Article pubs.acs.org/Organometallics
Single-Site Tetracoordinated Aluminum Hydride Supported on Mesoporous Silica. From Dream to Reality! Baraa Werghi, Anissa Bendjeriou-Sedjerari, Abdesslem Jedidi, Edy Abou-Hamad, Luigi Cavallo,* and Jean-Marie Basset* King Abdullah University of Science and Technology (KAUST), KAUST Catalysis Center (KCC), Thuwal 23955-6900, Saudi Arabia S Supporting Information *
ABSTRACT: The reaction of mesoporous silica (SBA15) dehydroxylated at 700 °C with diisobutylaluminum hydride, i-Bu2AlH, gives after thermal treatment a single-site tetrahedral aluminum hydride with high selectivity. The starting aluminum isobutyl and the final aluminum hydride have been fully characterized by FT-IR, advanced SS NMR spectroscopy (1H, 13C, multiple quanta (MQ) 2D 1H−1H, and 27Al), and elemental analysis, while DFT calculations provide a rationalization of the occurring reactivity. Trimeric i-Bu2AlH reacts selectively with surface silanols without affecting the siloxane bridges. Its analogous hydride catalyzes ethylene polymerization. Indeed, catalytic tests show that this single aluminum hydride site is active in the production of a high-density polyethylene (HDPE).
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INTRODUCTION Modification of a silica surface by the reaction of alkylaluminum compounds and MAO is the most versatile approach for the preparation of cocatalysts or activators for olefin polymerization.1 However, selective surface functionalization to afford a well-defined tricoordinated supported aluminum remains a huge challenge that has never been reached until now. For example, triisobutylaluminum (i-Bu3Al), although very bulky, reacts simultaneously with both isolated silanols, Si−OH, and siloxane groups, Si−O−Si, present on highly dehydroxylated silica (up to 500 °C).1i,2 We recently highlighted the complexity of the reaction of iBu3Al with SBA15700 surfaces.1i Surprisingly, i-Bu3Al reacts simultaneously not only with isolated Si−OH but also with “strained” siloxane bridges. This metathetical surface reaction leads to the formation of an adjacent silicon isobutyl, Si−iBu, and an [Al]−siloxy species. The complex geometry of the surface aluminum associated with the bulkiness of the Si−i-Bu in the proximity of the [Al]−i-Bu could explain the poor cocatalytic activity of the materials. These results showed that the expected “true” singlesite tricoordinated aluminum alkyl catalyst on surfaces is a “utopia”. To our knowledge, avoiding the reactivity of alkylaluminum compounds toward strained siloxane bridges has never been reported up to now for the challenging generation of welldefined tetra- and even tricoordinated Al species. Therefore, it was logical to use a nonhomoleptic Al derivative such as diisobutylaluminum hydride, i-Bu2AlH, which exists in solution in a trimeric form,3 in contrast to the monomeric i-Bu3Al.4 We report herein the first well-defined single-site tetrahedral isobutylaluminum supported complex, [(Si−O−Si)( © XXXX American Chemical Society
Si−O)2Al−i-Bu] (1), obtained by an unexpected and complex selective reaction of i-Bu2AlH with Si−OH of SBA15700 (Scheme 1). In contrast to what has been observed with iScheme 1. Design of a Supported Tetracoordinated Aluminum Site along with Silicon Hydride and Isobutyl
Bu3Al,1i where a mixture of tetra-, penta-, and octahedral Al geometry is observed, here a unique tetracoordinated Al site is formed. A controlled thermal treatment under vacuum leads to the corresponding tetracoordinated Al−H, which is active in ethylene polymerization.
