Ambient stable trigonal bipyramidal copper(III) complexes equipped with an exchangeable axial ligand

A stable trigonal bipyramidal copper(III) complex, [PPN][Cu(^TMSPS3)Cl] (1, wherein PPN represents bis(triphenylphosphine)iminium), was synthesized from CuCl₂/PPNCl via intramolecular copper(II) disproportionation. Under ambient conditions, the axial chloride of 1 is exchangeable in solution thus making 1 serve as an intermediate to prepare trigonal bipyramidal copper(III) derivatives, e.g., [PPN][Cu(^TMSPS3)(N₃)] (2) and [Cu(^TMSPS3)(DABCO)] (3). Diamagnetic complexes 1-3 were fully characterized by X-ray crystallography, NMR, UV-vis, and Cu K-edge absorption spectroscopy. A series of UV-vis titrations were performed to investigate the relative ligand affinity toward the [Cu(^TMSPS3)] moiety, verifying the 1:1 binding equilibrium between various ligands. Compared to known copper(III) compounds, Cu K-edge absorptions of 1-3 possess lower pre-edge energy and higher shakedown transition energy, which, respectively, attribute to the electron donation from ^TMSPS3^3- ligand and their trigonal ligand field.