The First X-ray Structural Evidence Demonstrating Thiolate Coordination in an Organocobalt B12 Model Complex: Implications for Methionine Synthase

Article abstract of DOI:10.1021/ic961019z

Enzyme-bound methyl-B₁₂ transfers its methyl group to homocysteine during methionine synthesis. However, treatment of several types of organocobalt B₁₂ models with arene- and alkanethiolates under ambient conditions leads only to thiolate ligation. The structure of [AsPh₄][EtSCo(DH)₂CH₃] (DH = monoanion of dimethylglyoxime). the first characterization by X-ray crystallography of an organocobalt complex containing a unidentate coordinated thiolate, demonstrates unambiguously the S-ligation of ethanethiolate to Co, trans to the CH₃ ligand. This compound contains a very long Co-S bond (2.342(2) ?). However, the length of the Co-C bond (2.005(7) ?) is typical; this result strongly supports reported FT-Raman spectroscopic data indicating that the thiolate-type ligand does not have a strong trans influence and does not significantly weaken the Co-C bond in the ground state. Since a strong trans influence alkyl ligand weakens the trans Co-C bond, we examined the effect of EtS^- on Co-((DO)(DOH)pn)(CH₃)₂ [(DO)(DOH)pn = N²,N^2′-propanediylbis(2,3-butanedione 2-imine 3-oxime) is an imine/oxime quadridentate ligand]. Even for this compound, no attack on the Co-C bond was observed, although independently synthesized EtSCo((DO)(DOH)pn)CH₃ was stable. Furthermore, thiolate did not cleave the Co-C bond of an organocobalt complex with a highly distorted Co-C group. Several new spectroscopic and ligand-exchange reactions were observed in this study. Ligand-responsive NMR shift trends in these other new complexes also indicate that thiolate ligands have a weak trans influence. ? Abstract published in Advance ACS Abstracts, January 1, 1997.

Full text of DOI:10.1021/ic961019z

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Inorg. Chem. 1997, 36, 307-313
307
The First X-ray Structural Evidence Demonstrating Thiolate Coordination in an
Organocobalt B12 Model Complex: Implications for Methionine Synthase
Suzette M. Polson, Lory Hansen, and Luigi G. Marzilli*
Department of Chemistry, Emory University, Atlanta, Georgia 30322
ReceiVed August 21, 1996X
Enzyme-bound methyl-B12 transfers its methyl group to homocysteine during methionine synthesis. However,
treatment of several types of organocobalt B12 models with arene- and alkanethiolates under ambient conditions
leads only to thiolate ligation. The structure of [AsPh4][EtSCo(DH)2CH3] (DH ) monoanion of dimethylglyoxime),
the first characterization by X-ray crystallography of an organocobalt complex containing a unidentate coordinated
thiolate, demonstrates unambiguously the S-ligation of ethanethiolate to Co, trans to the CH3 ligand. This compound
contains a very long Co-S bond (2.342(2) Å). However, the length of the Co-C bond (2.005(7) Å) is typical;
this result strongly supports reported FT-Raman spectroscopic data indicating that the thiolate-type ligand does
not have a strong trans influence and does not significantly weaken the Co-C bond in the ground state. Since
a strong trans influence alkyl ligand weakens the trans Co-C bond, we examined the effect of EtS- on Co-
((DO)(DOH)pn)(CH3)2 [(DO)(DOH)pn ) N2,N2′-propanediylbis(2,3-butanedione 2-imine 3-oxime) is an imine/
oxime quadridentate ligand]. Even for this compound, no attack on the Co-C bond was observed, although
independently synthesized EtSCo((DO)(DOH)pn)CH3 was stable. Furthermore, thiolate did not cleave the Co-C
bond of an organocobalt complex with a highly distorted Co-C group. Several new spectroscopic and ligand-
exchange reactions were observed in this study. Ligand-responsive NMR shift trends in these other new complexes
also indicate that thiolate ligands have a weak trans influence.
Introduction
Scheme 1. Possible Reactions of Thiolates with
Organocobalt Compounds
Methionine synthase (MS) is a methylcobalamin (MeCbl,
MeB12)-dependent enzyme that catalyzes the reaction that
converts homocysteine to methionine.1 One proposed mecha-
nism for this alkylation reaction involves demethylation of
MeCbl by homocysteine attack at the C of the CoIIICH3 group
of MeCbl to form a CoICbl intermediate.2 Thus, elucidating
the interaction of thiolates with Co(III) complexes is of
considerable importance for understanding the MeCbl function.
One potential mode for activating the Co(III)-C bond for
demethylation is to replace the ligand trans to the alkyl group
with a better donor.3-5 However, a FT-Raman study of the
influence of the trans ligand on the Co-C stretching frequency
suggests that significant Co-C bond weakening will occur only
if the trans ligand is a very strong donor, e.g. an alkyl ligand.4
Reactions of model systems have been used to address various
aspects of thiolate B12 chemistry, including three types of
reactions of thiolates with organocobalt compounds (Scheme
1): (i) attack at the organometallic carbon to form a thioether;
(ii) reaction (i) followed by cleavage of the Co-C bond; or
(iii) coordination at the Co trans to the alkyl group (thiolate
ligation). The reports of reactions of thiolates with organocobalt
complexes have been controversial. The reported result that
thiol-promoted Co-C bond cleavage (reaction ii) of organo-
cobaloximes (LCo(DH)2R, Chart 1) occurred under neutral to
acidic conditions6,7 could not be reproduced.8-10 Treatment of
MeCbl with the thiolate form of â-mercaptoethanol resulted in
methylation of the thiolate and Co-C bond cleavage (reaction
ii).11 Another reaction involving methylation of a thiolate by
an organocobalt compound (reaction ii) was reported recently12
for the five-coordinate model CH3CoIIIPc (Pc ) phthalocyanine).
The thiolate ligation reaction pathway (reaction iii) has been
invoked in several instances for MeCbl and organocobalt B12
X Abstract published in AdVance ACS Abstracts, January 1, 1997.
(1) Marzilli, L. G. In Bioinorganic Catalysis; Reedijk, J., Ed.; Marcel
Dekker, Inc.: New York, 1993; p 227.
(2) Matthews, R. G.; Drummond, J. T. Chem. ReV. 1990, 90, 1275.
(3) Puckett, J. M., Jr.; Mitchell, M. B.; Hirota, S.; Marzilli, L. G. Inorg.
Chem. 1996, 35, 4656.
(4) Hirota, S.; Polson, S. M.; Puckett, J. M., Jr.; Moore, S. J.; Mitchell,
M. B.; Marzilli, L. G. Inorg. Chem. 1996, 35, 5646.
(5) Drennan, C. L.; Huang, S.; Drummond, J. T.; Matthews, R. G.;
Ludwig, M. L. Science (Washington, D.C.) 1994, 266, 1669.
(6) Schrauzer, G. N.; Windgassen, R. J. J. Am. Chem. Soc. 1967, 89, 3607.
(7) Schrauzer, G. N.; Stadlbauer, E. A. Bioinorg. Chem. 1974, 3, 353.
(8) Frick, T.; Francia, M. D.; Wood, J. M. Biochim. Biophys. Acta 1976,
428, 808.
(9) Brown, K. L.; Kallen, R. G. J. Am. Chem. Soc. 1972, 94, 1894.
