Re-evaluating the Cu K pre-edge XAS transition in complexes with covalent metal–ligand interactions

摘要

Three [Me2NN]Cu(h2\ud-L2) complexes (Me2NN ¼ HC[C(Me)NAr]2; L2 ¼ PhNO (2), ArF\ud2N2 (3), PhCH]CH2 (4);\udAr ¼ 2,6-Me2-C6H3; ArF ¼ 3,5-(CF3)2-C6H3) have been studied by Cu K-edge X-ray absorption\udspectroscopy, as well as single- and multi-reference computational methods (DFT, TD-DFT, CASSCF,\udMRCI, and OVB). The study was extended to a range of both known and theoretical compounds bearing\ud2p-element donors as a means of deriving a consistent view of how the pre-edge transition energy\udresponds in systems with significant ground state covalency. The ground state electronic structures of\udmany of the compounds under investigation were found to be strongly influenced by correlation effects,\udresulting in ground state descriptions with majority contributions from a configuration comprised of a\udCu(II) metal center anti-ferromagentically coupled to radical anion O2, PhNO, and ArF\ud2N2 ligands. In\udcontrast, the styrene complex 4, which displays a Cu K pre-edge transition despite its formal d10 electron\udconfiguration, exhibits what can best be described as a Cu(I):(styrene)0 ground state with strong pbackbonding.\udThe Cu K pre-edge features for these complexes increase in energy from 1 to 4, a trend\udthat was tracked to the percent Cu(II)-character in the ground state. The unexpected shift to higher preedge\udtransition energies with decreasing charge on copper (QCu) contributed to an assignment of the\udpre-edge features for these species as arising from metal-to-ligand charge transfer instead of the\udtraditional Cu1s / Cu3d designation.