A thorough investigation of two novel hybrid materials, namely, (2,2′-Hbpy)3[PW12O40] and (4,4′-H2bpy)1.5[PW12O40]·1.5H2O built from Keggin phosphotungstic acid (PTA) and bipyridine, describes the impact of bipyridine isomers in their formation and physicochemical properties. The hybrids’ formation was confirmed by powder X-ray diffraction, while infrared spectroscopy (IR) proved the polyoxometalate (POM) structural preservation. The stoichiometric composition and thermal stability of the hybrids were solved by thermogravimetric analysis–mass spectrometry, which also revealed newly acquired hydrophobic properties. Raman and IR spectroscopies demonstrated that the POM skeleton units in both hybrids were distorted compared to the POM in PTA, which induced a decrease of their reduction potentials as observed by diffuse reflectance ultraviolet–visible spectroscopy (DR–UV–vis). The hybrids’ acidity was assessed by ammonia temperature-programmed desorption, which showed no remaining acid sites compared to the strong acidic character of the pristine PTA. The properties of the hybrids were tested in the epoxidation of cyclooctene in the presence of H2O2. The reaction was boosted when the hybrids were pre-activated with H2O2.

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The present study further explores the behavior of polyoxometalate-based hybrid compounds as catalysts for liquidphase cyclooctene epoxidation with H 2 O 2 . Precisely, it unveils the nature of the relevant active species derived from the hybrid based on Keggin polyoxometalate (POM) and bipyridines (bpy) of formula (2,2′-Hbpy) 3 [PW 12 O 40 ] (1). Whereas (i) it is generally accepted that the catalytic oxidation of organic substrates by H 2 O 2 involving Keggin HPAs proceeds via an oxygen transfer route from a peroxo intermediate and (ii) the catalytically active peroxo species is commonly postulated to be the polyperoxotungstate {PO 4 [W(O)(O 2 ) 2 ] 4 } 3− complex (PW 4 ), we show that the studied epoxidation reaction seems to be more sophisticated than commonly reported. During the catalytic epoxidation, 1 underwent a partial transformation into two oxidized species, 2 and 3. Compound 3 corresponding to 2,2′-bipyridinium oxodiperoxotungstate of formula [WO(O 2 ) 2 (2,2′-bpy)] was shown to be the main species responsible for the selective epoxidation of cyclooctene since 2 (in which the POM is associated with a protonated mono-Noxide derivative of 2,2′-bpy of formula (2,2′-HbpyO) 3 [PW 12 O 40 ]) exhibited no activity. The structures of 1, 2, and 3 were solved by single-crystal X-ray diffraction and were independently synthesized. The speciation of 1 was monitored under catalytic conditions by 1 H and 1 H DOSY NMR spectroscopies, where the formation in situ of 2 and 3 was revealed. A reaction mechanism is proposed that highlights the pivotal, yet often underestimated, role of H 2 O 2 in the reached catalytic performances. The active species responsible for the oxygen transfer to cyclooctene is a hydroperoxide intermediate species that is formed by the interaction between the anionic structure of the catalyst and H 2 O 2 . The latter operates as a "conservative agent" whose presence in the catalytic system is required to prevent the catalysts from deactivating irreversibly.