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 the 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 were confirmed by X-ray analysis. I–IV catalyze the reductive etherification of 2-hydroxy- and 4-hydroxy and 2-methoxy and 4-methoxybenzaldehyde and vanillin to their respective isopropyl ethers in isopropanol as a “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) into 2,5-bis(isopropoxymethyl)furane (BPMF), a potentially high-performance fuel additive. Similar results were achieved by using a combination of M(OPrⁱ)₄ and ligand R-POSS(OH)₃ as a 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.