Spectroscopic characterization of the oxo-transfer reaction from a bis(μ-oxo)dicopper(iii) complex to triphenylphosphine

Abstract

The oxygen-atom transfer reaction from the bis(μ-oxo)dicopper(III) complex [Cu^III₂(μ-O)₂(L)₂]²⁺1, where L = N,N,N′,N′ -tetraethylethylenediamine, to PPh₃ has been studied by UV-vis, EPR, ¹H NMR and Cu K-edge X-ray absorption spectroscopy in parallel at low temperatures (193 K) and above. Under aerobic conditions (excess dioxygen), 1 reacted with PPh₃, giving OPPh₃ and a diamagnetic species that has been assigned to an oxo-bridged dicopper(II) complex on the basis of EPR and Cu K-edge X-ray absorption spectroscopic data. Isotope-labeling experiments (¹⁸O₂) established that the oxygen atom incorporated into the triphenylphosphine oxide came from both complex 1 and exogenous dioxygen. Detailed kinetic studies revealed that the process is a third-order reaction; the rate law is first order in both complex 1 and triphenylphosphine, as well as in dioxygen. At temperatures above 233 K, reaction of 1 with PPh₃ was accompanied by ligand degradation, leading to oxidative N-dealkylation of one of the ethyl groups. By contrast, when the reaction was performed in the absence of excess dioxygen, negligible substrate (PPh₃) oxidation was observed. Instead, highly symmetrical copper complexes with a characteristic isotropic EPR signal at g = 2.11 were formed. These results are discussed in terms of parallel reaction channels that are activated under various conditions of temperature and dioxygen.