Selective Oxidation by H5[PV2Mo10O40] in a Highly Acidic Medium.

Chandan Kumar Tiwari,Mark Baranov,Ira A Weinstock,Alevtina Neyman,Ronny Neumann

doi:10.1021/acs.inorgchem.9b03747

Chandan Kumar Tiwari, Mark Baranov + Show 3 more

Open Access

https://doi.org/10.1021/acs.inorgchem.9b03747

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Abstract

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.

Highlights

While the protonation of intact 1 in 50% (9.4 M) aq H2SO4 could readily account for the more positive reduction potential observed in that solvent system,[8,9] the retention of high selectivity further suggested that the cluster anion remained otherwise largely intact and able to sequester electrons from radical-organic intermediates
To investigate the solution-state chemistry of 1 in 50% aq H2SO4, the first set of experiments involved the use of 51V NMR spectroscopy, in combination with cyclic voltammetry, to help explain the previously observed[9] ca. 0.5 V increase in reduction potential
The use of 115 mM 1 was critically important because this concentration was used in previously reported reactivity studies[9] and because of the speciation chemistry of metal cations, including those that form metal-oxide cluster anions (POMs) is a function of concentration.[10,11]

Summary

■ INTRODUCTION

The molybdovanadophosphate cluster anion, H5[PV2Mo10O40] (1), is a versatile catalyst for selective aerobic oxidations of organic and inorganic compounds via a range of mechanisms, from electron transfer (ET) to electrontransfer induced oxygen transfer (ET-OT), a homogeneous liquid-phase analogue of Mars−van Krevelen type reactions.[1−4]. While the protonation of intact 1 in 50% (9.4 M) aq H2SO4 could readily account for the more positive reduction potential observed in that solvent system,[8,9] the retention of high selectivity further suggested that the cluster anion remained otherwise largely intact and able to sequester electrons from radical-organic intermediates. As previously observed,[7−9] dilution of this thermodynamically stable oxidative system leads quantitatively to hydrolytic self-assembly of the component species into fully intact 1

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES