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.