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The First X-ray Structural Evidence Demonstrating Thiolate Coordination in an Organocobalt B12 Model Complex: Implications for Methionine Synthase
journal contribution
posted on 1997-01-29, 00:00 authored by Suzette M. Polson, Lory Hansen, Luigi G. MarzilliEnzyme-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.