Kinetics and mechanism of alkyl transfer from organocobalt(III) to nickel(I): Implications for the synthesis of acetyl coenzyme A by CO dehydrogenase

Article abstract of DOI:10.1021/ja963061z

Reaction of CH₃Co(dmgBF₂)₂L (dmgBF₂ = (difluoroboryl)dimethylglyoximato); L = py, PEt₃) with 2 equiv of [Ni(tmc)]OTf (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; OTf^- = CF₃SO₃^-) gave Co(dmgBF₂)₂py^-, [Ni(tmc)]OTf₂, and [Ni(tmc)CH₃]OTf in 80% yield. The overall transformation provides the first model for the transfer of a CH₃ group from methylcobalamin to the Ni-containing enzyme carbon monoxide dehydrogenase during acetyl coenzyme A synthesis. RRSS-[Ni(tmc)CH₃](BAr'₄) (BAr'₄^- = B(3,5-(CF₃)₂C₆H₃)₄^-) has been characterized by X-ray diffraction. The products and 1CH₃Co(dmgBF₂)₂L:2[Ni(tmc)]OTf stoichiometry of the reaction are consistent with a three-step mechanism initiated by electron transfer from [Ni(tmc)]OTf to CH₃Co(dmgBF₂)₂L. The second step is rapid CH₃-Co^- bond homolysis yielding Co(dmgBF₂)₂L^- and CH₃.; then the CH₃ radical is captured by the second equivalent of [Ni(tmc)]OTf, yielding [Ni(tmc)CH₃]OTf. Radical clock experiments have corroborated the production of free radicals. Reaction of (5-hexenyl)Co(dmgBF₂)₂L with [Ni(tmc)]OTf, followed by hydrolysis of the organonickel products, gave methylcyclopentane, consistent with the formation and cyclization of the 1-hexenyl radical. The second-order rate constants measured by stopped-flow experiments parallel the relative radical stabilities: R = CH₃ (2.43 x 10³ M^-1 s^-1) < C₂H₅ (7.88 x 10³ M^-1 s^-1) < CH(CH₃)₂ (19.2 x 10³ M^-1 s^-1), L = py (2.43 x 10³ M^-1 s^-1) < PFt₃ (5.01 x 10³ M^-1 s^-1). During the course of these studies the following molecules were also characterized by X-ray diffraction, CH₃Co(dmgBF₂)₂L, L = PEt₃, py, H₂O.

Full text of DOI:10.1021/ja963061z

1648
J. Am. Chem. Soc. 1997, 119, 1648-1655
Kinetics and Mechanism of Alkyl Transfer from
Organocobalt(III) to Nickel(I): Implications for the Synthesis of
Acetyl Coenzyme A by CO Dehydrogenase
M. S. Ram,† Charles G. Riordan,*,† Glenn P. A. Yap,‡ Louise Liable-Sands,‡
Arnold L. Rheingold,‡ Adam Marchaj,§ and Jack R. Norton§
Contribution from the Departments of Chemistry, Kansas State UniVersity,
Manhattan, Kansas 66506, UniVersity of Delaware, Newark, Delaware 19716, and
Colorado State UniVersity, Fort Collins, Colorado 80523
ReceiVed August 30, 1996X
Abstract: Reaction of CH3Co(dmgBF2)2L (dmgBF2 ) (difluoroboryl)dimethylglyoximato); L ) py, PEt3) with 2
equiv of [Ni(tmc)]OTf (tmc ) 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; OTf- ) CF3SO3-) gave
Co(dmgBF2)2py-, [Ni(tmc)]OTf2, and [Ni(tmc)CH3]OTf in 80% yield. The overall transformation provides the first
model for the transfer of a CH3 group from methylcobalamin to the Ni-containing enzyme carbon monoxide
-
dehydrogenase during acetyl coenzyme A synthesis. RRSS-[Ni(tmc)CH3](BAr′4) (BAr′4- ) B(3,5-(CF3)2C6H3)4
)
has been characterized by X-ray diffraction. The products and 1CH3Co(dmgBF2)2L:2[Ni(tmc)]OTf stoichiometry
of the reaction are consistent with a three-step mechanism initiated by electron transfer from [Ni(tmc)]OTf to CH3-
Co(dmgBF2)2L. The second step is rapid CH3-Co- bond homolysis yielding Co(dmgBF2)2L- and CH3•; then the
CH3 radical is captured by the second equivalent of [Ni(tmc)]OTf, yielding [Ni(tmc)CH3]OTf. Radical clock
experiments have corroborated the production of free radicals. Reaction of (5-hexenyl)Co(dmgBF2)2L with [Ni-
(tmc)]OTf, followed by hydrolysis of the organonickel products, gave methylcyclopentane, consistent with the
formation and cyclization of the 1-hexenyl radical. The second-order rate constants measured by stopped-flow
experiments parallel the relative radical stabilities: R ) CH3 (2.43 × 103 M-1 s-1) < C2H5 (7.88 × 103 M-1 s-1
)
< CH(CH3)2 (19.2 × 103 M-1 s-1), L ) py (2.43 × 103 M-1 s-1) < PEt3 (5.01 × 103 M-1 s-1). During the course
of these studies the following molecules were also characterized by X-ray diffraction, CH3Co(dmgBF2)2L, L )
PEt3, py, H2O.
Introduction
nickel ion is covalently linked by a bridging ligand, X (S2- or
RS- from a cysteine residue), to a [Fe4S4]2+/+ cluster. However,
the nickel primary coordination sphere ligand composition and
geometry appear different for the two sites. The nickel in cluster
A resides in a distorted planar S4 or S2N2 donor environment.
Spectroelectrochemical and EPR studies suggest that CO and
CH3 bind to the same state of the enzyme, which is one electron
reduced from the resting state.4 Vibrational spectroscopy has
provided a detailed picture of the CO and CH3 binding loci
within cluster A.5 CO is terminally bound to an iron and the
CH3 is bound to nickel as kinetically competent intermediates.
Consequently, Ragsdale, Spiro, and co-workers have proposed5a
a bimetallic catalytic mechanism reminiscent of the Monsanto
acetic acid cycle. Notable features of the mechanism include
nonenzymatic transfer of a methyl substituent from methylco-
balamin to nickel; bimetallic migratory insertion producing an
iron (or nickel) acetyl; and nucleophilic attack on the acetyl by
CoA or, alternatively, coordination of CoA to cluster A, yielding
a metal thiolate, followed by formal reductive elimination of
acetyl-CoA.
The nickel-containing enzyme carbon monoxide dehydroge-
nase (CODH), found in certain methanogenic, acetogenic, and
sulfate-reducing bacteria, enables these species to grow au-
totrophically by the catalysis of two distinct reactions, the
oxidation of CO to CO2 and the synthesis of acetyl-coenzyme
A (acetyl-CoA) via the Wood-Ljungdahl pathway.1 The latter
process, termed acetyl-CoA synthetase (ACS), requires the
multistep coupling of a CH3 group from a corrinoid protein (C/
Fe-SP) to CO and the thiolate, CoA.2 CODH has been subjected
to a complete array of spectroscopic studies which have resulted
in the development of working models for the active sites of
CO oxidation (cluster C) and acetyl-CoA synthesis (cluster A).3
Both contain unusual heterometallic clusters in which a single
* To whom correspondence should be addressed. E-mail:
Riordan@ksu.edu.
† Kansas State University.
‡ University of Delaware
§ Colorado State University.
X Abstract published in AdVance ACS Abstracts, January 15, 1997.
(1) (a) Ragsdale, S. W. Crit. ReV. Biochem. Mol. Biol. 1991, 26, 261-
300. (b) The Bioinorganic Chemistry of Nickel; Lancaster, J. R., Ed.; VCH
Publishers: New York, 1988; Chapters 13 and 14.
