Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes.

A series of organometallic copper complexes in formal oxidation states ranging from +1 to +3 have been characterized by a combination of Cu K-edge X-ray absorption (XAS) and Cu Kβ valence-to-core X-ray emission spectroscopies (VtC XES). Each formal oxidation state exhibits distinctly different XAS and VtC XES transition energies due to the differences in the Cu Z eff , concomitant with changes in physical oxidation state from +1 to +2 to +3. Herein, we demonstrate the sensitivity of XAS and VtC XES to the physical oxidation states of a series of N-heterocyclic carbene (NHC) ligated organocopper complexes. We then extend these methods to the study of the [Cu(CF 3 ) 4 ] - ion. Complemented by computational methods, the observed spectral transitions are correlated with the electronic structure of the complexes and the Cu Z eff . These calculations demonstrate that a contraction of the Cu 1s orbitals to deeper binding energy upon oxidation of the Cu center manifests spectroscopically as a stepped increase in the energy of both XAS and Kβ 2,5 emission features with increasing formal oxidation state within the [Cu n +(NHC 2 )] n+ series. The newly synthesized Cu(III) cation [Cu III (NHC 4 )] 3+ exhibits spectroscopic features and an electronic structure remarkably similar to [Cu(CF 3 ) 4 ] - , supporting a physical oxidation state assignment of low-spin d 8 Cu(III) for [Cu(CF 3 ) 4 ] - . Combining XAS and VtC XES further demonstrates the necessity of combining multiple spectroscopies when investigating the electronic structures of highly covalent copper complexes, providing a template for future investigations into both synthetic and biological metal centers.