Unearthing a Well-Defined Highly Active Bimetallic W/Ti Precatalyst Anchored on a Single Silica Surface for Metathesis of Propane.

Two compatible organometallic complexes, W(Me)6 (1) and TiNp4 (2), were successively anchored on a highly dehydroxylated single silica support (SiO2-700) to synthesize the well-defined bimetallic precatalyst [(≡Si-O-)W(Me)5(≡Si-O-)Ti(Np)3] (4). Precatalyst 4 was characterized at the molecular level using advanced surface organometallic chemistry (SOMC) characterization techniques. The strong autocorrelation observed between methyl of W and Ti in 1H-1H multiple-quantum NMR spectra demonstrates that W and Ti species are in close proximity to each other. The bimetallic precatalyst 4, with a turnover number (TON) of 9784, proved to be significantly more efficient than the silica-supported monometallic catalyst [(≡Si-O-)W(Me)5] (3), with a TON of 98, for propane metathesis at 150 °C in a flow reactor. The dramatic improvement in the activity signifies the cooperativity between Ti and W and indicates that the key step of alkane metathesis (C-H bond activation followed by ß-H elimination) occurs on Ti, followed by olefin metathesis, which occurs on W. We have demonstrated the influence and importance of proximity of Ti to W for achieving such a significantly high activity. This is the first report demonstrating the considerably high activity (TON = 9784) in propane metathesis at moderate temperature (150 °C) using a well-defined bimetallic system prepared via the SOMC approach.

SOMC-Designed Silica Supported Tungsten Oxo Imidazolin-2-iminato Methyl Precatalyst for Olefin Metathesis Reactions.

Synthesis, structure, and olefin metathesis activity of a surface complex [(≡Si-O-)W(═O)(CH3)2-ImDippN] (4) (ImDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) supported on silica by a surface organometallic chemistry (SOMC) approach are reported. The reaction of N-silylated 2-iminoimidazoline with tungsten(VI) oxytetrachloride generated the tungsten oxo imidazolin-2-iminato chloride complex [ImDippNW(═O)Cl3] (2). This was grafted on partially dehydroxylated silica pretreated at 700 °C (SiO2-700) to afford a well-defined monopodal surface complex [(≡Si-O-)W(═O)Cl2-ImDippN] (3). 3 underwent alkylation by ZnMe2 to produce [(≡Si-O-)W(═O)(CH3)2-ImDippN] (4). The alkylated surface complex was thoroughly characterized by solid-state NMR, elemental microanalysis, Raman, FT-IR spectroscopies, and XAS analysis. 4 proved to be an active precatalyst for self-metathesis of terminal olefins such as propylene and 1-hexene.

Synergy between Two Metal Catalysts: A Highly Active Silica-Supported Bimetallic W/Zr Catalyst for Metathesis of n-Decane.

A well-defined, silica-supported bimetallic precatalyst [≡Si-O-W(Me)5≡Si-O-Zr(Np)3] (4) has been synthesized for the first time by successively grafting two organometallic complexes [W(Me)6 (1) followed by ZrNp4 (2)] on a single silica support. Surprisingly, multiple-quantum NMR characterization demonstrates that W and Zr species are in close proximity to each other. Hydrogenation of this bimetallic catalyst at room temperature showed the easy formation of zirconium hydride, probably facilitated by tungsten hydride which was formed at this temperature. This bimetallic W/Zr hydride precatalyst proved to be more efficient (TON = 1436) than the monometallic W hydride (TON = 650) in the metathesis of n-decane at 150 °C. This synergy between Zr and W suggests that the slow step of alkane metathesis is the C-H bond activation that occurs on Zr. The produced olefin resulting from a ß-H elimination undergoes easy metathesis on W.

Alkane metathesis with the tantalum methylidene [(≡SiO)Ta(═CH2)Me2]/[(≡SiO)2Ta(═CH2)Me] generated from well-defined surface organometallic complex [(≡SiO)Ta(V)Me4].

