Strong σ-hole triel-bond between C5H5Tr (Tr B, Al, Ga) and N‐base (N‐base NCH, NH3, NC−): Cooperativity and solvation effects

Can H2 be Superacidic? A Computational Study of Triel-Bonded Brønsted Acids

The abundance of XIII group element compounds in science and industry together with their electron-deficient character gives rise to their influence on properties of the systems they interact with. This paper is an attempt to assess the strength, nature, and effect of formation of a triel bond on acidity. A wide set of Brønsted acids among others comprising hydrocarbons, halogen hydrides, and amines bonded with B, Al, and Ga trifluorides forming HX/TF3 was selected for the research. Various computational approaches (e.g., MP2, GFN2-xTB, SAPT2 + 3(CCD)δMP2, quantum theory of atoms in molecules analysis, and density overlap regions indicator) are used to describe the triel-bonded systems. Among other things, it was found that the electrostatics may not be the dominant contribution to the triel binding in some cases. Additionally, it was established that even weak Brønsted acids such as C2H2 or H2 may be superacidic if bonded to a Lewis acid (TF3) that is strong enough. The calculations indicate a significant covalent character of some of the studied HX/TF3 triel-bonded systems. Moreover, the effect of solvation of HX with TF3 as well as that of the reverse process on the acidity of the resulting system is thoroughly described.

Synthesis, Characterization, and Trapping of a Cyclic Diborylcarbene, an Electrophilic Carbene

A carbene bearing two geminal boryl substituents, called diborylcarbene (DBC), has been predicted to be highly Lewis acidic in sharp contrast to the well-studied persistent carbenes stabilized by π-donating substituents. Studies on DBC have been limited to either the base-trapping or theoretical calculations. Herein, we developed chemical equivalents for DBC, namely, K/X-diborylcarbenoids 2_X (X = F or Cl). Treatment of 2_F with Al(C₆F₅)₃ yielded [AlF(C₆F₅)₃]^--stabilized DBC 1-FAl, which showed a significant low-field shift of the carbenoid carbon from 169 ppm (doublet, coupling with ¹⁹F) to 242 ppm (singlet). The loss of halogen was also detected through electrospray ionization time-of-flight mass spectrometry analysis of 2_X only in the presence of Al(C₆F₅)₃. Generated DBC 1 from 1-FAl or 2_Cl was successfully trapped with excess amounts of trialkylphosphines (PR₃, R = Me or Et), which afforded the corresponding DBC-PR₃ adducts. In addition, the Lewis acidity of DBC 1 was evaluated both experimentally and theoretically to reveal that 1 is one of the most Lewis acidic species among neutral molecules.