We report the synthesis
of vanadium(V) oxo complex
1
with a pincer-type dianionic
mesoionic carbene (MIC) ligand
L
1
and the general formula [VOCl(L
1
)]. A comparison of the
structural (SC-XRD), electronic (UV–vis),
and electrochemical (cyclic voltammetry) properties of
1
with the benzimidazolinylidene congener
2
(general
formula [VOCl(L
2
)]) shows that the MIC is a stronger donor
also for early transition metals with low d-electron population. Since
electrochemical studies revealed both complexes to be reversibly reduced,
the stronger donor character of MICs was not only demonstrated for
the vanadium(V) but also for the vanadium(IV) oxidation state by isolating
the reduced vanadium(IV) complexes
[Co(Cp*)
2
][1]
and
[Co(Cp*)
2
][2]
([Co(Cp*)
2
] = decamethylcobaltocenium).
The electronic structures of the compounds were investigated by computational
methods. Complex
1
was found to be a moderate precursor
for salt metathesis reactions, showing selective reactivity toward
phenolates or secondary amides, but not toward primary amides and
phosphides, thiophenols, or aryls/alkyls donors. Deoxygenation with
electron-rich phosphines failed to give the desired vanadium(III)
complex. However, treatment of the deprotonated ligand precursor with
vanadium(III) trichloride resulted in the clean formation of the corresponding
MIC vanadium(III) complex
6
, which undergoes a clean
two-electron oxidation with organic azides yielding the corresponding
imido complexes. The reaction with TMS-N
3
did not afford
a nitrido complex, but instead the imido complex
10
.
This study reveals that, contrary to popular belief, MICs are capable
of supporting early transition-metal complexes in a variety of oxidation
states, thus making them promising candidates for the activation of
small molecules and redox catalysis.