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Structural, Spectroscopic, and Theoretical Characterization of Bis(μ-oxo)dicopper Complexes, Novel Intermediates in Copper-Mediated Dioxygen Activation

Journal of the American Chemical Society1996Vol. 118(46), pp. 11555–11574

Citations Over TimeTop 1% of 1996 papers

Samiran Mahapatra, J.A. Halfen, Elizabeth Wilkinson, Gaofeng Pan, Xuedong Wang, Victor G. Young, Christopher J. Cramer, Lawrence Que, William B. Tolman

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, LBn3; 1,4,7-triisopropyl-1,4,7-triazacyclononane, LiPr3; or 1-benzyl-4,7-diisopropyl-1,4,7-triazacyclononane, LiPr2Bn; X = variety of anions). The geometry of the [Cu2(μ-O)2]2+ core was defined by X-ray crystallography for [(d21-LBn3Cu)2(μ-O)2](SbF6)2 and by EXAFS spectroscopy for the complexes capped by LBn3 and LiPr3; 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 [(LBn3Cu)(LBn2HCu)(μ-OH)2](O3SCF3)2·2CH3CO, [(LiPr2HCu)2(μ-OH)2](BPh4)2·2THF, and [(LiPr2BnCu)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-1 18O-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 [CuIII2(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).

Citations Over TimeTop 1% of 1996 papers

Abstract

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