Crystal and Molecular Structure of a Tetracarbon Carborane, (CH₃)₄C₄B₈H₈, a New Type of Nido Cage System


The structure of 2,3,7,8-tetramethyl-2,3,7,8-tetracarbadodecaborane(12) [(CH3)4C4B8H8], a carborane formed by air oxidation of [(CH3)2C2B4H4] 2CoH or [(CH3)2C2B4H4] 2FeH2, has been determined from a single-crystal x-ray diffraction study. The molecule is a distorted icosahedron, in which two carbon-carbon bonds have been severed to create two four-sided open faces, and consists of two pyramidal (CH3)2C2B4H4 units fused face-to-face with the four skeletal carbon atoms forming a contiguous chain. The polyhedral cage contains two more electrons than the icosahedral C2B10H12 system and is isoelectronic with the C2B10H12 2- ion. The geometry of (CH3)4C4B8H8 differs sharply from those of the only other structurally established 12-vertex nido systems, (C6H5)2C2B10H 11 - and (η5-C5H5)Fe(CH3) 4C4B7H8, each of which has a single large open face. The compound crystallizes in the monoclinic space group P21/n with a = 8.398 (3) Å, b = 11.420 (5) Å, c = 13.458 (7) Å, β = 90.61 (4)°, dobsd = 1.05 g cm-3, and dcalcd = 1.04 g cm-3 for Z = 4. The structure was refined by full-matrix least-squares procedures to a final R value of 0.153. The molecule is discussed in relation to the theory of bonding in polyhedral cages and also in terms of the mechanism of its formation from the metallocarborane precursors.



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