A Model for the Active-Site Formation Process in DMSO Reductase Family Molybdenum Enzymes Involving Oxido−Alcoholato and Oxido−Thiolato Molybdenum(VI) Core Structures

New
bis­(ene-1,2-dithiolato)-oxido–alcoholato molybdenum­(VI) and
-oxido–thiolato molybdenum­(VI) anionic complexes, denoted as [MoVIO­(ER)­L2]– (E = O, S; L =
dimethoxycarboxylate-1,2-ethylenedithiolate), were obtained from the
reaction of the corresponding dioxido-molybdenum­(VI) precursor complex
with either an alcohol or a thiol in the presence of an organic acid
(e.g., 10-camphorsulfonic acid) at low temperature. The [MoVIO­(ER)­L2]– complexes were isolated and characterized,
and the structure of [MoVIO­(OEt)­L2]– was
determined by X-ray crystallography. The Mo­(VI) center in [MoVIO­(OEt)­L2]– exhibits a distorted octahedral geometry with
the two ene-1,2-dithiolate ligands being symmetry inequivalent. The
computed structure of [MoVIO­(SR)­L2]– is essentially
identical to that of [MoVIO­(OR)­L2]–. The electronic structures
of the resulting molybdenum­(VI) complexes were evaluated using electronic
absorption spectroscopy and bonding calculations. The nature of the
distorted Oh geometry
in these [MoVIO­(EEt)­L2]– complexes results
in a lowest unoccupied molecular orbital wave function that possesses
strong π* interactions between the Mo­(dxy) orbital and the cis S­(pz) orbital localized on one sulfur donor from a single ene-1,2-dithiolate
ligand. The presence of a covalent Mo–Sdithiolene bonding interaction in these monooxido Mo­(VI) compounds contributes
to their low-energy ligand-to-metal charge transfer transitions. A
second important d–p π bonding interaction derives from
the ∼180° Ooxo–Mo–E–C dihedral angle involving the alcoholate and thiolate donors,
and this contributes to ancillary ligand contributions to the electronic
structure of these species. The formation of [MoVIO­(OEt)­L2]– and [MoVIO­(SEt)­L2]– from the dioxidomolybdenum­(VI) precursor may be regarded as a model
for the active-site formation process that occurs in the dimethyl
sulfoxide reductase family of pyranopterin molybdenum enzymes.