Spectroscopic and Electronic Structural Studies of Blue Copper Model Complexes. 1. Perturbation of the Thiolate−Cu Bond
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J. Am. Chem. Soc., Vol: 122, Issue: 47
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- Inorganic Chemistry; Physical Chemistry
A tris(pyrazolyl)hydroborate triphenylmethylthiolate Cu(II) model complex (1) that reproduces structural and spectroscopic features of active sites of blue Cu proteins is characterized using low-temperature absorption, magnetic circular dichroism (MCD), X-ray absorption (XAS), and resonance Raman (rR) spectroscopies combined with DFT calculations to define its electronic structure. The electronic structure of 1 is further related to the oxidized Cu site in plastocyanin. The key spectral differences relative to plastocyanin include an increase in the intensity of the S pπ → Cu CT band and a decrease in the absorption intensity at 450 nm. The energies of d → d transitions in 1 decrease relative to plastocyanin, which reflects the more tetrahedral geometry of 1. S K-edge XAS measurements demonstrate a more covalent thiolate interaction in the HOMO of 1 (52% S p) than in plastocyanin (38% S p). The effects of the high thiolate covalency on the absorption and Raman spectral features for 1 are evaluated. Additional changes in the absorption spectrum of 1 relative to plastocyanin in the 450 nm and the near-infrared regions are due to differences in the electronic structure of the nitrogen ligands associated with the change from imidazole to pyrazole. Finally, XAS measurements at the Cu L- and K-edges indicate that the effective nuclear charge of Cu in 1 is higher than in plastocyanin, which likely results from misdirection of the ligating orbitals in the constrained tris(pyrazolyl)hydroborate ligand system. This reduces the donor interaction of this ligand with the copper which increases the covalency of the thiolate−Cu bond and can contribute to the electron-transfer properties of the blue copper site.