The First X-ray Structural Evidence Demonstrating Thiolate Coordination in an
Organocobalt B<sub>12</sub> Model Complex: Implications for Methionine Synthase
Suzette M. Polson
Lory Hansen
Luigi G. Marzilli
10.1021/ic961019z.s001
https://acs.figshare.com/articles/journal_contribution/The_First_X-ray_Structural_Evidence_Demonstrating_Thiolate_Coordination_in_an_Organocobalt_B_sub_12_sub_Model_Complex_Implications_for_Methionine_Synthase/3617241
Enzyme-bound methyl-B<sub>12</sub> transfers its methyl group to
homocysteine during methionine synthesis. However,
treatment of several types of organocobalt B<sub>12</sub> models with
arene- and alkanethiolates under ambient conditions
leads only to thiolate ligation. The structure of
[AsPh<sub>4</sub>][EtSCo(DH)<sub>2</sub>CH<sub>3</sub>] (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, <i>trans </i>to the CH<sub>3</sub> 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 <i>trans</i> influence and does not significantly
weaken the Co−C bond in the ground state.
Since
a strong <i>trans</i> influence alkyl ligand weakens the
<i>trans</i> Co−C bond, we examined the effect of
EtS<sup>-</sup> on Co((DO)(DOH)pn)(CH<sub>3</sub>)<sub>2</sub>
[(DO)(DOH)pn =
<i>N</i><sup>2</sup>,<i>N</i><sup>2‘</sup><i>-</i>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)CH<sub>3</sub> 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 <i>trans</i>
influence.
1997-01-29 00:00:00
trans influence alkyl ligand
trans influence
NMR
bond
CH 3 ligand
thiolate
Co
DH
organocobalt B 12 models