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

Abstract

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, which identified that [Nb­(O–O)2LA]− was more advantageous than [Nb­(O–O)2(OOH)2]− for the epoxidation of olefins. Due to the coordination between the α-hydroxy acids and the monomeric peroxoniobate anions, the functionalized ILs can efficiently catalyze the epoxidation of a wide range of olefins and allylic alcohols under very mild conditions. Additionally, the effect of solvents on the reaction is illustrated. It was found that methanol can lower the epoxidation barriers by forming a hydrogen bond with a peroxo ligand attached to the niobium center.

Ionic Liquid Stabilized Niobium Oxoclusters Catalyzing Oxidation of Sulfides with Exceptional Activity

“…HRTEM images exhibited that the Nb−OC@[TBA][PA] oxocluster aggregated slightly with an average diameter of 3.7 nm (Figure c,d), whereas the Nb−OC@[TBA][HPA] oxocluster even agglomerated more obviously with the average particle size of 3.4 nm (Figure e,f). This reflected that the propionate or 3‐hydroxypropionate IL could only generate labile coordination with the niobium site, leading to weak stabilization of oxoclusters, which was in agreement with our previous research . Interestingly, the IL [TBA][LA]‐stabilized oxocluster Nb−OC@[TBA][LA] existed as highly uniform nanoparticles with the average particle size of 2.6 nm (Figure g,h), which is distinctly different from that of niobic acid.…”

“…After coordination with the niobium site, these signals shifted slightly downfield and appeared at δ =182.39 and 68.46 ppm, respectively. This confirmed that the alcoholic group attached to the α‐carbon atom could be deprotonated, leading to formation of five‐membered rings composed of metal atom and lactic acid molecules . Meanwhile, the chemical shifts of other carbon atoms had not changed, which indicated α‐hydroxy carboxylate indeed has a stable coordination effect on niobium site.…”

“…Notably, the properties of Nb−OC@[TBA][LA] are distinctly different from those reported mononuclear peroxyniobate anion based ILs [TBA][Nb=O(O−O)(OH) 2 ] and [TBA][NbO(OH) 2 (LA)] . As shown in Figure S2, Supporting Information, the latter two ILs were miscible completely with methanol, and there was no obvious change after adding an equivalent amount of H 2 O 2 .…”

“…In addition, the vibrational peak belonging to ν (Nb=O) at approximately 885 cm −1 might be overlapped by σ(C−H) in the ILs. The bands near 545 cm −1 were characteristic for ν as (Nb−O) of the catalysts . The vibrational peak at 841 cm −1 might be assigned to be the stretching of the Nb‐O‐Nb bridge, indicating that the polyoxoniobate anion existed as a multinuclear instead of a mononuclear form.…”

“…The α‐hydroxyl carboxylate anion (e.g. lactate anion) acted as bidentate ligands, coordinating with the niobium site results in a five‐membered chelate ring . During the synthesis of the Nb oxocluster, the precursor peroxy‐niobic acid was transformed into polyoxoniobate anion due to condensation under slightly high temperature.…”