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

Abstract

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 [Cu^I₂{µ-N₂[CO(OBu^t)]₂}{µ-Ph₂P(CH₂)₆PPh₂}₂]⁺ was established at 10^19·7. 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 Å 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.