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.