RADICAL REDUCTIONS OF CU(II)-PENICILLAMINE COMPLEXES - EVIDENCE FOR A CU(II)-DISULFIDE ANION INTERMEDIATE

In basic solution Cu(II) ions form planar complexes of stoichiometry Cu(II).L2 with penicillamine molecules, where both the nitrogen and sulphur atoms of the thiol [(NH2)CH(CO2-)C(CH3)2S-] are involved in the coordination. The rates of reduction of this complex by the formate radical and the thiyl radical of penicillamine have been measured by pulse radiolysis. The rate constant for reaction [14] has been found to be 1.2 +/- 0.2 x 10(8)M-1 s-1 [14] .CO2- + Cu(II)(PenS-)2 --> CO2 + Cu(I)(PenS-)2 For the thiyl radical, which is not inherently reducing, reduction must involve the PenS therefore SPen- anion complex. [2] PenS therefore SPen- + Cu(II)(PenS-)2 --> (PenS)2 + Cu(I)(PenS-)2 Under the conditions of this study, ([PenS-] approximately 10(-2) M, pH = 10.0), this anion exists in free solution equilibrium with PenS. [1] PenS. + PenS- [GRAPHICS] PenS therefore SPen- K1 = 60.0 A systematic study of the dependence of the overall reduction rate on [PenS-] showed that the reaction proceeded rapidly even when the PenS therefore SPen- concentration was negligibly small. This finding can be explained by the formation of an internal PenS therefore SPen- anion from the reaction of free PenS. with one of the PenS- ligands of the Cu(II) complex: [3] PenS. + Cu(II)(PenS-)2 --> (PenS-)Cu(II)(PenS therefore SPen-) and the subsequent rapid transfer of an electron to Cu(II): [4] (PenS-)Cu(II)(PenS therefore SPen-) --> (PenS-)Cu(I)(PenSSPen) Analysis of the results gave k2 = 2.9 +/- 0.3 x 10(8) M-1 s-1 and k3 = 3.4 +/- 0.4 x 10(8) M-1 s-1.