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Tris(trifluoromethyl)borane carbonyl, (CF3)(3)BCO - Synthesis, physical, chemical and spectroscopic properties, gas phase, and solid state structure

TitleTris(trifluoromethyl)borane carbonyl, (CF3)(3)BCO - Synthesis, physical, chemical and spectroscopic properties, gas phase, and solid state structure
Publication TypeJournal Article
Year of Publication2002
AuthorsFinze, M, Bernhardt, E, Terheiden, A, Berkei, M, Willner, H, Christen, D, Oberhammer, H, Aubke, F
JournalJournal of the American Chemical Society
Date PublishedDec
Type of ArticleArticle
ISBN Number0002-7863

Tris(trifluoromethyl)borane carbonyl, (CF3)(3)BCO, 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)(3)BCO, is synthesized for comparison purposes by the isolation of (C6F5)(3)B 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 (CO)-C-13, the carbonyl substituent at boron undergoes exchange, which follows a first-order rate law. Its temperature dependence yields an activation energy (EA) 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)(3)BCO 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 C3, 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) Angstrom in the solid state and 1.617(12) Angstrom in the gas phase. A corresponding shift of nu(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)(x)BF3-x1 = 1-3. In the vibrational analysis 21 of the expected 26 fundamentals are observed experimentally. The B-11-, C-13-, and F-19-NMR data, as well as the structural parameters of (CF3)(3)BCO, are compared with those of related compounds.

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