(10) Agnes, G.; Hill, H. A. O.; Pratt, J. M.; Ridsdale, S. C.; Kennedy, F.
S.; Williams, R. J. P. Biochim. Biophys. Acta 1971, 252, 207.
(11) Hogenkamp, H. P. C.; Bratt, G. T.; Sun, S.-Z. Biochemistry 1985, 24,
6428.
(12) Galezowski, W.; Lewis, E. S. J. Phys. Org. Chem. 1994, 7, 90.
S0020-1669(96)01019-1 CCC: $14 00 © 1997 American Chemical Society

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308 Inorganic Chemistry, Vol. 36, No. 3, 1997
Polson et al.
Chart 1
Experimental Section
Materials and Methods. (Abbreviations: N-MeImd ) N-meth-
ylimidazole; py ) pyridine.) The following were prepared as previously
reported: [Co(DH)2py]2,16,17 pyCo(DH)2Cl,18 H2OCo(DH)2CH3,18 Co-
((DO)(DOH)pn)Cl2,19,20 [pyCo((DO)(DOH)pn)Cl]PF6,21 [H2OCo((DO)-
(DOH)pn)CH3]ClO4,18 [Me3PCo((DO)(DOH)pn)CH3]PF6,4 [BrCo(C1-
py)]ClO4,22 [CH3Co(C1py)]ClO4.22 [N-MeImdCo((DO)(DOH)pn)Cl]PF6
was prepared similarly to the procedure for the ClO4 salt.23 Me2PhPCo-
(DH)2CH3 was prepared from t-BupyCo(DH)2CH324 by a simple ligand-
exchange reaction. Caution! Perchlorate salts may be explosiVe and
should be handled in small quantities with appropriate precautions.25
3,5-Dimethylthiophenol (3,5-Me2PhSH), 2,6-dimethylthiophenol ) (2,6-
Me2PhSH), sodium p-toluenesulfinate dihydrate (Na[4-MePhSO2]‚
2H2O), p-thiocresol (4-MePhSH), potassium isopropylxanthate (K[i-
C3H7OCS2]), and sodium ethanethiolate (NaSEt, technical grade) were
obtained from Aldrich, AgNO3 was obtained from Apache Chemicals,
and [NEt4]OH (25% w/w solution in MeOH) was obtained from Sigma.
Analytical results (Atlantic Microlabs, Atlanta, GA) and selected NMR
data (GE QE300; referenced vs TMS) are listed in the Supporting
Information.
[AsPh4][EtSCo(DH)2CH3]‚H2O. In the drybox, a methanolic
solution of NaSEt (0.07 g in 1-2 mL) was added to a solution of H2-
OCo(DH)2CH3 (0.20 g, 0.62 mmol) in MeOH (7 mL), and the red
solution was stirred for 30 min. A solution of [AsPh4]Cl (0.28 g, 0.62
mmol in 1 mL of H2O) was added to the reaction mixture outside the
drybox, and the solution was quickly placed on a rotary evaporator to
minimize exposure to air. After most of the MeOH was removed, the
bright orange solid that precipitated was quickly isolated by vacuum
filtration, washed with H2O, and dried under vacuum for 5 h before
being reintroduced into the drybox. Yield: 0.35 g (75%).
[NEt4][3,5-Me2PhS] (Based on a Known Procedure26). In the
drybox, 3,5-Me2PhSH (4.50 g, 32.6 mmol) was added to anhydrous
MeOH (50 mL) in a three-necked flask. [NEt4]OH (27 mL of 25%
w/w solution in MeOH, 36 mmol) was then added. The covered flask
was removed from the drybox, and the mixture was heated under
nitrogen at reflux for 2.5 h. After the flask had cooled, MeOH was
removed under reduced pressure, and the flask was then reintroduced
into the drybox. The white solid was collected and recrystallized from
CH3CN/Et2O at -35 °C. Yield: 6.05 g (69%). [NEt4][4-MePhS] and
[NEt4][2,6-Me2PhS] were prepared similarly.
[AsPh4][3,5-Me2PhSCo(DH)2CH3]‚1.5H2O. In the drybox, metha-
nolic solutions of [NEt4][3,5-Me2PhS] (0.14 g, 0.51 mmol in 1 mL)
and H2OCo(DH)2CH3 (0.20 g, 0.62 mmol in 7 mL) were mixed, and
the red solution was stirred for 30 min. Outside the drybox, the solution
was diluted with water (20 mL) and treated with [AsPh4]Cl (0.28 g,
0.62 mmol in 1 mL of H2O). The solution was placed in a rotary
evaporator quickly to minimize exposure to air. After most of the
MeOH was removed, the bright orange solid that precipitated was
isolated quickly by vacuum filtration, washed with H2O, and dried under
vacuum. Yield: 0.08 g (16%).
models, although no characterized isolated complexes of this
type have been reported. The first reports of thiolato organo-
cobaloximes9 and imine/oxime-type organocobalt(III) com-
plexes13 {[LCo((DO)(DOH)pn)R]X, Chart 1} used UV-vis
1
measurements as evidence of thiolate ligation.9,13 Later, H
NMR evidence for a thiolato organocobaloxime was presented,
but the isolated complex could not be adequately characterized.14
It was suggested that CH3 transfer from CH3CoIIIPc was
inhibited by the formation of the inactive thiolato complex CH3-
CoIIIPc(SPh);12 however, this complex was not isolated. In
contrast, it was suggested that thiolate ligation did not occur in
MeCbl (thus replacing the 5,6-Me2Bzm) since no change was
observed in the 13C NMR spectrum of a sample of MeCbl
treated with the thiolate form of dithiothreitol.11
The mechanisms of several reactions involving Co species
and sulfur compounds are not fully understood, but these could
involve Co species with coordinated sulfur ligands. For
example, H2OCo(DH)2CH3 was used as a catalyst to cleave
disulfides in basic solution.14 In another system, CoCl2 in the
presence of organohalides appeared to mimic MeCbl-dependent
enzymes and produced thioethers from thiols.15
EtSCo((DO)(DOH)pn)CH3‚0.5H2O. In the drybox, a solution of
NaSEt (0.06 g in 1 mL of MeOH) was added to a methanolic solution
of [H2OCo((DO)(DOH)pn)CH3]ClO4 (0.20 g, 0.46 mmol in 8 mL).
After 15 min of stirring, degassed H2O was added to the solution. A
dark red solid formed as the MeOH was removed by rotary evaporation.
The solid was isolated by filtration, washed with a small amount of
H2O, and vacuum-dried for several hours. Yield: 0.09 g (51%).
We report evaluations of the interactions of thiolates with a
number of inorganic and organocobalt B12 model systems and
compare the resulting complexes to each other. FT-Raman
data12 suggest that thiolate ligands are not sufficiently strong
donors to weaken the Co-C bond in the ground state. This
result is consistent with the suggestion that thiolate ligation could
hinder attack at the Co-C bond, but there are no reported X-ray
data to confirm the conclusion of the Raman study. We present
the first X-ray evidence for an organocobalt B12 model contain-
(16) Schrauzer, G. N.; Windgassen, R. J. J. Am. Chem. Soc. 1967, 89, 1999.
(17) Schrauzer, G. N.; Windgassen, R. J. J. Am. Chem. Soc. 1966, 88, 3738.
