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, magnet ic circular dichroism (MCD), X-ray absorption (XAS), and resonance Raman (r R) spectroscopies combined with DFT calculations to define its electronic s tructure. The electronic structure of 1 is further related to the oxidized Cu site in plastocyanin. The key spectral differences relative to plastocya nin include an increase in the intensity of the S p pi --> Cu CT band and a decrease in the absorption intensity at similar to 450 nm. The energies, o f d --> d transitions in 1 decrease relative to plastocyanin, which reflect s 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 pl astocyanin (38% S p). The effects of the high thiolate;covalency drl:the ab sorption and Raman spectral features for I are evaluated. Additional change s in the absorption spectrum of 1 relative to plastocyanin in the similar t o 450 nm and the near-infrared regions are due to differences in the electr onic structure of the nitrogen ligands associated with the change from imid azole to pyrazole. Finally, XAS measurements at the Cu L- and K-edges indic ate that the effective nuclear charge of Cu in I is higher than in plastocy anin, which likely results from misdirection of the ligating orbitals in th e constrained tris(pyrazolyl)hydroborate Ligand system. This reduces the do nor interaction of this ligand with the copper which increases I : the cova lency of the thiolate-Cu bond and can contribute to the electron-transfer p roperties of the blue copper site.