‘Inverse cryptate’ structure of an exceptionally stable dicopper(I) semiquinonoid intermediate

Dinuclear diphosphinecopper(I) complexes of the bis(chelating)‘S-frame’ ligand di-tert-butyl azodiformate exhibit a remarkable kinetic and thermodynamic stability of the deep blue o-semiquinonoid intermediate as evident from its facile formation, stability towards air and protic media, and from the electrochemical potential range. The comproportionation constant of [CuI2{µ-N2[CO(OBut)]2}{µ-Ph2P(CH2)6PPh2}2]+ was established at 1019·7. The crystal structure of the tetraphenylborate salt has been determined. It shows an ‘inverse cryptate’ structure; two bridging diphosphine ligands span the two bridgehead copper(I) centres which are fixed at 4.82 A apart by the bis(chelating) azodicarboxylate anion radical. In contrast to the neutral (reduced) form of the complex, the dicationic oxidised state could only be spectroelectrochemically.