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The structures of 1,2-μ-(C4H6)-1,2-closo-C2B10H10 (“dihydrobenzocarborane”) and 1,2-μ-(C4H4)-1,2-closo-C2B10H10 (“benzocarborane”) determined previously (Wade et al., 1996) are analysed. This provides evidence for an enhanced structural carborane effect, whereby the cage carbon atoms in the latter bond less strongly with B3 and B6, presumably because their pπ orbitals are associated to some degree with the exopolyhedral C6 ring, affording it a small degree of aromaticity. The corresponding cobaltacarboranes 3-Cp-1,2-μ-(C4H6)-1,2-closo-CoC2B9H9 (1) and 3-Cp-1,2-μ-(C4H4)-1,2-closo-CoC2B9H9 (2) are synthesised, fully characterised by spectroscopic methods and studied crystallographically. Comparison of their molecular structures affords further evidence for the enhanced structural carborane effect in that the Ccage–Co3 and Ccage–B6 distances in the latter are both significantly longer than those in the former, whilst at the same time the C1–C2 distance significantly decreases on moving from 1 to 2. |
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