hypercloso‐Hexa(amino)hexaboranes: Structurally Related to Known hypercloso‐Dodecaboranes, Metastable with Regard to Their Classical Cycloisomers

B-6(NMe2)(6) (2a) is the first neutral hypercloso-hexaborane to be characterized by X-ray structural analysis. The geometry of 2a is in nice agreement with that of the C-i symmetric molecule computed at the B3LYP/6-311+G(center dot center dot) level of theory. Two B-3 triangles with long B center dot center dot center dot B distances in 2a are reminiscent of those in Hawthorne's benzyloxy-substituted hypercloso-dodecaboranes 6a,b. Upon heating to 200 degrees C, 2a transforms into Noth's classical cyclohexaborane 1a. Computations at the B3LYP/6-311+G(center dot center dot) + ZPE level of theory show 1a to be 21.6 kcal mol(-1) lower in energy than 2a, that is, the latter is metastable. hypercloso-Hexaborane B-6(NEt2)(6) (2b), which was reported to be thermodynamically more stable than 1b, is computed to be 22.4 kcal mol(-1) less stable than 1b. Pure 1b is shown here not to transform into 2b upon standing in solution, which is in contrast to reports in the literature for a mixture containing 1b. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)