Quantitative Electromerism of a Well-defined Mononuclear Copper Complex Through Reversible Intramolecular Metal-Ligand Electron Transfer.

Copper amine oxidases are enzymes that exhibit in their active site a mononuclear copper complex and a 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor; in the oxidative half of the catalytic cycle, the enzymes regulate their activity by a temperature-dependent electron transfer equilibrium between the Cu complex with the reduced, aminoquinol form of the cofactor and the reactive Cu complex with the corresponding oxidized, semiquinone form of the cofactor. Here, we report the first mononuclear copper complex with redox-active ligands showing quantitative, reversible electromerism between a Cu eletromer with reduced, neutral ligand and a Cu electromer with an oxidized, radical monocationic ligand. The Cu form, being exclusively present at low temperature, exhibits a lower enthalpy (like the enzymes), but the Cu complex exhibits a higher entropy and is exclusively present at room temperature in CH Cl solution. Further analysis, based on six different copper complexes, discloses a large solvent effect on electromerism.