582307-32-2Relevant academic research and scientific papers
Structure and reactivity of bis(silyl) dihydride complexes (PMe3)3Ru(SiR3)2(H)2: Model compounds and real intermediates in a dehydrogenative C-Si bond forming reaction
Dioumaev, Vladimir K.,Yoo, Bok R.,Procopio, Leo J.,Carroll, Patrick J.,Berry, Donald H.
, p. 8936 - 8948 (2007/10/03)
A series of stable complexes, (PMe3)3Ru(SiR3)2(H)2 ((SiR3)2 = (SiH2Ph)2, 3a; (SiHPh2)2, 3b; (SiMe2-CH2CH2SiMe2), 3c), has been synthesized by the reaction of hydridosilanes with (PMe3)3Ru(SiMe3)H3 or (PMe3)4Ru(SiMe3)H. Compounds 3a and 3c adopt overall pentagonal bipyramidal geometries in solution and the solid state, with phosphine and silyl ligands defining trigonal bipyramids and ruthenium hydrides arranged in the equatorial plane. Compound 3a exhibits meridional phosphines, with both silyl ligands equatorial, whereas the constraints of the chelate in 3c result in both axial and equatorial silyl environments and facial phosphines. Although there is no evidence for agostic Si-H interactions in 3a and 3b, the equatorial silyl group in 3c is in close contact with one hydride (1.81(4) A) and is moderately close to the other hydride (2.15(3) A) in the solid state and solution (ν(Ru...H...Si) = 1740 cm-1 and ν(RuH) = 1940 cm-1). The analogous bis(silyl) dihydride, (PMe3)3Ru(SiMe3)2(H)2 (3d), is not stable at room temperature, but can be generated in situ at low temperature from the 16e- complex (PMe3)3Ru(SiMe3)H (1) and HSiMe3. Complexes 3b and 3d have been characterized by multinuclear, variable temperature NMR and appear to be isostructural with 3a. All four complexes exhibit dynamic NMR spectra, but the slow exchange limit could not be observed for 3c. Treatment of 1 with HSiMe3 at room temperature leads to formation of (PMe3)3-Ru(SiMe2CH2 SiMe3)H3 (4b) via a CH functionalization process critical to catalytic dehydrocoupling of HSiMe3 at higher temperatures. Closer inspection of this reaction between -110 and -10 °C by NMR reveals a plethora of silyl hydride phosphine complexes formed by ligand redistribution prior to CH activation. Above ca. 0 °C this mixture converts cleanly via silane dehydrogenation to the very stable tris(phosphine) trihydride carbosilyl complex 4b. The structure of 4b was determined crystallographically and exhibits a tetrahedral P3Si environment around the metal with the three hydrides adjacent to silicon and capping the P2Si faces. Although strong Si...HRu interactions are not indicated in the structure or by IR, the HSi distances (2.00(4) - 2.09(4) A) and average coupling constant (JSiH = 25 Hz) suggest some degree of nonclassical SiH bonding in the RuH3Si moiety. The least hindered complex, 3a, reacts with carbon monoxide principally via an H2 elimination pathway to yield mer-(PMe3)3(CO)Ru(SiH2Ph)2, with SiH elimination as a minor process. However, only SiH elimination and formation of (PMe3)3(CO)Ru(SiR3)H is observed for 3b-d. The most hindered bis(silyl) complex, 3d, is extremely labile and even in the absence of CO undergoes SiH reductive elimination to generate the 16e- species 1 (ΔHSiH-elim = 11.0 ± 0.6 kcal·mol-1 and ΔSSiH-elim = 40 ± 2 cal·mol-1·K-1; ΔHSiH-elim? = 9.2 ± 0.8 kcal·mol-1 and ΔSSiH-elim? = 9 ± 3 cal·mol-1·K-1). The minimum barrier for the H2 reductive elimination can be estimated, and is higher than that for silane elimination at temperatures above ca. -50 °C. The thermodynamic preferences for oxidative additions to 1 are dominated by entropy contributions and steric effects. Addition of H2 is by far most favorable, whereas the relative aptitudes for intramolecular silyl CH activation and intermolecular SiH addition are strongly dependent on temperature (ΔHSiH-add = -11.0 ± 0.6 kcal·mol-1 and ΔSSiH-add = -40 ± 2 cal·mol-1·K-1; ΔHβ-CH-add = -2.7 ± 0.3 kcal·mol-1 and ΔSβ-CH-add = -6 ± 1 cal·mol-1·K-1).
