Proton-induced cis-trans conversion of a platinum(II) center coordinated by L-cysteinatocobalt(III) metalloligands

Treatment of Lambda(L)-[Co(L-cys-N,S)(en)(2)](+) (L-H(2)cys = L-cysteine) with [PtCl4](2-) in water, followed by the addition of acid, gave an S-bridged (Co2PtII)-Pt-III trinuclear complex ([1](4+)), which was reversibly converted to its deprotonated complex ([2](2+)) in an aqueous solution. While [1](4+) formed only a trans isomer, [2](2+) existed as a mixture of trans and cis isomers. The selective formation of a cis isomer was achieved by treatment of [1](4+) or [2](2+) with phthalic acid in water, which afforded a unique (Co4Pt2II)-Pt-III hexanuclear complex ([3](4+)). Complex [3](4+) was reverted back to [1](4+) by treatment with aqueous HCl, accompanied by the complete cis-to-trans conversion.