The First X-ray Structural Evidence Demonstrating Thiolate Coordination in an Organocobalt B₁₂Model Complex:  Implications for Methionine Synthase

Abstract

Enzyme-bound methyl-B 12 transfers its methyl group to homocysteine during methionine synthesis. However, treatment of several types of organocobalt B 12 models with arene-and alkanethiolates under ambient conditions leads only to thiolate ligation. The structure of [AsPh 4 ][EtSCo(DH) 2 CH 3 ] (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 CH 3 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 EtSon Co-((DO)(DOH)pn)(CH 3 ) 2 [(DO)(DOH)pn ) N 2 ,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)CH 3 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 ligandexchange 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.

Synthesis, characterization, and thermodynamics of tertiary phosphine cobalt(III) Schiff-base complexes

“…The properties of these Salen complexes were examined with the aim to gain an insight into the chemistry of the related biological molecules. Some extensive works have been done on these models (4)(5)(6)(7)(8). Recently the kinetics of ligand replacement reactions of the axial H 2 O in the trans-[Co(en) 2 (H 2 O)(Me)] 2+ complex have been studied by Hamza et al (9).…”

The Thermodynamics of Tertiary Phosphine Cobalt (III) Schiff Base Complexes

“…Extensive work has been done on these models. [4][5][6][7][8][9] Recently the kinetic of ligand replacement reactions of the axial H 2 O in the trans -[Co(en) 2 (H 2 O)(Me)] 2+ complex have been studied by Hamza et al 10 The equilibrium of the adduct formation of [Co (Salen) X] + , (X= tertiary phosphines) with NO 2 has been studied. It was shown that the equilibrium constants decrease according to the following trend: PPh 3 < PEtPh 2 < PBu 3 .…”

Synthesis and Characterization of Cobalt(III) Complex of 1,5-bis(2-Hydroxybenzamido)-3- aza pentane and its Interaction with Glutathione: A Kinetic and Spectrophotometric Study

“…The observed shift of peak position of cysteine protons i.e. Cys-CHα peak from 4.5 → 4.3 ppm and Cys-CH2β peak from 2.98→3.33 ppm is consistent with direct thiolate ligation to the cobalt centre and after binding of GSH through S atom the cysteine protons are conformationally adjacent to the cobalt ion appeared to be strongly affected [40,41]. Similar observation also reported by Sun et al [42] in their study of interaction of Sb(III)tartarate with glutathione.…”