Hydrosilation reactions represent an important class of chemical transformations and there has been considerable recent interest in expanding the scope of these reactions by developing new catalysts. A major theme to emerge from these investigations is the development of catalysts with electrophilic character that transfer electrophilicity to silicon via Si-H activation. This type of mechanism has been proposed for catalysts ranging from Group 4 transition metals to Group 15 main group species. Additionally, other electrophilic silicon species, such as silylene complexes and η 3 -H2SiRR' complexes, have been identified as intermediates in hydrosilation reactions. In this Review, different types of catalysts are compared to highlight the range of hydrosilation mechanisms that feature electrophilic silicon centers, and the importance of these catalysts to the development of new hydrosilation reactions is discussed.
New η(3)-silane σ-complexes [PhBP(Ph)(3)]RuH(η(3)-H(2)SiRR') (RR' = PhMe, Ph(2)) were synthesized. Lewis bases [THF, 4-(dimethylamino)pyridine, and PMe(3)] coordinate to the silicon centers of these complexes to form stable adducts. The base adducts, [PhBP(Ph)(3)]Ru(μ-H)(3)SiRR'(base), feature three nonclassical Ru-H-Si interactions and hexacoordinate silicon centers, as determined by multinuclear NMR spectroscopy, X-ray crystallography, and computational investigations.
Hydridosilicate anions ([ArSiH(4)](-) and [SiH(6)](2-)) were stabilized as ligands in diruthenium Si-H σ-complexes [{(PhBP(Ph(3))Ru}(2)(μ-Cl)(μ-η(3),η(3)-H(4)SiAr)] (Ar = 2-MeOC(6)H(4), Mes, Ph) and [{(PhBP(Ph)(3))Ru}(2)(μ-η(4),η(4)-H(6)Si)] (see picture). These complexes were formed under mild conditions and characterized by single-crystal X-ray diffraction (see picture), NMR and IR spectroscopy, and computational techniques.
Complexes [PhBP(Ph)3]RuH(η(3)-H2SiRR') (R,R' = Me,Ph, 1a; RR' = Ph2, 1b) react with XylNC (Xyl = 2,6-dimethylphenyl) to form Fischer carbene complexes [PhBP(Ph)3]Ru(H)═[C(H)(N(Xyl)(η(2)-H-SiRR'))] (2a,b) that feature a γ-agostic Si-H bond. The ruthenium isocyanide complexes [PhBP(Ph)3]Ru(H)(CNXyl)(η(2)-HSiHRR') (6a,b) are not intermediates as they do not convert to 2a,b. Experimental and theoretical investigations indicate that XylNC is activated by initial coordination to the silicon center in 1a,b, followed by 1,1-insertion into an Si-H bond of the coordinated silane and then rearrangement to 2a,b.
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