(18) Schubert, E. Masters Thesis, Emory University, 1994.
(19) Parker, W. O., Jr. Ph.D. Thesis, Emory University, 1987.
(20) Costa, G.; Mestroni, G.; de Savorgnani, E. Inorg. Chim. Acta 1969,
3, 323.
(21) Gerli, A.; Marzilli, L. G. Inorg. Chem. 1992, 31, 1152.
(22) Gerli, A.; Sabat, M.; Marzilli, L. G. J. Am. Chem. Soc. 1992, 114,
6711.
1
ing a thiolate ligand and discuss the use of H NMR spectros-
copy to detect thiolate ligation. Finally, we examine the ligand-
responsive NMR shift trends in order to assess the electronic
trans influence of thiolate ligands.
(23) Polson, S. M.; Cini, R.; Pifferi, C.; Marzilli, L. G. Inorg. Chem. 1997,
36, 314.
(24) Trogler, W. C.; Stewart, R. C.; Epps, L. A.; Marzilli, L. G. Inorg.
Chem. 1974, 13, 1564.
(25) Raymond, K. N. Chem. Eng. News 1983, 61 (Dec 5), 4.
(26) Darensbourg, D. J. Inorg. Chem. 1988, 27, 3636.
(13) Pellizer, G.; Tauszik, G. R.; Costa, G. J. Chem. Soc., Dalton Trans.
1973, 317.
(14) Brown, K. L.; Jinkerson, D. J. Organomet. Chem. 1979, 164, 203.
(15) Chowdhury, S.; Samuel, P. M.; Das, I.; Roy, S. J. Chem. Soc., Chem.
Commun. 1994, 1993.

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An Organocobalt B12 Model Complex
Inorganic Chemistry, Vol. 36, No. 3, 1997 309
3,5-Me2PhSCo((DO)(DOH)pn)CH3. In the drybox, a solution of
[H2OCo((DO)(DOH)pn)CH3]ClO4 (0.20 g, 0.46 mmol) in MeOH (15
mL) was treated with [NEt4][3,5-Me2PhS] (0.14 g, 0.51 mmol) in
MeOH (1 mL). After 30 min of stirring, the solution was exposed to
air, and H2O (5 mL) was added. The volume of the red solution was
reduced (to <5 mL), and precipitation began. After the mixture was
chilled for 2 h (-35 °C), the dark pink solid that formed was isolated,
washed quickly with H2O, and vacuum-dried. Residual salts were
removed by washing with cold H2O. Recrystallization was ac-
complished from MeOH/H2O. The compound is reasonably stable to
air oxidation. Yield: 82%.
[Me2PhPCo((DO)(DOH)pn)CH3]PF6‚0.5H2O. In the drybox, a
suspension of [H2OCo((DO)(DOH)pn)CH3]PF6 (0.25 g, 0.52 mmol)
in CH2Cl2 (35 mL) was treated with PMe2Ph (0.109 mL, 0.77 mmol)
to give a bright orange solution, which became clear yellow after being
stirred overnight. Solvent was removed under reduced pressure until
the volume was ∼5 mL. The yellow, air-stable solid that precipitated
was collected and washed with ether. Yield: 0.23 g (74%).
pyCo(DH)2(3,5-Me2PhS). Method 1 (Based on a Known Proce-
dure6). A solution of 3,5-Me2PhSH (0.30 g, 1.9 mmol in 2 mL of
MeOH) was added to a suspension of [Co(DH)2py]2 (0.64 g, 1.25 mmol)
in MeOH (15 mL) in the drybox. After 1.5 h of stirring the solution,
a dark green solid precipitated. The solid was isolated, washed with
Et2O, and vacuum-dried. Yield: 0.76 g (79%). 4-MePhSCo(DH)2py
(80%) and 2,6-Me2PhSCo(DH)2py (53%) were prepared similarly.
Method 2. In the drybox, a solution of [NEt4][3,5-Me2PhS] (0.18
g, 0.68 mmol in 1 mL of MeOH) was added to a suspension of pyCo-
(DH)2Cl (0.25 g, 0.62 mmol) in MeOH (20 mL). The solution darkened
within seconds and was stirred overnight. The green powder that
formed was collected, washed with Et2O, and dried under vacuum.
Yield: 0.20 g (64%). 4-MePhSCo(DH)2py was prepared similarly in
39% yield.
[N-MeImdCo((DO)(DOH)pn)(3,5-Me2PhS)]PF6. In the drybox,
a solution of [NEt4][3,5-Me2PhS] (0.13 g, 0.47 mmol in 1 mL of MeOH)
was added to a suspension of [N-MeImdCo((DO)(DOH)pn)Cl]PF6 (0.25
g, 0.45 mmol) MeOH (20 mL). The solution darkened within seconds.
After the solution was stirred overnight, it was chilled to -35 °C until
a green solid formed (in a few days). The solid was collected by
vacuum filtration, washed with Et2O, and dried under vacuum. Yield:
0.19 g (64%). Recrystallization was accomplished from MeOH/ Et2O.
[N-MeImdCo((DO)(DOH)pn)(4-MePhS)]PF6 was prepared similarly.
[N-MeImdCo((DO)(DOH)pn)(4-MePhSO2)]PF6. A suspension of
[N-MeImdCo((DO)(DOH)pn)Cl]PF6 (0.25 g, 0.45 mmol) in MeOH (25
mL) was treated with Na[4-MePhSO2]‚2H2O (0.11 g, 0.50 mmol). The
pink suspension turned to a red solution, and an orange solid precipitated
after 10 min. After 1 h, the solid was collected, washed with H2O,
and dried under vacuum. Yield: 0.19 g (62%).
[i-C3H7OCS2Co(C1py)]ClO4. A solution of [BrCo(C1py)]ClO4 (0.5
g, 0.88 mmol) in a 1:1 MeOH:H2O mixture (50 mL) was treated with
AgNO3 (0.15 g, 0.88 mmol). The solution was stirred overnight and
then filtered to remove AgBr. K[i-C3H7OCS2] (0.16 g, 0.90 mmol)
was added to the filtrate. The brown solid that precipitated immediately
was isolated and then recrystallized from MeOH. Yield: 0.16 g (37%).
[4-MePhSO2Co(C1py)]ClO4. A solution of [BrCo(C1py)]ClO4 (0.5
g, 0.88 mmol) in a 1:1 MeOH:H2O (50 mL) mixture was treated with
AgNO3 (0.15 g, 0.88 mmol). The solution was stirred overnight and
then filtered to remove AgBr. Na[4-MePhSO2]‚2H2O, (0.19 g, 0.90
mmol) was added to the filtrate. MeOH was removed by rotary
evaporation, and the aqueous solution was extracted with CHCl3. The
combined organic extracts were dried with MgSO4, and the solvent
was removed by rotary evaporation. Adding MeOH to the residue
produced a suspension, from which a golden yellow solid was isolated
by filtration and washed with Et2O. Yield: 0.10 g (18%).
