The first member of the eleven-vertex azadicarbaborane series, 1,6,9-NC2B8H13, and its N-alkyl derivatives

Abstract

Reactions between closo-1,2-C₂B₈H₁₀ (1) and amines of general formulation R¹R²NH (where R¹, R² = H, H; Me, H; t-Bu, H and Et, Et) resulted in a straightforward cluster expansion and formation of the 11-vertex arachno-azadicarbaboranes of the 1,1-R¹,R²⁻1,6,9-NC₂B₈H₁₁ (2) cluster constitution (where R¹, R² = H, H 2a; Me, H 2b; t-Bu, H 2c and Et, Et 2d) in yields 10–75%, depending on the nature of the amine used. The reactions are the first example of a direct closo to arachno transformation in the area of cluster-boron compounds. Compounds 2b and 2c were isolated in two isomeric forms anti- and syn- that differ in the positioning of the t-Bu substituent with respect to the bridging hydrogen site. Deprotonation of compounds 2 generally leads to removal of the bridging proton and formation of the [1,1-R¹,R²⁻1,6,9-NC₂B₈H₁₁]⁻ (2⁻) anions that, in the case of the monoalkylated Me and t-Bu derivatives, adopt only an anti configuration. The structure of anti-2c was determined by X-ray diffraction analysis and the geometries of the parent compound and the corresponding syn and anti isomers were optimised at the RMP2/6-31G* level. The composition of all compounds is consistent with the results of mass spectrometry and multinuclear (¹H and ¹¹B) spectroscopy complemented by two-dimensional [¹¹B–¹¹B]-COSY and ¹H{¹¹B(selective)} NMR measurements. Experimental ¹¹B chemical shifts generally show acceptable agreement with theoretical values calculated by GIAO methods, in particular at GIAO-MP2/II, where possible.