Approaching the Silylium (R₃Si⁺) Ion: Trends with Hexahalo (Cl, Br, I) Carboranes as Counterions

A systematic investigation has been made into the degree of silylium ion character in species of the type i-Pr3SiY where Y- is a weakly coordinating hexahalo-carborane anion, X6-CB11H6-, with X = Cl, Br, I. By X-ray crystal structural criteria such as the average C−Si−C angle (117.3(4)°, 117.0(13)°, and 115.6(9)° for X = Cl, Br, I, respectively) and the downfield 29Si chemical shift (115, 110, and 97 ppm for X = Cl, Br, I, respectively) the hexachloro-carborane anion is the least coordinating. Thus, the “ion-like” species i-Pr3Siδ+(Cl6-CB11H6)δ- becomes the closest structural approach to date to a condensed phase silylium ion. With the aid of ab initio molecular orbital theory and silicon chemical shift calculations we address the question of how much silylium ion character this species represents. From structural, energetic, and NMR criteria (where expectation values for the condensed phase 29Si chemical shifts are revised downward) we conclude that i-Pr3Siδ+(Cl6-CB11H6)δ- may have >50% silylium ion character.