a coordinatively saturated species or proceed with
dissociation of a ligand to eliminate from a lower coordinate
intermediate.4 In order to investigate the potential role of ligand
dissociation in CÀC bond coupling at cobalt, the influence of
added phosphine on 1-Me2 was examined by a series of kinetic
experiments. Ethane elimination from 1-Me2 in benzene-d6
solution was monitored by 1H NMR spectroscopy at 50 °C over
measurements. This confirms that loss of PMe3 from 1-Me2 to
access a coordinatively unsaturated complex is not rate limiting,
and the CÀC bond formation likely represents the slowest event
in the elimination reaction.
MethylÀmethyl site exchange in 1-Me2 was similarly analyzed
in the 2D EXSY NMR spectra and found to occur with a rate
constant of 6.9(7) sÀ1 (27 °C). Significantly, this rate is indis-
tinguishable from the measured rate of PMe3 loss from 1-Me2
(9(2) sÀ1) and was found to be independent of the concentration
of added phosphine (up to 0.5 M free PMe3). The precision of
methyl site exchange rate measurements were considerably
greater than those for free/bound PMe3 exchange, owing to
the superior signal dispersion of the methyl resonances and the
ability to record EXSY NMR spectra in the absence of added
phoshine.13 The interchange of the two CoÀCH3 sites was
monitored over a 45 °C range and established activation para-
meters of ΔHq = 23(2) kcal/mol and ΔSq = 21(6) eu. The
absence of [PMe3] dependence and the substantially favorable
entropy of activation are consistent with a site exchange pathway
where dissociation of PMe3 from 1-Me2 is rate limiting and
followed by fast Berry pseudorotation14 to exchange the two
methyl ligands (Figure 1). Notably, the mechanism in Figure 1
does not require interchange of the Co-PMe3 sites and is
consistent with the selectivity of PMe3 loss from 1-Me2.
Although methylÀmethyl site exchange in 1-Me2 bears little
direct influence on the rate or mechanism of ethane formation,
the Eyring parameters for this process can be taken as a measure
of the activation parameters for phosphine dissociation from 1-
Me2 as PMe3 loss appears rate limiting for the intramolecular site
exchange.
Investigations into the kinetics of PMe3 exchange with 1-Me2
provide a convincing case for the presence of a five-coordinate
intermediate in route to ethane reductive elimination. However,
greater insight into the mechanism of the CÀC bond forming
event itself was pursued. There are at least three possible path-
ways for CÀC bond reductive elimination at strong field cobalt
which could begin with PMe3 loss (Figure 2).4 In path A, the five-
coordinate intermediate undergoes cobaltÀmethyl bond clea-
vage to form a methyl radical which could then either abstract a
methyl group from the resulting cobalt(II) complex or recom-
bine with another methyl radical.15 Path B invokes concerted
CÀC bond formation, likely proceeding via a three-centered
transition structure en route to ethane loss.4dÀk Finally, path C
would proceed through a sequence including α-hydride
three half-lives and afforded a rate constant of 3.1(5) Â 10À5 sÀ1
.
The observed rate constant was confirmed by independent
measurement of 2.4(7) Â 10À5 sÀ1 (50 °C) using UVÀvis
spectrophotometry and the method of initial rates.12 These rates
correspond to a barrier of ca. 25 kcal/mol and are within a range
commonly observed for ethane elimination from d6-octhedral
complexes.4a,j,11 Attempts to monitor ethane elimination from 1-
Me2 in the presence of 2 equiv of free PMe3 failed to produce
detectable alkane formation over 3 days at 50 °C. Thermolysis at
temperatures in excess of 90 °C resulted in slow decomposition
into an intractable mixture. Thermolysis of 1-Me2 in coordinat-
ing solvent (acetonitrile-d3) also failed to produce reductive
elimination at elevated temperatures.13 Notably, addition of
excess nBu4NI yielded no significant change in the rate of ethane
extrusion (within the saturation limit of the salt in benzene). The
dramatic inhibition of CÀC bond coupling by added PMe3 and
donor solvent is consistent with a pre-dissociation pathway for
elimination, and implicates formation of an unobserved five-
coordinate intermediate prior to ethane loss (Figure 1). The role
of phosphine loss from cobalt as a rate influencing event in
reductive elimination motivated further investigation into the
ligand dissociation process.
