Charge-transfer spectra of model Cu(II) chromophores

Abstract

Electronic ‘tuning’ of copper proteins for specific O 2-transport, redox, and oxygenase functions is determined by the number, nature and geometrical arrangements of the protein side-chain donor ligands which bind the copper ion(s) at the active site(s). The resulting energies and orientation of the upper occupied Cu(II) orbitals and D-vacancy in the oxidized proteins generally can be inferred from the ESR and ligand field (LF) spectra. At higher energies than those absorption, Cu(II) chromophores exhibit ligand to metal charge-transfer (LMCT) absorption. These absorptions reflect a potentially important yet little studied aspect of electronic structure: the relative overlaps of the σ and π-symmetry ligand donor orbitals with the Cu(II) d-vacancy. We review recent studies of LMCT spectra exhibited by structurally unambiguous mode Cu(II) chromophores containing ligation such as thiolate, thioether, imidazole and imidazolate [1]. These results facilitate the understanding of corresponding LMCT absorptions exhibited by various copper proteins. Replacement of the Cu(II) in the model complexes and proteins by the less oxidizing Ni(II) and Co(II) ions results in characteristics blue-shifts of these LMCT absorptions, and support an LMCT rather than assignment. Owing to the LF-dependency of the metal ion d-vacancy energy, the LMCT absorptions of the planar Cu(II) and Ni(II) chromophores are blue-shifted relative to their approximately tetrahedral analogs. Cu(II)imidazole complexes exhibit LMCT absorption from two ring π orbitals, one having primarily carbon 2p character (π 1,HOMO) and the other having primarily nitrogen 2p character (π 2). The calculated (INDO/S method) orbital energy increases induced by successive ring methylations are correlated with red shifts of imidazole → Cu(II) LMCT exhibited by this series of complexes [2]. Spectroscopic effects associated with the orientation of the imidazole rings relative to the Cu(II) d-vacancy are also discussed. Imidazolate ligation results in a characteristics additional LMCT absorption at relatively low energy. The intense (ϵ > 1000) near UV LMCT absorption associated with equatorial Cu(II)thioether bonding disappears when, as for plastocyanin, the Cu(II)-thioether bond becomes elongated (apical) and is oriented normal to the d-vacancy. The syntheses and LMCT spectra of model M(II)-thiolate complexes (M = Cu, Co, Ni) are described briefly.