Model studies using mononuclear nickel species have repro-
duced many of the proposed steps in the catalytic cycle.6 Holm
has employed Sacconi’s NS3 tripodal ligand (NS3 ) N(CH2-
(2) (a) Ljungdahl, L. G. Annu. ReV. Microbiol. 1986, 40, 415. (b) Fuchs,
G. FEMS Microbiol. ReV. 1986, 39, 181-213. (c) Wood, H. G.; Ragsdale,
S. W.; Pezacka, E. Trends Biochem. Sci. 1986, 11, 14.
(4) Lu, W.; Harder, S. R.; Ragsdale, S. W. J. Biol. Chem. 1990, 265,
3124-3133.
(5) (a) Qiu, D.; Kumar, M.; Ragsdale, S. W.; Spiro, T. G. Science 1994,
264, 817-819. (b) Kumar, M.; Qiu, D.; Spiro, T. G.; Ragsdale, S. W.
Science 1995, 270, 628-630.
(6) (a) Stavropoulos, P.; Carrie´, M.; Muetterties, M. C.; Holm, R. H. J.
Am. Chem. Soc. 1990, 112, 5385-5386. (b) Stavropoulos, P.; Carrie´, M.;
Muetterties, M. C.; Holm, R. H. J. Am. Chem. Soc. 1991, 113, 8485-
8492. (c) Matsunaga, P.; Hillhouse, G. L. Angew. Chem., Int. Ed. Engl.
(3) (a) Cramer, S. P.; Eidsness, M. K.; Pan, W.; Morton, T. A.; Ragsdale,
S. W.; DerVartanian, D. V.; Ljungdahl, L. G.; Scott, R. A. Inorg. Chem.
1987, 26, 2477-2479. (b) Bastian, N. R.; Diekert, G.; Niederhoffer, E. G.;
Teo, B.; Walsh, C. P.; Orme-Johnson, W. H. J. Am. Chem. Soc. 1988, 110,
5581-5582. (c) Lindahl, P. A.; Ragsdale, S. W.; Mu¨nck, E. J. Biol. Chem.
1990, 265, 3880-3888. (d) Fan, C.; Gorst, C. M.; Ragsdale, S. W.; Hoffman,
B. M. Biochemistry 1991, 30, 431-435. (e) Xia, J.; Dong, J.; Wang, S.;
Scott, R. A.; Lindahl, P. A. J. Am. Chem. Soc. 1995, 117, 7065-7070.
S0002-7863(96)03061-2 CCC: $14.00 © 1997 American Chemical Society

Alkyl Transfer from Organocobalt(III) to Nickel(I)
J. Am. Chem. Soc., Vol. 119, No. 7, 1997 1649
CH2SR)3, R ) Pri, But) to prepare tbp Ni(NS3)X+ derivatives
including X ) CH3. The methyl complex reacts with CO to
Co(dmgBF2)2(H2O)2 was prepared in THF rather than Et2O as described
by Bakac and Espenson.12
-
RRSS- and RSRS-[Ni(tmc)CH3](X) (X ) OTf-, B(C6H5)4
,
yield isolable (NS3)NiC(O)CH3+, which subsequently reacts
+ 6a,b
B(C6F5)4-, B(3,5-(CF3)2C6H3)4-(BAr′4-), and PF6-). The synthesis
of each isomer proceeded similarly. [Ni(tmc)](B(C6F5)4)2 and [Ni(tmc)]-
(BAr′4)2 were prepared by methathesis of [Ni(tmc)]OTf2 with the
corresponding Na salt in acetone. The final products were washed with
hexanes containing 5% EtOH and then with hexanes containing 5%
Et2O. [Ni(tmc)](B(C6F5)4)2 and [Ni(tmc)](BAr′4)2 are soluble in THF.
The appropriate [Ni(tmc)](X)2 (1.0 g, 1.6 mmol) was reacted with
(CH3)2Mg (50% excess) in 1:1 THF-Et2O. The Mg salts were
separated by filtration and the solvents were removed in vacuo. [Ni-
(tmc)CH3](BAr′4) was recrystallized from Et2O-hexanes. [Ni(tmc)-
CH3](BAr′4) and [Ni(tmc)CH3](B(C6F5)4) were soluble in THF, Et2O,
and (CH3)2CO. In general, the RRSS derivatives were less soluble than
the RSRS species. Yields: 60%. UV-vis (THF), λmax, nm (ꢀ, M-1
cm-1): 352 (1700), 705 (40). 1H NMR ((CD3)2SO, 27 °C): RSRS-
[Ni(tmc)CH3][B(C6H5)4] δ 123.10, 101.28, 75.07, 51.83, 32.57, 7.19
(m, B(C6H5)4), 6.93 (m, B(C6H5)4), 6.80 (m, B(C6H5)4), -3.47, -7.47,
-332 (Ni-CH3); RRSS-[Ni(tmc)CH3][OTf], 13C δ 163.3, 139.4, 99.43,
74.96, 50.13, 34.83, 31.37, 17.10, -3.17, -5.40, -7.40, -11.6, -14.2,
-332 (Ni-CH3).
with thiols producing CH3C(O)R, Ni(0), and HNS3
.
The
latter reaction is proposed to occur via nucleophilic attack of
the thiolate on the acetyl ligand. Hillhouse has used planar
(bpy)NiSCH2CH2CH2 to produce thialactones under a CO
atmosphere.6c Recently, Holm has demonstrated CO insertion
into the CH3-Ni bond of (bpy)NiCH3(SR) yielding the first
stable acetyl(thiolato)nickel complex.6d Additional exposure to
CO results in reductive elimination of CH3C(O)SR. The latter
two studies provide precedent for nickel-mediated intramolecular
thioester formation.
While these studies provide a model for several of the
stoichiometric transformations relevant to ACS activity, prece-
dents for alkyl transfer from cobalt to nickel have not existed
in the organometallic literature. The studies contained herein
describe a successful strategy that provides the first example
of methyl transfer from CH3Co to Ni(I) yielding a stable CH3-
Ni species.7 Stopped-flow and radical clock experiments have
suggested a detailed mechanism. The results obtained on these
model systems are critically evaluated in the context of the
parameters of ACS catalysis. Additionally, we have elucidated
the structures of the organocobalt complexes and RRSS-[Ni-
(tmc)CH3](BAr′4) by X-ray diffraction.
RCo(dmgBF2)2L (R ) CH3, CH2CH3, CH(CH3)2, (CH2)4CHdCH2;
L ) py, PEt3, H2O). These complexes were originally prepared by
Schrauzer14 from CH3Co(dmgH)2L and BF3•Et2O. We found it more
efficient to prepare these complexes from Co(dmgBF2)2(H2O)2 in the
presence of L by standard reduction/alkylation procedures as exempli-
fied for CH3Co(dmgBF2)2py: Co(dmgBF2)2(H2O)2 (1.0 g, 2.4 mmol)
was suspended in 40 mL of CH3OH in a Schlenk flask under Ar. NaOH
(0.2 mL, 50%) was added followed by addition of py (0.2 mL, 2.5
mmol). The mixture was cooled to 10 °C and NaBH4 (0.15 g, 4.0
mmol) was added. The mixture was stirred for 5 min during which
time the color changed from brown to blue-green. (CH3)2SO4 (0.8 mL,
8.4 mmol) was added and the solution turned yellow over 10 min. The
remaining procedures were carried out under ambient conditions. An
additional 1.0 mL of py was added and the solution stirred for 5 min.
Fifty milliliters of H2O was added, and the product was filtered and
washed with 3 × 20 mL of H2O. Alternately, CH3Co(dmgBF2)2PEt3
was prepared from CH3Co(dmgBF2)2H2O by reaction with PEt3 in CH2-
Cl2. The (5-hexenyl)Co derivatives were prepared from 1-bromo-5-
hexene in CH3CN. Products were recrystallized from CH2Cl2-Et2O.
