Further studies of the Enhanced Structural Carborane Effect: tricarbonylruthenium and related derivatives of benzocarborane, dihydrobenzocarborane and biphenylcarborane.

Abstract

Detailed comparison of the molecular structures of [1,2-μ-(C4H4)-3,3,3-(CO)3-3,1,2-closo-RuC2B9H9] (1) and [1,2-μ-(C4H6)-3,3,3-(CO)3-3,1,2-closo-RuC2B9H9] (2) reveals evidence for an Enhanced Structural Carborane Effect in 1 arising from the involvement of the cage pπ orbitals in the exopolyhedral ring to some degree. A minor co-product in the synthesis of 2 is [η-{1,2-μ-(C4H6)}-3,3-(CO)2-3,1,2-closo-RuC2B9H9] (3). Compounds 2 and 3 are readily interconverted, since heating 2 to reflux in THF or reaction with Me3NO affords 3 which readily reacts with CO to regenerate 2. The η-ene bonding in 3 is also displaced by PMe3, P(OMe)3 and t-BuNC to yield [1,2-μ-(C4H6)-3,3-(CO)2-3-PMe3-3,1,2-closo-RuC2B9H9] (4), [1,2-μ-(C4H6)-3,3-(CO)2-3-P(OMe)3-3,1,2-closo-RuC2B9H9] (5) and [1,2-μ-(C4H6)-3,3-(CO)2-3-t-BuNC-3,1,2-closo-RuC2B9H9] (6), respectively. Structural studies of 4-6, focussing on the Exopolyhedral Ligand Orientation of the {Ru(CO)2L} fragment relative to the C2B3 carborane face, are discussed in terms of the structural trans effects of PMe3, P(OMe)3 and t-BuNC relative to that of CO. An improved synthesis of [1,2-μ-(C6H4)2-1,2-closo-C2B10H10], "biphenylcarborane", is reported following which the first transition-metal derivatives of this species, [1,2-μ-(C6H4)2-3-Cp-3,1,2-closo-CoC2B9H9] (7) and [1,2-μ-(C6H4)2-3,3,3-(CO)3-3,1,2-closo-RuC2B9H9] (8), are prepared. Comparisons of the structures of 7 and 8 with the corresponding benzocarborane derivatives [1,2-μ-(C4H4)-3-Cp-3,1,2-closo-CoC2B9H9] and 1, respectively, suggest that Clar's rule for aromaticity can be applied to polycyclic aromatic hydrocarbons fused onto carborane cages.