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

Mahapatra, S; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Young, V. G., Jr.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B.
J. Am. Chem. Soc. 1996, 118, 12555.

A description of the structure and bonding of novel bis(mu-oxo)dicopper complexes and their bis(mu-hydroxo)dicopper decomposition products was derived from combined X-ray crystallographic, spectroscopic, and ab initio theoretical studies. The compounds [(LCu)2(mu-O)2]X2 were generated from the reaction of solutions of [LCu(CH3CN)]X with O2 at -80oC (L = 1,4,7-tribenzyl-1,4,7-triazacyclononane, LBn3, 1,4,7-triisopropyl-1,4,7-triazacyclononane, LiPr3, 1-benzyl-4,7-diisopropyl-1,4,7-triazacyclononane, LiPr2Bn; X = variety of anions). The geometry of the [Cu2(mu-O)2]2+ core was defined by X-ray crystallography for [(d21-LBn3Cu)2(mu-O)2](SbF6)2 and by EXAFS spectroscopy for the complexes capped by LBn3 and LiPr3; notable dimensions include short Cu-O (approx. 1.80 angstroms) and Cu-Cu (approx. 2.8 angstroms) distances like those reported for analogous M2(mu-O)2 (M = Fe or Mn) rhombs. The core geometry is contracted compared to those of the bis(mu-hydroxo)dicopper(II) compounds that result upon decomposition of the bis(mu-oxo) complexes upon warming. X-ray structures of the decomposition producets 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(mu-oxo)dicopper complexes in solution revealed important topological and electronic structural features of the intact [Cu2(mu-O)2]2+ core. The bis(mu-oxo)dicopper unit is diamagnetic, undergoes a rapid fluxional process involving interchange of equatorial and axial N-donor ligand environments, and exhibits a diagnostic approx. 600 cm-1 18O-sensitive feature in Raman spectra. Ab initio calculations on a model system [(NH3)6Cu2(mu-O)2]2+ predicted a closed-shell singlet ground-state structure that agrees well with the bis(mu-oxo)dicopper geometry determined by experiment and helps to rationalize many of its physicochemical properties. Based on 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(mu-O)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).

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