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

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.