A Well-Defined Terminal Vanadium(III) Oxo Complex

The ubiquity of vanadium oxo complexes
in the V+ and IV+ oxidation states has contributed to a comprehensive
understanding of their electronic structure and reactivity. However,
despite being predicted to be stable by ligand-field theory, the isolation
and characterization of a well-defined terminal mononuclear vanadium­(III)
oxo complex has remained elusive. We present the synthesis and characterization
of a unique terminal mononuclear vanadium­(III) oxo species supported
by the pentadentate polypyridyl ligand 2,6-bis­[1,1-bis­(2-pyridyl)­ethyl]­pyridine
(PY5Me2). Exposure of [VII(NCCH3)­(PY5Me2)]2+ (1) to either dioxygen or selected
O-atom-transfer reagents yields [VIV(O)­(PY5Me2)]2+ (2). The metal-centered one-electron
reduction of this vanadium­(IV) oxo complex furnishes a stable, diamagnetic
[VIII(O)­(PY5Me2)]+ (3) species. The vanadium­(III) oxo species is unreactive toward H-
and O-atom transfer but readily reacts with protons to form a putative
vanadium hydroxo complex. Computational results predict that further
one-electron reduction of the vanadium­(III) oxo species will result
in ligand-based reduction, even though pyridine is generally considered
to be a poor π-accepting ligand. These results have implications
for future efforts toward low-valent vanadyl chemistry, particularly
with regard to the isolation and study of formal vanadium­(II) oxo
species.