Yields: greater than 80%. Electronic spectra agreed with those
reported.12 1H NMR (CD3NO2, 27 °C): ((CH3)2CH)Co(dmgBF2)2py
δ 7.85(d, py, 2 H), 7.73 (t, py, 1 H), 7.38 (t, py, 2 H), 2.54 (s, CH3, 12
H), 2.19 (sept, CH, 1 H), 0.38 (d, CH(CH3)2, 6 H); (CH3-
CH2)Co(dmgBF2)2py δ 7.90 , 7.42 (py, 5 H), 2.45 (s, CH3, 12 H), 2.18
(q, CH2CH3, 2 H), 0.26 (t, CH2CH3, 3 H). 1H NMR ((CD3)2CO, 27
°C): (5-hexenyl)Co(dmgBF2)2py δ 8.04 (d, py, 2 H), 7.91 (t, py, 1 H),
7.41 (t, py, 2 H), 5.75 (m, CH, 1 H), 4.92 (m, CHCH2, 2 H), 2.46 (s,
CH3, 12 H), 1.96 (q, CH2, 2 H), 1.26 (q, CH2, 2 H), 0.88 (m, CH2, 2
H); (5-hexenyl)Co(dmgBF2)2PEt3 δ 5.72 (m, CH, 1 H), 4.87 (m,
CHCH2, 2 H), 2.40 (d, 5JP-H ) 4.27 Hz, CH3, 12 H), 2.13 (m, CH2, 2
H), 1.94 (q, CH2, 2 H), 1.63 (m, 2JP-H ) 8.3 Hz, PCH2, 6 H), 1.24 (q,
Experimental Section
Materials and Methods. All manipulations were carried out under
N2 or Ar using standard Schlenk and glovebox techniques. All solvents
were purified as previously described.8 NMR solvents were obtained
from Cambridge Isotope Labs and were used without further purifica-
tion. PEt3 and 1-bromo-5-hexene were used as received from Aldrich.
NMR spectra were recorded on a 400-MHz Varian Unity Plus equipped
with a Sun workstation. Samples of [Ni(tmc)CH3](X) required 10 mM
concentrations and 2K transients to obtain adequate signal-to-noise
ratios. Twenty hertz line broadening was used for processing the
1
paramagnetic H NMR spectra. One hertz line broadening was used
to process the 2H, 13C, and 19F data. GC-MS experiments were
performed on a Hewlett-Packard 5989A mass spectrometer interfaced
with a HP 5890-II gas chromatograph. Cyclic voltammetric experi-
ments were performed on a BAS 50W voltammetric analyzer employing
cells housed inside a Vacuum Atmospheres glovebox.8 Experiments
at sweep rates exceeding 2 V/s were performed on a BAS 100
voltammetric analyzer using a 50-µm Pt wire working electrode.9
Potentials were referenced to internal ferrocene/ferrocenium (Fc/Fc+,
E1/2 ) +801 mV vs NHE in THF). (CH3)2Mg was prepared by stirring
CH3MgBr in Et2O with 1 equiv of dioxane; the MgBr2 was filtered off
and (CH3)2Mg was used as a stock solution. NaBAr′4 and NaB(C6F5)4
were prepared according to Brookhart10 and characterized by 19F NMR
((CH3)2CO, 27 °C): NaBAr′4 δ 10.2 (s, CF3); NaB(C6F5)4 δ -94.6 (t,
8 F), -91.4 (t, 4 F), -62.4 (d, 8 F). RRSS- and RSRS-[Ni(tmc)]OTf2
3
CH2, 2 H), 0.94 (m, JP-H ) 13.4 Hz, PCH2CH3, 9 H), 0.64 (q, CH2,
2 H). In coordinating solvents such as CH3CN or (CH3)2SO, CH3Co-
(dmgBF2)2py exists as a 1:1 equilibrium mixture of CH3Co(dmgBF2)2py
and CH3Co(dmgBF2)2S. 1H NMR ((CD3)2SO, 27 °C): CH3Co-
(dmgBF2)2((CH3)2SO) δ 2.12 (s, CH3, 12 H), 0.88 (s, CH3Co, 3 H).
CH3Co(dmgBF2)2PEt3 showed no evidence of ligand exchange in these
solvents.
-
were synthesized as described by Barefield.11 The PF6 salts were
prepared from the OTf- salts as previously described.8 Ni(tmc)-
(OTf)•NaOTf was prepared as previously described7 and was character-
ized by its electronic spectrum in THF (ꢀ(352 nm) ) 3700 M-1 cm-1).
1994, 1748-1750. (d) Tucci, G. C.; Holm, R. H. J. Am. Chem. Soc. 1995,
117, 6489-6496. (e) Sellmann, D.; Schillinger, H.; Knoch, F.; Moll, M.
Inorg. Chim. Acta 1992, 198, 351. (f) Sellmann, D.; Ha¨ussinger, D.; Knoch,
F.; Moll, M. J. Am. Chem. Soc. 1996, 118, 5368-5374.
(7) Ram, M. S.; Riordan, C. G. J. Am. Chem. Soc. 1995, 117, 2365-
2366.
(8) Ram, M. S.; Riordan, C. G.; Ostrander, R.; Rheingold, A. L. Inorg.
Chem. 1995, 34, 5884-5892.
(9) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry, A
Series of AdVances; Bard, A. J., Ed.; Marcel Dekker: New York, 1987;
Vol. 15, p 328.
Conversion of CH3Co(dmgBF2)2py to CH3Co(dmgBF2)2H2O.
CH3Co(dmgBF2)2py was washed three times with 20 mL of 6 M HClO4
and subsequently washed with two times with 20 mL of H2O. CH3-
Co(dmgBF2)2H2O was precipitated from 1:1 ethyl acetate-(CH3)2CO
upon addition of 1:1 Et2O-hexanes. The aquo complex is highly
soluble in (CH3)2CO. This procedure is efficient in eliminating Co-
(dmgBF2)2(H2O)2, a common contaminant in the RCo(dmgBF2)2py
complexes.
(10) Brookhart, M.; Grant, B.; A. F. Volpe, J. Organometallics 1992,
11, 3920-3922.
(11) (a) Wagner, F.; Barefield, E. K. Inorg. Chem. 1973, 12, 2435-
2439. (b) Wagner, F.; Barefield, E. K. Inorg. Chem. 1976, 15, 408-417.
(12) Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1984, 106, 5197-
5202.
(13) Lin, S.; Juan, B. HelV. Chim. Acta 1991, 74, 1725.
(14) Schrauzer, G. N. Acc. Chem. Res. 1968, 1, 97-103.

1650 J. Am. Chem. Soc., Vol. 119, No. 7, 1997
Table 1. Crystallographic Data
Ram et al.
[Ni(tmc)CH3](BAr′4)
CH3Co(dmgBF2)2(PEt3)
CH3Co(dmgBF2)2(py)•2CH3NO2
CH3Co(dmgBF2)2(H2O)
formula
C47H47BF24N4Ni
1193.4
emerald green block
orthorhombic
Pbca
19.036(2)
19.890(2)
27.737(2)
10502(1)
8
C15H30B2CoF4N4O4P
517.9
brown block
tetragonal
I41/a
31.654(4)
31.654(4)
18.331(7)
18336(3)
32
C16H26B2CoF4N7O8
601.0
orange block
orthorhombic
Pbca
19.648(2)
9.789(2)
35.969(5)
5158(1)
C9H17B2CoF4N4O5
417.82
red block
orthorhombic
Pbca
12.551(3)
12.952(3)
19.390(2)
3152(1)
formula weight
color, habit
crystal system
space group
a, Å
b, Å
c, Å
V, Å3
Z
8
8
T, K
296
247
298
248
λ, Å (Mo KR)
F(calcd), g cm-3
µ, cm-1
Cu KR (1.54178)
1.510
Mo KR (0.71073)
1.499
Mo KR (0.71073)
1.548
Mo KR (0.71073)
1.761
16.53
8.78
7.49
11.63
R(F), R(wF2)a
0.0847, 0.2011a
0.0586, 0.1365a
0.0732, 0.1596a
0.0523, 0.1373a
2
2
a R(F) ) ∑∆/Σ(Fo), ∆ ) |Fo - Fc|; R(wF2) ) ∑[w(Fo - Fc2)2]/∑[(wFo )2]1/2
.
