Deuteration of closo-1,2- and 1,7-C2B10H12 using C6D6/AlCl3: Mechanistic considerations

Abstract

Regioselective deuteration of 1-X-C2B10H12 (X = 2, 7) cage systems with C6D6/AlCl3 is correlated to ab initio calculational results on a [C2B10H13]+ intermediate. Full geometry optimizations of pertinent [C2B10H13]+ isomers, derived from each of the two 1-X-C2B10H12 carborane isomers, result in cage geometries not unlike the (nearly) icosahedral starting carborane. Each isomer contains a BH2 group having an acute H-B-H angle, long B–H bonds, and a very short H · · · H distance, hinting that the pertinent boron shares the electrons of a hydrogen molecule σ pair. It is suggested that the structural differences between the BH2 group of [C2B10H13]+ and the CH2 group of the benzenium ion, [C6H7]+ (the intermediate strongly intimated upon protonation of benzene), can be explained, in part, by (a) the availability of the π-ring electrons for bonding to the (extra) proton in the latter and (b) the unavailability of π electrons from the carborane. Thus, the C2B10H12 cage is most probably very reluctant to give up a cage electron pair in order to assist in bonding to an (externally bound) proton. Instead, it is more probable that “hydridic” B–H sigma electrons could very well play the important role in providing bonding to the attacking proton. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:95–102, 1998