A new
air-stable catalyst for the oxidative dehydrogenation of benzylic
alcohols under ambient conditions has been developed. The synthesis
and characterization of this compound and the related monomeric and
dimeric V(IV)- and V(V)-pinF (pinF = perfluoropinacolate)
complexes are reported herein. Monomeric V(IV) complex (Me4N)2[V(O)(pinF)2] (1) and dimeric (μ-O)2-bridged V(V) complex (Me4N)2[V2(O)2(μ-O)2(pinF)2] (3a) are prepared
in water under ambient conditions. Monomeric V(V) complex (Me4N)[V(O)(pinF)2] (2) may
be generated via chemical oxidation of 1 under an inert
atmosphere, but dimerizes to 3a upon exposure to air.
Complexes 1 and 2 display a perfectly reversible
VIV/V couple at 20 mV (vs Ag/AgNO3), whereas
a quasi-reversible VIV/V couple at −865 mV is found
for 3a. Stoichiometric reactions of 3a with
both fluorenol and TEMPOH result in the formation of (Me4N)2[V2(O)2(μ-OH)2(pinF)2] (4a), which contains
two V(IV) centers that display antiferromagnetic coupling. In order
to structurally characterize the dinuclear anion of 4a, {K(18C6)}+ countercations were employed, which formed
stabilizing K···O interactions between the counterion
and each terminal oxo moiety and H-bonding between the oxygen atoms
of the crown ether and μ-OH bridges of the dimer, resulting
in {K(18C6)}2[V2(O)2(μ-OH)2(pinF)2] (4b). The formal
storage of H2 in 4a is reversible and proton-coupled
electron transfer (PCET) from crystals of 4a regenerates 3a upon exposure to air over the course of several days. Furthermore,
the reaction of 3a (2%) under ambient conditions with
excess fluorenol, cinnamyl alcohol, or benzyl alcohol resulted in
the selective formation of fluorenone (82% conversion), cinnamaldehyde
(40%), or benzaldehyde (7%), respectively, reproducing oxidative alcohol
dehydrogenation (OAD) chemistry known for VO
x
surfaces and demonstrating, in air, the thermodynamically
challenging selective oxidation of alcohols to aldehydes/ketones.
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