Redox-active ligand-mediated oxidative addition and reductive elimination at square planar cobalt(III): Multielectron reactions for cross-coupling

Article abstract of DOI:10.1021/ja106212w

Square planar cobalt(III) complexes with redox-active amidophenolate ligands are strong nucleophiles that react with alkyl halides, including CH ₂Cl₂, under gentle conditions to generate stable square pyramidal alkylcobalt(III) complexes. The net electrophilic addition reactions formally require 2e^- oxidation of the metal fragment, but there is no change in metal oxidation state because the reaction proceeds with 1e ^- oxidation of each amidophenolate ligand. Although the four-coordinate complexes are very strong nucleophiles, they are mild outer-sphere reductants. Accordingly, addition of alkyl- or phenylzinc halides to the five-coordinate organometallic complexes regenerates the square planar starting materials and extrudes C-C coupling products. The net 2e^- reductive elimination reaction also occurs without a oxidation state change at the cobalt(III) center. Together these reactions comprise a complete, well-defined cycle for cobalt Negishi-like cross-coupling of alkyl halides with organozinc reagents.

Full text of DOI:10.1021/ja106212w

Published on Web 09/29/2010
Redox-Active Ligand-Mediated Oxidative Addition and Reductive
Elimination at Square Planar Cobalt(III): Multielectron Reactions for
Cross-Coupling
Aubrey L. Smith,† Kenneth I. Hardcastle,‡ and Jake D. Soper*,†
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, and X-ray
Crystallography Center, Department of Chemistry, Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322
Received July 13, 2010; E-mail: jake.soper@chemistry.gatech.edu
Abstract: Square planar cobalt(III) complexes with redox-active
amidophenolate ligands are strong nucleophiles that react with
alkyl halides, including CH2Cl2, under gentle conditions to gener-
ate stable square pyramidal alkylcobalt(III) complexes. The net
electrophilic addition reactions formally require 2e- oxidation of
the metal fragment, but there is no change in metal oxidation state
because the reaction proceeds with 1e- oxidation of each
amidophenolate ligand. Although the four-coordinate complexes
are very strong nucleophiles, they are mild outer-sphere reduc-
tants. Accordingly, addition of alkyl- or phenylzinc halides to the
five-coordinate organometallic complexes regenerates the square
planar starting materials and extrudes C-C coupling products.
The net 2e- reductive elimination reaction also occurs without a
oxidation state change at the cobalt(III) center. Together these
reactions comprise a complete, well-defined cycle for cobalt
Negishi-like cross-coupling of alkyl halides with organozinc re-
agents.
Figure 1. Solid-state structures of (a) CoIII(CH2Cl)(isqPh)2 drawn with 50%
probability ellipsoids. The [isqPh]•- ligand hydrogen atoms are omitted for
clarity. (b) Schematic of selected bond lengths (Å) drawn to correspond to
Figure 1a.
The utility of palladium in cross-coupling catalysis derives from
its ability to mediate 2e- oxidative addition and reductive elimina-
tion steps for selective assembly of C-C bonds.1 Because later
first-row metal ions typically exist in oxidation states that vary by
only 1e-, they are often prone to radical reactions with organic
substrates.2 Accordingly, development of base metal catalysts for
selective cross-coupling cycles requires strategies to effect 2e- redox
reactions over potentially competing 1e- pathways.3 An emerging
approach to multielectron chemistry employs redox “noninnocent”
ligands as reservoirs of electrons for bond-making and bond-
breaking reactions at coordinatively unsaturated metals.2,4 We have
utilized this strategy to prepare square planar cobalt complexes that
are strong nucleophiles but mild 2e- reductants. We report here
how these properties facilitate well-defined 2e- pseudo-oxidative
addition and reductive elimination reactions at cobalt(III), compris-
ing both steps of a catalytic cycle for Negishi-like cross-coupling
of alkyl halides with alkyl- and arylzinc reagents.
