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A Silica-supported Monoalkylated Tungsten Dioxo Complex Catalyst for Olefin Metathesis Niladri Maity, Samir Barman, Yury Minenkov, Samy Ould-Chikh, Edy Abou-Hamad, Tao Ma, Ziyauddin S. Qureshi, Luigi Cavallo, Valerio D'Elia, Bruce C. Gates, and Jean-Marie Basset ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04304 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018
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A Silica-Supported Monoalkylated Tungsten Dioxo Complex Catalyst for Olefin Metathesis Niladri Maity,a,‡ Samir Barman,a,‡ Yury Minenkov,a Samy Ould-Chikh,a EdyAbou-Hamad,a Tao Ma,b Ziyauddin S. Qureshi,a,c Luigi Cavallo,a Valerio D’Elia,a,d* Bruce C. Gates,b* Jean-Marie Basseta* a
King Abdullah University of Science & Technology, KAUST Catalysis Center (KCC), 23955-
6900 Thuwal, Saudi Arabia b
Department of Chemical Engineering, University of California at Davis, Davis, California 95616,
United States. c
Center of Research Excellence in Petroleum Refining and Petrochemicals, King Fahd University
of Petroleum & Minerals, Dhahran 31261, Saudi Arabia d
Department of Materials Science and Engineering, School of Molecular Science and Engineering,
Vidyasirimedhi Institute of Science and Technology (VISTEC), 21210, Payupnai, WangChan, Rayong, Thailand.
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A well-defined silica-supported monoalkylated tungsten dioxo complex [(Si–O–)W(=O)2(CH2t
Bu)] was prepared by treatment of highly dehydroxylated silica (SiO2-700: silica treated at 700 ºC
under high vacuum) with an ionic precursor complex [NEt4][W(=O)3(CH2-tBu)]. The identity of the resulting neutral monoalkylated tungsten dioxo surface complex was established by means of elemental microanalysis and spectroscopic studies (IR, solid-state NMR, Raman, and X-ray absorption spectroscopies). The supported tungsten complex was found to act as a pre-catalyst for the self-metathesis of 1-octene in a batch reactor. The mechanistic implications of this reaction are discussed with the support of DFT calculations highlighting the potential occurrence of thus-far unexplored mechanistic pathways.
KEYWORDS: olefin metathesis, tungsten dioxo complex, mechanistic study, metallacyclobutane intermediate, tungsten alkylidene
Introduction
Since its discovery in 1955, attributed to researchers at E. I. du Pont de Nemours and Company,12
olefin metathesis has assumed a crucial role in both laboratory and industrial organic synthesis,
being important in industry for the valorization of unsaturated hydrocarbons by the formation of new C=C bonds.3-5 The solid catalysts employed in industrial processes are largely supported oxides of tungsten, molybdenum, or rhenium, with WO3/SiO2 playing a major role.6-7 This catalyst is generally prepared by incipient wetness impregnation of porous silica with tungsten salts, a method that generates a variety of surface species and therefore allows only little control over the structure and the degree of isolation of the supported tungsten complexes.6 In this context, it has been shown that the preparation of silica-supported tungsten oxo pre-catalysts by the surface organometallic chemistry (SOMC) approach8-9 can afford well-defined, isolated, and essentially 2
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identical tungsten oxo alkyl or tungsten oxo alkylidene surface complexes that are able to mimic the active site of WO3/SiO2 (Chart 1, structures a-g).10
Chart 1. Well-defined silica-supported tungsten oxo alkyl (a-d, in order, from references 10, 11, 12, 13) alkylidene (e, f, from references 14, 15, respectively) and cationic alkylidene (g, reference 16) complexes as olefin metathesis pre-catalysts. (h) Tungsten oxo alkylidene intermediate. Proposed isolated tungsten dioxo (i) and oxo (j) structures on silica prepared in reference 17. (k) Proposed structure [(Si–O–)W(=O)2(CH2-tBu)] of the pre-catalyst prepared in this work. 3
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The application of such discrete pre-catalysts in olefin metathesis has led to a dramatic increase in the catalytic turnover relative to that characterizing the traditional WO3/SiO2 catalyst.10,
12, 16
Significantly, these recent advances have been focused almost exclusively on the synthesis of mono-oxo tungsten complexes. This emphasis is justified by the necessity to preserve available coordination sites for alkyl substituents at the metal center for the generation of the crucial tungsten alkylidene intermediate after grafting of the precursor to the silica surface.10 But we stress that the active site of the WO3/SiO2 catalyst, before activation, has been proposed to consist predominantly of a tungsten dioxo surface complex ((Si-O-)2W(=O)2, Chart 1, i).6,17 From this complex, the ((Si-O-)2W=O(=CHR)) pre-catalyst (Chart 1, h),12,
18
a precursor of the crucial
metallacyclobutane intermediate,19-20 could be generated via a pseudo-Wittig reaction with the reactant olefin during catalyst pre-activation by the formation of oxygenates;21-24 the latter process is not being fully understood.6, 25 Therefore, we posited that the grafting or generation of welldefined, isolated tungsten dioxo complexes on the silica surface could help to advance the understanding of this class of catalyst, in prospect representing (a) an original synthetic approach for the synthesis and investigation of olefin metathesis catalysts and (b) an opportunity to access yet unexplored surface structures and gain deeper insight into the mechanistic pathways of active species generation. To date, the preparation of heterogeneous catalysts containing exclusively well-defined tungsten dioxo complexes has not been reported. Mougel et al. recently grafted [W(=O)2(OSi(O-tBu)3)2(DME)] on dehydroxylated silica affording i (Chart 1) but it was formed in a 1:1 mixture with a tungsten mono-oxo complex j.17 In planning to synthesize exclusively surface tungsten bis-oxo moieties, we observed the limitation that there are only a few reported tungsten dioxo precursors. Among them, the commercially 4
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available WO2Cl2 is insoluble in most solvents and lacks any organic component. Therefore, the characterization of the products of grafting of this compound would be hindered by the lack of opportunity for application of solid state (SS) 1H and 13C NMR spectroscopies, which, on par with IR spectroscopy, are powerful components of the classical toolkit of SOMC.26-27 Other reported tungsten dioxo complexes incorporate large chelating ligands that could deactivate the metal center and/or alter its reactivity in olefin metathesis.28 Therefore, we turned our attention to the tungsten trioxo complex [NEt4][W(=O)3(CH2-tBu)](1). In this study we demonstrate that the grafting of the [NEt4][WO3(CH2-tBu)] (1)29 precursor on highly dehydroxylated silica (SiO2-700; silica that had been partially dehydroxylated at 700 C under a dynamic vacuum of 10-5 Torr) leads to novel monoalkylated tungsten dioxo surface complexes (2) as the only grafting products. We show that this surface moiety is active in olefin catalysis at moderate temperature and we investigate the formation of the crucial tungsten oxo alkylidene intermediate with the support of DFT calculations by exploring new possible mechanistic pathways in olefin metathesis.
Experimental and computational methods General information. All air and moisture sensitive samples were handled in a glove box and by standard Schlenk techniques under argon atmosphere. The treatments involving the surface organometallic complexes in this study were carried out using high-vacuum lines (
