The crystal and molecular structure of tetramethylammonium 1-dimethylamino-1-carba-undecahydro-closo-dodecaborate

Abstract

The title compound crystallizes in the trigonal system and in the P3 space group. The unit cel parameters are a = b = 2 717.1(8) pm, c = 1 222.5(3) pm, V = 7 816 . 106 pm3. Z = 18 density calculated Dc = 0.995 Mg m-3. Radiation used CuKα, λ = 154.18 pm, intensities measured at room temperature. Mr = 260.34, linear absorption coefficient μ = 0.342 mm-1, F(000) = 2 520. Final R = 0.077 for 3 096 observed independent reflections. There is an unusual number of six isomers in the asymmetric unit. The structure is built up from CB11H11·N(CH3)2 anions and N(CH3)4 cations. The dimethylamino group attached to the carbon atom of the carbaborane icosahedron in not planar (as usual in aromatic compounds), but has an almost tetrahedral form, which reveals the presence of the sp3 hybridization of the orbitals of the nitrogen atom. The C-N-C bond angles are 117.6(3)°, 112.7(3)° and 111.8(3)° on average. The comparison of the monocarbaborane CB11H11 with the B12H12 framework showed an overall contraction of the first one - the averaged C-B bond lengths are shorter by 6 pm and the B-B bond lengths by up to 3 pm then the B-B bond lengths in B12H12. Cartesian coordinates of the averaged framework are given. The mutual arrangement of the ions simulates fairly well the space group R3, but the symmetry is reduced to P3 due to the presence of the orientational (but not positional) disorder of the anions and of the cations. The centroids of the anions form a nearly perfect primitive cubic point lattice. The cations are positioned in the proximity of the center of the cube formed by the anions and consequently, their central nitrogen atoms simulate the same cubic point lattice. The arrangement of the building units resembles the B2 structure type with one exception: the positions on the 3-fold axes (two vertices of each cube) are vacant.