S-Bridged Polynuclear Complexes. II. Formation and Some Properties of [M{Co( l -cys-N,S)3}2]-Type Complexes (M=Fe(III), Co(III), and Ni(II); l -cys= l -cysteinate)

Three trinuclear complexes with ΔLll- and/or ΛLll-fac(S)-[Co(l-cys-N,S)3] terminals, and ΔLllΔLll-,ΔLllΛLll-, and ΛLllΛLll-[Fe{Co(l-cys-N,S)3}2]3−, ΔLllΛLll-[Co{Co(l-cys-N,S)3}2]3− and ΔLllΔLll-[Ni{Co(l-cys-N,S)3}2]4−, were newly prepared. These complexes were characterized from their electronic absorption, CD, and 13C NMR spectra. Each reaction of ΔLll-fac(S)-[Co(l-cys-N,S)3]3− with Fe(II) and Co(II) was accompanied by the inversion of the absolute configuration of the one or both terminals to give the ΔLllΛLll and ΛLllΛLll isomers, while the reaction with Fe(III), Co(III), or Ni(II) proceeded with the retention of the terminal to give the ΔLllΔLll isomer. The voltammetric study revealed that the formal potential of ΔLllΔLll-[Fe{Co(l-cys-N,S)3}2]3− (−0.16 V) is more anodic than that of ΔLllΔLll-[Co{Co(l-cys-N,S)3}2]3− (−0.43 V). These redox potentials are discussed in relation to the formation ratios of the isomers for the trinuclear complexes.