Silyl and σ-silane ruthenium complexes: Chloride substituent effects on the catalysed silylation of ethylene

Article abstract of DOI:10.1039/c0dt00065e

Silylation of ethylene by the chlorosilanes HSiMe₂Cl and HSiMeCl₂ was catalysed by the bis(dihydrogen) complex RuH ₂(η²-H₂)₂(PCy₃) ₂ (1). Dehydrogenative silylation leading to the formation of the corresponding vinylsilanes was in competition with hydrosilylation. The rate and selectivity of the reactions were influenced by the number of chloro substituents and the ethylene pressure. A comparative mechanistic study was performed in toluene-d₈ with the two chlorosilanes. Reaction of 1 with an excess of HSiMe₂Cl (10 equiv.) produced the σ-silane complexes RuH₂(η²-H₂)(η²- HSiMe₂Cl)(PCy₃)₂ (2Me₂Cl), RuH ₂(η²-HSiMe₂Cl)₂(PCy ₃)₂ (3Me₂Cl) and the silyl complex RuCl(SiMe₂Cl)(η²-H₂)(PCy₃) ₂ (4Me₂Cl), all characterised by multinuclear NMR spectroscopy. Complexes 2Me₂Cl and 3Me₂Cl adopt a cis configuration for the two bulky phosphine ligands as a result of stabilising SISHA (Secondary Interactions between Silicon and Hydrogen Atoms) interactions. Complex 4Me₂Cl resulted from the stoichiometric reaction of HSiMe₂Cl with 1 producing RuHCl(η²-H ₂)(PCy₃)₂in situ which further reacted with evolution of H₂ and formation of 4Me₂Cl. When reacting 1 with 10 equiv. of HSiMeCl₂, the corresponding complexes 3MeCl ₂ and 4MeCl₂ were detected as well as traces of 2MeCl ₂. The reactivity toward ethylene was then examined. Under catalytic conditions (excess silane in toluene-d₈, ethylene atmosphere) only two compounds could be characterised: free PCy₃ and the new (η⁶-aryl)(disilyl) complexes of the general formula Ru(η⁶-C₆D₅CD₃)(SiMe _3-nCl_n)₂(PCy₃) (6Me _3-nCl_n-d₈, n = 1,2). The X-ray structure of 6MeCl₂ was obtained on a single-crystal at 160 K. When only 2 equiv. of HSiMe₂Cl were added, the ethylene(silyl) complex RuH(SiMe ₂Cl)(C₂H₄)(PCy₃)₂ (7Me₂Cl) was obtained in addition to the organic products resulting from catalytic hydrogenation, hydrosilylation and dehydrogenative silylation, i.e. C₂H₆ (major one), C₂H₃SiMe ₂Cl and C₂H₅SiMe₂Cl. In the case of 2 equiv. of HSiMeCl₂, upon ethylene addition, 7MeCl₂ was formed in minority compared to a new disilyl complex Ru(SiMeCl₂) ₂(PCy₃)₂ (8MeCl₂) characterised by NMR spectroscopy and X-ray diffraction on a single crystal at 160 K. In 8MeCl₂, a formal 14-electron species, stabilisation through two agostic C-H bonds of the cyclohexyl groups was ascertained by DFT calculations.