abstract12
Inorg. Chem., 35 (21), 6170 -6178, 1996. ic960360g S0020-1669(96)00360-6
Copyright © 1996 American Chemical Society
Existence of C,3-Me2–closo-1,2-C2B3H3 Refuted by the Ab Initio/IGLO, GIAO-MP2/NMR Method. Attempted Repetition of the Synthesis
Matthias Hofmann, Mark A. Fox, Robert Greatrex,* Paul von Ragué Schleyer,* Joseph W. Bausch, and Robert E. Williams*
Computer-Chemie-Centrum, Institut für Organische Chemie der Universität Erlangen-Nürnberg, Henkestrasse 25, D-91054 Erlangen, Germany, School of Chemistry, University of Leeds, Leeds LS2 9JT, England, and Loker Hydrocarbon Research Institute, University of Southern California, University Park, Los Angeles, California, 90089-1661
Received April 3, 1996
Abstract:
The 11B chemical shift assignment for the closo-carborane C,3-Me2-1,2-C2B3H3 structure (1a or 1b) is refuted by ab initio/IGLO, GIAO/NMR evaluations. Attempts to reproduce the literature NMR data by examining the computed chemical shifts of many alternative structures failed. Rearrangement of the 1,2-C2B3H5 isomer into the much more stable 1,5-C2B3H5 is computed to involve only a modest energy barrier. Repetition of the thermal (but not the electric discharge) experiments failed to give a compound with the spectroscopic data reported earlier. The original experimental data remain unexplained. Nevertheless, the elimination of the C,3-Me2-1,2-C2B3H3 structural assignment removes the sole exception to four established structural patterns of closo-carboranes: (a) carbons tend to occupy sites with the lowest coordination, (b) carbons tend to be nonadjacent in the most thermodynamically stable closo-carboranes, (c) the 11B NMR of boron atoms with exo-terminal alkyl groups are always found at lower field than those of otherwise identical boron atoms with exo-terminal hydrogens, and (d) exo-terminal 
(1H)’s normally parallel the (11B) of the boron nuclei to which they are bound.