Table 1. Crystallographic Data for [AsPh4][EtSCo(DH)2CH3]
empirical formula
fw
space group
a (Å)
C38H54AsCoN4O7S
844.76
P212121
9.091(2)
b (Å)
17.098(6)
26.505(2)
4120(2)
c (Å)
V (Å3)
Z
4
d(calc)
abs coeff
1.36 mg m-3
1.31 mm-1
radiation
Mo KR (λ ) 0.710 73 Å)
temperature (K)
min/max transm
abs structure param
refinement method
final R indices [I > 2σ(I)]
R indices (all data)
173
0.267/0.355
0.00(2)
full-matrix least-squares on F2
R ) 4.34%; Rw ) 9.96%
R ) 5.51%; Rw ) 11.34%
suspension of [BrCo(C1py)]ClO4 (0.085 g, 0.15 mmol) in MeOH (18
mL). An immediate color change to dark green was observed. The
solution was stirred for 30 min, and the green precipitate that formed
was isolated, washed with Et2O, and vacuum-dried. Yield: 0.03 g
(30%).
Co((DO)(DOH)pn)(i-C3H7OCS2)2. To a solution of Co((DO)-
(DOH)pn)Cl2 (0.53 g, 1.45 mmol) in a 5:3 MeOH:H2O solution (40
mL) was added AgNO3 (0.49 g, 2.9 mmol). After the mixture was
stirred for 30 min, AgCl was removed by filtration, and the filtrate
was diluted with MeOH to a volume of 200 mL. A solution of K[i-
C3H7OCS2] (0.50 g, 2.9 mmol in 10 mL of H2O) was then added. The
fine brown powder that precipitated was collected on a filter and washed
with H2O and Et2O. Yield: 0.52 g (63%).
Co((DO)(DOH)pn)(4-MePhSO2)2. A solution of Co((DO)(DOH)-
pn)Cl2 (1.00 g, 2.7 mmol) in a 5:4 MeOH:H2O mixture (90 mL) was
treated with AgNO3 (0.92 g, 5.4 mmol). The solution, stirred for 30
min, was filtered to remove AgCl. The filtrate was diluted with MeOH
to a volume of 200 mL, and a solution of Na[4-MePhSO2]‚2H2O (1.16
g, 5.4 mmol, in 10 mL of H2O) was added. The solution was allowed
to evaporate slowly in the hood. The large red crystals that formed
were collected after 1 day and washed with H2O and Et2O. Yield:
0.70 g (42%).
X-ray Crystallography. An orange-red crystal of [AsPh4]-
[EtSCo(DH)2CH3] (0.26 × 0.44 × 0.45 mm) recrystallized from MeOH/
Et2O was used for data collection. Intensity data were collected at
-100 °C on a Siemens P4 instrument using an LT2 low-temperature
device. The crystal system and high-angle cell constants were
determined by automatic reflection selection, indexing, and least-squares
refinement (XSCANS Version 2.0). Three check reflections were
measured every 97 reflections; there was no significant deviation in
intensity. Intensity data were corrected for Lorentz and monochromator
polarization and absorption (semiempirical method based on azimuthal
scans). The structure of [AsPh4][EtSCo(DH)2CH3] was solved by direct
methods, and all non-hydrogen atoms were refined anisotropically by
full-matrix least-squares procedures on F2 using SHELXL-93. The H
atoms were generated at calculated positions (d(O-H) ) 0.85 Å; d(C-
H) ) 0.96 Å). All H atoms were constrained using a riding model
with isotropic thermal parameters fixed at 20% greater than that of the
bonded atom. Crystal data and refinement parameters are presented
in Table 1, and selected bond lengths and bond angles appear in Table
2.
Results and Discussion
X-ray Crystallography. The structure of [AsPh4]-
[EtSCo(DH)2CH3] (Figure 1) reveals coordination of ethanethi-
olate, trans to the CH3 group. This is the first report of an
organocobalt complex containing a unidentate coordinated
thiolate characterized by X-ray crystallography. [HgI2]-
[pyCo(DH)2SCH3] is the only other unidentate (alkanethiolato)-
cobalt(III) complex characterized by X-ray crystallography.27
[4-MePhSCo(C1py)]ClO4. In the drybox, a solution of [NEt4][4-
MePhS] (0.04 g, 0.16 mmol, in 1 mL of MeOH) was added to a
suspension of [BrCo(C1py)]ClO4 (0.10 g, 0.16 mmol) in MeOH (20
mL). The color changed immediately to dark green. After 2 h of
stirring, the solution was chilled to -35 °C. The green needles that
formed were isolated after 1 day, washed with MeOH and Et2O, and
vacuum-dried. Yield: 0.04 g (40%).
[3,5-Me2PhSCo(C1py)]ClO4. In the drybox, a solution of [NEt4][3,5-
Me2PhS] (0.04 g, 0.15 mmol in 1 mL of MeOH) was added to a
(27) Kergoat, R.; Kubicki, M. M.; Guerchais, J. E.; Howard, J. A. K. J.
Chem. Res., Synop. 1982, 340.

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310 Inorganic Chemistry, Vol. 36, No. 3, 1997
Polson et al.
Table 3. X-ray Crystal Structural Data (Bond Distances in Å) for
Organocobalt and Cobalt Thiolato and Thioether Complexes
Co-N
Co-S (trans to S) ref
Co-C
Organometallic Complexes
[AsPh4][EtSCo(DH)2CH3]
2.005(7) 2.342(2)
2.026(3) 2.2482(9) 2.011(3) 28
1.969(5) 2.203(2)
2.039(10) 2.216(4)
a-c
[Co(aeaps)2]S2O6
Co(Me3L′)a-c
29
30
CoCS(Cp*)(CO)a,b,d
Coordination Complexes
[HgI2][CH3SCo(DH)2py]
[Co(L-Cys)(dien)]3(ClO4)3
2.286
1.989(5) 27
2.013(8) 31
32
b
2.266(3)
2.346(3)
2.330(3)
2.305(4)
2.289(4)
2.206(3)
2.211(3)
2.206(3)
2.204(3)
[Na⊂222][Co(SC6HF4)2(TPP)]
[Co(SPh)2(phen)2]ClO4
1.97(1)
1.97(1)
33
34
Figure 1. Perspective drawing of [AsPh4][EtSCo(DH)2CH3] with 50%
probability for the thermal ellipsoids.
[Ph4P][Co(SPh-i-Pr3)4]
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for
[AsPh4][EtSCo(DH)2CH3]
Bond Lengths (Å)
a Carbon is chelated. b Sulfur is chelated. c Thioether coordination.
d Co-C-S three-membered ring. Abbriations: aeaps ) 2-aminoethyl
3-aminopropyl sulfide; Me3L′ ) a dithiapropyl-substituted triazene
1-oxide; salenC2(SEt) ) N,N′-disalicylidene-2-methyl-4-(ethylthio)-1,2-
butanediamine; Cp* ) pentamethylcyclopentadiene; L-CysH ) L-
cysteine; dien ) diethylenetriamine; phen ) 1,10-phenanthroline;
i-Pr3PhSH ) 2,4,6-triisopropylbenzenethiol; SC6HF4 ) 2,3,5,6-tet-
rafluorobenzenethiol; TPP ) meso-tetraphenylporphyrin; Na⊂222 )
222 cryptated sodium.
