Item

Abstract

A two step synthesis to the isocyanotris(trifluoromethyl)borate anion, [(CF₃)₃BNC]^-, and its isomerization to the cyanotris(trifluoromethyl)borate anion, [(CF₃)₃BCN]^-, at temperatures above 150 °C are presented. In the first step (CF₃)₃BNCH was obtained by reacting (CF₃)₃BCO with hydrogen cyanide followed by deprotonation of the HCN adduct with Li[N(SiMe₃)₂] in toluene. The thermal behavior of K[(CF₃)₃BNC] and K[(CF₃)₃BCN] were investigated by differential scanning calorimetry (DSC), and K[BF₄] was identified as a major solid decomposition product. The enthalpy of the isocyanide−cyanide rearrangement, ΔH_iso = −35 ± 4 kJ mol^-1, was obtained from DSC measurements, and the activation energy, E_a = 180 ± 20 kJ mol^-1, from kinetic measurements. The isomerization was modeled as an intramolecular reaction employing DFT calculations at the B3LYP/6-311+G(d) level of theory yielding a reaction enthalpy of ΔH_iso = −36.1 kJ mol^-1 and an activation energy of E_a = 155.7 kJ mol^-1. The solid-state structures of K[(CF₃)₃BNC] and K[(CF₃)₃BCN] were determined by single-crystal X-ray diffraction. Both salts are isostructural and crystallize in the orthorhombic space group Pnma (no. 62). In the crystals the borate anions possess C_s symmetry, while for the energetic minimum C₃ symmetry is predicted by DFT calculations. The borate anions have been characterized by IR and Raman spectroscopy as well as by NMR spectroscopy. The assignment of the IR and Raman bands is supported by their calculated wavenumbers and intensities. The spectroscopic and structural properties of both borate anions are compared to the properties of the isoelectronic borane carbonyl (CF₃)₃BCO and the [B(CF₃)₄]^- anion as well as to those of other related species.