posted on 2020-03-05, 16:33authored byChandan
Kumar Tiwari, Mark Baranov, Alevtina Neyman, Ronny Neumann, Ira A. Weinstock
Dissolution
of the polyoxometalate (POM) cluster anion H5[PV2Mo10O40] (1; a
mixture of positional isomers) in 50% aq H2SO4 dramatically enhances its ability to oxidize methylarenes, while
fully retaining the high selectivities typical of this versatile oxidant.
To better understand this impressive reactivity, we now provide new
information regarding the nature of 1 (115 mM) in 50%
(9.4 M) H2SO4. Data from 51V NMR
spectroscopy and cyclic voltammetry reveal that as the volume of H2SO4 in water is incrementally increased to 50%,
V(V) ions are stoichiometrically released from 1, generating
two reactive pervanadyl, VO2+, ions, each with
a one-electron reduction potential of ca. 0.95 V (versus Ag/AgCl),
compared to 0.46 V for 1 in 1.0 M aq H2SO4. Phosphorus-31 NMR spectra obtained in parallel reveal the
presence of PO43–, which at 50% H2SO4 accounts for all the P(V) initially present
in 1. Addition of (NH4)2SO4 leads to the formation of crystalline [NH4]6[Mo2O5(SO4)4]
(34% yield based on Mo), whose structure (from single-crystal X-ray
diffraction) features a corner-shared, permolybdenyl [Mo2O5]2+ core, conceptually derived by acid condensation
of two MoO3 moieties. While 1 in 50% aq H2SO4 oxidizes p-xylene to p-methylbenzaldehyde with conversion and selectivity both
greater than 90%, reaction with VO2+ alone gives
the same high conversion, but at a significantly lower selectivity.
Importantly, selectivity is fully restored by adding [NH4]6[Mo2O5(SO4)4], suggesting a central role for Mo(VI) in attenuating the (generally)
poor selectivity achievable using VO2+ alone.
Finally, 31P and 51V NMR spectra show that intact 1 is fully restored upon dilution to 1 M H2SO4.