Trivalent copper stabilised by acetylacetone dithiocarbazate Schiff base ligands: structural, spectroscopic and electrochemical properties

The copper coordination chemistry of N₂S₂ Schiff base ligands derived from acetylacetone and S-methyl or S-benzyl dithiocarbazate (H₃acacsR, R = Me, Bn) reveals a rich variety of products depending on the reaction conditions. The free ligands spontaneously cyclise to their pyrazoline isomers but ring-open upon complexation with Cu^II. In the absence of oxygen, the ligands form Cu^IIN₂S₂ complexes ([Cu^II(HacacsR)]) that have been characterised electrochemically, spectroscopically and structurally. Intermediates in the complexation reaction are observed with time-resolved UV-Vis spectroscopy. Upon exposure to air, a number of different complexes are formed. Facile oxidation of [Cu^II(HacacsR)] to the trivalent analogue [Cu^III(acacsR)] occurs in air. This compound is the precursor to two further oxidation reactions; one to the ketone [Cu^II(acacsRO)] where a carbonyl group has been installed at the apical C atom of the acetylacetone moiety and another to afford the novel dinuclear complex [(Cu^III(acacsR))₂]. The presence of excess base (Et₃N) favours formation of the dimer.