EXSY NMR spectra of 1-Me2 with 2 equiv of added PMe3
(mixing time 150 ms; 27 °C) exhibited strong exchange correla-
tions between free phosphine and the PMe3 ligand bound trans
to methyl, as well as between the two CoÀCH3 sites. A rate of
PMe3 dissociation from 1-Me2 (k1; Figure 1) of 9(2) sÀ1 was
determined by quantitative EXSY NMR analysis over multiple
mixing times with a free PMe3 concentration of 0.053 M.13
Significantly, the rate of phosphine loss is dramatically faster than
the overall rate of ethane elimination (3.1(5) Â 10À5 sÀ1), even
accounting for the variation in temperature between the
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elimination, methyl migratory insertion into a methylidene, and
CÀH bond reductive elimination.4a,b Each pathway would likely
finish with rapid scavenging of the free phosphine by a low-
coordinate cobalt complex to generate 1. Significantly, an inter-
system crossing event is required at some point along each mecha-
nism to account for conversion of diamagnetic 1-Me2 to the S = 1
cobalt(I) monoiodide species. Definitive determination of when this
spin-state change occurs is not possible with the experimental data in
hand; however, it is plausible that the crossing event occurs
subsequent to the loss of the strong-field methyl ligand(s).
These mechanistic pathways were examined by kinetic and
isotopic-labeling experiments beginning with the α-hydride
elimination/methyl migratory insertion route (path C). This
mode of CÀC bond formation has previously found only limited
experimental support for alkane elimination at late transition
metals4a,b,16 and was excluded for 1-Me2 on the basis of isotope
effects of 1.2(2) (by NMR) and 1.3(1) (by UVÀvis) at 50 °C for
1-Me2/1-d6Me2 (1-d6Me2 = cis,mer-(PMe3)3Co(CD3)2I).13 An
isotope effect of this magnitude is inconsistent with the sub-
stantially larger effect anticipated for a reaction pathway which
involves breaking a CÀH/D bond during α-hydride elimination
and forming a CÀH/D bond in ethane reductive elimination,17
even if a modest inverse equilibrium isotope effect is incurred for
a slow alkane dissociation event.18 In addition, the absence of
observable quantities of methane or ethylene which could form
from transient cobalt(III) methyl methylidene hydride and
cobalt(III) ethyl hydride species, respectively, argues against
path C as the mechanism of ethane formation.
Radical (path A) and concerted (path B) mechanisms both
have considerable precedents for CÀC bond coupling at late
transition metals and were regarded as probable pathways for
ethane extrusion.4dÀk,15 Initial investigations of these pathways
included monitoring the rate of reductive elimination over a
46 °C range which afforded Eyring parameters of ΔHq = 31.4(8)
kcal/mol and ΔSq = 17(3) eu.13 However, this favorable entropy
of activation could be consistent with either methyl radical
extrusion from 1-Me2 (path A) or a late, highly dissociative
transition state for concerted CÀC bond elimination (path B).
Therefore, detection of a free methyl radical intermediate during
elimination was attempted using 1,4-cyclohexadiene (CHD) as a
trapping agent.19 Thermolysis of 1-Me2 in the presence of 5
equiv of CHD (ca. 0.25 M) yielded less than 8% methane as the
alkane product, suggesting the presence of an outer-sphere
methyl radical intermediate was minimal. Still, the observation
of small quantities of methane does not completely rule out
contributions from a tightly caged radical or rapid radical re-
bound version of path A, which can be difficult to confirm
experimentally.20 These observations motivated additional study
of the radical pathway by crossover experiments.
