Spectroscopic comparison of the five-coordinate [Cu(SMeIm)(HB(3,5-iPr2pz)3)] with the four-coordinate [Cu(SCPh3)(HB(3,5-iPr2pz)3)]: effect of coordination number increase on a blue copper type site

A variety of spectroscopic techniques have been applied to characterize and compare two model complexes with relevance to red and blue copper centers in metalloproteins, [Cu(SMeIm)(HB(3,5-iPr2pz)3)] (1) and [Cu(SCPh3)(HB(3,5-iPr2pz)3)] (2), which are five- and four-coordinate, respectively. The key spectral differences in low temperature absorption and magnetic circular dichroism (MCD) include an increase in the Sσ→Cu charge transfer (CT) band intensity at 370 nm and a decrease in the absorption intensity at ∼570 nm for the Sπ→Cu CT. The energies of the d→d transitions in 1 are increased relative to 2 reflecting a more tetragonal geometry and a stronger ligand field. S K-edge X-ray absorption spectroscopy (XAS) measurements demonstrate a less covalent thiolate Cu interaction in the HOMO of 1 (15% Sp) compared to 2 (52% Sp). XAS at the Cu L-edge indicates that the Cu d-character in the HOMO of 1 has increased relative to that of 2. The electronic perturbation resulting from the increased coordination number has been evaluated. The thiolate rotates in the NNS plane resulting in increased σ overlap with the Cu. Additionally, the Cu–S bond length increases. The associated reduced covalency of the thiolate can contribute to the function of perturbed blue copper sites in proteins.