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Imidazole modified Merrifield resin and (3-Aminopropyl)trimethoxysilane-modified graphene oxide supported oxidovanadium(IV) radical complexes PS-im-[VIVO(tbnC )(acac)] (1) and GO-ATPMS-[VIVO(tbnO )(acac)] (2) were synthesized and characterized by various spectroscopic, thermal and chemical techniques. The radical nature of 1 and 2 was established by trapping experiments in addition to EPR spectroscopy. In EPR analysis, complex 2 shows a prominent signal with g = 2.005, characteristic of an oxygen-centered radical. The neat complex [VIVO(tbnC )(acac)] (A) displays an EPR signal at g = 2.0025, typical of carbon-centered radical. On the contrary, such characteristic EPR signal of a radical is absent in complex 1, presumably due to spin pairing. XPS analysis confirms the +4 oxidation state of vanadium in fresh as well as recycled catalysts 1 and 2. Both the supported complexes show excellent catalytic activity towards a variety of aliphatic alcohols. Comparatively, the polymer-supported complex displays better substrate conversion than the graphene oxide-supported complex. However, 2 shows better selectivity towards aldehydes, whereas carboxylic acids are obtained as major products in the presence of 1. Interestingly, catalyst 1 is almost equally effective towards all the examined alcohols, but its effectiveness reduces slightly for longer carbon chain alcohols. On the other hand, catalyst 2 shows better substrate conversion for the alcohols with a longer carbon chain. During the catalytic oxidation of alcohols, the active intermediate species oxidoperoxidovanadium(V) complex ([VO(O2)(tbn)(acac-H)]−) was detected by FT-IR, UV–vis, and LC–MS analysis. |
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