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

Abstract

Silylation of ethylene by the chlorosilanes HSiMe(2)Cl and HSiMeCl(2) was catalysed by the bis(dihydrogen) complex RuH(2)(eta(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(2)Cl (10 equiv.) produced the sigma-silane complexes RuH(2)(eta(2)-H(2))(eta(2)-HSiMe(2)Cl)(PCy(3))(2) (2Me(2)Cl), RuH(2)(eta(2)-HSiMe(2)Cl)(2)(PCy(3))(2) (3Me(2)Cl) and the silyl complex RuCl(SiMe(2)Cl)(eta(2)-H(2))(PCy(3))(2) (4Me(2)Cl), all characterised by multinuclear NMR spectroscopy. Complexes 2Me(2)Cl and 3Me(2)Cl 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(2)Cl resulted from the stoichiometric reaction of HSiMe(2)Cl with 1 producing RuHCl(eta(2)-H(2))(PCy(3))(2)in situ which further reacted with evolution of H(2) and formation of 4Me(2)Cl. 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 (eta(6)-aryl)(disilyl) complexes of the general formula Ru(eta(6)-C(6)D(5)CD(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(2)Cl were added, the ethylene(silyl) complex RuH(SiMe(2)Cl)(C(2)H(4))(PCy(3))(2) (7Me(2)Cl) was obtained in addition to the organic products resulting from catalytic hydrogenation, hydrosilylation and dehydrogenative silylation, i.e. C(2)H(6) (major one), C(2)H(3)SiMe(2)Cl and C(2)H(5)SiMe(2)Cl. 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.