Well-defined silica supported aluminum hydride: another step towards the utopian single site dream?

Baraa Werghi,Anissa Bendjeriou-Sedjerari,Luigi Cavallo,Jean-Marie Basset,Julien Sofack-Kreutzer,Abdesslem Jedidi,Edy Abou-Hamad

doi:10.1039/c5sc02276b

Baraa Werghi, Anissa Bendjeriou-Sedjerari + Show 5 more

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https://doi.org/10.1039/c5sc02276b

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Abstract

Reaction of triisobutylaluminum with SBA15700 at room temperature occurs by two parallel pathways involving either silanol or siloxane bridges. It leads to the formation of a well-defined bipodal [([triple bond, length as m-dash]SiO)2Al-CH2CH(CH3)2] 1a, silicon isobutyl [[triple bond, length as m-dash]Si-CH2CH(CH3)2] 1b and a silicon hydride [[triple bond, length as m-dash]Si-H] 1c. Their structural identity was characterized by FT-IR and advanced solid-state NMR spectroscopies (1H, 13C, 29Si, 27Al and 2D multiple quantum), elemental and gas phase analysis, and DFT calculations. The reaction involves the formation of a highly reactive monopodal intermediate: [[triple bond, length as m-dash]SiO-Al-[CH2CH(CH3)2]2], with evolution of isobutane. This intermediate undergoes two parallel routes: transfer of either one isobutyl fragment or of one hydride to an adjacent silicon atom. Both processes occur by opening of a strained siloxane bridge, [triple bond, length as m-dash]Si-O-Si[triple bond, length as m-dash] but with two different mechanisms, showing that the reality of "single site" catalyst may be an utopia: DFT calculations indicate that isobutyl transfer occurs via a simple metathesis between the Al-isobutyl and O-Si bonds, while hydride transfer occurs via a two steps mechanism, the first one is a β-H elimination to Al with elimination of isobutene, whereas the second is a metathesis step between the formed Al-H bond and a O-Si bond. Thermal treatment of 1a (at 250 °C) under high vacuum (10-5 mbar) generates Al-H through a β-H elimination of isobutyl fragment. These supported well-defined Al-H which are highly stable with time, are tetra, penta and octa coordinated as demonstrated by IR and 27Al-1H J-HMQC NMR spectroscopy. All these observations indicate that surfaces atoms around the site of grafting play a considerable role in the reactivity of a single site system.

Highlights

The reaction involves the formation of a highly reactive monopodal intermediate: [^SiO–Al– [CH2CH(CH3)2]2], with evolution of isobutane. This intermediate undergoes two parallel routes: transfer of either one isobutyl fragment or of one hydride to an adjacent silicon atom. Both processes occur by opening of a strained siloxane bridge, ^Si–O–Si^ but with two different mechanisms, showing that the reality of “single site” catalyst may be an utopia: DFT calculations indicate that isobutyl transfer occurs via a simple metathesis between the Al-isobutyl and O–Si bonds, while hydride transfer occurs via a two steps mechanism, the first one is a b-H elimination to Al with elimination of isobutene, whereas the second is a metathesis step between the formed Al–H bond and a O–Si bond
These results indicate that TIBA reacts with single silanols (Scheme 2, pathway a) and with strained siloxane bridges (Scheme 2, pathway b).[41,54]
Phase isobutane plus isobutene is close to the amount of grafted Al in principle in agreement with a protonolysis of one Al-isobutyl group and the microanalysis gives 8C/gra ed Al (Tables S1 and S2, Electronic supplementary information (ESI)†) – surprisingly the hydrolysis of the material gives only one mole of isobutane per gra ed Al indicating that a ^Si-isobutyl, which is not hydrolyzed, is already formed at room temperature simultaneously to Al-isobutyl

Summary

Introduction

There is a tendency to rationalize the understanding of heterogeneous catalysis by making well-de ned model compounds.[1,2,3,4,5,6,7,8] The study of the reactivity of these models allows the identi cation of some elementary steps of heterogeneous catalysis.[9,10,11,12,13,14] One of the simplest ways to make such models compounds is to react the surface silanols (^Si–OH) of partially dehydroxylated silica with a metal-alkyl to obtain in principle well de ned gra ed complexes.[2,9,15,16] it appeared progressively that depending on the degree of dehydroxylation of a support, mono, bi and multipodal species can be generated.[2,9,17,18,19] there is growing evidence that the gra ed complexes interact with the surrounding atoms present on the surface (Scheme 1).[2,9,20]the silica dehydroxylation treatment leads to a highly strained reactive surface that may participate in anchoring reactions.[21,22,23,24,25] The resulting surface organometallic chemistry became more associated to an “ensemble” of surfaces atoms that makes this chemistry increasingly difficult to fully understand, approaching to some extent the complexity of classical. The reaction of 1 equivalent of TIBA (1 M/hexane)/silanol with SBA15700 leads to the formation of several compounds: [(^SiO)2–Al–CH2CH(CH3)2] 1a, silicon isobutyl [^Si–CH2CH(CH3)2] 1b and silicon hydride [^Si–H] 1c (Scheme 2). Reaction of TIBA led to the formation of silicon hydride(s) species characterized by a broad n(Si–H) band at 2100–2200 cmÀ1.39,55,56 Based on DFT calculations (vide infra), the presence of these silicon hydrides is explained by a b-H elimination from one of the isobutyl fragment to the Al center of [^SiO–Al–[CH2CH(CH3)2]2.

Results

Conclusion