To evaluate the importance of the axial ligand in blue Cu centers, the electronic structure of a three-coordinate model compound LCuSCPh3 (2, L = β-diketiminate ligand) is defined using low-temperature absorption, magnetic circular dichroism (MCD), X-ray absorption spectroscopy (XAS), and resonance Raman (rR) profiles coupled with density functional calculations. Using these excited-state spectroscopic methods the electronic structure of 2 is compared to that of a four-coordinate blue Cu model compound LCuSCPh3 (1, L = tris(pyrazolyl)hydroborate ligand) and the three-coordinate blue Cu center in fungal laccase. The spectral features of 2 are substantially altered from those of 1 and reflect a trans influence of the β-diketiminate ligand that involves a strong interaction with the Cu dx2-y2 orbital, concomitant with a decreased S pπ interaction with the Cu dx2-y2 orbital. The lack of an axial ligand coupled with the influence of this equatorial donor leads to many of the perturbed spectral features of 2 relative to 1 including: the shift of the S pπ → Cu CT transition to lower energy and its reduced intensity, the stronger ligand field, the larger nitrogen covalency in the HOMO at the expense of thiolate covalency, and the decreased excited-state distortion. From XAS, it also is clear that loss of the axial pyrazole ligand in 2 relative to 1 leads to an increase in the effective nuclear charge of the three-coordinate Cu in 2, which demonstrates that the axial ligand modulates the electronic structure. From a comparison of 2 to fungal laccase it is evident that even in the absence of an axial ligand the electronic structure can be modulated by differential charge donation from the equatorial ligands. These studies show that elimination of the axial ligand of the blue copper site can affect the covalency of the thiolate−Cu bond and potentially modify the electron-transfer properties of the site.
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