The First X-ray Structural Evidence Demonstrating Thiolate Coordination in an Organocobalt B<sub>12</sub> Model Complex:  Implications for Methionine Synthase

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.