Co-N(4)
Co-N(2)
Co-N(1)
1.852(6)
1.853(6)
1.876(6)
Co-N(3)
Co-C(1)
Co-S
1.886(7)
2.005(7)
2.342(2)
Bond Angles (deg)
N(4)-Co-N(2)
N(4)-Co-N(1)
N(2)-Co-N(1)
N(4)-Co-N(3)
N(2)-Co-N(3)
N(1)-Co-N(3)
N(4)-Co-C(1)
N(2)-Co-C(1)
N(1)-Co-C(1)
N(3)-Co-C(1)
N(4)-Co-S
179.8(3)
98.5(3)
81.4(3)
81.5(3)
98.6(3)
177.4(3)
88.1(3)
91.7(3)
87.2(3)
90.3(3)
94.0(2)
86.2(2)
N(1)-Co-S
92.3(2)
90.2(2)
N(3)-Co-S
C(1)-Co-S
177.9(2)
117.0(5)
121.8(5)
117.2(5)
125.1(5)
116.3(5)
121.7(5)
116.8(5)
124.9(5)
107.6(3)
C(4)-N(1)-Co
O(1)-N(1)-Co
C(6)-N(2)-Co
O(2)-N(2)-Co
C(8)-N(3)-Co
O(3)-N(3)-Co
C(10)-N(4)-Co
O(4)-N(4)-Co
C(2)-S-Co
for CD3-, which is known to have a very large trans influence.
Thus, the similar Co-C bond length for [AsPh4][EtSCo(DH)2-
CH3] compared to those of other cobaloximes is consistent with
the Raman data.
The Co-S bond of [AsPh4][EtSCo(DH)2CH3] is significantly
longer than those in known examples of (alkanethiolato)-
organocobalt complexes (Table 3). However, most of the latter
have chelated sulfur, and relatively short bond lengths can be
attributed to the chelate effect. The Co-S bond of the
unidentate thiolate ligand in the related inorganic cobalt complex
[HgI2][pyCo(DH)2SCH3]27 is lengthened by the interaction of
S with Hg. However, the Co-S bond in [AsPh4][EtSCo(DH)2-
CH3] is longer, further evidence of the large trans influence of
CH3. There are other examples of long Co-S bonds (Table
3), but these involve ArS complexes and these aryl ligands
appear to be weaker donors.
Interaction of Organocobalt Complexes with Thiolates.
We now discuss our investigations into the interactions of
various B12 models with two kinds of thiolate ligands: arene-
thiolates and the more reactive alkanethiolates. These experi-
ments often led to the isolation of inorganic and organocobalt
thiolato complexes. 1H NMR spectroscopy has proved useful
in indicating coordination of thiolates, in identifying oxidized
S-containing organic products and in assessing the potential
formation of thioethers. A downfield shift of the CH2 signal
from free EtS- (Table 4) indicates oxidation to the disulfide. A
signal at ∼2.1 ppm (s, 3H, SCH3) is expected for thioethers.
We also prepared complexes containing other sulfur ligands in
order to assess the effects of S-ligation on spectra.
N(2)-Co-S
The three other reported cases of organocobalt compounds
containing a Co-S bond were not B12 models. In these, the
coordinated carbon was part of a chelate, with cis Co-C and
Co-S bonds (Table 3).27-34 Only one contained a thiolate, in
a three-membered Co-C-S ring.30 The other two contained a
Co-S thioether bond.28,29
The 2.005(7) Å Co-C bond for [AsPh4][EtSCo(DH)2CH3]
falls within the normal distance range for cobaloximes of the
type LCo(DH)2CH3.35 Examples of typical Co-C bond
lengths: 1.990(5) (L ) H2O), 1.998(5) (L ) py), 2.009(7) (L
) N-MeImd), 2.011(3) Å (L ) PMe3).35 We recently found
by FT-Raman spectroscopy that the Co-CH3 stretching fre-
quency (νCo-CH ), a parameter related to Co-C bond strength,
3
was only slightly dependent on the nature of the trans ligand,
L.4 For the primary series studied, [LCo((DO)(DOH)pn)CH3]0/+
(Chart 1), the frequency decreased from 505 to 455 cm-1, with
stronger electron-donating character of the trans L in the
following order: Cl-, N-MeImd, py, 3,5-Me2PhS-, PMe3,
- 4
CD3
.
However, νCo-CH values for most of the compounds
3
were clustered near 500 cm-1, with the lowest value by far found
(28) Bjerrum, M. J.; Gajhede, M.; Larsen, E.; Springborg, J. Inorg. Chem.
1988, 27, 3960.
(29) Chakraborty, P.; Karmakar, S.; Chandra, S. K.; Chakravorty, A. Inorg.
Chem. 1994, 33, 816.
(30) Herbig, J.; Kohler, J.; Nuber, B.; Stammler, H. G.; Ziegler, M. L. J.
Organomet. Chem. 1993, 444, 107.
(31) Okamoto, K.; Umehara, H.; Nomoto, M.; Einaga, H.; Hidaka, J. Bull.
Chem. Soc. Jpn. 1987, 60, 1709.
(32) Doppelt, P.; Fischer, J.; Weiss, R. J. Am. Chem. Soc. 1984, 106, 5188.
(33) Chadha, R. K.; Kumar, R.; Lopez-Grado, J. R.; Tuck, D. G. Can. J.
Chem. 1988, 66, 2151.
(34) Fikar, R.; Koch, S. A.; Millar, M. M. Inorg. Chem. 1985, 24, 3311.
(35) Bresciani-Pahor, N.; Forcolin, M.; Marzilli, L. G.; Randaccio, L.;
Summers, M. F.; Toscano, P. J. Coord. Chem. ReV. 1985, 63, 1.
(a) Cobaloximes. Formation of the organocobaloxime thi-
olato complex [CH3SCo(DH)2CH3]- in 1:1 CD3OD:D2O solu-
tion was suggested14 on the basis of the upfield shift of the 1H
NMR signal for CH3S- relative to the signal for free CH3S-.
Loss of CH3SH under reduced pressure from the isolated
compound [AsPh4][CH3SCo(DH)2CH3] prevented its charac-
terization by elemental analysis.14 We successfully prepared
[AsPh4][EtSCo(DH)2CH3] by reaction of H2OCo(DH)2CH3 with
NaSEt (reaction iii), and we were able to characterize the

+
+
An Organocobalt B12 Model Complex
Inorganic Chemistry, Vol. 36, No. 3, 1997 311
Table 4. 1H NMR Shifts (ppm) of Sulfur Species and of
Cobaloximes in CD3OD
pn)CH3]ClO4 in CD3OD with NaSEt (1 equiv) resulted in the
slow formation of ∼25% EtSCo((DO)(DOH)pn)CH3 after 2
days (reaction iii). After 4 days, no further reaction was evident.
With time, the imine CH3 1H NMR signal disappeared. We
attribute this phenomenon, observed in several other cases, to
the known D-exchange of this type of methyl group.36,37 We
treated a CD3OD solution of Co((DO)(DOH)pn)(CH3)2 with 1
equiv of NaSEt. As mentioned above, only the alkyl ligand
has a sufficiently strong trans influence to significantly weaken
the Co-C bond, as indicated both by FT-Raman spectroscopy
and by X-ray structural data.4,38 In addition to the D-exchange
of the imine CH3 protons, EtSCo((DO)(DOH)pn)CH3 formed
as a minor product after 1 day (total 20% after 2 days).
