Back to Table of Contents

45990-AC3
New Silanimine and NSi-H-M Agostic Complexes for Coupling with Petroleum Derived Products

Georgii I. Nikonov, Brock University

The final goal of this project is to develop new synthetic methodologies for the coupling of silanes with petroleum derived products. Specifically, it is aimed at the preparation of new silanimine and agostic silylamido complexes and investigation of their reactivity towards unsaturated organic substrates.

The specific results of the current grant period are:

1)      For the first time we accomplished the coupling of two silane molecules with an imido complex. The product, (ArN=)(ArN-SiHPh-H...)Mo(SiH₂Ph)(PMe₃) (1), features both the agostic silylamido group and silyl functionality. Complex 1 was characterized by NMR and IR spectroscopy and by X-ray diffraction.

2)      By labelling experiments with PhSiD₃ and TolSiH₃, we showed that complex 1 reversibly eliminates silane PhSiH₃ to generate a reactive silanimine intermediate (ArN=)(ArN-SiHPh)Mo(PMe₃) (2) (Scheme 1). EXSY NMR confirms this by showing that the silyl ligand is an exchange with external silane.

Scheme 1

3)      We discovered that 1 is an effective catalyst for hydrosilylation of aldehyde and ketones and alcoholysis of silane. This is the first example of catalytic behavior of an agostic silyl compound. Moreover, we achieved catalytic hydrosilylation of nitriles which is selective in the first step, the products being silylated imines. Hydrosilylation of nitriles is rarely done catalytically and in no previous case selectively. Complex 1 also catalyzes slowly silane redistribution when excess PhSiH₃ is present.

4)      We performed stoichiometric reactions of 1 with olefins and nitriles and in both cases observed products of Si-C coupling (Scheme 2). The reaction goes via silane elimination to give intermediate 2, which then adds olefin to the silanimine ligand to give a five- membered metalacycle. Beta-hydride abstraction affords a silylated olefin/hydride complex which was isolated in the case of substituted olefins, such as styrene. With ethylene two equivalents are added to give an ethyl complex, which can be converted to the hydride (ArN=)(ArN-SiHPh-CH=CH₂)Mo(H)(PMe₃) by the reaction with PhSiH₃. The latter product easily reacts with olefins RCH=CH₂ to give alkyl derivatives (ArN=)(ArN-SiHPh-CH=CH₂)Mo(CH₂CH₂R)(PMe₃).

Scheme 2

5)      By reacting the complex (^tBuN)₂Mo(PMe₃)₂ with H₃SiPh we achieved for the first time the coupling of three silane molecules with an imido complex. The product, (^tBuN=){h²- ^tBuN(SiHPh)₂}Mo(H)(SiH₂Ph)(PMe₃)₂ (3) was characterized by NMR spectroscopy and X-ray diffraction. Note that 3 is formally a silyl hydride derivative of Mo(VI) compound, i.e. has a very unusual oxidation state.

This result proves conclusively that an imido ligand can be transferred from the metal to silicon, which may be a step in our proposed tricomponent coupling reaction of silane, amine, and an unsaturated substrate. Thus, so far we observed three individual steps of the proposed catalytic cycle: silane/imido coupling, coupling of the agostic silylamide with unsaturated substrates (proceeding via a silanimine), detachment of the functionalized imido ligand from the metal. The remaining goal is to assemble these steps into a cycle on one specific metal center.

6)      We prepared new imido hydride precursors, (RN=)MoCl(H)(PMe₃)₃ (R=Ar and Ar'), which were then used to synthesize tripod-supported derivatives (^PhL₃)(Ar'N=)Mo(H)(PMe₃) (4, ^PhL₃ = PhB(N-metylimidozolydene)), (Tp*)(Ar'N=)Mo(H)(PMe₃) (5, Tp* = tris(3,5-dimethylpyrazolyl)borate), and (Tp)(ArN=)Mo(H)(PMe₃) (6, Tp = tris(pyrazolyl)borate). For comparison purposes, we also prepared their Cp analogs Cp(RN=)Mo(H)(PMe₃) (R = Ar' (7) and Ar (8)).

7)      We studied reactions of the new tripodal complexes with silanes. Complexes 4 and 5 either do not react with silanes or give intractable mixtures of products (at elevated) temperatures, most likely due to the bulkiness of the tripodal ligands. However, complex 5 catalyses hydrosilylation of cyclohexanone by H₃SiPh at 70 ºC. In contrast, less sterically congested complex 8 reacts with one and two equivs of H₃SiPh to give the silyl complexes Cp(ArN=)Mo(SiH₂Ph)(PMe₃) and Cp(ArN=)Mo(H)(SiH₂Ph)₂, respectively. The unexpectedly high oxidation state VI of molybdenum is observed in the latter product.

Back to top