Reconversion to CH3Co(dmgBF2)2py. Pyridine (0.3 g) was added
to aqueous CH3Co(dmgBF2)2H2O. After 15 min a yellow precipitate
formed and was isolated by filtration and washed with 1:1 Et2O-
hexanes containing 1% py. Yield: 80%. The pyridine complex is
moderately soluble in (CH3)2CO, CH2Cl2, (CH3)2SO, and CH3CN. H
NMR ((CD3)2CO, 27 °C): δ 8.06 (d, py, 2 H), 7.92 (t, py, 1 H), 7.43
(t, py, 2 H), 2.44 (s, CH3, 12 H), 1.25 (s, CH3Co, 3 H).
methylcyclopentane (2.05 min) were resolved on a nonpolar RTX-1
column (30 m × 0.25 mm, 50 °C). In (CH3)2CO, it was necessary to
ramp the oven temperature (30 °C, 2 min-50 °C/min-50 °C, 8 min-
50 °C/min-300 °C) to minimize interference from the solvent. Under
these conditions retention times were 4.10 min for 1-hexene and 5.10
min for methylcyclopentane. In both cases, GC peaks were analyzed
by mass selection. While both 1-hexene and methylcyclopentane
displayed m/e 84, 69, and 56 amu, the ratio of intensities of m/e 84 to
69 were diagnostic for the two compounds: 0.65 for 1-hexene and
0.25 for methylcyclopentane.
1
Na[Co(dmgBF2)2(L)] (L ) py, PEt3). Co(dmgBF2)2(H2O)2 (500
mg, 1.2 mmol) was added to PEt3 (1.25 mL, 1 M in THF) in 20 mL of
THF. The mixture was vigorously stirred over Na/Hg (60 g 0.33%)
for 20 min. The THF solution was filtered through Celite. The product
was precipitated by addition of 20 mL of Et2O and 100 mL of hexanes
and subsequently washed with Et2O (3 × 20 mL). Recrystallization
from THF-Et2O yielded 75% of Na[Co(dmgBF2)2(PEt3)]. Na[Co-
(dmgBF2)2(py)] was prepared in an analogous manner. Na[Co-
(dmgBF2)2(PEt3)]: 1H NMR ((CD3)2CO, 27 °C) δ 1.89 (s, CH3, 12
H), 1.34 (q, PCH2, 6 H), 0.75 (t, PCH2CH3, 9 H); UV-vis (acetone),
Crystallographic Studies. Crystal, data collection, and refinement
parameters are given in Table 1. Suitable crystals were selected and
mounted with epoxy cement to glass fibers. The unit-cell parameters
were obtained by least-squares refinement of the angular settings of
24 reflections (20° e 2θ e 25°). The systematic absences in the
diffraction data are uniquely consistent with the space group Pbca for
RRSS-[Ni(tmc)CH3](BAr′4), CH3Co(dmgBF2)2py, and CH3Co(dmg-
BF2)2H2O and I41/a for CH3Co(dmgBF2)2PEt3. The structures were
solved using direct methods, completed by subsequent difference
Fourier syntheses, and refined by full-matrix least-squares procedures.
Semiempirical absorption corrections were applied to RRSS-[Ni(tmc)-
CH3](BAr′4) but were not required for CH3Co(dmgBF2)2PEt3, CH3Co-
(dmgBF2)2py, and CH3Co(dmgBF2)2H2O because there was less than
10% variation in the ψ-scan intensities. In the anion of RRSS-[Ni-
(tmc)CH3](BAr′4) the phenyl groups were fixed as rigid groups and
the fluorine atoms of the perfluoromethyl groups were constrained to
an idealized tetrahedron to conserve data. The carbon atom, C(9), in
RRSS-[Ni(tmc)CH3](BAr′4) is disordered over two positions with an
occupancy distribution of 60/40. The asymmetric unit of CH3Co-
(dmgBF2)2PEt3 consisted of two independent, but chemically equivalent
molecules. There are two nitromethane solvent molecules in the
asymmetric unit of CH3Co(dmgBF2)2py. The pyridine ring in CH3-
Co(dmgBF2)2py was fixed as a rigid group to conserve data and refined
isotropically along with the two solvent molecules. All other non-
hydrogen atoms were refined with anisotropic displacement coefficients.
The hydrogen atoms in RRSS-[Ni(tmc)CH3](BAr′4) were omitted and
all other hydrogen atoms were treated as idealized contributions. All
software and sources of the scattering factors are contained in the
SHELXTL (5.1) program libraries (G. Sheldrick, Siemens XRD,
Madison, WI).
λ
max, nm (ꢀ, M-1 cm-1) 622 (9500). 1H NMR ((CD3)2SO, 27 °C): δ
8.06 (d, py, 2 H), 7.92 (t, py, 1 H), 7.43 (t, py, 2 H), 1.76 (s, CH3, 12
H); UV-vis (acetone), λmax, nm (ꢀ, M-1 cm-1) 640 (9500).
Reactions of [Ni(tmc)CH3]OTf. [Ni(tmc)CH3]OTf was character-
ized by its reactions with H2O (D2O) and HgCl2. Hydrolysis of
[Ni(tmc)CD3](OTf) (prepared from (CD3)2Mg) with H2O in (CH3)2SO
yielded [Ni(tmc)OH](OTf)15 and CD3H. 1H NMR ((CD3)2SO, 27 °C):
δ 0.14 (sept, 2JH-D ) 1.95 Hz, CD3H). [Ni(tmc)CH3]OTf (60 mg, 0.13
mmol) reacted with HgCl2 (40 mg, 0.14 mmol) in 2 mL of (CH3)2SO
to yield a red solution and white precipitate. 1H NMR of red solution
2
199
((CD3)2SO, 27 °C): δ 0.75 (t, J
) 222 Hz, CH3HgCl).
Hg-H
Stopped-Flow Experiments. The stopped-flow system was con-
figured and used as described elsewhere16 with the following modifica-
tions. An On-Line Instrument Systems RSM 1000 rapid scanning
monochromator was used to scan the UV-vis region at 1-ms intervals
over an adjustable 300-nm wavelength range. The system was
controlled and the spectral and kinetic data were analyzed with OLIS
RSM software on a Dell 4100 DM microcomputer. The reagent
solutions were prepared and handled under Ar at 25 ( 0.1 °C. [Ni-
(tmc)]OTf was always present in at least 10-fold excess. For each set
of reagent concentrations, data from about six kinetic runs were
collected and the observed rate constants averaged.
GC-MS Experiments. Methane was separated on a 10" Carbo-
sphere column (130 °C) with N2 as carrier gas using a TCD. In a
representative experiment, CH3Co(dmgBF2)2L (1.0 mM) was reacted
with [Ni(tmc)]OTf (2.0 mM) in (CH3)2SO followed by hydrolysis in a
5-mL glass vial with a screw cap injection valve. CH4 was produced
in greater than 80% yield based on standard reactions (CH3MgBr
hydrolysis) at similar concentrations. Organic products from reactions
of (5-hexenyl)Co(dmgBF2)2L with [Ni(tmc)]OTf in (CH3)2SO for L )
py (or (CH3)2CO for L ) PEt3) followed by hydrolysis were monitored
as follows. In (CH3)2SO, 1-hexene (retention time ) 1.75 min) and
Results and Discussion
Choice of Model Complexes. As cluster A in CODH
appears to consist of a complex assembly of minimum stoichi-
ometry Ni-X-Fe4S4, we deliberately began these studies with
a monomeric nickel complex void of sulfur ligands. This
approach allows us to systematically evaluate the role of the
nickel primary coordination sphere and the covalent attachment
(15) Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1986, 108, 713-
718.
(16) Eisenberg, D. C.; Lawrie, C. J. C.; Moody, A. E.; Norton, J. R. J.
Am. Chem. Soc. 1991, 113, 4888-4895.

Alkyl Transfer from Organocobalt(III) to Nickel(I)
J. Am. Chem. Soc., Vol. 119, No. 7, 1997 1651
and consequently exhibits seven resonances for the tmc protons.
