C O M M U N I C A T I O N S
In conclusion, dipic V complexes are able to catalyze the aerobic
oxidative C-C bond cleavage of pinacol. Reaction under anaerobic
conditions allows for isolation of a VIII µ-oxo dimer, which provides
strong support for the potential involvement of VIII in aerobic
oxidation reactions. Experiments exploring the scope and mecha-
nism of this reaction in more detail, including reactions with lignin
and cellulose, are currently underway.
Acknowledgment. This work was supported by the NSF via
the Center for Enabling New Technologies through Catalysis
(CENTC). We thank Professors S. L. Scott and P. C. Ford (UCSB)
for helpful discussions.
Figure 2. Molecular structure of trivalent [(dipic)V(pyr)2]2(µ-O) (4)
(thermal ellipsoids at 50% probability, H atoms, and cocrystallized pyr
omitted for clarity). Selected bond lengths (Å): V1-O5 ) 1.797(1), V1-O3
) 2.027(2), V1-O1 ) 2.030(2), V1-N1 ) 2.097(2), V1-N2 ) 2.151(2),
V1-N3 ) 2.154(2).
Supporting Information Available: Experimental procedures and
details, characterization data for 2-6, and X-ray crystallographic data
and CIF files for 2-4. This material is available free of charge via the
proposed as intermediates in the vanadium-catalyzed aerobic
oxidation of activated alcohols,11 VIII products have never been
isolated from such reactions. This reaction appears to be highly
ligand-dependent; when the VIV complex VIV(O)(acac)2 was treated
with pinacol under the same reaction conditions employed to isolate
4 (pyr-d5, 100 °C), formation of acetone was observed in less than
30% yield after 1 week (whereas reaction of 3 with pinacol to give
4 was complete in 48 h under these conditions).
Both complexes 3 and 4 could be oxidized by air, suggesting
potential catalytic reactivity of (dipic)V toward polyalcohols.
Reaction of a pyridine solution of 4 with air was followed by
UV-vis spectroscopy, and yielded 3 after 1 h at room tempera-
ture.14 Only trace oxidation of the VIV complex 3 occurred under
air in pyr-d5 solution at room temperature over several weeks, but
when the reaction was conducted at 100 °C, slow formation of cis-
dioxo VV complex 5 was observed (50% yield after 4 days).
Complex 5 could also be prepared by reaction of 1a or 2 with H2O
in pyridine (eq 3).16
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+
-
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+
one-electron pathway is implicated for VO2 in HClO4, where
reaction with cyclobutanol was found to give primarily ring-opened
products (94%), with less than 1.5% cyclobutanone formed.9
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