Ruthenium tris(σ-B-H) borate complexes: synthesis, structure, and reactivity

The coordination of the B-H bond to a transition metal is fundamentally intriguing due to its connection with the transition metal-mediated B-H bond activation mechanism and catalysis. Most of the reported examples are σ-borane/borate complexes containing one or two σ-B-H bonds. However, examples of compounds containing three σ-B-H bonds are still much rarer. Herein, we report a facile approach for the synthesis of rare transition metal tris(σ-B-H) borate complexes by the salt elimination protocol using [Cp*RuCl]₄ and lithium trihydroborates Li[ArBH₃] containing 2,6-disubstituted aryl groups. These complexes have remarkable thermal stability in solution. In addition, this novel class of compounds has the unusual σ-complex that features three labile σ-B-H bonds. Our studies on these new species demonstrate that the coordination σ-B-H bond can undergo H/D exchange with D₂ and ligand substitution reactions with PPh₃, PCy₃ and IPr₂Me₂. These results provide new approaches for the synthesis of both tris(σ-B-H) borates and bis(σ-B-H) borates.