However, it is unlikely that EtS- caused the displacement of
the CH3 group, since Co((DO)(DOH)pn)(CH3)2 is known to
decompose in protic solvents to give methane and [(solvent)-
Co((DO)(DOH)pn)CH3]+.39 In a control experiment with no
NaSEt present, we observed ∼15% decomposition to [MeO-
HCo((DO)(DOH)pn)CH3]+. In contrast, in CD3CN we ob-
served no reaction for Co((DO)(DOH)pn)(CH3)2 treated with
up to 6 equiv of NaSEt, except for partial D-exchange of the
imine CH3 protons.
S-ligand
signals
oxime
CH3
axial
CH3
EtS-
EtSSEt
2.50, 1.26
2.68, 1.29
H2OCo(DH)2CH3
2.23
2.15
1.88
1.96
2.17
0.67
0.61
0.66
0.80
0.76
[AsPh4][EtSCo(DH)2CH3]
[AsPh4][3,5-Me2PhSCo(DH)2CH3]
[4-MePhSO2Co(DH)2CH3]- a
[i-PrOCS2Co(DH)2CH3]- a
1.89, 1.06
6.68, 6.62
7.29, 7.16
5.62
a Prepared in situ.
compound by elemental analysis and X-ray crystallography (see
1
above). The EtS H NMR signals for [AsPh4][EtSCo(DH)2-
CH3] in CD3OD solution are upfield of the signals for EtS-
(by 0.6 (CH2) and 0.2 (CH3) ppm, Table 4). Addition of NaSEt
(0.5 equiv) to the solution of [AsPh4][EtSCo(DH)2CH3] CD3-
OD did not change the spectrum of the complex, and the EtS-
signals were in the normal position; therefore, EtS does not
appear to undergo fast exchange.
The methyl group of MeB12 was reported to transfer to
dithiothreitol, reaction ii (t1/2 ) 1.5 h at 43 °C).11 In contrast,
the MeB12 model CH3CoPc transferred a CH3 group to ben-
zenethiolate at ambient temperature with t1/2 ) 0.4 s.12 Wishing
to test for modes of reactivity of thiolates with organoco-
baloximes other than S-ligation, we selected a cobaloxime with
a strongly coordinating axial ligand trans to the methyl group
to block the ligand-exchange reaction with EtS- (reaction iii).
Treatment of a CD3OD solution of Me2PhPCo(DH)2CH3 with
EtS- (5 equiv) resulted in only <10% displacement of Me2-
PhP to form [EtSCo(DH)2CH3]- after 5 days, with no NMR
evidence of any other reactions. Thus, alkylation of the thiolate
(reaction ii) was not favored under the reaction conditions. We
also treated a cobaloxime having a good leaving group on the
organometallic carbon, N-MeImdCo(DH)2CH2Br, with NaSEt
(c) Lariat-Type Imine/Oximes. The lariat-type complexes
([RCo(C1py)]X, Chart 1)21 are a new class of imine/oxime-type
B
12 models that contain a pyridyl group covalently attached to
the equatorial ligand. We studied the formation of organocobalt
1
thiolato imine/oxime lariat-type complexes by H NMR spec-
1
troscopy. The H NMR shift of the pyridyl R-H can be used
to indicate whether the pendant pyridyl was bound to Co. A
shift of the coordinated pyridyl R-H signal well upfield of 8.55
ppm is due largely to the influence of Co anisotropy.40
A CD3OD solution of [CH3Co(C1py)]ClO4 was treated with
1.1 equiv of NaSEt, and the pyridyl R-H signal shifted from
8.38 ppm in [CH3Co(C1py)]ClO4 to 8.52 ppm, a shift consistent
with the thiolato ligand replacing pyridyl (reaction iii, Table
5). The -CH2S quartet appeared at 1.65 ppm, a shift similar
to that for EtSCo((DO)(DOH)pn)CH3 (Supporting Information).
The equatorial imine CH3 protons underwent D-exchange after
a brief exposure to NaSEt in CD3OD. After 2 days, signals for
a second, closely related species arose (Table 5), consistent with
two slightly different organocobalt thiolato species. Further-
more, after 2 weeks, the two new Co-CH3 signals became equal
in intensity, a result also consistent with methyl transfer. Using
additional thiolate initially (2 equiv) did not alter the course of
the reaction. Most likely, the second species is a geometric
isomer that formed as a result of Co(II)-catalyzed methyl
transfer.41 Realkylation of the Co(II) compound could occur
on the opposite side of the equatorial ligand, on the side of the
pendant pyridyl moiety (Scheme 2). Such a situation would
arise whenever a strong unidentate ligand replaced the pendant
pyridyl. Thus, we studied the effect of the treatment of [CH3-
Co(C1py)]ClO4 with 1 equiv of PMe2Ph in CD3OD. After only
1
(2.5 equiv). The H NMR of the CD3OD solution indicated
that, even after 4 days, only extensive N-MeImd replacement
to form [EtSCo(DH)2CH2Br]- occurred (upfield EtS signals
observed at 1.89 (q, SCH2) and 1.05 ppm (t, CH3)); there was
no evidence for the formation of [EtSCo(DH)2CH2SEt]- (reac-
tion i).
We prepared organocobalt complexes using other S-donor
ligands. An arenethiolato complex, [AsPh4][3,5-Me2PhSCo-
(DH)2CH3], was prepared similarly to the EtS- analog. In
addition, we prepared complexes containing innocent S-donor
ligands in situ, [i-C3H7OCS2Co(DH)2CH3]- and [4-MePhSO2-
Co(DH)2CH3]-. The different S-donors did not significantly
1
affect the position of the H NMR Co-CH3 signal compared
to other axial ligands (Table 4), although an upfield shift of the
oxime CH3 signal was observed for the S-donor ligands
containing an aromatic ring (3,5-Me2PhS- and 4-MePhSO2-).
(b) Imine/Oximes. We obtained the neutral organocobalt
imine/oxime compound EtSCo((DO)(DOH)pn)CH3 by anaero-
bic treatment of a MeOH solution of [H2OCo((DO)(DOH)pn)-
CH3]PF6 with NaSEt (reaction iii). Air-stable 3,5-Me2PhSCo-
((DO)(DOH)pn)CH3 was prepared similarly using [NEt4][3,5-
Me2PhS]. The 1H NMR spectrum of EtSCo((DO)(DOH)pn)CH3
in CD3OD exhibited thiolato ligand signals significantly upfield
of the free thiolate values, as expected.
1
1 h, the resulting H NMR spectrum contained two slightly
different [CH3Co(C1py)PMe2Ph]ClO4 species, Viz., two CH3
1
doublets at 0.98 (3 Hz) and 1.03 ppm (4 Hz), due to H-31P
coupling (comparable values for [PhMe2PCo((DO)(DOH))CH3]-
PF6 were 1.01 ppm (4 Hz)). In addition, other data were
(36) Pellizer, G.; Tauszik, G. R.; Tauzher, G.; Costa, G. Inorg. Chim. Acta
1973, 7, 60.
(37) Polson, S. M.; Hansen, L.; Marzilli, L. G. J. Am. Chem. Soc. 1996,
118, 4804.
(38) Calligaris, M. J. Chem. Soc., Dalton Trans. 1974, 1628.
(39) Witman, M. W.; Weber, J. H. Synth. React. Inorg. Met.-Org. Chem.
1977, 7, 143.
(40) Marzilli, L. G.; Gerli, A.; Calafat, A. M. Inorg. Chem. 1992, 31, 4617.
