Carbonium ion salts. Part 16. Molecular orbital study of tropylium ions with borane anion and carborane substituents (hemiousene compounds)

Abstract

We have extended our ab initio study of substituted tropylium ions to examine several known and postulated hemiousene compounds where the ring has a substituent derived from a polyhedral borane anion, carborane, or carborane anion. Tropyliumyl derivatives of the borane anions closo-B 10H 10 2−, closo-B 12H 12 2−, and closo-CB 9H 10 − are Type III substituted tropylium ions with substituent to ring charge-transfer in the HOMO–LUMO (HL) transition. An additional boron electron pair of the closo-cage in 2-C 7H 6–B 10H 9 − and C 7H 6–B 12H 11 − is completely delocalized over cage and ring. Tropyliumyl derivates of the carborane and carborane anions closo-C 2B 10H 12 and closo-CB 11H 12 − are Type II substituted tropylium ions with polarization of ring π-system by substituent and little if any charge-transfer HL interaction. The tropyliumyl derivative of nido-C 2B 9H 12 − is in a class by itself; however both theory and experiment demonstrate that it has strong HL charge-transfer like a Type III ion.