1H nuclear magnetic resonance study of the small closo-carboranes 1,5-dicarba-closo-pentaborane(5), carba-closo-hexaborane(7), 1,2-dicarba-closo-hexaborane(6), 1,6-dicarba-closo-hexaborane(6), and 2,4-dicarba-closo-heptaborane(7)

Abstract

Boron-11 decoupled proton n.m.r. spectra and data are reported for the closo-carboranes 1,5-C2B3H5, CB5H7, 1,2-C2B4H6, 1,6-C2B4H6, and 2,4-C2B5H7. The 1H n.m.r. results are in agreement with the structural assignments for these carboranes and are compatible with previously obtained boron-11 n.m.r. results. Fine structure, attributed to homonuclear long-range coupling, is observed in the decoupled spectrum of each of these closed polyhedral compounds. The largest long-range proton–proton spin coupling (10–12 Hz) is found between trans situated nuclei in 1,5-C2B3H5, CB5H7, and in 1,2-C2B4H6. An estimate of orbital hybridization in C2B3H5 from 11B13C, H13C, and H11B coupling data indicates that most of the cage-orbital s-character is to be found in the C–B bonds and very little in the B–B bonds. Unambiguous chemical shift assignments are made for all the proton resonances except for the 2,3 and 4,5 positions of CB5H7.