(41) Toscano, P. J.; Marzilli, L. G. Prog. Inorg. Chem. 1984, 31, 105.
In order to test for an alternative reaction of thiolates with
imine/oxime organocobalt complexes, we examined by 1H NMR
complexes with a strongly coordinating axial ligand trans to
the methyl to block the ligand-exchange reaction (iii). While
treatment of Me2PhPCo(DH)2CH3 with NaSEt resulted in ∼10%
L displacement after 5 days, treatment of [Me3PCo((DO)(DOH)-

+
+
312 Inorganic Chemistry, Vol. 36, No. 3, 1997
Table 5. 1H NMR Shifts (ppm) of [CH3Co(C1py)]ClO4 in CD3OD Treated with NaSEt
Polson et al.
EtS (CH3)
NaSEt
none
R
â
γ
axial CH3
0.84
N-CH2
imine CH3
2.32
oxime CH3
2.21
EtS (CH2S)
1.65
8.38
7.22
7.39
7.32
7.73
4.26, 3.93
1 equiva
8.52
7.83
0.58
0.51
3.93, 3.46
4.00, 3.32
b
b
2.22d
2.20d
1.04
7.45
7.32 sh
7.45 sh
1 equiv,
8.52 sh
7.83 sh
1.76
with timec
2.68e
1.29e
a Complete reaction. b Signal disappeared (D-exchange). c Signals of a second new species formed from 1 equiv of solution with time. After 2
weeks, the signals of the two species were similar in intensity. d Signals diminished significantly in intensity with time. e EtSSEt. sh ) shoulder.
Scheme 2
oxime CH3 signal (δ 2.26 (s, 12H)) suggested that the
bis(thiolato) complex [Co(DH)2(SEt)2]- formed. The 0.8 ppm
upfield shift of the SCH2 signal compared that of to free NaSEt
supports this conclusion. The minor S-containing product
(signals at 2.69 (q) and 1.29 (t) ppm) accounts for <5% of the
S-containing signals initially, increasing to 20% overnight. The
downfield shift of these signals compared to those of NaSEt
suggests that EtSSEt formed (Table 4). Thus, alkane- and
arenethiolates reacted differently with Co(III) complexes, and
the above redox method (eq 2) is the only successful procedure
reported for the preparation of an alkanethiolato inorganic
cobaloxime complex.
consistent with Scheme 2; Viz., the two R-H signals had
downfield signals at 8.51 and 8.66 ppm.
(d) Distorted Model. We studied the reaction of NaSEt with
the organocobalt complex [N-MeImdCo(N-CH2-CHEL)]ClO4
An EP value (an electronic parameter) for the ArS ligand
was determined in order to assess its electronic trans influence
quantitatively. The EP value is derived from the 13C shift for
the γ-C for complexes of the type pyCo(DH)2R(or X),42 ranging
from -1.56 for X ) Cl (weak trans influence) to +0.24 for
CH2OCH3 (strong trans influence). We expected the EP value
to be close to the EP value of -0.75 that we calculated for
t-BupyCo(DH)2S(t-Bu).35,43 We assigned 13C NMR signals of
pyCo(DH)2(3,5-Me2PhS) at 138.12 ppm and of pyCo(DH)2(4-
MePhS) at 138.14 ppm to the py γ-C and used 138.13 ppm to
calculate an EP value of -0.65. This result means that the ArS-
ligand has a moderate electronic trans influence, similar to that
of the alkyl donors -CH2CF3 and -CH2CN.
1
(Chart 1)37 by H NMR. This compound contains a distorted
coordination environment, and there is a relatively open avenue
for attack on the carbon attached to Co. However, addition of
2.4 equiv of NaSEt to a CD3OD solution of [N-MeImdCo(N-
CH2-CHEL)]ClO4 resulted only in thiolate displacement of
N-MeImd (reaction iii). The signals for the free N-MeImd
integrated for 1 equiv (relative to the olefin and axial methylene
protons). Prolonged exposure of [N-MeImdCo(N-CH2-CHEL)]-
ClO4 to NaSEt led to decomposition, with formation of ethyl
1
disulfide identified on the basis of its H NMR signals.
Inorganic (Thiolato)cobalt(III) Complexes. (a) Co-
baloximes. We further explored thiolate chemistry relevant to
cobalt by preparing a number of inorganic B12 model complexes.
We prepared cobaloximes containing unidentate N-donors trans
to an arenethiolate ligand by the reported redox method (eq 1).6
Alkanethiolates can also be incorporated into a cobaloxime
containing an N-donor axial ligand by treatment of cobal(II)-
oxime (“CoII(DH)2” generated in situ in basic solution) in the
presence of py with dimethyl disulfide (eq 2).6
(b) Imine/Oximes. Although Co(II) imine/oxime complexes
are known,44,45 these have not been used to prepare thiolate
derivatives (cf. eqs 1 and 2). We found routes for the synthesis
of thiolato imine/oxime-type complexes starting with Co(III)
compounds (eq 4). We treated a MeOH solution of [N-
[LCo((DO)(DOH)pn)Cl]PF6 + RS- f
[LCo((DO)(DOH)pn)SR]PF6 + Cl- (4)
[Co(DH)2py]2 + 2PhSH f 2pyCo(DH)2(PhS) + H2 (1)
MeImdCo((DO)(DOH)pn)Cl]PF6 with [NEt4][ArS] to produce
complexes of the type [N-MeImdCo((DO)(DOH)pn)(ArS)]PF6
(ArS ) 4-MePhS, 3,5-Me2PhS). Our attempts using this method
to prepare arenethiolate complexes containing N-donors weaker
than N-MeImd or alkanethiolate complexes with any N-donor
failed. For example, even the bulky arenethiolate [NEt4][2,6-
Me2PhS] (1 equiv) displaced py from [pyCo((DO)(DOH)pn)-
“CoII(DH)2” + py + 0.5CH3SSCH3 f pyCo(DH)2SCH3
(2)
We wished to test whether compounds of the type pyCo-
(DH)2SR (R ) alkyl, aryl) could also be prepared from a
Co(III) precursor. We found that addition of tetraalkylammo-
nium salts of arenethiolates to solutions of cobaloximes such
as pyCo(DH)2Cl led to selective substitution of the halide (eq
3). However, treatment of a suspension of pyCo(DH)2Cl in
1
Cl]PF6, as suggested by the lack of py signals in the H NMR
spectrum of the isolated product. In addition, we treated a
suspension of [N-MeImdCo((DO)(DOH)pn)Cl]PF6 in CD3OD
with the alkanethiolate NaSEt (1 equiv). Integration of the
dominant thiolate signals (δ 1.59 (q, 4H), 1.00 (t, 6H)) suggested
that the bis(thiolato) complex Co((DO)(DOH)pn)(SEt)2 had
pyCo(DH)2Cl + [NEt4][ArS] f
pyCo(DH)2(ArS) + [NEt4]Cl (3)
(42) Zangrando, E.; Bresciani-Pahor, N.; Randaccio, L.; Charland, J.-P.;
Marzilli, L. G. Organometallics 1986, 5, 1938.
(43) Stewart, R. C.; Marzilli, L. G. Inorg. Chem. 1977, 16, 424.
(44) Costa, G.; Mestroni, G.; Pellizer, G.; Tauzher, G.; Licari, T. Inorg.
Nucl. Chem. Lett. 1969, 5, 515.
