Structural, spectroscopic, and theoretical characterization of bis(μ-oxo)dicopper complexes, novel intermediates in copper-mediated dioxygen activation

Abstract

A description of the structure and bonding of novel bis(μ-oxo)dicopper complexes and their bis(μ-hydroxo)dicopper decomposition products was derived from combined X-ray crystallographic, spectroscopic, and ab initio theoretical studies. The compounds [(LCu)2(μ-O)2]X2 were generated from the reaction of solutions of [LCu(CH3CN)]X with O2 at -80 °C (L = 1,4,7-tribenzyl-1,4,7-triazacyclononane, L(Bn3); 1,4,7-triisopropyl-1,4,7-triazacyclononane, L(iPr3); or 1-benzyl-4,7-diisopropyl-1,4,7-triazacyclononane, L(iPr2Bn); X = variety of anions). The geometry of the [Cu2(μ-O)2]2+ core was defined by X-ray crystallography for [(d21-L(Bn3)Cu)2(μ-O)2](SbF6)2 and by EXAFS spectroscopy for the complexes capped by L(Bn3) and L(iPr3); notable dimensions include short Cu-O (~1.80 Å) and Cu···Cu (~2.80 Å) distances like those reported for analogous M2(μ-O)2 (M = Fe or Mn) rhombs. The core geometry is contracted compared to those of the bis(μ-hydroxo)dicopper(II) compounds that result from decomposition of the bis(μ-oxo) complexes upon warming. X-ray structures of the decomposition products [(L(Bn3)Cu)(L(Bn2H)Cu)(μ-OH)2](O3SCF3)2·2CH3CO, [(L(iPr2H)Cu)2(μ-OH)2](BPh4)2·2THF, and [(L(iPr2Bn)Cu)2(μ-OH)2](O3SCF3)2 showed that they arise from N-dealkylation of the original capping macrocycles. Manometric, electrospray mass spectrometric, and UV-vis, EPR, NMR, and resonance Raman spectroscopic data for the bis(μ-oxo)dicopper complexes in solution revealed important topological and electronic structural features of the intact [Cu2(μ-O)2]2+ core. The bis(μ-oxo)dicopper unit is diamagnetic, undergoes a rapid fluxional process involving interchange of equatorial and axial N-donor ligand environments, and exhibits a diagnostic ~600 cm-118O-sensitive feature in Raman spectra. Ab initio calculations on a model system, {[(NH3)3Cu]2(μ-O)2}2+, predicted a closed-shell singlet ground-state structure that agrees well with the bis(μ-oxo)dicopper geometry determined by experiment and helps to rationalize many of its physicochemical properties. On the basis of an analysis of the theoretical and experimental results (including a bond valence sum analysis), a formal oxidation level assignment for the core is suggested to be [Cu(III)(μ-O2-)2]2+, although a more complete molecular orbital description indicates that the oxygen and copper fragment orbitals are significantly mixed (i.e., there is a high degree of covalency).