Mechanism of Formation of Copper(II) Chloro Complexes Revealed by Transient Absorption Spectroscopy and DFT/TDDFT Calculations

Copper­(II) complexes are extremely labile with typical ligand exchange rate constants on the order of 106–109 M–1 s–1. As a result, it is often difficult to identify the actual formation mechanism of these complexes. In this work, using UV–vis transient absorption when probing in a broad time range (20 ps to 8 μs) in conjunction with DFT/TDDFT calculations, we studied the dynamics and underlying reaction mechanisms of the formation of extremely labile copper­(II) CuCl4 2– chloro complexes from copper­(II) CuCl3 – trichloro complexes and chloride ions. These two species, produced via photochemical dissociation of CuCl4 2– upon 420 nm excitation into the ligand-to-metal-charge-transfer electronic state, are found to recombine into parent complexes with bimolecular rate constants of (9.0 ± 0.1) × 107 and (5.3 ± 0.4) × 108 M–1 s–1 in acetonitrile and dichloromethane, respectively. In dichloromethane, recombination occurs via a simple one-step addition. In acetonitrile, where [CuCl3]− reacts with the solvent to form a [CuCl3CH3CN]− complex in less than 20 ps, recombination takes place via ligand exchange described by the associative interchange mechanism that involves a [CuCl4CH3CN]2– intermediate. In both solvents, the recombination reaction is potential energy controlled.