Steric Selection of Anion Binding Sites by Organoantimony(V)\nPnictogen Bond Donors: An Experimental and Computational Study

Catecholatostiboranes\nhave emerged as useful Lewis acids in several\napplications. To better understand the factors that control the properties\nof these species, we examined the Lewis acidities of (<i>o</i>-C<sub>6</sub>Cl<sub>4</sub>O<sub>2</sub>)Sb(<i>o</i>-Tol)<sub>3</sub> (<b>2</b>, Tol = tolyl) and (<i>o</i>-C<sub>6</sub>Cl<sub>4</sub>O<sub>2</sub>)Sb(<i>p</i>-Tol)<sub>3</sub> (<b>3</b>), two triarylcatecholatostiboranes that differ\nby the nature of the aryl substituent. Fluoride anion binding studies\nindicate that <b>3</b> is more Lewis acidic than <b>2</b>, a factor readily assigned to the steric crowding around antimony\nin the case of the <i>o</i>-tolyl derivative. But, while <b>3</b> binds F<sup>–</sup> trans to a Sb–C<sub>aryl</sub> bond as is typical of catecholatostiboranes, <b>2</b> prefers\nbinding trans to a Sb–O bond. Computational analyses of <b>2</b> and <b>3</b> reveal the existence of several σ\nholes, and an activation strain analysis is employed to elucidate\nthe origin of these stiboranes’ anion-binding geometry preferences.