The RRSS isomer, lacking a mirror plane, displays 13 tmc
resonances Alternatively, [Ni(tmc)CD3]OTf was prepared from
CD3Co(dmgBF2)2py and [Ni(tmc)]OTf. The 2H NMR spectrum
contained a single sharp resonance at δ -332 (∆ν1/2 ) 140
Hz). Such a high-field resonance for the methyl protons is
consistent with the spin state S ) 1, which provides a significant
Fermi contact contribution to the chemical shift. (To date, all
five-coordinate derivatives of [Ni(tmc)(X)]OTf are high spin,
even with strong field apical ligands.8) The 1H NMR spectrum
of the products also contained resonances attributed to [Ni(tmc)]-
(OTf)2. Because many of the species in solution were para-
magnetic, it was not possible to quantify the Ni-containing
products by NMR. The fact that one product was [Ni(tmc)-
CH3]OTf was further corroborated by its characteristic reactions
with electrophiles, H2O and HgCl2. Addition of H2O to a
(CH3)2SO solution of [Ni(tmc)CD3]OTf yielded [Ni(tmc)OH]-
OTf and CD3H quantitatively.19 Alternatively, addition of
HgCl2 to a (CH3)2SO solution of [Ni(tmc)CH3]OTf resulted in
an immediate color change to red with precipitation of an off-
Figure 1. 1H NMR spectra of CH3Co(dmgBF2)2PEt3 (10 mM) in
acetone-d6 before (A) and after (B) addition of 2 equiv of [Ni(tmc)]OTf:
-
(*) CH3Co(dmgBF2)2PEt3, (#) Co(dmgBF2)2PEt3
.
1
white solid. The H NMR spectrum of the solution contained
of a Fe4S4 cluster to nickel,17 respectively, in determining the
course of the reactions. [Ni(tmc)](OTf)2 was chosen because
it is the only nickel complex in which the Ni(I) derivative has
been crystallographically characterized and the corresponding
methylnickel complex is stable;6b both the starting material and
desired product of methyl transfer are isolable species. In
choosing a suitable organocobalt complex, we required a system
that allowed for variation of R and L substituents on cobalt
and did not contain acidic hydrogens. The former criterion is
paramount in the systematic evaluation of the effect of the steric
and electronic parameters of the alkyl group and trans axial
ligand in driving the kinetics of the reaction. The latter arises
from the rapid reaction of [Ni(tmc)]OTf with protons, resulting
in oxidation to Ni(II) with liberation of H2.8 (For example,
[CH3Co([14]aneN4)(H2O)](OTf)2 and CH3Co(dmgH)2py oxidize
[Ni(tmc)]OTf in preference to methyl transfer.) Therefore, we
replaced the protons of the pseudomacrocycle in CH3Co-
2
199
a resonance for CH3HgCl (d 0.76, J Hg-H ) 222 Hz), in 50%
yield based on CH3Co(dmgBF2)2py.
The products and stoichiometry of the reaction are consistent
with a three-step mechanism triggered by single electron transfer
(SET) from [Ni(tmc)]OTf to CH3Co(dmgBF2)2L, eqs 1-3. This
-
e
CH3Co(dmgBF2)2L + Ni(tmc)+ transfer8
CH3Co(dmgBF2)2L- + Ni(tmc)2+ (1)
•
CH3Co(dmgBF2)2L- f Co(dmgBF2)2L- + CH3 (2)
•
+
Ni(tmc)+ + CH3 f Ni(tmc)CH3
(3)
mechanism is analogous to that proposed by Espenson and
Bakac for the reaction of CH3I with [Ni(tmc)]ClO4. In this
regard, the analogy of methylcobalamin serving as Nature’s
“CH3I equivalent” appears valid. The reduced CH3Co(dmg-
BF2)2L- is susceptible to facile CH3-Co homolysis which
generates free radicals and Co(I), Co(dmgBF2)2L-.20 In the final
step, the radical is captured by a second equivalent of [Ni(tmc)]-
OTf to yield [Ni(tmc)CH3]OTf. This colligation rate has been
measured by Espenson and Bakac in aqueous media for
1-hexenyl•. At 25 °C, the rate of capture of 1-hexenyl• by [Ni-
+
(dmgH)2py with (formally) BF2 to yield the macrocyclic
complex, CH3Co(dmgBF2)2py, first described by Schrauzer.18
Addition of CH3Co(dmgBF2)2L (L ) PEt3, py) to [Ni(tmc)]-
OTf resulted in an immediate color change from light green to
blue-green. The electronic spectrum of the products was
dominated by Co(dmgBF2)2L-, with a λmax at 640 nm (ꢀ ) 9
× 103 M-1cm-1), produced in greater than 80% yield. Spectral
titration of [Ni(tmc)]OTf with CH3Co(dmgBF2)2py or CH3Co-
(dmgBF2)2PEt3 displayed a limiting absorbance at 640 nm with
[[Ni(tmc)]OTf]/[CH3Co(dmgBF2)2L] equal to 2, establishing the
stoichiometry of the reaction. NMR spectroscopy was necessary
to determine the other reaction products, Figure 1. [Ni(tmc)-
CH3]OTf displayed a characteristic signal at δ -332 for the
Ni-CH3.13
(tmc)]ClO4 is 6 × 107 M-1 s-1 15
.
To corroborate the production
of free radicals in the present system, we employed (5-hexenyl)-
Co(dmgBF2)2L derivatives in the reaction with [Ni(tmc)]OTf.
1-Hexenyl Radical Probe. The 5-hexenyl radical undergoes
cyclization to the cyclopentylmethyl radical in H2O with a rate
constant of 2.2 × 105 s-1 at 25 °C.21 Consequently, the
corresponding 1-halo-5-hexenes have been used as probes. In
instances where both the cyclized and uncyclized products are
observed, the rate constant for radical capture has been
determined.22 (5-hexenyl)Co(dmgBF2)2L was prepared by
addition of 1-bromo-5-hexene to in situ generated solutions of
Co(dmgBF2)2L-. Alkylation of Co(I) derivatives proceeds
through a SN2 pathway resulting in no detectable rearrangement
This assignment was corroborated by two experiments. First,
[Ni(tmc)CH3]OTf was prepared independently from [Ni(tmc)]-
(OTf)2 and (CH3)2Mg according to Barefield.19 The RRSS and
RSRS isomers displayed identical 1H NMR chemical shifts for
the Ni-CH3 protons. The isomeric identity was established
by the number of resonances for the tmc protons (Experimental
Section). The RSRS isomer contains a mirror plane of symmetry
(17) In this context, we are aware of one model complex that contains
a Ni ion covalently linked to an Fe4S4 cluster. This report contains a crystal
structure but no spectroscopic nor catalytic data. Osterloh, F.; Saak, W.;
Haase, D.; Pohl, S. J. Chem. Soc., Chem. Commun. 1996, 777-778.
(18) Schrauzer, G. N.; Windgassen, R. J. J. Am. Chem. Soc. 1966, 88,
3738-3743.
(20) (a) Lexa, D.; Save´ant, J. J. Am. Chem. Soc. 1978, 100, 3220-3222.
(b) Martin, B. D.; Finke, R. G. J. Am. Chem. Soc. 1990, 112, 2419-2420.
(c) Elliot, C. M.; Hershenhart, E.; Finke, R. G.; Smith, B. L. J. Am. Chem.
Soc. 1981, 103, 5558-5566.
(21) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. Soc.
1981, 103, 7739-7742.
(19) D’Aniello, M. J.; Barefield, E. K. J. Am. Chem. Soc. 1976, 98,
1610-1611.
(22) Samuels, G. J.; Espenson, J. H. Inorg. Chem. 1979, 18, 2587.

1652 J. Am. Chem. Soc., Vol. 119, No. 7, 1997
Ram et al.
Table 2. Second-Order Rate Constants for the Reaction
RCo(dmgBF2)2L + 2[Ni(tmc)](OTf) at 25 °Ca
Table 3. Electrochemical Data for CH3Co(dmgBF2)2L (L ) PEt3,
py)
b
R
L
solvent
10-3k2 (M-1 s-1
)
L
Ef, Va
CH3
py
py
py
PEt3
py
py
py
PEt3
(CH3)2SO
(CH3)2SO
(CH3)2SO
(CH3)2CO
THF
THF
THF
2.43(4)
7.88(4)
19.2(10)
5.01(18)
4.51(16)c
11.1(23)
56(5)
PEt3
py
-1.02
-1.10
quasi-irrev
quasi-irrev
CH3CH2
(CH3)2CH
CH3
a Versus NHE in THF with internal Fc/Fc+ as indicated in the
Experimental Section.
