Towards Supported Nitrogen Containing Fragments on Silica Surface for Catalytic Applications

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Abstract

This thesis shed lights on metal-nitrogen organometallic fragments supported on silica surface for catalytic applications. It Focuses on group IV and V metal transition as a well-defined single-site catalysts, specifically titanium, hafnium, and tantalum to utilize them in the development of selective heterogeneous catalysis for imine metathesis, hydroamination and hydroaminoalkylation of olefins and alkynes. Developing new metal-nitrogen containing fragments by using easily available and abundant precursors which is silica SiO2 and metal amides complexes. Here, we describe metal fragments starting with hafnium-nitrogen fragment. All the catalysts were prepared by reacting homoleptic metal amido of group IV and V with partially dehydroxylated silica. In most cases the resulting surface amido is monopodal and leads to well defined single site catalysts precursors. In particular with Hf we have isolated hafniaaziridine 2.1 [(≡Si-O-)Hf(η2-MeNCH2)(η1-NMe2)(η1-HNMe2)], imido, and amido fragments 2.3 [(≡Si-O-)Hf(=NMe)(η1-NMe2)], and two intermediates the five-membered ring 2.2 [≡Si-O-Hf(HNMe2)(η2-NMeCH2CH(C6H13)CH2)(NMe2)] and 2.4 [(≡Si-O-)Hf(=NCH2Ar) (η1-NMe2)]. For tantalum 3.1 [(≡Si-O-)Ta=NtBu)(η1-NMeEt)2]; we have isolated two intermediates after treating 3.1 catalyst with aniline substrate lead to isolating 3.3 [(≡Si-O-)Ta(η1σ-NEtMe)2(η1σ-NHtBu)(NHC6H10)], and upon treating with 1-octyne lead to isolating 3.2 [(≡Si-O-)Ta(η1σ-NEtMe)2(η2-NtBuC=CC7H13)]. For titanium-nitrogen fragments, we isolated on silica SiO2-200 4.1 [(≡Si-O2-)Ti(NMe2)2 (η1-HNMe2)] and on SiO2-700 the titaniaaziridine 4.2a [≡Si-O-Ti(NMe2)3] 4.2b, [(≡Si-O-)Ti(η2-MeNCH2)(η1-NMe2)(η1-HNMe2)], the imido, and amido fragments 4.4 [(≡Si-O-)Ti(=NMe)(η1-NMe2)], and the five-membered ring intermediate 4.3 [≡Si-O-Ti(HNMe2)(η2-NMeCH2CH(C6H13)CH2)(NMe2)]. Research in this area has led to isolating several intermediates containing nitrogen fragments, this is the strength of surface organometallic chemistry which allows a deeper understanding of catalytic phenomena which could not be approached either in homogeneous catalysis or in classical heterogeneous catalysis. A molecular level characterization of the surface nitrogen containing fragments have been characterized by SOMC tools such as FTIR and EXAFS spectroscopy, elemental analysis, solid-state single and multiple quantum NMR, advanced DNP-SENS and DFT. A catalytic cycle was proposed based not only on the isolation of intermediates but also based on DFT calculations.

Citation

Aljuhani, M. A. (2019). Towards Supported Nitrogen Containing Fragments on Silica Surface for Catalytic Applications. KAUST Research Repository. https://doi.org/10.25781/KAUST-3Z199