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RESULTS AND DISCUSSION 1. Reaction of i-Bu2AlH with SBA15700. The hexagonally ordered mesoporous SBA155 with a surface area of 810 m2/g (calcined SBA15) was subjected to a dehydroxylation at 700 °C under vacuum (10−5 mbar) for 16 h to afford SBA15700. SBA15700 (1.8 mmol/g of Si−OH, obtained by titration with MeLi)6 was treated with 1 equiv/silanol of i-Bu2AlH (1 M/ cyclohexane) at room temperature in pentane for 1 h. Almost instantaneously, 1.2 ± 0.2 molecules of isobutane per grafted Received: June 6, 2016
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DOI: 10.1021/acs.organomet.6b00454 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Bu3Al. Indeed, the intensity of the ν(Si−H) band is more important than that observed after reaction of i-Bu3Al with SBA15700.1i This information suggests that silicon hydride species are formed by a selective reaction of i-Bu2AlH with the silanols (FT-IR) by its isobutyl fragment followed by a hydride transfer to the neighboring siloxane bridge (Si−O−Si) leading to Si−O−Al and Si−H (Figure 1). It is noteworthy that no Al−H surface species appears in the FTIR spectrum (1940, 1660, and 1610 cm−1).8a,9 The 1H MAS spectrum of 1 displays three clear resonances (Figure S3 in the Supporting Information). The signals at 0.2, 0.9, and 1.85 ppm are assigned on the basis of the literature1i,2b to [(SiO−)2-Al-CH2CH(CH3)2], [(SiO−)2Al−CH2CH(CH3)2] (this proton signals overlap with [Si−CH2CH(CH3)2]) and [(SiO)2Al−CH2CH(CH3)2] (overlapping with [Si−CH2CH(CH3)2]). To be sure of these interpretations, DQ and TQ analyses needed to be done. The 2D double-quantum (DQ) 1H−1H MAS NMR spectrum in Figure 2 shows for the isobutyl group [CH2CH-
aluminum is selectively evolved. This corresponds also to 0.9 molecule of isobutane per silanol. To our surprise, no H2 was detected. Note that isobutene is not detected in the gas phase, in opposition to what is observed with i-Bu3Al.1i The C/Al ratio of 6.6 is attributed mainly to the formation of an “ill defined” aluminum isobutyl (a C/Al ratio of 8 is expected for the formation of a monopodal Al or 4 for a bipodal Al). Further proof of the surface stoichiometry came from hydrolysis of these materials at room temperature, which gave 1 ± 0.2 isobutane molecules per Al for 1 (Table S1 in the Supporting Information). The chemistry of aluminum and silicon alkyl teaches us the great reactivity of Al alkyl species toward water and the poor reactivity of silicon alkyls with water.7 The main conclusion of these hydrolysis reactions is that only one isobutyl is left on aluminum after grafting. Therefore, in comparison with the reaction of i-Bu3Al with the same SBA15700, these elemental and gas-phase analyses are drastically different.1i These analytical data led us to suggest that i-Bu2AlH reacts selectively with Si−OH via its isobutyl fragment rather than via its hydride. Further evidence is provided through solid-state spectroscopy analysis and DFT calculations. It is noteworthy that the textural parameters of the resulting mesoporous materials 1, large specific surface area (560 m2/g), and wellordered hexagonal structure remain intact (Figures S1 and S2 and Table S2 in the Supporting Information) after the different treatment. The FT-IR spectra of the SBA15700 and 1 are shown in Figure 1. First, an almost complete consumption of the Si−
Figure 1. IR spectra of (i) SBA15700, (ii) the sample in (i) after reaction with 1 equiv of i-Bu2AlH/Si−OH (1), and (iii) subtraction of the spectrum of (ii) from that of (i).
Figure 2. (i) DQ rotor-synchronized 2D 1H MAS NMR spectrum of 1 and (ii) 1H TQ MAS spectrum.
OH (∼92%) vibrational bands (ν(OH) at 3741 cm−1) is observed after the reaction. Then, the characteristic vibrational bands of isobutyl groups appear at 2950 (νas(CH3)), 2865 (νs(CH2)), 1465 (δas(CH3)), and 1365 cm−1 (δs(CH3)). Surprisingly, an intense ν(Si−H) band at 2181 cm−1 is observed, 8 definitively present in a higher amount in comparison to the remaining SiOH (∼8%) in a hydrogenbonded environment. The FT-IR spectrum of 1 features a tremendous difference from that obtained after reaction of i-
(CH3)2] a strong expected autocorrelation on the 2:1 diagonal centered around 0.2−0.9 ppm in F2 and 0.4−1.8 ppm. Outside the diagonal, a correlation between Al−CH, (1.9 ppm) and Al− CH3 (0.9 ppm) is observed at 2.8 ppm in the F2 dimension. Although the characteristic signal of the Si−H in the 1H MAS spectrum does not appears clearly, a strong correlation is observed at 6.8 ppm in the F2 dimension. It is clearly attributed to a correlation between the proton resonance of [( SiO−)2Al−CH2CH(CH3)2] and the proton resonance of B
DOI: 10.1021/acs.organomet.6b00454 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics silicon hydride [Si−H] (δH(−CH−) + δH(Si−H) = 1.9 + 4.9) (Scheme 2a). A through-space correlation is also observed as
ppm is unambiguously attributed to a SBA15700 supported AlIV isobutyl, never reported up to now (Figure 3).
Scheme 2. Schematic Showing the Observed Proximities of (a) [(SiO)2Al−CH2CH(CH3)2] and [Si−H] and (b) [(SiO)2Al−CH2CH(CH3)2] and [Si−H]
expected between the proton resonance assigned to [Si-H] and one proton resonance of the [(SiO−)2Al−CH2CH(CH3)2] at 5.8 ppm (δH(Si-H) + δH(−CH−) = 0.9 + 4.9). In other words, [Si−H] and [(SiO)2Al−CH2CH(CH3)2] are in close proximity (
