Emmanuel Callens

WMe6 Tamed by Silica: ≡Si–O–WMe5 as an Efficient, Well-Defined Species for Alkane Metathesis, Leading to the Observation of a Supported W–Methyl/Methylidyne Species

The synthesis and full characterization of a well-defined silica-supported ≡Si-O-W(Me)5 species is reported. Under an inert atmosphere, it is a stable material at moderate temperature, whereas the homoleptic parent complex decomposes above -20 °C, demonstrating the stabilizing effect of immobilization of the molecular complex. Above 70 °C the grafted complex converts into the two methylidyne surface complexes [(≡SiO-)W(≡CH)Me2] and [(≡SiO-)2W(≡CH)Me]. All of these silica-supported complexes are active precursors for propane metathesis reactions.

Nature and Structure of Aluminum Surface Sites Grafted on Silica from a Combination of High-Field Aluminum-27 Solid-State NMR Spectroscopy and First-Principles Calculations

The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.

[(SiO)TaVCl2Me2]: A Well‐Defined Silica‐Supported Tantalum(V) Surface Complex as Catalyst Precursor for the Selective Cocatalyst‐Free Trimerization of Ethylene

Over the last 50 years, the production of linear a-olefins by ethylene oligomerization has gained increasing interest in industrial and academic research. Currently, numerous studies in this field have been reported and developed into industrial processes: titanium-based catalysts for the dimerization of ethylene (Alpha-butol, IFP); chromium-based catalysts for the trimerization of ethylene (Phillips Petroleum), and more recently chromium-bearing PNP ligand for ethylene tetramerization (SASOL). Along with these nowclassical systems, which require co-catalysts such as MAO, Sen also reported the use of tantalum pentachloride in combination with an alkylating agent, such as SnMe4, ZnMe2, AlMe3, or MeLi. In this case, the catalysts is assumed to be Ta species formed in situ by the reduction of TaMe2Cl3 in the presence of ethylene. In this context, Mashima and coworkers have shown that Ta active species can be alternatively formed by reduction of TaCl5 by 3,6-bis(trimethylsilyl)-1,4-cyclohexadiene derivatives. However the catalytic performances of these systems require the addition of cocatalysts. In several instances, site isolation on the oxide surface has been highly beneficial to the design of efficient catalysts, compared to inactive or rapidly deactivating molecular analogues, which is in particular due to the absence of bimolecular reactions between supported metal complexes. When considering the reaction intermediates, and in the view of developing well-defined silica supported species, we targeted the immobilization of Me3TaCl2 [9] onto an inorganic carrier, silica, by surface organometallic chemistry (SOMC) as a catalyst precursor. Herein, we report the synthesis and the surface characterization of the grafted organometallic species ( SiO)TaCl2Me2 2 in view of its application in ethylene oligomerization. Furthermore, mechanistic studies on this selective catalytic process have been successfully achieved thanks to the dynamic reactor used. They indicate three different pathways for the initiation process. SBA-15 was selected because of its ordered mesoporous network with large surface area (Supporting Information, Figures S1, S2). This porous silica was subjected to partial dehydroxylation under vacuum at 700 8C to afford SBA15(700), which features mostly isolated silanols, as indicated on the IR spectrum by the characteristic sharp peak at 3747 cm 1 (Supporting Information, Figure S3). SBA-15(700) was reacted with TaCl2Me3 1 (Supporting Information, Figure S4), and the resulting powder was characterized to determine the organometallic species on the surface prior to its catalytic application. Elemental analysis gave 14.8% Ta, 1.65% C, and 0.39 % H, with a ratio of Ta/C/Cl = 1:1.97:1.88 (theoretical: Ta/C/Cl = 1:2:2). H-MAS NMR spectrum of 2 unexpectedly displays two major signals at 1.27 ppm and 0.85 ppm with a broad peak at 1.90 ppm and a very weak signal at 0.03 ppm, which is probably due to methane or a trace amount of SiMe (see below for further comments, and the Supporting Information, Figure S5). The NMR signal at 1.9 ppm most likely corresponds to the small amount of unreacted silanols, in agreement with IR spectroscopy results. Two peaks appear at 1.27 and 0.85 ppm that would be consistent with two inequivalent methyl groups coming from one species or indicating the presence of two distinct species. Proton double (DQ)and triple (TQ)-quantum correlation spectra under 22 kHz MAS (Supporting Information, Figure S6) confirm that these two signals correspond to methyl groups, most likely from two different species in view of the absence of correlation of diagonal peak. Autocorrelation peaks are observed on the diagonal of the 2D DQ spectrum for all the protons (notably, this shows that unreacted silanols are in close proximity to each other and most likely located in micropores). A strong autocorrelation [*] Y. Chen, E. Callens, E. Abou-Hamad, J.-M. Basset KAUST Catalysis Center King Abdullah University of Science and Technology Thuwal 23955-6900 (Kingdom of Saudi Arabia) E-mail: jeanmarie.basset@kaust.edu.sa

Triisobutylaluminum: bulkier and yet more reactive towards silica surfaces than triethyl or trimethylaluminum

Triisobutylaluminum reacts with silica yielding three different Al sites according to high-field aluminum-27 NMR and first principle calculations: a quadruply grafted dimeric surface species and two incorporated Al(O)x species (x = 4 or 5). This result is in stark contrast to the bis-grafted species that forms during Et3Al silica grafting. Thus the isobutyl ligands, which render R3Al monomeric, lead to greater reactivity towards the silica surface.

Cyclooctane Metathesis Catalyzed by Silica‐Supported Tungsten Pentamethyl [(SiO)W(Me)5]: Distribution of Macrocyclic Alkanes

Metathesis of cyclic alkanes catalyzed by the new surface complex [(≡SiO)W(Me)5] affords a wide distribution of cyclic and macrocyclic alkanes. The major products with the formula C(n)H(2n) are the result of either a ring contraction or ring expansion of cyclooctane leading to lower unsubstituted cyclic alkanes (5≤n≤7) and to an unprecedented distribution of unsubstituted macrocyclic alkanes (12≤n≤40), respectively, identified by GC/MS and by NMR spectroscopies.

Striking difference between alkane and olefin metathesis using the well-defined precursor [Si–O–WMe5]: indirect evidence in favour of a bifunctional catalyst W alkylidene–hydride

Metathesis of linear alkanes catalyzed by the well-defined precursor (Si–O–WMe5) affords a wide distribution of linear alkanes from methane up to triacontane. Olefin metathesis using the same catalyst and under the same reaction conditions gives a very striking different distribution of linear α-olefins and internal olefins. This shows that olefin and alkane metathesis processes occur via very different pathways.