Activation of small molecules by ambiphilic NHC-stabilized phosphinoborenium cation: formation of boreniums with B-O-C, B-O-B, and B-O-P structural motifs

Abstract

The reactivity of the phosphinoborenium cation supported by a 1,3,4,5-tetramethylimidazolin-2-ylidene ligand toward small molecules was explored. The phosphinoborenium cation exhibited dual Lewis acid–base properties due to the presence of the Lewis acidic boron center and the Lewis basic phosphido ligand connected by a covalent bond. The reaction of the title cation with CO₂ led to the insertion of a CO₂ molecule into the P-B bond. The obtained borenium CO₂-adduct underwent hydrolysis, forming an N-heterocyclic carbene stabilized diborenium dication bearing a B-O-B functionality. The activation of N₂O proceeded via the insertion of an oxygen atom into the B-P bond of the parent cation, yielding a borenium cation with a phosphinite moiety. An alternative synthetic pathway to borenium cations with a B-O-P skeleton was achieved via the activation of secondary phosphine oxides by the phosphinoborenium cation. Furthermore, borenium cations and diborenium dications with B-O-C structural motifs were obtained from the reaction of the title compound with perfluorinated tert-butyl alcohol and hydroquinone, respectively. The structure of the obtained borenium cations is discussed based on multinuclear NMR spectroscopy, X-ray diffraction, and density functional theory calculations.