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

摘要

Silylation of ethylene by the chlorosilanes HSiMe_2Cl and HSiMeCl_2 was catalysed by the bis(dihydrogen) complex RuH _2(η~2-H_2)_2(PCy_3) _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-d_8 with the two chlorosilanes. Reaction of 1 with an excess of HSiMe_2Cl (10 equiv.) produced the σ-silane complexes RuH_2(η~2-H_2)(η~2- HSiMe_2Cl)(PCy_3)_2 (2Me_2Cl), RuH _2(η~2-HSiMe_2Cl)_2(PCy _3)_2 (3Me_2Cl) and the silyl complex RuCl(SiMe_2Cl)(η~2-H_2)(PCy_3) _2 (4Me_2Cl), all characterised by multinuclear NMR spectroscopy. Complexes 2Me_2Cl and 3Me_2Cl 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 4Me_2Cl resulted from the stoichiometric reaction of HSiMe_2Cl with 1 producing RuHCl(η~2-H _2)(PCy_3)_2in situ which further reacted with evolution of H_2 and formation of 4Me_2Cl. When reacting 1 with 10 equiv. of HSiMeCl_2, the corresponding complexes 3MeCl _2 and 4MeCl_2 were detected as well as traces of 2MeCl _2. The reactivity toward ethylene was then examined. Under catalytic conditions (excess silane in toluene-d_8, ethylene atmosphere) only two compounds could be characterised: free PCy_3 and the new (η~6-aryl)(disilyl) complexes of the general formula Ru(η~6-C_6D_5CD_3)(SiMe _(3-n)Cl_n)_2(PCy_3) (6Me _(3-n)Cl_n-d_8, n = 1,2). The X-ray structure of 6MeCl_2 was obtained on a single-crystal at 160 K. When only 2 equiv. of HSiMe_2Cl were added, the ethylene(silyl) complex RuH(SiMe _2Cl)(C_2H_4)(PCy_3)_2 (7Me_2Cl) was obtained in addition to the organic products resulting from catalytic hydrogenation, hydrosilylation and dehydrogenative silylation, i.e. C_2H_6 (major one), C_2H_3SiMe _2Cl and C_2H_5SiMe_2Cl. In the case of 2 equiv. of HSiMeCl_2, upon ethylene addition, 7MeCl_2 was formed in minority compared to a new disilyl complex Ru(SiMeCl_2) _2(PCy_3)_2 (8MeCl_2) characterised by NMR spectroscopy and X-ray diffraction on a single crystal at 160 K. In 8MeCl_2, a formal 14-electron species, stabilisation through two agostic C-H bonds of the cyclohexyl groups was ascertained by DFT calculations.