Proton-Induced Cis−Trans Conversion of a Platinum(II) Center Coordinated by l-Cysteinatocobalt(III) Metalloligands

Treatment of ΛL-[Co(l-cys-N,S)(en)2]+ (l-H2cys = l-cysteine) with
[PtCl4]2- in water, followed by the addition of acid, gave an
S-bridged CoIII2PtII 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 CoIII4PtII2 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.