Formation and Reactivity of Copper Acetylacetone Bis(Thiosemicarbazone) Complexes

The complexation reaction mechanism of acetylacetone bis‐thiosemicarbazone ligands (H 2 acacR) with Cu II is explored using a variety of physical methods. The complexes form via a complicated multistep mechanism that is initiated by ring opening of the pyrazoline form of the ligand and leads, ultimately in air, to an oxidised ketone form of the ligand. Tetradentate N 2 S 2 coordinated forms of the intermediate [Cu(acacR)] are stable only under anaerobic conditions. Upon exposure to air these complexes are cleanly oxidised to the ketone complex [Cu(acacRO)] as shown by X‐ray crystallography, electrochemistry, UV/Vis and EPR spectroscopy. The behavior of these complexes contrasts with those of closely related bis‐dithiocarbazate Schiff bases which stabilise Cu III .