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

Catecholatostiboranes have emerged as useful Lewis acids in several applications. To better understand the factors that control the properties of these species, we examined the Lewis acidities of (<i>o</i>-C₆Cl₄O₂)Sb(<i>o</i>-Tol)₃ (<b>2</b>, Tol = tolyl) and (<i>o</i>-C₆Cl₄O₂)Sb(<i>p</i>-Tol)₃ (<b>3</b>), two triarylcatecholatostiboranes that differ by the nature of the aryl substituent. Fluoride anion binding studies indicate that <b>3</b> is more Lewis acidic than <b>2</b>, a factor readily assigned to the steric crowding around antimony in the case of the <i>o</i>-tolyl derivative. But, while <b>3</b> binds F^- trans to a Sb-C_aryl bond as is typical of catecholatostiboranes, <b>2</b> prefers binding trans to a Sb-O bond. Computational analyses of <b>2</b> and <b>3</b> reveal the existence of several σ holes, and an activation strain analysis is employed to elucidate the origin of these stiboranes' anion-binding geometry preferences.