Photophysical and Lewis acidic properties of triarylboranes with meta-substituted 2-R-o-carboranes

Triarylboranes incorporating meta-substituted 2-R-o-carboranes (R = H, Me, iBu, Ph, 4-CF3C6F4) were prepared to investigate the effects of the meta-2-R-o-carborane on Lewis acidity. X-ray crystal structures revealed that the carboranyl C C bond distances increased with increasing steric and electron-withdrawing effects of the 2-R group. UV/Vis absorption titrations with fluoride ions showed high binding constants (K > 107 M−1 in THF and K = 3.9–6.6 × 103 M−1 in THF/H2O (9:1 vol)). Fluoride ion affinity was greater for the 2-aryl (4-CF3C6F4)-substituted triarylborane than 2-H and -alkyl (Me)-substituted triarylboranes, which was supported by anodic shifts in the reduction potentials of 2-aryl-o-carborane-substituted triarylboranes. Comparison of fluoride binding constants and reduction potentials of meta- and para-substituted triarylboranes indicated slightly lower Lewis acidity of meta-substituted triarylboranes. In contrast to para-substituted triarylboranes, the Lewis acidity of meta-substituted triarylboranes is likely to be primarily enhanced by the inductive effect of the 2-R-o-carborane.