CH3
CH3CH2
(CH3)2CH
CH3
derivatives, k2 for L ) PEt3 is ca. four times greater than k2
for L ) py. As discussed below, this difference in rate constants
reflects the difference in one-electron reduction potentials of
the cobalt complexes, with the L ) PEt3 complex more easily
reduced.
THF
17.0(3)
a Minor secondary spectral changes, slower by at least a factor of
10, were observed in some cases. b Errors refer to standard deviations
based on an average of six runs per [[Ni(tmc)]OTf]. c A nontrivial
intercept value of 74 s-1 was observed when the kobs values were plotted
against [[Ni(tmc)]OTf].
d[Co(dmgBF2)2L-]
) k2[Ni(tmc)OTf][RCo(dmgBF2)2L]
of the olefin.14 Reaction of (5-hexenyl)Co(dmgBF2)2L with 2
equiv of [Ni(tmc)]OTf was carried out in (CH3)2SO for L ) py
and acetone for L ) PEt3. In each case, the products were
hydrolyzed and the volatile organics were analyzed by GC-
MS. For L ) py, methylcyclopentane was the only product
detected. However, for L ) PEt3, methylcyclopentane and
several hexadiene isomers were present in the GC-MS.
Production of methylcyclopentane provides compelling evidence
for a free radical intermediate. Despite attempts over a range
of concentrations, we were unable to find conditions which
produced both uncyclized and cyclized products. Consequently,
the rate constant for 5-hexenyl radical capture by [Ni(tmc)]OTf
in organic solvents could not be determined.
Kinetics. Stopped-flow measurements were conducted with
an excess of [Ni(tmc)]OTf. Under these conditions pseudo-
first-order kinetics were observed for at least 3 half-lives by
monitoring the growth of Co(dmgBF2)2L- at the wavelength
of largest absorbance change, near λmax ) 628 nm for L ) PEt3
and λmax ) 640 nm for L ) py. The kobs values obtained from
these single-wavelength measurements agreed with those ob-
tained from a global fit of the data from 450 to 750 nm. Over
this 300-nm range isosbestic behavior was observed. Each kobs
value was determined by taking the mean of typically six values
from individual runs. The reaction obeyed a second-order rate
law (eq 4); second-order rate constants are listed in Table 2.
The trend of rate constants, R ) CH3 < CH3CH2 < (CH3)2CH,
L ) py, is consistent with the relative stabilities of the
corresponding alkyl radicals and is therefore in agreement with
our proposed mechanism.23 Bakac and Espenson have observed
a similar trend for RX reduction by [Ni(tmc)]ClO4.15 The rate
constants they measured in H2O are less than an order of
magnitude lower than our values in (CH3)2SO. This remarkable
agreement lends further support for similar mechanisms for both
systems.
dt
(4)
Cyclic Voltammetry of CH3Co(dmgBF2)2L Complexes.
We previously reported the CV for the L ) py derivative in
(CH3)2SO at a sweep rate of 20 mV/s.7 Under these conditions,
a chemically irreversible reduction at -990 mV (vs Ag/AgCl)
was attributed to an EC mechanism in which CH3Co(dmgBF2)2-
py is reduced to CH3Co(dmgBF2)2py-. The CH3-Co bond of
the latter species rapidly homolyzes to yield Co(dmgBF2)2py-
•
and CH3 . In accord with this mechanism, an anodic sweep
revealed an oxidative process at -430 mV which was present
only following the reductive process. The oxidative process is
assigned to the Co(II)/Co(I) couple by comparison with an
independently prepared sample of Co(dmgBF2)2py. The other
alkylcobalt complexes exhibited identical reduction potentials
under these conditions. Our qualitative CV observations agree
well with those reported for methylcobalamin20a and meth-
ylcobinamide.20b In those studies, Lexa and Save´ant and Finke,
respectively, reported rate constants for the CH3-Co(II) bond
homolysis (khom) of 4400 s-1. To compare these rate constants
to the present system, CV experiments were performed in THF
at fast sweep rates using microelectrodes. Instrumental limita-
tions prevented sweep rates greater than 150 V/s from being
accessed. These conditions were sufficient to observe up to
50% of the anodic current, i.e. ic/ia ) 2, for the back oxidation
of CH3Co(dmgBF2)2L-. By measuring the ic/ia ratio as a
function of sweep rate, values of khom were extracted as
described previously.8 The rates of khom ) 500 (L ) py) and
300 s-1 (L ) PEt3) are an order of magnitude less than those
of methylcobalamin and methylcobinamide. Furthermore, by
sweeping at these higher rates, the formal potentials for the
reductive process may be determined, Table 3. The py complex
is 80 mV more difficult to reduce than the PEt3 complex, and
correspondingly, the rate of reduction by [Ni(tmc)]OTf is a
factor of 4 slower. While the relative potentials of the [Ni-
(tmc)]OTf and CH3Co(dmgBF2)2L indicate the initial electron
transfer step is endergonic by at least 290 and 370 mV (values
are approximations because the CH3Co(III/II) formal potentials
are not known) for L ) PEt3 and py, respectively, the CH3-
Co(II) homolysis and CH3-Ni formation are exergonic26 and
more than compensate for the unfavorable energetics of the first
step.
Nucleophilic displacement of R-Co by [Ni(tmc)]OTf would
be expected to result in a trend opposite that in Table 2. For
example, Castro and Stolzenberg independently determined that
Ni(OEiBC)-, a Ni(I) tetrapyrrole, reacted with haloalkanes in
the order CH3 > primary > secondary > tertiary.25 Further-
more, the small solvent dependence a factor of ca. 2 for THF
vs DMSO of the rate constants in Table 2 argues against an
SN2 mechanism. Reactions of CH3I with metal complexes of
unit charge exhibit pronounced solvent effects, i.e. for CH3I +
CpFe(CO)2SPh-, kDMSO/kacetone is 12.24 Finally, for the methyl
Cyclic Voltammetry of [Ni(tmc)CH3]OTf. [Ni(tmc)CH3]-
OTf displayed a chemically irreversible oxidation at 50 mV
(sweep rate, 50 mV/s) yielding a species which upon subsequent
cathodic scanning was reduced at -1.0 V, Figure 2. The latter
(23) Shi, S.; Bakac, A.; Espenson, J. H. Inorg. Chem. 1991, 30, 3410-
3414.
(24) Ashby, M. T.; Enemark, J. H.; Lichtenberger, D. L. Inorg. Chem.
1988, 27, 191-197.
(25) (a) Helvenston, M. C.; Castro, C. E. J. Am. Chem. Soc. 1992, 114,
8490-8496. (b) Lahiri, G. K.; Schussel, L. J.; Stolzenberg, A. M. Inorg.
Chem. 1992, 31, 4991-5000.
(26) The Ni-Bzl BDE in [Ni(tmc)Bzl]OTf has been determined to be
18 kcal/mol by kinetic measurements. J. Halpern and M. Schofield, private
communication.

