X-ray absorption edge spectroscopy and computational studies on LCuO2 species:: Superoxide-CuII versus peroxide-CuIII bonding

The geometric and electronic structures of two mononuclear CuO2 complexes, [Cu(O-2){HB(3-Ad-5-(i)Prpz)(3)}] (1) and [Cu(O-2)(beta-diketiminate)] (2), have been evaluated using Cu K- and L-edge X-ray absorption spectroscopy (XAS) studies in combination with valence bond configuration interaction (VBCI) simulations and spin-unrestricted broken symmetry density functional theory (DFT) calculations. Cu K- and L-edge XAS data indicate the Cu(II) and Cu(III) nature of 1 and 2, respectively. The total integrated intensity under the L-edges shows that the Psi(*)(LUMO)'s in 1 and 2 contain 20% and 28% Cu character, respectively, indicative of very covalent ground states in both complexes, although more so in 1. Two-state VBCI simulations also indicate that the ground state in 2 has more Cu (vertical bar 3d(8)>) character. DFT calculations show that the Psi(*)(LUMO)'s in both complexes is dominated by O-2(n-) character, although the O-2(n-) character is higher in 1. It is shown that the ligand L plays an important role in modulating Cu-O-2 bonding in these LCuO2 systems and tunes the ground states of 1 and 2 to have dominant Cu(II)-superoxide- like and Cu(III)-peroxide-like character, respectively. The contributions of ligand field (LF) and the charge on the absorbing atom in the molecule (Q(mol)(M)) to L- and K-edge energy shifts are evaluated using DFT and time-dependent DFT calculations. It is found that LF makes a dominant contribution to the edge energy shift, while the effect of Q(mol)(M) is minor. The charge on the Cu in the Cu(III) complex is found to be similar to that in Cu(II) complexes, which indicates a much stronger interaction with the ligand, leading to extensive charge transfer.