Mononuclear NiIII-Alkyl Complexes (Alkyl = Me and Et): Relevance to the Acetyl-CoA Synthase and Methyl-CoM Reductase

Mononuclear, distorted trigonal bipyramidal [PPN][Ni-III(R)-(P(C6H3-3-SiMe3-2-S)(3))] (R = Me (1); R = Et (2)) were prepared by reaction of [PPN][(NiCl)-Cl-III(P(C6H3-3-SiMe3-2-S)(3))] and CH3MgCl/C2H5MgCl, individually. EPR, SQUID studies as well as DFT computations reveal that the Ni-III in 1 has a low-spin d(7) electronic configuration in a distorted trigonal bipyramidal ligand field. The Ni-C bond lengths of 1.994(3) angstrom in 1 and 2.015(3) angstrom in 2 are comparable to that in the Ni-III-methyl state of MCR (similar to 2.04 angstrom) (Sarangi, R.; Dey, M.; Ragsdale, S. W. Biochemistry 2009, 48, 3146). Under a CO atmosphere, CO triggers homolytic cleavage of the Ni-III-CH3 bond in 1 to produce Ni-II-thiolate carbonyl [PPN][Ni-II(CO)(P(C6H3-3-SiMe3-2-S)(3))] (3). Additionally, protonation of 1 with phenylthiol generates Ni-III-thiolate [PPN][Ni-III(SPh)-(P(C6H3-3-SiMe3-2-S)(3))] (4).