Surprisingly facile decomposition of the dication : a metal-mediated Hunsdiecker reaction of a succinimidyl ester?

A series of dicationic organoiridium complexes [Cp*Ir(arene)]2+, (4,5 and6) have been prepared in which the π-bonded phenyl ring is attached to an alkyl chain terminated by a succinimidyl ester, thus forming Bolton-Hunter reagents for protein labelling. In solution, the benzyl derivative,4, underwent facile decomposition. Loss of the succinimidyl moiety followed by decarboxylation led to [Cp*Ir(η-MeOC6H4Me)][BF4]2 (7). This product was characterized spectroscopically and by X-ray crystallography. Compound7 crystallizes in the space group P21/a with a = 13.567(5)Å, b = 17.664(1)Å, c = 9.044(5)Å, β = 90.15°, V = 2167Å3and Z = 4. A rationale for this surprisingly facile decomposition invokes stabilization of the intermediate benzyl radical by the iridium.