Alkyl Transfer from Organocobalt(III) to Nickel(I)
J. Am. Chem. Soc., Vol. 119, No. 7, 1997 1653
Table 4. Selected Bond Distances (Å) and Bond Angles (deg) for
[Ni(tmc)CH3](BAr′4), CH3Co(dmgBF2)2(PEt3),
CH3Co(dmgBF2)2py•2CH3NO2, and CH3Co(dmgBF2)2(H2O)
[Ni(tmc)CH3](BAr′4)
Ni-C(105)
Ni-N(1)
Ni-N(2)
Ni-N(3)
Ni-N(4)
2.04(1)
2.16(1)
2.14(1)
2.22(1)
2.10(1)
N(1)-Ni-N(2)
N(2)-Ni-N(3)
N(2)-Ni-N(4)
N(3)-Ni-N(4)
N(1)-Ni-N(4)
N(1)-Ni-C(105)
N(2)-Ni-C(105)
N(3)-Ni-C(105)
N(4)-Ni-C(105)
84.2(5)
91.7(5)
150.6(4)
84.9(4)
90.7(5)
97.9(5)
103.4(5)
99.1(5)
106.0(5)
CH3Co(dmgBF2)2(PEt3)
Co-C(15)
Co′-C(15′)
Co-N(1)
Co′-N(1′)
Co-N(2)
Co′-N(2′)
Co-N(3)
Co′-N(3′)
Co-N(4)
Co′-N(4′)
Co-P
2.036(5)
2.038(4)
1.857(4)
1.859(4)
1.858(4)
1.862(4)
1.862(4)
1.859(4)
1.869(4)
1.852(4)
2.363(1)
2.371(2)
N(1)-Co-N(2)
N(1′)-Co′-N(2′)
N(1)-Co-N(3)
N(1′)-Co′-N(3′)
N(2)-Co-N(4)
N(2′)-Co′-N(4′)
N(3)-Co-N(4)
N(3′)-Co′-N(4′)
C(15)-Co-P
C(15′)-Co′-P′
N(1)-Co-P
81.8(2)
81.5(2)
98.3(2)
98.2(2)
98.2(2)
98.5(2)
81.2(2)
81.5(2)
179.6(2)
179.2(2)
90.5(1)
92.6(1)
96.0(1)
91.0(1)
89.0(1)
92.6(1)
94.4(1)
91.5(1)
Figure 2. Cyclic voltammogram of [Ni(tmc)CH3](OTf) in DMSO.
Parameters are indicated in the Experimental Section. Peak assignments
are contained in the text.
Co′-P′
N(1′)-Co′-P′
N(2)-Co-P
N(2′)-Co′-P′
N(3)-Co-P
N(3′)-Co′-P′
N(4)-Co-P
N(4′)-Co′-P′
CH3Co(dmgBF2)2py•2 CH3NO2
Co-C(14)
Co-N(1)
Co-N(2)
Co-N(3)
Co-N(4)
Co-N(5)
2.007(8)
1.876(7)
1.851(7)
1.874(6)
1.846(7)
2.119(4)
N(1)-Co-N(2)
N(1)-Co-N(3)
N(2)-Co-N(4)
N(3)-Co-N(4)
C(14)-Co-N(1)
C(14)-Co-N(2)
C(14)-Co-N(3)
C(14)-Co-N(4)
C(14)-Co-N(5)
81.3(3)
175.1(3)
176.2(3)
82.2(3)
89.4(3)
85.2(3)
85.8(3)
91.1(3)
179.7(3)
Figure 3. PLUTO drawing of the cation of [Ni(tmc)CH3](BAr′4). The
minor isomer and hydrogen atoms are omitted for clarity.
event was assigned to the Ni(II)/Ni(I) couple.8 The anodic peak
was attributed to an EC mechanism entailing oxidation to [Ni-
(tmc)CH3]2+ followed by CH3-Ni(III) homolysis yielding [Ni-
•
(tmc)]2+ and CH3 . Efforts to observe the back reduction in
CH3Co(dmgBF2)2H2O
competition with homolysis by sweeping at scan rates up to
150 V/s were unsuccessful. Consequently, the lower limit for
CH3Ni(III) homolysis is at least khom ) 800 s-1. This value is
an order of magnitude greater than for the homolysis of another
CH3Ni(III), [CH3Ni([14]aneN4)](ClO4)2.27 The difference in
rates may reflect the greater steric crowding at nickel and
consequently weaker CH3Ni bond, in the N-methylated complex.
Molecular Structure of [Ni(tmc)CH3](BAr′4). Attempts to
determine the molecular structure of [Ni(tmc)CH3](X), X )
OTf-, PF6-, failed due to crystal twinning which resulted in
data sets which could not be satisfactorily refined. Therefore,
a large counteranion was sought which would significantly alter
the crystal packing of the molecule. Crystals of [Ni(tmc)CH3]-
(BAr′4) grown from THF-hexanes proved suitable for X-ray
analysis. The crystal structure consists of discrete ions with
Co-C(9)
Co-N(1)
Co-N(2)
Co-N(3)
Co-N(4)
Co-O(5)
2.000(6)
1.866(4)
1.864(5)
1.854(5)
1.878(4)
2.127(4)
N(1)-Co-N(2)
N(1)-Co-N(3)
N(2)-Co-N(4)
N(3)-Co-N(4)
C(9)-Co-N(1)
C(9)-Co-N(2)
C(9)-Co-N(3)
C(9)-Co-N(4)
C(9)-Co-O(5)
81.6(2)
179.9(2)
179.2(2)
82.0(2)
92.1(2)
87.7(2)
87.9(2)
93.0(2)
177.4(2)
ing 2.15(2) Å and the CH3 in the apical position, Ni-C 2.04(1)
Å. The Ni is displaced from the N4 plane toward the alkyl
ligand by 0.43 Å. The Ni-N distances are similar to those of
other five-coordinate Ni(tmc)X+ derivatives28 such as the
arylthiolates recently prepared by us, RSRS-[Ni(tmc)SC6H5]PF6,
Ni-Nav, 2.15(1) Å,8 and in accord with the high-spin state. The
Ni-C bond length is at the upper limits of the range for other
Ni(II)-C(sp3) lengths, 1.81-2.02 Å.29 Recently, Latos-Gra-
zynski has reported the only other structure of a paramagnetic
organonickel(II) complex, (21-CH3CTPP)Ni.30 The N3C coor-
+
no close contacts between Ni(tmc)CH3 and BAr′4-. A view
of the cation is presented in Figure 3 and selected metric
parameters are contained in Table 4. Despite starting with
isomerically pure RSRS-[Ni(tmc)](BAr′4)2, the Ni(tmc)CH3+ ion
shows a 60:40 distribution of the RSRS and RRSS isomers of
the tmc ligand. This is manifest in the central methylene carbon
of one of the six-membered rings, C(9), which is disordered
over two positions with the above occupancies. The geometry
about the Ni ion is square pyramidal, with the four amine
nitrogens providing basal ligation with Ni-N distances averag-
(28) (a) Wagner, F.; Mocella, M. T.; D’Aniello, M. J.; Wang, A. H.-J.;
Barefield, E. K. J. Am. Chem. Soc. 1974, 96, 2627-2635. (b) D’Aniello,
M. J.; Mocella, M. T.; Wagner, F.; Barefield, E. K.; Paul, I. C. J. Am.
Chem. Soc. 1975, 97, 192-194. (c) Lincoln, S. F.; Hambley, T. W.;
Pisaniello, D. L.; Coates, J. H. Aust. J. Chem. 1984, 37, 713-723. (d) Crick,
I. S.; Hoskins, B. F.; Tregloan, P. A. Inorg. Chim. Acta 1986, 114, L33-
L34.
(27) (a) Sauer, A.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1988, 27,
4578. (b) Kelley, D. G.; Marchaj, A.; Bakac, A.; Espenson, J. H. J. Am.
Chem. Soc. 1991, 113, 7583-7587.
(29) Jolly, P. W. In ComprehensiVe Organometallic Chemistry; Wilkin-
son, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol.
6, p 37.

1654 J. Am. Chem. Soc., Vol. 119, No. 7, 1997
Ram et al.
