L-Edge X-ray Absorption Spectroscopy and DFT Calculations on Cu2O2 Species: Direct Electrophilic Aromatic Attack by Side-on Peroxo Bridged Dicopper(II) Complexes

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• 作者：Munzarin F. Qayyum ; Ritimukta Sarangi ; Kiyoshi Fujisawa ; T. Daniel P. Stack ; Kenneth D. Karlin ; Keith O. Hodgson ; Britt Hedman ; Edward I. Solomon
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：November 20, 2013
• 年：2013
• 卷：135
• 期：46
• 页码：17417-17431
• 全文大小：863K
• 年卷期：v.135,no.46(November 20, 2013)
• ISSN：1520-5126

文摘

The hydroxylation of aromatic substrates catalyzed by coupled binuclear copper enzymes has been observed with side-on-peroxo-dicopper(II) (P) and bis-渭-oxo-dicopper(III) (O) model complexes. The substrate-bound-O intermediate in [Cu(II)₂(DBED)₂(O)₂]²⁺ (DBED = N,N鈥?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鈥揙 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 卤 6%) corresponding to higher covalency in the O species compared to the P species (52 卤 4%) reflects a stronger bonding interaction of the bis-渭-oxo core with the Cu(III) centers. DFT calculations show that 10鈥?0% Hartree鈥揊ock (HF) mixing for P and 38% for O species are required to reproduce the Cu鈥揙 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鈥?/sup>)₂ species. Rather, there is direct electron donation from the aromatic ring into the peroxo 蟽* orbital of the Cu(II)₂(O₂2鈥?/sup>) species, leading to concerted C鈥揙 bond formation with O鈥揙 bond cleavage. Thus, species P is capable of direct hydroxylation of aromatic substrates without the intermediacy of an O-type species.}