Spectroscopy and structure of thiolate and thioether complexes of copper(II) and the relationship of their redox chemistry to that of certain copper proteins

Abstract

Several complexes of copper and zinc(II) have been synthesized, with use of multidentate ligands entailing thiolate, phenolate, imine, and thioether donor atoms. The thioethers include tridentate and linear and macrocyclic tetra-and pentadentate ligands. The copper(II) complex of L-2, a potential N2S (pyridyl, imine, thioether) donor, crystallizes in the space group P21/c, with cell constants Z = 4, a = 9.212 (7) Å, b = 14.439 (6) Å, c = 15.117 (9) Å, β = 95.60 (6), V= 2001 Å3, and pMlcd = 1.748 g-cm3. The structure, refined to Rw= 7.4%, contains elongated pseudooctahedral Cu(II) ions, with the slightly distorted equator occupied by an H2O and the nitrogen and sulfur atoms of the tridentate ligand. Perchlorates are weakly bound on the axial positions. Electron spin resonance, optical absorption, and UV difference spectroscopy have been used to delineate the coordination of the thiolate and thioether complexes in the solid state and in nonaqueous solution. Binding of the terminal thioether donors in these oligodentate ligands is solvent dependent. Coordination is optimized in MeNO2, but in certain of the complexes, the thioether sulfur is noncoordinated in dimethylformamide, where coordinative disproportionation occurs, as for Cu(L-2)2+, which yields Cu(L-2)22+. Monomeric thiolates have been prepared and characterized as having “normal” ESR parameters. The redox thermodynamics of the complexes has been examined by dc polarography and cyclic voltammetry. Most of the compounds generate stable Cu(I) states, the macrocyclic CuIN2S2systems being notably so. The consequence of transforming a thiolato-copper complex to its thioether analogue is that the E1/2is elevated by about 0.5 V. This information is used to deduce the E°' of a tetragonal [CunN2S2]+moiety and thus comment on the possible roles of structural distortion and ligand transmutation at the type-1 active sites of copper proteins. © 1984, American Chemical Society. All rights reserved.