The ability of amidophenolate ligands to deliver electrons for
bond-forming redox reactions at square planar cobalt(III) centers
was previously demonstrated in reactions of [CoIII(apAr)2]- (where
[apAr]2- is [apPh]2- ) 2,4-di-tert-butyl-6-(phenylamido)phenolate
or [apiPr]2- ) 2,4-di-tert-butyl-6-(2,6-diisopropylphenylamido)phe-
nolate) with chlorine electrophiles.5 During the course of these
studies, we observed that addition of excess CH2Cl2 to purple
CH3CN solutions containing the [CoIII(apPh)2]- anion gave quantita-
tive conversion to a bright green material over hours at 25 °C.
Electrospray ionization mass spectra (ESI-MS) of THF solutions
containing the green product showed a molecular ion at 698 m/z
that was shifted to 700 m/z when the reaction was performed with
CD2Cl2 in place of CH2Cl2, corresponding to the molecular weights
of the [CoIII(apPh)2]- reactant and a [CH2Cl] or [CD2Cl] fragment.
Green crystals of the protio material were obtained from concen-
trated CH3CN-CH2Cl2 solution. The X-ray structure contains a
square pyramidal chloromethyl complex (Figures 1, S1). As
compared to [CoIII(apPh)2]-, the aminophenol ligand metrical data
exhibit systematic geometrical changes that are characteristic of a
change in ligand oxidation state. In particular, the contracted C-O
and C-N bond distances and the quinoid-like pattern of four long
and two short C-C bond distances about the aminophenol ring
(Figure 1b) match those typical for [isqPh]•- ([isqPh]•- ) 2,4-di-
tert-butyl-6-(phenylimino)semiquinonate),6 suggesting that the prod-
uct is best described as CoIII(CH2Cl)(isqPh)2. This formulation
implies that the reaction occurs with a net 2e- oxidation of the
[CoIII(apPh)2]- fragment, but the oxidation state of the cobalt center
is unchanged because each ligand supplies one electron (Scheme
1a).
THF solutions of CoIII(CH2Cl)(isqPh)2 are stable for days in the
dark at 25 °C but degrade in light to cleanly afford CoIII(apPh)(isqPh),
the product of Co-CH2Cl homolysis.7 Accordingly, when moni-
tored at wavelengths >700 nm, the reaction of CH2Cl2 with
[CoIII(apPh)2]- in CH3CN exhibits clean pseudo-first-order kinetics
with an isosbestic point at 911 nm (Figure S2).
† Georgia Institute of Technology.
‡ Emory University.
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14358 J. AM. CHEM. SOC. 2010, 132, 14358–14360
10.1021/ja106212w  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
Addition of 1 equiv of EtBr to Na[CoIII(apPh)2] in CH3CN
halides react with [CoIII(apPh)2]- to afford the corresponding
alkylcobalt(III) complexes, and the relative rates of the reactions
parallel the lability of the halide. For instance, addition of 5 equiv
of CH3I to 0.01 M [CoIII(apPh)2]- in CH3CN affords quantitative
conversion to CoIII(CH3)(isqPh)2 in seconds, whereas complete
reaction with CH2Cl2 takes ∼12 h under the same conditions. The
analogous reaction with CH2ClBr requires ∼30 min and gives
exclusively the CoIII(CH2Cl)(isqPh)2 product, as determined by ESI-
MS. [CoIII(apPh)2]- reacts with CH3OTf (OTf - ) trifluoromethane-
sulfonate) to afford CoIII(CH3)(isqPh)2 at a rate comparable to that
of the fast reaction with CH3I. (2) All of the addition reactions are
slower with more sterically demanding carbon centers. Accordingly,
reactions with EtI or EtBr form CoIII(Et)(isqPh)2 in <1 min and ∼1
h, respectively, and reaction of [CoIII(apPh)2]- with isopropyl iodide
requires 3 h. PhCH2Br reacts with [CoIII(apPh)2]- to give
CoIII(CH2Ph)(isqPh)2 in 1 d, and >2 d are required for complete
reaction with PhCH2Cl. (3) No addition reaction occurs at sp2-
hybridized carbon centers. [CoIII(apPh)2]- is inert to iodobenzene
and vinyl bromide over weeks at 25 °C. (4) The addition reactions
are highly sensitive to steric bulk at the cobalt center. For example,
all alkyl halide additions to [CoIII(apiPr)2]- are significantly slowed
relative to [CoIII(apPh)2]-. All of these observations are collected
in Table S1.
