L-edge X-ray absorption spectroscopy and DFT calculations on Cu2O2 species: direct electrophilic aromatic attack by side-on peroxo bridged dicopper(II) complexes.

Abstract

The hydroxylation of aromatic substrates catalyzed by coupled binuclear copper enzymes has been observed with side-on-peroxo-dicopper(II) (P) and bis-μ-oxo-dicopper(III) (O) model complexes. The substrate-bound-O intermediate in [Cu(II)2(DBED)2(O)2](2+) (DBED = N,N'-di-tert-butyl-ethylenediamine) was shown to perform aromatic hydroxylation. For the [Cu(II)2(NO2-XYL)(O2)](2+) complex, only a P species was spectroscopically observed. However, it was not clear whether this O-O bond cleaves to proceed through an O-type structure along the reaction coordinate for hydroxylation of the aromatic xylyl linker. Accurate evaluation of these reaction coordinates requires reasonable quantitative descriptions of the electronic structures of the P and O species. We have performed Cu L-edge XAS on two well-characterized P and O species to experimentally quantify the Cu 3d character in their ground state wave functions. The lower per-hole Cu character (40 ± 6%) corresponding to higher covalency in the O species compared to the P species (52 ± 4%) reflects a stronger bonding interaction of the bis-μ-oxo core with the Cu(III) centers. DFT calculations show that 10-20% Hartree-Fock (HF) mixing for P and ~38% for O species are required to reproduce the Cu-O bonding; for the P species this HF mixing is also required for an antiferromagnetically coupled description of the two Cu(II) centers. B3LYP (with 20% HF) was, therefore, used to calculate the hydroxylation reaction coordinate of P in [Cu(II)2(NO2-XYL)(O2)](2+). These experimentally calibrated calculations indicate that the electrophilic attack on the aromatic ring does not involve formation of a Cu(III)2(O(2-))2 species. Rather, there is direct electron donation from the aromatic ring into the peroxo σ* orbital of the Cu(II)2(O2(2-)) species, leading to concerted C-O bond formation with O-O bond cleavage. Thus, species P is capable of direct hydroxylation of aromatic substrates without the intermediacy of an O-type species.