Fluorine and alkyl substituent effects on the gas-phase Lewis acidities of boranes by ion cyclotron resonance spectroscopy

Abstract

Formation of Lewis acid-base adducts R_3BF^- and R_2FBF^- in reactions of SF_5^- and SF_6^- with neutral boranes R_3B (R = CH_3, C_2H_5, i-C_3H_7, and F) is examined using trapped-ion cyclotron resonance techniques. Fluoride transfer reactions observed in binary mixtures of the various boranes (in the presence of traces of SF_6 acting as the source of F^-) establish the Lewis acidity order BF_3 > (i-C_3H_7)_2FB > (i-C_3H_7)_ 3B > (C_2H_5)_2FB > (C_2H_5)_3B > (CH_3)_2FB > (CH_3)_3B > SF_4 in the gas phase with F^- as reference base. Quantitative estimates of adduct bond dissociation energies D[R_3B-F^-] and heats of formation of adducts R_3BF^- are derived. Variations in Lewis acidity resulting from alkyl and fluoro substitution on boron are discussed in terms of properties characteristic of substituents and the anion reference base.

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