Zirconium and hafnium polyhedral oligosilsesquioxane complexes – green homogeneous catalysts in the formation of bio-derived ethers via a MPV/etherification reaction cascade

Abstract

The polyhedral oligosilsesquioxane complexes {[(isobutyl)7Si7O12]ZrOPri∙(HOPri)}2 (I), {[(cyclohexyl)7Si7O12]ZrOPri∙(HOPri)}2 (II), {[(isobutyl)7Si7O12]HfOPri∙(HOPri)}2 (III) and {[(cyclohexyl)7Si7O12]HfOPri∙(HOPri)}2 (IV), were synthesized in good yields from reactions of M(OPri)4 (M = Zr, Hf) with R-POSS(OH)3 (R = isobutyl, cyclohexyl), resp. I-IV were characterized by 1H, 13C and 29Si NMR spectroscopy and their dimeric solid-state structures confirmed by X-ray analysis. I-IV catalyze the reductive etherification of 2-hydrox- and 4-hydroxy, 2-methoxy and 4-methoxybenzaldehyde and vanillin to their respective isopropyl ethers in isopropanol as “green” solvent and reagent. I-IV are durable and robust homogeneous catalysts operating at temperatures of 100-160°C for days without significant loss of catalytic activity. Likewise, I-IV selectively catalyze the conversion of 5-hydroxymethylfurfural (HMF) to 2,5-bis(isopropoxymethyl)furane (BPMF), a potential high-performance fuel additive. Similar results were achieved using a combination of M(OPri)4 and ligand R-POSS(OH)3 as catalyst system demonstrating the potential of this “in situ” approach for applications in biomass transformations. A tentative reaction mechanism for the reductive etherification of aldehydes catalysed by I-IV is proposed