Neutral and Anionic Silyl Hydride Derivatives of the Tantalum Imido Fragment Cp*(DippN)Ta (Cp* = η⁵-C₅Me₅; Dipp = 2,6-ⁱPr₂C₆H₃). Reactive σ-Bonds and Intramolecular C−H Bond Activations Involving the Silyl Ligands

Abstract

The syntheses and reactivities of alkyl, silyl, and hydride complexes of the type Cp*(DippN)TaRR‘ (Cp* = η5-C5Me5; Dipp = 2,6-iPr2C6H3; R = silyl, alkyl; R‘ = alkyl, hydride) are described. The compounds Cp*(DippN)Ta(SiR3)Cl (SiR3 = Si(SiMe3)3 (1), SiPh3 (2), SiHMes2 (3); Mes = 2,4,6-Me3C6H2), prepared from Cp*(DippN)TaCl2 and (THF) n LiSiR3, react with H2 to yield the dimeric hydride [Cp*(DippN)TaCl(μ-H)]2 (4). The reaction of 4 with (THF)3LiSi(SiMe3)3 (2 equiv) gave the 16-electron silyl hydride complex Cp*(DippN)Ta[Si(SiMe3)3]H (6), which is Lewis acidic and reversibly binds halide ions to afford anionic Ta(V) complexes of the type A+{Cp*(DippN)Ta(X)[Si(SiMe3)3]H}- (A = Li(THF)3, X = Cl (5); A = NBu4, X = Br (7) and I (8)). For complexes 5, 7, and 8, NMR and X-ray data suggest the presence of weak three-center interactions involving Ta, Si, and H. Silyl hydride 6 reacts with xylyl isocyanide (xylyl = 2,6-dimethylphenyl) and acetonitrile to give the corresponding η2-silaimine and azomethine insertion products, respectively, while treatment with acetone yields the stable isopropoxide product arising from insertion into the Ta−H bond. When complex 6 is treated with ethylene or diphenylacetylene, elimination of HSi(SiMe3)3 occurs, along with formation of a five-membered tantalacycle resulting from coupling of the unsaturated substrate at Ta. Hydrogenolysis of 6 yields HSi(SiMe3)3 and a dimeric dihydride [Cp*(DippN)TaH(μ-H)]2 (14). At room temperature, 6 rearranges (t 1/2 = 8.5 h) to an unusual alkyl hydride species resulting from C−H bond activation, Cp*(DippN)Ta[CH2Si(SiMe3)2SiMe2H]H (15). This complex exhibits a γ-agostic Si−H interaction with the metal center. An attempt to prepare Cp*(DippN)Ta(SiHMes2)H produced the alkyl hydride product Cp*(DippN)Ta[η2-CH2(2-SiH2Mes-3,5-Me2C6H2)]H (20), which apparently results from decomposition of the expected silyl hydride. The alkyl hydride complex [Cp*(DippN)TaMe(μ-H)]2 (25) was prepared by the hydrogenolysis of Cp*(DippN)Ta[Si(SiMe3)3]Me (23), whereas Cp*(DippN)Ta(CH2CMe3)H (26) was obtained by treatment of 4 with NpMgCl. Complex 26 possesses an α-agostic C−H interaction, and the corresponding deuteride 26- d slowly scrambles deuterium into the methylene positions.

Alkyl–η2-alkene niobocene and tantalocene complexes with the allyldimethylsilyl–η5-cyclopentadienyl ligand: synthesis, NMR studies and DFT calculations

“…15 it was reported that this is also the region where the hydride signal of terminal Nb-H 16 and Ta-H 17 bonds are observed for this type of dicyclopentadienyl imido complexes. The difference between the 13 C quaternary and methyl carbon resonances of the tertbutylimido group is 30.8 ppm suggesting a linear disposition of the Nb-N-tBu ligand in complex 15.…”

Non‐Innocent Behaviour of Imido Ligands in the Reactions of Silanes with Half‐Sandwich Imido Complexes of Nb and V: A Silane/Imido Coupling Route to Compounds with Nonclassical SiH Interactions

“…[1] For example, Cp/imido, bisA C H T U N G T R E N N U N G (imido) and alkoxyimido ligand sets have been widely used for the preparation of metallocene-like catalysts for olefin polymerization, [2] dehydrogenative polymerization of silanes [3] and olefin metathesis. [4] In addition to its role as a supporting ligand for reactive groups elsewhere in a metals coordination sphere, such as silyl and hydride ligands, [3,5] the unsaturated and polar M=NR bond itself can act as a potent reaction site towards addition reactions with saturated and unsaturated substrates.…”

“…[1] For example, Cp/imido, bisA C H T U N G T R E N N U N G (imido) and alkoxyimido ligand sets have been widely used for the preparation of metallocene-like catalysts for olefin polymerization, [2] dehydrogenative polymerization of silanes [3] and olefin metathesis. [4] In addition to its role as a supporting ligand for reactive groups elsewhere in a metals coordination sphere, such as silyl and hydride ligands, [3,5] the unsaturated and polar M=NR bond itself can act as a potent reaction site towards addition reactions with saturated and unsaturated substrates. [6] 3 ) 2 ] (M= V, Nb) with silanes afford a plethora of products, depending on the nature of the metal, substitution at silicon and 3 ) 2 ] bearing a penta-coordinate silicon centre, which then rearranges into the final products through a SiÀH or SiÀCl bond activation process.…”

“…[4] In addition to its role as a supporting ligand for reactive groups elsewhere in a metals coordination sphere, such as silyl and hydride ligands, [3,5] the unsaturated and polar M=NR bond itself can act as a potent reaction site towards addition reactions with saturated and unsaturated substrates. [6] 3 ) 2 ] (M= V, Nb) with silanes afford a plethora of products, depending on the nature of the metal, substitution at silicon and 3 ) 2 ] bearing a penta-coordinate silicon centre, which then rearranges into the final products through a SiÀH or SiÀCl bond activation process. DFT calculations show that this imido-silane adduct is additionally stabilized by a Si À H···M agostic interaction.…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…101 Silyl derivatives of group 5 organometallic complexes have been investigated including intramolecular C-H activation of silyl ligands by tantalum() imido moieties 102 and the formation of an unusual imido-bridged 1,3-disilaniocyclobutane 11 from reaction between [Nb(η 5 -C 5 H 5 )(H)(SiMe 2 I) 2 ] and H 2 NEt 2 . 103 Hydrogenation of aromatics 104 and intramolecular hydride transfers to aromatic rings have been studied and an investigation into dihydrogen addition to [Ta(CH 2 )-(η 5 -C 5 Me 5 ) 2 (H)] using parahydrogen has also been reported.…”