similarly yields a green, light-sensitive, square pyramidal ethyl
complex suitable for analysis by X-ray diffraction. As in the reaction
with CH2Cl2 described above, systematic changes in the C-O,
C-N, and C-C bond distances (Figure 2) indicate that the
amidophenolate chelates in [CoIII(apPh)2]- are each oxidized by 1e-,
so the product is best formulated as CoIII(Et)(isqPh)2. Solid CoIII-
(Et)(isqPh)2 is stable for >2 d in the dark at 25 °C, and the UV-vis
spectrum of a THF solution is unchanged over 4 h at 25 °C,
suggesting that the five-coordinate alkyl complexes are relatively
inert to ꢀ-hydrogen elimination.
Four mechanisms that are consistent with the UV-vis kinetics
were considered for generation of CoIII(R)(isqAr)2 and X- from
[CoIII(apAr)2]- and R-X (Scheme 2). Two invoke six-coordinate
oxidative addition [CoIIIX(R)(isqPh)2] products as unobserved
intermediates followed by rapid loss of X-, but the data above
suggest that both of these pathways involving Co-X bond
formation can likely be ruled out. In particular, the rapid reaction
of [CoIII(apPh)2]- with CH3OTf suggests that a radical mechanism
(iV) of initial X• transfer is unlikely, and substrates such as CH3I
and CH3OTf do not typically react by a concerted addition
mechanism (iii).8
Figure 2. Solid-state structures of (a) CoIII(Et)(isqPh)2 drawn with 50%
probability ellipsoids. The [isqPh]•- ligand hydrogen atoms are omitted for
clarity. (b) Schematic of selected bond lengths (Å) drawn to correspond to
Figure 2a.
A comparison of the relative rates of organohalide addition to
[CoIII(apPh)2]- revealed several pertinent trends: (1) Primary alkyl
Scheme 2
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C O M M U N I C A T I O N S
[CoIII(apAr)2]- complexes are 200-400 mV less reducing than most
cobaloxime(I) species.5 This permits reactions of CoIII(Et)(isqPh)2
with organozinc reagents to give the products of 2e- reductive
elimination, also without a change in cobalt oxidation state. The
redox-active ligand-mediated reactions are therefore complementary
to recently reported ligand-derived oxidative addition and reductive
elimination at d0 metal complexes.4b-d Notably, the surprising
proclivity for 2e- reactions over 1e- redox forms a basis for
development of new well-defined first-row metal catalysts for
selective cross-coupling cycles.
