Spectroscopic and Electronic Structural Studies of Blue Copper Model Complexes. 1. Perturbation of the Thiolate−Cu Bond

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.