Identifying Catalytically Active Mononuclear Peroxoniobate Anion of Ionic Liquids in the Epoxidation of Olefins

The organic carboxylic acid coordinated monomeric peroxoniobate-based ionic liquids (ILs) [TBA][NbO(OH)2(R)] (TBA = tetrabutylammonium; R = lactic acid (LA), glycolic acid (GLY), malic acid (MA)) were prepared and fully characterized by elemental analysis, NMR, IR, Raman, TGA, 93Nb NMR, and HRMS. These IL catalysts exhibited not only high catalytic activity for the epoxidation of olefins under very mild reaction conditions, as the turnover frequency of [TBA][NbO(OH)2(LA)] reached up to 110 h–1, but also satisfactory recyclability in the epoxidation by using only 1 equiv of hydrogen peroxide as an oxidant. Meanwhile, this work revealed that the ILs underwent structural transformation from [NbO(OH)2(R)]− to [Nb(O–O)2(R)]− (R = LA, GLY, MA) in the presence of H2O2 by a subsequent activity evaluation, characterization, and first-principles calculations. Moreover, the organic carboxylic acid coordinated monomeric peroxoniobate-based ILs were investigated using density functional theory (DFT) calculations, whic...