Spectroscopic and Electronic Structural Studies of Blue Copper Model Complexes. 2. Comparison of Three- and Four-Coordinate Cu(II)−Thiolate Complexes and Fungal Laccase

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.