(45) Murakami, Y.; Hisaeda, Y.; Fan, S.; Matsuda, Y. Bull. Chem. Soc.
Jpn. 1989, 62, 2219.
CD3OD with 1 equiv of NaSEt did not proceed as in eq 3.
Instead, the 1H NMR spectrum of the resulting brown-red
solution contained signals for two different S-containing
compounds. The integration of the dominant thiolate signals
(δ 1.74 (q, 4H, SCH2), 1.00 (t, 6H, CH3)) relative to the single

+
+
An Organocobalt B12 Model Complex
Inorganic Chemistry, Vol. 36, No. 3, 1997 313
Table 6. 1H NMR Shifts (ppm) of Lariat-Type Complexes [RCo(C1py)]ClO4 (or PF6) in CDCl3
R
â
γ
axial CH3
...
N-CH2
imine CH3
2.59
oxime CH3
2.44
a
[ClCo(C1py)]ClO4
7.80
7.25
7.33
7.05
7.21
7.06
7.38
7.14
7.40
7.75
4.42, 4.20
[4-MePhSO2Co(C1py)]ClO4
[4-MePhSCo(C1py)]ClO4
[CH3Co(C1py)]ClO4
7.88
8.09
8.31
7.63
7.62
7.65
...
4.58, 4.28
4.40, 4.24
4.36, 3.93
2.46
2.25
2.37
2.08
2.12
2.23
...
0.79
a Reference 40.
formed (unchanged starting material was still suspended). Thus,
the reaction (eq 4) was successful only for L ) N-MeImd and
RS- ) ArS- and failed for all N-donors when RS- ) EtS-.
(c) Lariat-Type Imine/Oximes. We prepared without
complication the inorganic thiolato lariat-type complexes [ArSCo-
(C1py)]ClO4 (ArS ) 4-MePhS, 3,5-Me2PhS) by treating [BrCo-
(C1py)]ClO4 with the appropriate [NEt4][ArS] reagent (1 equiv).
is a reasonable explanation for this result. The negative oxygens
of the 4-MePhSO2 ligand are repelled by the oxime oxygens.
As a consequence, the ring would be preferentially oriented
away from the oxime region and toward the imine region.
Conclusions
The alkanethiolate ligand is comparable to alkylphosphines
in its affinity for axial coordination to B12 models. It is a much
stronger ligand than biologically relevant N-heterocycles such
as benzimidazole and imidazole. The principal reaction fol-
lowing addition of thiolates to organocobalt complexes is
S-ligation, not Co-C bond cleavage. This finding holds also
for both the dialkyl complex Co((DO)(DOH)pn)(CH3)2, with
strong trans ligands, and for a complex with a highly distorted
alkyl group sterically exposed to attack by the thiolate.
The influence of the thiolate ligand on the length of the trans
Co-C bond has been shown to be small in this first crystal-
lographic determination of the structure of this type of com-
pound. NMR shift trends are also consistent with the new
structural results and the previously reported FT- Raman data
and indicate that thiolates have a moderate trans influence. It
is thus likely that thiolates will not significantly influence the
Co-CH3 bond in methylcobalamin. Therefore, thiolate ligation
by homocysteine is unlikely to be an important step in the
enzymatic processes involving methionine synthase. In fact,
the protein may act to prevent thiolate ligation since the results
presented here and elsewhwere in the studies with CH3-
CoIIIPc12 suggest that such ligation may impede attack at the
Co-C bond.
1
The H NMR shift of the R-H signal in a CDCl3 solution of
[4-MePhSCo(C1py)]ClO4 was 8.09 ppm, or 0.46 ppm upfield
of the free py R-H C1py signal (Table 6). Therefore, the pendant
pyridyl is bound to Co in a CDCl3 solution of [4-MePhSCo(C1-
py)]ClO4. The formation and successful isolation of [ArSCo(C1-
py)]ClO4 contrast with our results for the quadridentate imine/
oximes (complete replacement of py). Treatment of [BrCo-
(C1py)]ClO4 with the more reactive thiolate NaSEt (1 equiv)
resulted in the formation of brown needles. At least two species
were present in the 1H NMR spectrum (in CDCl3) of the product,
and many signals were broad. In addition, there are signals at
2.70 (q) and 1.32 (t) ppm, which match those of EtSSEt in this
solvent (Supporting Information). Thus, although conditions
may exist which allow a product to be formed and isolated,
alkanethiolates cause synthetic problems even for lariat-type
complexes.
As mentioned above, the shifts of 1H NMR signals, particu-
larly for protons close to Co, are influenced by Co anisotro-
py.40,46 The effect is to shift the signals of the equatorial ligand
protons in the direction opposite to those of the axial ligand
protons. The axial L ligand signals are most useful since these
are not affected by the anisotropy of the R or X axial ligand,
whereas the equatorial ligand signals are so influenced. The
shift trends tend to parallel the trans influence. For example,
the [ClCo(C1py)]ClO4 R-H signal is well upfield of that for
[CH3Co(C1py)]ClO4 (in CDCl3). The equatorial methyl signals
have the opposite relationship (Table 6). The shift of the R-H
signal for [4-MePhSCo(C1py)]ClO4 is between those of the Cl
and CH3 analogs, consistent with a moderate trans influence.
The R-H signal for an analog containing an oxidized S-donor,
[4-MePhSO2Co(C1py)]ClO4, is closer to that of the Cl analog.
The signals of three of the four equatorial CH3 groups for these
two S-donor complexes were upfield of the corresponding
signals for both the Cl and CH3 complex (Table 6). We attribute
this upfield shift to the proximity of the anisotropic aromatic
The structure of [AsPh4][EtSCo(DH)2CH3] revealed that the
Co(III)-S bond trans to the Co-C bond is unusually long
compared to other Co(III)-S containing alkanethiolate com-
plexes. However, relevant structural information is limited to
this compound, and additional data are needed to confirm this
result.
Acknowledgment. This work was supported by NIH Grant
GM 29225 (L.G.M.). Instrument purchases were funded by the
NIH and NSF. L.G.M. was supported in part by Emory’s F.
M. Bird Exchange Fellowship during a stay at St. Andrews
University, Scotland. The NSF (Grant ASC-9527186) supported
our use of the Internet for remote collaborative research. We
thank Mr. Hao Ni for the preparation of [AsPh4][3,5-Me2PhSCo-
(DH)2CH3].
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rings to the equatorial ligand. In contrast, the H shift of the
imine CH3 of the 4-MePhSO2 complex is significantly downfield
from both the 4-MePhS and the CH3 complexes (Table 6). This
imine CH3 shift for the [N-MeImdCo((DO)(DOH)pn)4-MePhSO2]-
PF6 complex is also relatively downfield. These shifts suggest
the anisotropic effect of the aromatic ring is nearly absent. There
Supporting Information Available: A packing diagram and tables
of crystallographic data, bond lengths, bond angles, atomic coordinates,
and displacement coefficients for [AsPh4][EtSCo(DH)2CH3], along with
1
tables of elemental analyses and H NMR data (14 pages). Ordering
information is given on any current masthead page.
(46) Parker, W. O., Jr.; Zangrando, E.; Bresciani-Pahor, N.; Marzilli, P.
IC961019Z
A.; Randaccio, L.; Marzilli, L. G. Inorg. Chem. 1988, 27, 2170.