By grafting TaMe5 on Aerosil700, a stable, well-defined, silica-supported tetramethyl tantalum(V) complex, [(≡SiO)TaMe4], is obtained on the silica surface. After thermal treatment at 150 °C, the complex is transformed into two surface tantalum methylidenes, [(≡SiO)2Ta(═CH2)Me] and [(≡SiO)Ta(═CH2)Me2], which are active in alkane metathesis and comparable to the previously reported [(≡SiO)2TaHx]. Here we present the first experimental study to isolate and identify a surface tantalum carbene as the intermediate in alkane metathesis. A systematic experimental study reveals a new reasonable pathway for this reaction.

Effect of support on metathesis of n-decane: drastic improvement in alkane metathesis with WMe5 linked to silica-alumina.

[WMe6 ] (1) supported on the surface of SiO2 -Al2 O3(500) (2) has been extensively characterized by solid-state NMR spectroscopy, elemental analysis, and gas quantification, which clearly reveal the formation of a mixture of monopodal and bipodal species with the migration of methyl from W to Al. The supported species SiO2 -Al2 O3(500) (2) transformed at 120 °C into two types of carbynic centers, one of which is cationic and the other neutral. These species are very efficient for the metathesis of n-decane. Comparison with already-synthesized neutral bipodal tungsten indicates that the high increase in activity is due to the cationic character of the grafted tungsten.

Single site silica supported tetramethyl niobium by the SOMC strategy: synthesis, characterization and structure-activity relationship in the ethylene oligomerization reaction.

A silica supported tetramethyl niobium complex [([triple bond, length as m-dash]SiO)NbMe4] 2 has been isolated by the surface alkylation of [([triple bond, length as m-dash]SiO-)NbCl3Me] 1 with dimethyl zinc in pentane. 1 can be easily synthesized by grafting NbCl3Me2 onto the surface of partially dehydroxylated silica by the SOMC strategy. Precise structural analysis was carried out using FTIR, advanced solid state NMR, elemental analysis and mass balance techniques (gas quantification after treating 2 with degassed water). Complex 1 was found to be active in the ethylene oligomerization reaction, producing up to C30, whereas to our surprise complex 2 selectively dimerized ethylene into 1-butene in the absence of a co-catalyst at the same conversion level.

Synthesis and characterization of a homogeneous and silica supported homoleptic cationic tungsten(vi) methyl complex: application in olefin metathesis.

A method for the synthesis of a homogeneous cationic tungsten(vi)pentamethyl complex [(WMe5)+(C6F5)3BMe-] from neutral tungstenhexamethyl (WMe6) and a silica supported cationic tungstentetramethyl complex [([triple bond, length as m-dash]Si-O-)WMe4+ (C6F5)3BMe-] from a neutral silica supported tungstenpentamethyl complex [([triple bond, length as m-dash]Si-O-)WMe5] is described. In both cases a direct demethylation using the B(C6F5)3 reagent was used. The aforesaid complexes were characterized by liquid or solid state NMR spectroscopy. Interestingly, the homogeneous cationic complex [(WMe5)+(C6F5)3BMe-] shows moderate activity whereas the supported cationic complex [([triple bond, length as m-dash]Si-O-)WMe4+(C6F5)3BMe-] exhibits good activity in olefin metathesis reactions.

Low temperature activation of methane over a zinc-exchanged heteropolyacid as an entry to its selective oxidation to methanol and acetic acid.

A Zn-exchanged heteropolyacid supported onto silica (Zn-HPW/SiO2) activates methane at 25 °C into Zn-methyl. At higher temperatures and with CH4/O2 or CH4/CO2, it gives methanol and acetic acid respectively.

The use of a well-defined surface organometallic complex as a probe molecule: [(≡SiO)Ta(V)Cl2Me2] shows different isolated silanol sites on the silica surface.

Ta(V)Cl2Me3 reacts with silica(700) and produces two different [(≡SiO)Ta(V)Cl2Me2] surface organometallic species, suggesting a heterogeneity of the highly dehydroxylated silica surface, which was studied with a combined experimental and theoretical approach.