Scheme 3
The two remaining mechanisms both invoke conversion of
[CoIII(apAr)2]- directly to the five-coordinate CoIII(R)(isqAr)2 product
without Co-X bond formation, but these represent limits of the
potential 1e- vs 2e- redox pathways. The observed increase in
reaction rate with leaving group lability, I- > Br- > Cl-, does not
distinguish mechanism (i) versus (ii), and previously reported
reactions of CoIII(apAr)(isqAr) with sources of net [Cl•] to generate
CoIIICl(isqAr)2 demonstrate the ability of CoIII(apAr)(isqAr) to function
as a radical trap.5 However, the sum of the other experimental
observations does not support an electron transfer (ET) mechanism
(ii). For example, the [CoIII(apiPr)2]- anion is a better 1e- reductant
than [CoIII(apPh)2]-,5 but it reacts with alkyl halides at a significantly
decreased rate. Additionally, its slow reactions with 2° alkyl halides
as compared to 1° haloalkanes are inconsistent with a mechanism
of initial outer-sphere ET (ii). Instead, this sensitivity to steric
hindrance at carbon, as well as to steric encapsulation of the
cobalt(III) center by the [apiPr]2- ligands, is most consistent with
the SN2-type mechanism (i) wherein Co-R bond formation requires
direct attack of the nucleophilic cobalt center on the alkyl halide
electrophile.9,10
The accessibility of the alkylcobalt(III) complexes prompted us
to pursue reductive C-C bond-forming reactions with organozinc
compounds. Treating CoIII(Et)(isqPh)2 with PhZnBr (2-10 equiv)
in a 1:1 CH3CN/THF solution results in an immediate discharge
of the green color and formation of the reduced cobalt fragment
[CoIII(apPh)2]- (Scheme 1b).11 Complete consumption of 0.01 M
CoIII(Et)(isqPh)2 requires >6 equiv of PhZnBr, presumably because
phenyl transfer is sterically disfavored (Scheme 3b). GC-MS
analysis of the reaction mixtures shows the expected cross-coupling
product ethylbenzene in 10-15% yield (Table S2). The analogous
reactions with hexylzinc bromide similarly afford n-octane in
5-15% yield. Both reactions give small quantities of the corre-
sponding homocoupling byproducts, biphenyl (<2%) and dodecane
(<7%). Notably, addition of greater excesses of RZnX gives
nonstatistical product distributions, with increased yields of the
cross-coupling products relative to homocoupling.12 These data
imply that the C-C bond-forming reactions do not occur by radical
Co-Et homolysis at CoIII(Et)(isqPh)2. Studies to elaborate the
reductive elimination mechanism, and to optimize conditions for
catalytic cross-coupling (Scheme 3), are in progress.
Acknowledgment. We thank the ACS Petroleum Research Fund
(45130-G3), the NSF (CAREER CHE-0844693), and the Georgia
Institute of Technology for financial support. We acknowledge
David Bostwick for mass spectrometry.
Supporting Information Available: Complete synthetic and ex-
perimental details; selected UV-vis absorption spectra; collected
reaction rates for organohalide addition to [CoIII(apPh)2]-; tabulated
yields from organozinc halide coupling reactions; X-ray crystallographic
data and files in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
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In sum, the ability of redox-active ligands to facilitate both 2e-
pseudo-oxidative addition and reductive elimination reactions at
mononuclear square planar cobalt(III) complexes is predicated on
two properties: (1) The [CoIII(apAr)2]- species are very strong
nucleophiles, reminiscent of “supernucleophilic” cobaloxime(I)
complexes and square planar iridium(I) compounds that undergo
SN2-type oxidative addition of R-X.7,13 However, these cobalt(III)
anions are unusual nucleophiles. They have diradical S ) 1 ground
states,5 and their reactions with haloalkanes occur without a change
in oxidation state at the cobalt(III) centers because the redox-active
aminophenol-derived ligands in [CoIII(apAr)2]- supply both of the
necessary redox equivalents for the net 2e- transformation. (2) The
(9) The slow reaction of [CoIII(apPh)2]- with PhCH2X likely further highlights
the sensitivity of the reaction to steric hindrance.
(10) Reactions with haloalkanes that are probes of radical intermediates (e.g.,
(bromomethyl)cyclopropane) were inconclusive because the 1H NMR
spectrum of the organometallic alkylcobalt(III) products are broadened by
1
trace quantities of the S )
/
CoIII(apAr)(isqAr) decomposition product.
2
(11) The speciation of reduced cobalt complex is concentration dependent.
[CoIII(apPh)2]- reacts rapidly with PhZnBr to form an equilibrium adduct
that we tentatively formulate as [CoIII(Ph)(apPh)2]2-
.
(12) In both reactions the balance of the cobalt-derived ethyls was not found.
These may form butane, which is not quantitated by the GC-MS method
used.
(13) For example, see: Labinger, J. A.; Osborn, J. A. Inorg. Chem. 1980, 19,
3230–3236, and references therein.
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