Ligand Noninnocence of Thiolate/Disulfide in Dinuclear Copper Complexes: Solvent-Dependent Redox Isomerization and Proton-Coupled Electron Transfer

Copper thiolate/disulfide interconversions are related to the functions of several important proteins such as human Sco1, Cu–Zn superoxide dismutase (SOD1), and mammalian zinc-bonded metallothionein. The synthesis and characterization of well-defined synthetic analogues for such interconversions are challenging yet provide important insights into the mechanisms of such redox processes. Solvent-dependent redox isomerization and proton-coupled electron transfer mimicking these interconversions are observed in two structurally related dimeric μ,η2:η2-thiolato Cu(II)Cu(II) complexes by various methods, including X-ray diffraction, XAS, NMR, and UV–vis. Spectroscopic evidence shows that a solvent-dependent equilibrium exists between the dimeric μ-thiolato Cu(II)Cu(II) state and its redox isomeric μ-disulfido Cu(I)Cu(I) form. Complete formation of μ-disulfido Cu(I)Cu(I) complexes, however, only occurs after the addition of 2 equiv of protons, which promote electron transfer from thiolate to Cu(II) and formation of disulfide and Cu(I) via protonation of the coordinating ligand. Proton removal reverses this reaction. The reported unusual reductive protonation/oxidative deprotonation of the metal centers may serve as a new chemical precedent for how related proteins manage Cu ions in living organisms.