Zwitterionic mixed-carbene coinage metal complexes: Synthesis, structures and photophysical studies

A series of three zwitterionic mixed-ligand bis(carbene) complexes of copper (1), silver (2) and gold (3) have been synthesized, based on a combination of ethyl-substituted cyclic (alkyl)(amino)carbene (Et2CAAC) and an anionic methyl-malonate-derived N-heterocyclic carbene (maloNHC). The crystal structures confirm the linear two-coordinate geometry without close intermolecular contacts. The compounds show blue-white phosphorescence consistent with a wide HOMO-LUMO energy gap (2.87–3.07 eV) estimated by cyclic voltammetry. The excited state lifetimes of crystalline powders decrease in the series Cu > Ag > Au, from 400 µs for copper 1 to 50 µs for the gold complex 3. DFT calculations indicate a large change in the transition dipole moment on excitation, of up to 16 D.