Spectroscopic and electronic structural studies of blue copper model complexes. 2. Comparison of three- and four-coordinate Cu(II)-thiolate complexesand fungal laccase

To evaluate the importance of the axial ligand in blue Cu centers, the elec tronic: structure of a three-coordinate model compound LCuSCPh3 (2, L, beta -diketiminate ligand) is defined using low-temperature absorption, magneti c circular dichroism (MCD), X-ray absorption spectroscopy (XAS), and resona nce 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 cente r in fungal laccase. The spectral features of 2 are substantially altered f rom those of 1 and reflect a. trans influence of the beta -diketiminate lig and that involves a strong interaction with the Cu d(x2-y2) orbital, concom itant with a decreased S p pi interaction with the Cu d(x2-y2) orbital. The lack of an axial ligand coupled with the influence of this equatorial dono r leads I to many of the perturbed spectral features of 2 relative to 1 inc luding: the shift of the,S p pi --> Cu CT transition to lower energy and it s reduced intensity, the stronger ligand field, the larger nitrogen covalen cy in the HOMO at the expense of thiolate covalency, and the decreased exci ted-state distortion. From XAS, it also is clear that loss of the axial pyr azole ligand in 2 relative to 1 leads to an increase in the effective nucle ar charge of the three-coordinate Cu in 2, which demonstrates that the axia l ligand modulates, the electronic structure. From a. comparison of 2 to fu ngal 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 li gand of the blue copper site can affect the covalency of the thiolate-Cu bo nd and potentially modify the electron-transfer properties of the site.