Department of Chemistry, Stanford University , Stanford, California 94305, United States.
J Am Chem Soc. 2013 Nov 20;135(46):17417-31. doi: 10.1021/ja4078717. Epub 2013 Nov 11.
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 (DBED = N,N'-di-tert-butyl-ethylenediamine) was shown to perform aromatic hydroxylation. For the Cu(II)2(NO2-XYL)(O2) 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). 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.
偕氧双铜(II)（P）和双-μ-氧双铜(III)（O）模型配合物催化的芳香族底物的羟化作用已被观察到。在 Cu(II)2(DBED)2(O)2（DBED=N,N'-二-叔丁基乙二胺）中，底物结合的-O 中间体被证明可以进行芳香族羟化。对于 Cu(II)2(NO2-XYL)(O2) 配合物，仅在光谱上观察到 P 物种。然而，尚不清楚该 O-O 键是否会沿着芳基二甲苯连接物的羟化反应坐标裂解，以通过 O 型结构进行。准确评估这些反应坐标需要对 P 和 O 物种的电子结构进行合理的定量描述。我们对两种经过良好表征的 P 和 O 物种进行了 Cu L 边缘 XAS，以实验定量测量其基态波函数中的 Cu 3d 特征。与 P 物种（52±4%）相比，O 物种中较低的每个空穴 Cu 特征（40±6%）对应于更高的键合共价性，反映了双-μ-氧核与 Cu(III)中心的更强键合相互作用。DFT 计算表明，对于 P 物种，需要 10-20%的 Hartree-Fock（HF）混合，对于 O 物种，需要约 38%的 HF 混合，以重现 Cu-O 键合；对于 P 物种，这种 HF 混合对于两个 Cu(II)中心的反铁磁耦合描述也是必需的。因此，使用 B3LYP（20%HF）来计算 Cu(II)2(NO2-XYL)(O2)中 P 的羟化反应坐标。这些经过实验校准的计算表明，对芳环的亲电攻击不涉及形成 Cu(III)2(O(2-))2 物种。相反，芳环中的电子直接捐赠给 Cu(II)2(O2(2-))物种的过氧 σ*轨道，导致与 O-O 键裂解同时形成 C-O 键。因此，P 物种能够直接对芳香族底物进行羟化，而无需 O 型物种的中介。
