Oxo–Mo(IV)(dithiolene)thiolato Complexes: Analogue of Reduced Sulfite Oxidase

A series of [MoIVO­(mnt)­(SR)­(N–N)]− (mnt = maleonitriledithiolate; R = Ph, nap, p-Cl-Ph, p-CO2H-Ph, and p-NO2-Ph; N–N = 2,2′-bipyridine
(bipy) and 1,10-phenanthroline (phen)) complexes analogous to the
reduced active site of enzymes of the sulfite oxidase family has been
synthesized and their participation in electron transfer reactions
studied. Equatorial thiolate and dithiolene ligations have been used
to closely simulate the three sulfur coordinations present in the
native molybdenum active site. These synthetic analogues have been
shown to participate in electron transfer via a pentavalent EPR-active
Mo­(V) intermediate with minimal structural change as observed electrochemically
by reversible oxidative responses. The role of the redox-active dithiolene
ligand as an electron transfer gate between external oxidants and
the molybdenum center could be envisaged in one of the analogue systems
where the initial transient EPR signal with ⟨g⟩ = 2.008 is replaced by the appearance of a typical Mo­(V)-centered
EPR (⟨g⟩ = 1.976) signal. The appearance
of such a ligand-based transient radical at the initial stage has
been supported by the ligand-centered frontier orbital from DFT calculation.
A stepwise rationale has been provided by computational study to show
that the coupled effects of the diimine bite angle and the thiolato
dihedral angle determine the metal- or ligand-based frontier orbital
occupancy. DFT calculation has further supported the similarity between
the reduced, semireduced, and oxidized resting state of the molybdenum
center in Moco of SO with the synthesized complexes and their corresponding
one-electron and fully oxidized species.