X-ray absorption edge spectroscopy and computational studies on LCuO ₂ species: Superoxide-Cu^II versus peroxide-Cu ^III bonding

The geometric and electronic structures of two mononuclear CuO₂ complexes, [Cu(O₂){HB(3-Ad-5-ⁱPrpz)₃}] (1) and [Cu(O₂)(β-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 ψ _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 (|3d⁸〉) character. DFT calculations show that the ψ_LUMO* in both complexes is dominated by O ₂^n- character, although the O₂^n- character is higher in 1. It is shown that the ligand L plays an important role in modulating Cu-O₂ 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.