Table 5. Comparative Bond Lengths (Å) for CH3Co(dmgX)2L
Complexes
X
L
Co-C
Co-Nav
Co-L
BF2
H
BF2
H
py
py
H2O
H2O
PEt3
2.007(8)
1.998(5)
2.000(6)
1.990(5)
2.036(5)
2.038(5)
2.011(3)
2.019(3)
2.026(1)
1.861(13)
1.899(8)
1.865(9)
1.890(9)
1.861(8)
1.858(8)
1.879(8)
1.882(8)
1.890(10)
2.119(4)
2.068(3)
2.127(4)
2.058(3)
2.363(4)
2.371(2)
2.291(1)
2.295(1)
2.418(1)
BF2
H
H
PMe3
PPh3
distinct conformations appear to be a consequence of solid state
effects. In solution at room temperature, facile equilibration
1
between the two isomers occurs on the H and 19F NMR time
scales. The plane of the pyridine ring in CH3Co(dmgBF2)2py
is perpendicular to the plane defined by the cobalt and two boron
atoms. The pyridine orientation is offset by 90° from that
typically observed in XCo(dmgH)2py structures.31a,32 The steric
influence of the BF2 groups is most likely responsible for this
change. Rotation of the pyridine by 90° would result in short
van der Waals’ contacts between the fluorine and the pyridine
ortho hydrogen. Table 5 lists comparative cobalt-ligand bond
lengths for the bis(difluoroboryl)dimethylglyoximato and di-
methylglyoximato complexes. The Co-C bond distances are
approximately 2.00 Å and invariant to the other ligands on
cobalt. The Co-N distances for the (dmgBF2)2 macrocycle are
shorter by ca. 0.03 Å than for the corresponding (dmgH)2
derivatives. This finding is consistent with the electronic spectra
which display shifts to higher energy for the absorption bands
of the (dmgBF2)2 complexes. While the Co-C bond length is
insensitive to these changes at the cobalt,33 the Co-L length
responds significantly. The (dmgBF2)2 complexes have longer
Co-L lengths, e.g., CH3Co(dmgH)2(H2O), Co-O, 2.058(3) Å;
CH3Co(dmgBF2)2(H2O), Co-O, 2.127(4) Å.
Summary
The following are the principal findings and conclusions of
this study.
(1) Methyl transfer from CH3Co(dmgBF2)2L to [Ni(tmc)]OTf
occurs in high yield by a radical mechanism involving free
methyl radicals. Support for the mechanism includes radical
clock experiments using (5-hexenyl)Co(dmgBF2)2L and kinetic
measurements which are in accord with rate-limiting electron
transfer. The trend of second-order rate constants for various
alkyl groups parallels the relative stability of the corresponding
free radicals. The rate constants are several orders of magnitude
greater than the most efficient SN2 methyl transfer reactions.34
Curiously, the rates are similar to the value of 1.2 s-1 at 25 °C
measured by Ragsdale for the reaction of methylated C/Fe-SP
with reduced CODH.5b
Figure 4. Thermal ellipsoid plots of (A) CH3Co(dmgBF2)2PEt3, (B)
CH3Co(dmgBF2)2py•2CH3NO2, and (C) CH3Co(dmgBF2)2H2O. Ther-
mal ellipsoids are drawn at the 40% probability level. The solvent
molecules in B and the hydrogen atoms are omitted for clarity.
dination core is essentially planar with the short Ni-C length
of 2.005(6) Å reflecting constraints imposed by the macrocycle.
Molecular Structures of CH3Co(dmgBF2)2L (L ) py, PEt3,
H2O). The molecular structure of each derivative is displayed
in Figure 4 with selected metric parameters contained in Table
4. This study is the first report of the structures of organocobalt
complexes containing the bis(difluoroboryl)dimethylglyoximato
macrocycle. The corresponding methylcobaloxime derivatives,
L ) py, PPh3, PMe3, and H2O, have been crystallographically
characterized previously.31 In CH3Co(dmgBF2)2py•2CH3NO2
and CH3Co(dmgBF2)2H2O the macrocycles are in the extended
chair conformations, i.e. one BF2 resides below the plane defined
by the four dimethylglyoximato nitrogens and one is above said
plane. CH3Co(dmgBF2)2PEt3 crystallized with two independent
molecules in the asymmetric unit cell. In general, the metric
parameters for each are identical within experimental error;
however, one molecule is in an extended chair conformation
and the other in an extended boat (latter not shown). These
(2) The electron transfer pathway is accessible in the present
system due to favorable thermodynamicssthe electron transfer
(31) (a) Bigotto, A.; Zangrando, E.; Randaccio, L. J. Chem. Soc., Dalton
Trans. 1976, 96-104. (b) Bresciani-Pahor, N.; Calligaris, M.; Randaccio,
L.; Marzilli, L. G. Inorg. Chim. Acta 1979, 32, 181-187. (c) Bresciani-
Pahor, N.; Randaccio, L.; Toscano, P. G.; Sandercock, A. C.; Marzilli, L.
G. J. Chem. Soc., Dalton Trans. 1982, 129-134. (d) McFadden, D. L.;
McPhail, A. T. J. Chem. Soc., Dalton Trans. 1974, 363-366.
(32) (a) Lenhert, G. P. J. Chem. Soc., Chem. Commun. 1967, 980. (b)
Adams, W. W.; Lenhert, G. P. Acta Crystallogr. 1973, B29, 2412.
(33) (a) Rossi, M.; Glusker, J. P.; Randaccio, L.; Summers, M. F.;
Toscano, P. J.; Marzilli, L. G. J. Am. Chem. Soc. 1985, 107, 1729-1738.
(b) Lo´pez, C.; Alvarez, S.; Font-Bard´ıa, M. J. Organomet. Chem. 1991,
414, 245-259.
(30) Chmielewski, P. J.; Latos-Grazynski, L.; Glowiak, T. J. Am. Chem.
Soc. 1996, 118, 5690-5701.
(34) Lewis, E. S.; Kukes, S.; Slater, C. D. J. Am. Chem. Soc. 1980, 102,
1619-1623.

Alkyl Transfer from Organocobalt(III) to Nickel(I)
J. Am. Chem. Soc., Vol. 119, No. 7, 1997 1655
step is only slightly endergonic as determined by the relative
formal potentials of Ni(II/I) and CH3Co(III/II). This reaction
is energetically compensated by the CH3-Ni bond forming step
which is enthalpically favored26 and therefore results in an
overall process which is thermodynamically driven. The
energetics of the enzyme are significantly different than those
of the present model. Electron transfer is undoubtedly more
endergonic (although the CH3Co(III/II) potential for the C/Fe-
SP has not been reported its value in other cobalamins is -1.36
V20a) due to the more modest cluster A reduction potential.
However, again CH3-Ni bond making may compensate for this
enthalpic cost. Alternatively, Nature may have chosen meth-
ylcobalamin as the penultimate methyl donor for ACS precisely
because its reduction potential is inaccessible by cluster A at
kinetically relevant rates. The enzyme may be protecting itself
from radical pathways and favoring two-electron events such
as nucleophilic displacements. Clearly, a complete understand-
ing of the mechanistic parameters of ACS awaits our ability to
evaluate the role of the Fe4S4 cluster in catalysis.17 Studies in
progress are designed to evaluate the role of thiolate ligands to
nickel and covalent attachment of a Fe4S4 cluster in modulating
the methyl transfer reaction.
length of 2.04(1) Å is within the range of other Ni(II)-C(sp3)
bond lengths and shows little or no effect from the high-spin
state of the metal.
Acknowledgment. This paper is dedicated to Professor
James H. Espenson on the occasion of his 60th birthday for his
many significant contributions to our understanding of alkyl
transfer reactions. C.G.R. gratefully acknowledges the financial
support of the National Science Foundation (OSR-9255223 and
NYI Award, 1994-99), and the donors of The Petroleum
Research Fund, administered by the American Chemical Society
(27076-G3). A.M. and J.R.N. acknowledge the financial support
of the National Institutes of Health (GM48537). We thank Dr.
B. Plashko for assistance with the GC-MS experiments and Prof.
M. M. Collinson for the use of the BAS 100 voltammetric
analyzer.
Supporting Information Available: Tables containing the
structure determination summary, atomic coordinates, bond
lengths, and bond angles for [Ni(tmc)CH3](BAr′4), CH3Co-
(dmgBF2)2(PEt3), CH3Co(dmgBF2)2(py)•2CH3NO2, and CH3-
Co(dmgBF2)2(H2O) (32 pages). See any current masthead page
for ordering and Internet access instructions.
(3) The structure of a paramagnetic organonickel complex
has been determined by X-ray diffraction. The Ni-C bond
JA963061Z