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.