Contrasting reactivity behaviour of the [RuHCl(CO)(PNP)] complex with electrophilic reagents XOTf (X = H, CH₃, Me₃Si)

Abstract

A new ruthenium pincer complex [RuHCl(CO)(PNP)] (PNP = PhCH₂N(CH₂CH₂PPh₂)₂) (1) was synthesized and characterized. The reactivity of complex 1 with electrophilic reagents XOTf (X = H, CH₃, and Me₃Si; OTf = CF₃SO₃) was studied by variable temperature NMR spectroscopy with an aim to observe and characterize sigma complexes of type [Ru(η²-HX)Cl(CO)(PNP)][OTf] (X = H (2), CH₃ (3), Me₃Si (4)). Reaction of complex 1 with HOTf resulted in the formation of the dihydrogen complex, [Ru(η²-H₂)Cl(CO)(PNP)[OTf] (2). On the other hand, the reaction between complex 1 and MeOTf and Me₃SiOTf resulted in the direct elimination of MeCl and Me₃SiCl via a S_N2 type of reaction without the intermediacy of the respective sigma complexes 3 and 4. This contrasting reactivity behaviour has been rationalized taking into consideration the approach of the relatively bulky electrophiles CH₃⁺ and Me₃Si⁺ onto the hydride moiety of the ruthenium fragment, which is sterically hindered.