Rearrangement Reactions of the Transient Lewis Acids (CF3)3B and (CF3)3BCF2:  An Experimental and Theoretical Study

• Published in: Inorganic chemistry Vol. 43; no. 2; pp. 490 - 505
• Main Authors: Finze, Maik, Bernhardt, Eduard, Zähres, Manfred, Willner, Helge
• Format: Journal Article
• Language: English
• Published: American Chemical Society 26.01.2004
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0350640

Short-lived (CF3)3B and (CF3)3BCF2 are generated as intermediates by thermal dissociation of (CF3)3BCO and F- abstraction from the weak coordinating anion [B(CF3)4]-, respectively. Both Lewis acids cannot be detected because of their instability with respect to rearrangement reactions at the B−C−F moiety. A cascade of 1,2-fluorine shifts to boron followed by perfluoroalkyl group migrations and also difluorocarbene transfer reactions occur. In the gas phase, (CF3)3B rearranges to a mixture of linear perfluoroalkyldifluoroboranes C n F2 n +1BF2 (n = 2−7), while the respective reactions of (CF3)3BCF2 result in a mixture of linear (n = 2−4) and branched monoperfluoroalkyldifluoroboranes, e.g., (C2F5)(CF3)FCBF2. For comparison, the reactions of [CF3BF3]- and [C2F5BF3]- with AsF5 are studied, and the products in the case of [CF3BF3]- are BF3 and C2F5BF2 whereas in the case of [C2F5BF3]-, C2F5BF2 is the sole product. In contrast to reports in the literature, it is found that CF3BF2 is too unstable at room temperature to be detected. The decomposition of (CF3)3BCO in anhydrous HF leads to a mixture of the new conjugate Brønsted−Lewis acids [H2F][(CF3)3BF] and [H2F][C2F5BF3]. All reactions are modeled by density functional calculations. The energy barriers of the transition states are low in agreement with the experimental results that (CF3)3B and (CF3)3BCF2 are short-lived intermediates. Since CF2 complexes are key intermediates in the rearrangement reactions of (CF3)3B and (CF3)3BCF2, CF2 affinities of some perfluoroalkylfluoroboranes are presented. CF2 affinities are compared to CO and F- affinities of selected boranes showing a trend in Lewis acidity, and its influence on the stability of the complexes is discussed. Fluoride ion affinities are calculated for a variety of different fluoroboranes, including perfluorocarboranes, and compared to those of the title compounds.