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 p_π 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.