Toward Transition-Metal-Templated Construction of Arylated B4 Chains by Dihydroborane Dehydrocoupling.

The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH2 ] (Dur=2,3,5,6-Me4 C6 H) and [(Me3 Si)2 NBH2 ] led to the formation of bridging borylene complexes of the form [(Cp*RuH)2 BR] (Cp*=C5 Me5 ; 1 a: R=Dur; 1 b: R=N(SiMe3 )2 ) through oxidative addition of the B-H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF3 )2 -C6 H3 BH2 ] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)4 [(Cp*Ru)2 B4 H5 (3,5-(CF3 )2 C6 H3 )4 ] (4), through an unusual, incomplete threefold dehydrocoupling process. A comparative theoretical investigation of the bonding in a simplified model of 4 and the analogous complex nido-[1,2(Cp*Ru)2 (μ-H)B4 H9 ] (I) indicates that there appear to be no classical σ-bonds between the boron atoms in complex I, whereas in the case of 4 the B4 chain better resembles a network of three B-B σ bonds, the central bond being significantly weaker than the other two.