Ruthenium‐Templated Construction of an Arylated B4 Chain by Dihydroborane Dehydrocoupling

The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH(2)] (Dur=2,3,5,6-Me4C6H) and [(Me3Si)(2)NBH2] led to the formation of bridging borylene complexes of the form [(Cp*RuH)(2)BR] (Cp*=C5Me5; 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)-C6H3BH2] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)(4)[(Cp*Ru)(2)B4H5(3,5-(CF3)(2)C6H3)(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)(mu-H)B4H9] (I) indicates that there appear to be no classical sigma-bonds between the boron atoms in complex I, whereas in the case of 4 the B-4 chain better resembles a network of three B-B sigma bonds, the central bond being significantly weaker than the other two.