Tris(trifluoromethyl)borane Carbonyl, (CF3)3BCOSynthesis, Physical, Chemical and Spectroscopic Properties, Gas Phase, and Solid State Structure

Abstract

Tris(trifluoromethyl)borane carbonyl, (CF₃)₃BCO, is obtained in high yield by the solvolysis of K[B(CF₃)₄] in concentrated sulfuric acid. The in situ hydrolysis of a single bonded CF₃ group is found to be a simple, unprecedented route to a new borane carbonyl. The related, thermally unstable borane carbonyl, (C₆F₅)₃BCO, is synthesized for comparison purposes by the isolation of (C₆F₅)₃B in a matrix of solid CO at 16 K and subsequent evaporation of excess CO at 40 K. The colorless liquid and vapor of (CF₃)₃BCO decomposes slowly at room temperature. In the gas phase t_1/2 is found to be 45 min. In the presence of a large excess of ¹³CO, the carbonyl substituent at boron undergoes exchange, which follows a first-order rate law. Its temperature dependence yields an activation energy (E_A) of 112 kJ mol^-1. Low-pressure flash thermolysis of (CF₃)₃BCO with subsequent isolation of the products in low-temperature matrixes, indicates a lower thermal stability of the (CF₃)₃B fragment, than is found for (CF₃)₃BCO. Toward nucleophiles (CF₃)₃BCO reacts in two different ways: Depending on the nucleophilicity of the reagent and the stability of the adducts formed, nucleophilic substitution of CO or nucleophilic addition to the C atom of the carbonyl group are observed. A number of examples for both reaction types are presented in an overview. The molecular structure of (CF₃)₃BCO in the gas phase is obtained by a combined microwave−electron diffraction analysis and in the solid state by single-crystal X-ray diffraction. The molecule possesses C₃ symmetry, since the three CF₃ groups are rotated off the two possible positions required for C_3v symmetry. All bond parameters, determined in the gas phase or in the solid state, are within their standard deviations in fair agreement, except for internuclear distances most noticeably the B−CO bond lengths, which is 1.69(2) Å in the solid state and 1.617(12) Å in the gas phase. A corresponding shift of ν(CO) from 2267 cm^-1 in the solid state to 2251 cm^-1 in the gas phase is noted in the vibrational spectra. The structural and vibrational study is supported by DFT calculations, which provide, in addition to the equilibrium structure, confirmation of experimental vibrational wavenumbers, IR-band intensities, atomic charge distribution, the dipole moment, the B−CO bond energy, and energies for the elimination of CF₂ from (CF₃)_xBF_3-x, x = 1−3. In the vibrational analysis 21 of the expected 26 fundamentals are observed experimentally. The ¹¹B-, ¹³C-, and ¹⁹F-NMR data, as well as the structural parameters of (CF₃)₃BCO, are compared with those of related compounds.