Tetranuclear Hafnium(IV) and Zirconium(IV) Cationic Complexes Sandwiched between Two Di-Lacunary Species of α-Keggin Polyoxometalates: Lewis Acid Catalysis of the Mukaiyama–Aldol Reaction

Abstract

Abstract The syntheses, X-ray structures, and homogeneous Lewis acid catalytic activities for the Mukaiyama–aldol reaction of four tetranuclear HfIV and ZrIV cluster cations, which are sandwiched between two 1,2-di-lacunary α-Keggin polyoxometalates (POMs) and between two 1,4-di-lacunary POMs, are described, i.e., [[{M(H2O)}2{M(H2O)2}2(µ-OH)3(µ3-OH)2](α-1,2-PW10O37)2]7− (M = Hf 1 and M = Zr 2) and [{M4(H2O)4(µ-OH)2(µ3-O)2}(α-1,4-PW10O37)2]8− (M = Hf 3 and M = Zr 4), respectively. Evaluated was homogeneous Lewis acid catalysis of the Mukaiyama–aldol reaction in aqueous/CH3CN mixed media at room temperature under air by water-soluble sodium or lithium salts of sandwich-structured Hf/Zr-containing Keggin and Dawson POMs. In particular, the sodium salts of tetranuclear Hf/Zr cluster cations sandwiched between two di-lacunary α-Keggin POMs, i.e., Na-1–Na-4, showed the highest activities, compared with other cluster cations. The present POM-based sandwich-structured compounds gave products with high stereoselectivity, i.e., high anti-selectivity, regardless of high or low activities. The excellent stabilities of Na-1–Na-4 as catalysts, i.e., with no reduced activities, were confirmed even after reusing several times.