Cationic closo carboranes - promising weakly coordinating ions

Abstract

The unexplored carbon rich cationic closo carboranes, C₃B_n-3H_n⁺¹ (n=5, 6, 7, 10, 12) are investigated theoretically. The position isomers were calculated at the B3LYP/6-31G* level, and the charge distribution in the cluster is estimated by NBO analysis. The criterion of ring-cap orbital overlap compatibility along with the number of BC, CC, and BB bonds help in explaining the stability order in each category. The most stable isomer is the one with maximum ring-cap orbital overlap and largest number of BC bonds. The order of relative stability among the trigonal bipyramid is 1c>1b>1a', where the stability is proportional to the number of CH caps over the small three-membered ring. The C₃B₃H₆⁺ isomer with the one allyl C3 group (2b) is more favorable than the one with a cyclopropenyl group (2a). Among the C₃B₄H₇⁺ isomers the stability order is 3e>3d>3c>3b>3a, which mostly depends on the ring-cap orbital overlap. In the bicapped square antiprism (4) where there is large number of isomers, the order follows the rule of ring cap compatibility and the number of BC bonds. The order of 5e>5d>5c>5b>5a obtained from the calculations is in perfect agreement with the above sited rules. Equations (1)-(5) devised for estimating the stability of isomers of C₃B_n-3H_n⁺ indicate an increase in stability with cage size. The mono-positive charge of the isomers is distributed throughout the cage, making them suitable candidates as weakly electrophillic cations.