Transfer of a quadridentate N2S2 ligand from M2+ (M = Cr, Mn, Fe, Co or Ni) to Cu2+:: Unexpected reactivities of bimetallic intermediates

A series of complexes of stoichiometry [MX2(dipyS)] {dipyS = bis(2-picolyl)-1,3-dithiopropane); M = Cr, Mn, Fe, Co, Ni, X = Cl; M = Ni or Cu, X = NO3} and [VOCl(dipyS)]Cl have been prepared and characterised, including the X-ray crystal structure of [Ni(ONO2)(2)(dipyS)]. The kinetics of the transfer of dipyS from these complexes (M = Cr, Mn, Fe, Co or Ni) to Cu2+, to form [Cu(dipyS)](2+), have been studied in MeOH. For M = Ni, the kinetics are consistent with a mechanism involving rate-limiting dissociation of the initial pyridyl-M bond. Subsequent binding of Cu2+, the pendant pyridyl-residue (or binding Cl- to the vacant site on M) is followed by the complete transfer of dipyS from M to Cu. For M = Cr, Mn or Co, the same mechanism is believed to operate, but in these cases intermediates in the dipyS transfer to Cu2+ have been detected spectroscopically. Evidence is presented that these intermediates have Cu2+ bound to a pendant pyridyl-group on [MCl2(dipyS)] and that the subsequent complete transfer of dipyS involves rate-limiting dissociation of a M-S bond. For M = Fe, e.p.r. spectroscopy shows that the complex is a dimer in solution. However, the transfer reaction with Cu2+ involves an analogous intermediate to that with M = Cr, Mn or Co, but only at high concentrations of Cu2+. Unexpectedly, the binding of Cu2+ inhibits the transfer of dipyS from Fe to Cu. The electronic factors which give rise to this behaviour are discussed.