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, LBn 3 ; 1,4,7-triisopropyl-1,4,7-triazacyclononane, LiPr 3 ; or 1-benzyl-4,7-diisopropyl-1,4,7-triazacyclononane, LiPr 2 Bn; X = variety of anions). The geometry of the [Cu2(μ-O)2]2+ core was defined by X-ray crystallography for [(d 21-LBn 3 Cu)2(μ-O)2](SbF6)2 and by EXAFS spectroscopy for the complexes capped by LBn 3 and LiPr 3 ; 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 [(LBn 3 Cu)(LBn 2 HCu)(μ-OH)2](O3SCF3)2·2CH3CO, [(LiPr 2 HCu)2(μ-OH)2](BPh4)2·2THF, and [(LiPr 2 BnCu)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 [CuIII 2(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).

The Reaction of μ‐η2:η2‐Peroxo‐ and Bis(μ‐oxo)dicopper Complexes with Flavonol

“…[46,47] After removal of excess dioxygen by purging with argon at Ϫ80°C for 10 min, a solution of flavonol (0.119 g, 0.5 mmol) in CH 2 Cl 2 (5 mL) was added, resulting in an instantaneous color change from red-brown to intense green. The reaction mixture was stirred at Ϫ80°C for 1 h, and then warmed to room temperature.…”

Biomimetic Applications of Metal Systems Supported by Scorpionates

“…A first hypothesis of the enzyme's catalytic mechanism, proposed by Averill (Hulse et al, 1989;Averill, 1996), contemplates the initial reduction of the metal center (with electrons shuttled from the T1 site) and subsequent binding of nitrite to Cu(I) by means of the N atom (η 1 -N coordination). This η 1 -N coordination is supported by experimental evidence on model complexes (Halfen & Tolman, 1994;Mahapatra et al, 1996). Subsequent protonation of the substrate would give water and a copper-nitrosyl complex, presumably an unstable {CuNO} 10 species (Enemark & Feltham, 1974), wherein NO is reasonably considered to be η 1 -N end-on coordinated to the metal.…”

A projected approximation to strongly contracted N-electron valence perturbation theory for DMRG wavefunctions

“…This is the price to pay for the low computational scaling with the number of active orbitals of the current algorithm. 36,39,[65][66][67][68][69][70][71][72][73][74] In the course of this work, a series of projected DMRG-SC-NEVPT2 calculations were conducted on "structure 7" from Ref. 74 that features an O-O bond distance of 1.60 Å.…”