Paramagnetic dioxovanadium(IV) molecules inside the channels of zeolite BEA - EPR screening of VO2 reactivity toward small gas-phase molecules

Interaction of small gas-phase molecules (NO, N2O, O-2, CO) with VO2 radicals inside the channels of a dealuminated SiBEA zeolite was investigated by means of electron paramagnetic resonance (EPR), infrared (IR), and mass (QMS) spectroscopies to provide direct insights into the chemistry of a unique paramagnetic state of vanadium - VO2 molecules. A facile way of forming VO2 inside the channels of SiBEA via thermal reduction of VO2+ precursor cations was shown. Dioxovanadium(IV) was identified based on its unusual EPR signal which, as compared with the typical monooxovanadium(IV) (VO2+ cation), is featured by rhombic symmetry and a positive A(iso) value leading to a hyperfine splitting as large as 32 mT. VO2 molecules exhibit reducing properties transforming N2O and O-2 into vanadium intrachannel cage adducts comprising of reactive oxygen species (O- and O-2(-), respectively). Interaction with CO led to its oxidation to CO2, while paramagnetic NO acted as a scavenger for VO2 radicals producing diamagnetic adducts. The observed reactivity was rationalized in terms of spin-pairing, electron transfer, and oxygen transfer processes. As a result new chemical pathways of vanadium reactivity were demonstrated which were not observed so far either in the homogeneous molecular systems or supported vanadium materials.