Electronic Structural Analysis of Copper(II)–TEMPO/ABNO Complexes Provides Evidence for Copper(I)–Oxoammonium Character

Copper/aminoxyl
species are proposed as key intermediates in aerobic
alcohol oxidation. Several possible electronic structural descriptions
of these species are possible, and the present study probes this issue
by examining four crystallographically characterized Cu/aminoxyl halide
complexes by Cu K-edge, Cu L2,3-edge, and Cl K-edge X-ray
absorption spectroscopy. The mixing coefficients between Cu, aminoxyl,
and halide orbitals are determined via these techniques with support
from density functional theory. The emergent electronic structure
picture reveals that Cu coordination confers appreciable oxoammonium
character to the aminoxyl ligand. The computational methodology is
extended to one of the putative intermediates invoked in catalytic
Cu/aminoxyl-driven alcohol oxidation reactions, with similar findings.
Collectively, the results have important implications for the mechanism
of alcohol oxidation and the underlying basis for cooperativity in
this co-catalyst system.