Synthesis and Fluoride Binding Properties of Tris-pyridinium Borane

A noel multi-cationic borane, tri-N-methylpyridinium substituted triarylborane, [BAr3N]I-3 ([2]I-3) (Ar-N=4-(4-C5H4N-Me)-2,6-Me-2-C6H2) was prepared from the corresponding neutral tris-pyridyl borane, BAr3 (2a) (Ar = 4-(4-C5H4N)-2,6-Me-2-C6H2). The crystal structure of 2a determined by X-ray diffraction study reveals the presence of tri-coordinate boron center with peripheral pyridyl moieties. The fluoride ion affinity of the cationic borane, [2]I-3 was investigated UV-vis absorption titrations and was compared with that of neutral 2a. While 2a binds fluoride with the binding constant of 1.9 x 10(2) M-1 in THF/H2O (9:1 v/v) mixture, [2]I-3 shows a very high binding constant (K=1.0 x 10(8) M-1) that is greater by six orders of magnitude than that of 2a in the same medium. This result indicates that the fluorophilicity of triarylborane can be drastically enhanced by multiple pyridinium substitutions.