Heterolytic activation of H-X (X = H, Si, B, and C) bonds: an experimental and theoretical investigation

Abstract

The highly electrophilic, coordinatively unsaturated, 16-electron [Ru(P(OH)₃)(dppe)₂][OTf]₂ (dppe = Ph₂PCH₂CH₂PPh₂) complex 1 activates the H-H, the Si-H, and the B-H bonds, in H₂(g), EtMe₂SiH and Et₃SiH, and H₃B·L (L = PMe₃, PPh₃), respectively, in a heterolytic fashion. The heterolysis of H₂ involves an η²-H₂ complex (observable at low temperatures), whereas the computations indicate that those of the Si-H and the B-H bonds proceed through unobserved η¹-species. The common ruthenium-containing product in these reactions is trans-[Ru(H)(P(OH)₃)(dppe)₂][OTf], 2. The [Ru(P(OH)₃)(dppe)₂][OTf]₂ complex is unique with regard to activating the H-H, the Si-H, and the B-H bonds in a heterolytic manner. These reactions and the heterolytic activation of the C-H bond in methane by the model complex [Ru(POH)₃)(H₂PCH₂CH₂PH₂)₂][Cl][OTf], 4, have been investigated using computational methods as well, at the B3LYP/LANL2DZ level. While the model complex activates the H-H, the Si-H, and the B-H bonds in H₂, SiH₄, and H₃B·L (L = PMe₃, PPh₃), respectively, with a low barrier, activation of the C-H bond in CH₄ involves a transition state of 57.5 kcal/mol high in energy. The inability of the ruthenium complex to activate CH₄ is due to the undue stretching of the C-H bond needed at the transition state, in comparison to the other substrates.