Toward Transition-Metal-Templated Construction of Arylated B 4 Chains 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-Me ₄ C ₆ H) and [(Me ₃ Si) ₂ NBH ₂ ] led to the formation of bridging borylene complexes of the form [(Cp*RuH) ₂ BR] (Cp*=C ₅ Me ₅ ; 1 a: R=Dur; 1 b: R=N(SiMe ₃ ) ₂ ) through oxidative addition of the B-H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF ₃ ) ₂ -C ₆ H ₃ BH ₂ ] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF) ₄ [(Cp*Ru) ₂ B ₄ H ₅ (3,5-(CF ₃ ) ₂ C ₆ H ₃ ) 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)B ₄ H ₉ ] (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 B ₄ chain better resembles a network of three B-B σ bonds, the central bond being significantly weaker than the other two.
(© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)