Figure 3. Elimination crossover from 1-d3Me3.
isotopologues and the change in ratio over the reaction are
inconsistent with a standard outer-sphere radical mechanism for
ethane elimination (path A).
The result of radical-trapping and crossover experiments auger
poorly for outer-sphere versions of path A. However, the
observation of crossover in the ethane product is also incon-
sistent with a straightforward concerted pathway (path B) with-
out an explanation for the origins of CH3CH3 and CD3CD3.
Note that the intramolecular methylÀmethyl site exchange
described for 1-Me2 would not result in crossover from 1-
d3Me2, and given the selective formation of only three isotopo-
logues of ethane, the crossover event is unlikely to involve CÀH
bond cleavage. Isotopic scrambling following reductive elimina-
tion was excluded by thermolysis of 1-Me2 in the presence of
CD3CD3, which yielded no detectable CD3CH3 product. Closer
1
examination of phosphorus-decoupled H NMR spectra of 1-
d3Me2 at low conversion revealed two new CoÀCH3 signals
shifted slightly downfield from those of 1-d3Me2.13 The new
signals were attributed to formation of 1-Me2 prior to reductive
elimination, and the assignment was confirmed by independently
mixing samples of 1-Me2 and 1-d3Me2. The small change in
CoÀCH3 resonances of the two isotopologues is ascribed to an
isotopic perturbation of the chemical shifts.21 This observation
confirms that the apparent crossover from 1-d3Me2 is in fact a
result of intermolecular methyl group exchange prior to ethane
elimination22 and renders concerted elimination (path B) as the
most likely of the considered pathways for ethane formation. The
mechanism of intermolecular methyl group exchange remains
the subject of investigation but does not appear to be intimately
linked to reductive elimination. Preliminary experiments indicate
that the degree of intermolecular exchange is not inversely
related to [PMe3] or coordinating solvent (conditions which
obviate ethane formation). Further examination of the mecha-
nism for this process is ongoing.
In conclusion, selective reductive elimination of ethane from a
dimethylcobalt(III) complex has been observed with preliminary
mechanistic studies indicating a pathway where dissociation of a
phosphine ligand precedes a largely concerted CÀC bond
formation. This mechanistic model provides a rare comparison
to the more intensely studied platinum group metal reductive
elimination reactions and may assist in developing base-metal
alternatives for processes now dominated by precious or toxic
metals. Commonalities between the pathway of elimination from
1-Me2 and the most commonly espoused mechanisms for CÀC
bond-coupling at precious metals are likely a result of the strong-
field ligands supporting 1-Me2. However, the observation
of intermolecular methyl group exchange, the necessity of a
spin-state change during the reaction of 1-Me2, and the
In order to monitor the origins of the two methyl fragments
in the ethane product, the cis,mer-(PMe3)3Co(CD3)(CH3)I
(1-d3Me2) isotopologue was prepared. Consistent with the
observation of rapid site exchange between methyl ligands, 1-
d3Me2 appeared as an essentially equimolar mixture of two isoto-
1
pomers by H NMR spectroscopy. Thermolysis of 1-d3Me2 at
60 °C for 3 h yielded complete conversion to a mixture of
CH3CH3, CD3CH3, and CD3CD3, with a 1:4 ratio of CH3CH3
to CD3CH3 (Figure 3). Moreover, careful monitoring of the
reaction at lower conversions revealed more selective formation
of CD3CH3 at short reaction times (1:9 at ca. 15% conv) with
a drop in selectivity occurring over the course of the reaction.
Both the deviation from a 1:2:1 statistical distribution of product
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precedent for single-electron events in many other organocobalt
complexes offer intriguing deviations from the typical heavier
metal congeners. Efforts to identify the role the of the inter-
system crossing event and to extend this avenue of study to
weaker field or redox-variable platforms are current research
targets which should provide further insight into key base-metal-
promoted transformations.
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Corresponding Author
wb36@brown.edu
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We gratefully acknowledge funding by Brown University and
its Richard B. Salomon Faculty Research Award. H.X. has been
partially supported by funds from the U.S. Department of Energy
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