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β-Agostic Silylamido and Silyl-Hydrido Compounds of Molybdenum and Tungsten

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journal contribution
posted on 19.10.2009, 00:00 by Stanislav K. Ignatov, Andrey Y. Khalimon, Nicholas H. Rees, Alexei G. Razuvaev, Philip Mountford, Georgii I. Nikonov
Reactions of bis(imido) compounds (RN)2Mo(PMe3)n (n = 2, R = tBu; n = 3, R =2,6-dimethylphenyl (Ar′) and 2,6-diisopropylphenyl (Ar)) and (RN)2W(PMe3)3 (R = 2,6-dimethylphenyl and 2,6-diisopropylphenyl) with silanes afford four types of products: the β-agostic silylamido compounds (RN)(η3-RN−SiR′2−H···)MCl(PMe3)2 (M = Mo and W), mono(imides) (RN)MCl2(PMe3)3 (M = Mo and W), silyl hydride bis(imido) derivative (ArN)2W(PMe3)(H)(SiMeCl2), and Si−Cl···W bridged product (ArN)(η2-ArN−SiHMeCl−Cl···)WCl(PMe3)2. Reactions of molybdenum compounds (RN)2Mo(PMe3)m (m = 2 or 3) with mono- and dichlorosilanes HSiClnR′3−n (R′ = Me, Ph; n = 1,2) afford mainly the agostic compounds (RN)(η3-RN−SiR′2−H···)MoCl(PMe3)2, accompanied by small amounts of mono(imido) derivatives (RN)MoCl2(PMe3)3. In contrast, the latter compounds are the only transition metal products in reactions with HSiCl3, the silicon co-product being the silanimine dimer (RNSiHCl)2. The reaction of (ArN)2W(PMe3)3 with HSiCl2Me under continuous removal of PMe3 affords the silyl hydride species (ArN)2W(PMe3)(SiMeCl2)H, characterized by NMR and X-ray diffraction. This product is stable in the absence of phosphine, but addition of catalytic amounts of PMe3 causes a fast rearrangement into the Si−Cl···W bridged product (ArN)(η2-ArN−SiHMeCl−Cl···)WCl(PMe3)2. The mechanism of silane addition to complexes (RN)2Mo(PMe3)n has been elucidated by means of density functional theory calculations of model complexes (MeN)2Mo(PMe3)n (n = 1−3). Complex (MeN)2Mo(PMe3)2 is found to be the most stable form. It undergoes facile silane-to-imido addition reactions that afford silylamido hydride complexes stabilized by additional Si···H interactions. The ease of this addition increases from HSiMe2Cl to HSiCl3, consistent with experimental observations. The most stable final products of silane addition are the agostic complexes (MeN)(η3-MeN−SiR2−H···)MoCl(PMe3)2 (R2 = Me2, MeCl, Cl2) and Cl-bridged silylamido complexes (MeN)(η2-MeN−SiRH−Cl···)MoCl(PMe3)2 (R = Me or Cl). In the case of HSiMeCl2 addition the former is the most stable, but for HSiCl3 addition the latter is the preferred product. In all cases, the isomeric silyl hydride species (MeN)2Mo(PMe3)(H)(SiClR2) are less stable by about 6 kcal mol−1. Silane additions to the imido ligand of the tris(phosphine) (MeN)2Mo(PMe3)3 afford octahedral silylamido hydride derivatives. The different isomers of these addition products are destabilized relative to (MeN)2Mo(PMe3)3 only by 7−24 kcal mol−1 (for the HSiMe2Cl additions), but since the starting tris(phosphine) is 14.8 kcal mol−1 less stable than (MeN)2Mo(PMe3)2, silane addition to the latter is a more preferred pathway. A double phosphine dissociation pathway via the species (MeN)2Mo(PMe3) was ruled out because this complex is by 24.7 kcal mol−1 less stable than (MeN)2Mo(PMe3)2.