Redox-Innocent Metal-Assisted Cleavage of S–S Bond in a Disulfide-Containing Ligand

Due
to their redox capabilities, thiols have an important role in biological
oxidative/reductive processes through the formation of disulfides
or their oxidation to into sulfenic, sulfinic, or sulfonic derivatives
being also relevant for specific enzyme activities. The mechanisms
of these biological pathways often involve metal ion(s). In this case,
deciphering metal-assisted transformation of the S–S bond is
of primary interest. This report details the reactivity of the disulfide-containing
2,6-bis­[(bis­(pyridylmethyl)­amino)­methyl]-4-methylmercaptophenyldisulfide
(LMe(BPA)S–S) ligand with Cu­(II) using different
experimental conditions (anaerobic, H2O-only, H2O/O2, or O2-only). Crystallographic snapshots
show the formation of tetranuclear disulfide, dinuclear sulfinate,
and sulfonate complexes. Mechanistic investigations using Zn­(II) as
control indicate a non-metal-redox-assisted process in all cases.
When present, water acts as nucleophile and attacks at the S–S
bond. Under anhydrous conditions, a different pathway involving a
direct O2 attack at the disulfide is proposed.