Dioxygen Activation and Catalytic Reduction to Hydrogen Peroxide by a Thiolate-Bridged Dimanganese(II) Complex with a Pendant Thiol

Herein, we describe an uncommon example
of a manganese–thiolate
complex, which is capable of activating dioxygen and catalyzing its
two-electron reduction to generate H2O2. The
structurally characterized dimercapto-bridged MnII dimer
[MnII2(LS)­(LSH)]­ClO4 (MnII2SH) is formed by reaction of the LS ligand (2,2′-(2,2′-bipyridine-6,6′-diyl)­bis­(1,1-diphenylethanethiolate))
with MnII. The unusual presence of a pendant thiol group
bound to one MnII ion in MnII2SH is evidenced both
in the solid state and in solution. The MnII2SH complex reacts
with dioxygen in CH3CN, leading to the formation of a rare
mono-μ-hydroxo dinuclear MnIII complex, [(MnIII2(LS)2(OH)]­ClO4 (MnIII2OH), which has also been structurally characterized. When MnII2SH reacts with O2 in the presence of a proton source,
2,6-lutidinium tetrafluoroborate (up to 50 equiv), the formation of
a new Mn species is observed, assigned to a bis-μ-thiolato dinuclear
MnIII complex with two terminal thiolate groups (MnIII2), with
the concomitant production of H2O2 up to ∼40%
vs MnII2SH. The addition of a catalytic amount of MnII2SH to an air-saturated solution of MenFc
(n = 8 or 10) and 2,6-lutidinium tetrafluoroborate
results in the quantitative and efficient oxidation of MenFc by O2 to afford the respective ferrocenium
derivatives (MenFc+, with n = 8 or 10). Hydrogen peroxide is mainly produced during
the catalytic reduction of dioxygen with 80–84% selectivity,
making the MnII2SH complex a rare Mn-based active catalyst for
two-electron O2 reduction.