Perturbation of 1JC,F Coupling in Carbon-Fluorine Bonds on Coordination to Lewis Acids: A Structural, Spectroscopic, and Computational Study

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.

Distorted Copper(II) Complex with Unusually Short CF···Cu Distances

We find a Cu(II)-(L-CF₃)₂ complex (L-CF₃ = 2,2,2-trifluoro-N-[2-(pyridin-2-yl)propan-2-yl]acetamide) with a distorted "seesaw" geometry. It has the shortest crystallographic CF···Cu distances yet reported, to the best of our knowledge (<2.6 Å), for which computational and experimental data indicate a secondary bonding interaction. A comparison with a CCl₃ version and one without ligand backbone gem-dimethyl groups suggests a steric origin for the distorted geometry, resulting from the specific ligand interactions.