Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy.

Chemical Science View Article Online Open Access Article. Published on 15 August 2016. Downloaded on 24/02/2017 15:28:29. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. EDGE ARTICLE Cite this: Chem. Sci., 2017, 8, 284 View Journal | View Issue Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy† Eva Pump,a Jasmine Viger-Gravel,b Edy Abou-Hamad,a Manoja K. Samantaray,a Bilel Hamzaoui,a Andrei Gurinov,c Dalaver H. Anjum,c David Gajan,d Anne Lesage,d Anissa Bendjeriou-Sedjerari,*a Lyndon Emsley*b and Jean-Marie Basset*a Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) is an emerging technique that allows access to high-sensitivity NMR spectra from surfaces. However, DNP SENS usually requires the use of radicals as an exogenous source of polarization, which has so far limited applications for organometallic surface species to those that do not react with the radicals. Here we show that reactive surface species can be studied if they are immobilized inside porous materials with suitably small Received 30th May 2016 Accepted 12th August 2016 windows, and if bulky nitroxide bi-radicals (here TEKPol) are used as the polarization source and which cannot enter the pores. The method is demonstrated by obtaining significant DNP enhancements from DOI: 10.1039/c6sc02379g highly reactive complelxes [(^Si–O–)W(Me)5] supported on MCM-41, and effects of pore size (6.0, 3.0 www.rsc.org/chemicalscience and 2.5 nm) on the performance are discussed. Introduction Heterogeneous catalysis is ubiquitous today and is central to solving many of the key problems facing chemistry including energy and environmental issues that contribute to a sustainable world.1,2 However, many heterogeneous catalysts contain numerous types of active sites which makes it difficult to reect the intrinsic efficiency of the catalyst. To develop structure– activity relations that would lead to “Catalysis by Design”, Surface Organometallic Chemistry (SOMC) has been introduced to provide a single-site strategy3–5 by creating well dened surface organometallic fragments (SOMF) that are presumed to be part of the catalytic cycle.5 To achieve this goal, the surface complexes need to be unambiguously characterized, usually through advanced spectroscopic techniques such as FT-IR, EXAFS and multi-dimensional solid-state NMR spectroscopy. By combining these tools, structure–activity relationships can be determined.4,5 For example, in the discovery of alkane a King Abdullah University of Science and Technology (KAUST), KAUST Catalysis Center (KCC), Thuwal, 23955-6900, Saudi Arabia. E-mail: ; b Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. E-mail: lyndon.emsley@ep.ch c Imaging and Characterization Lab. King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia d Institut de Sciences Analytiques (CNRS/ENS-Lyon/UCB-Lyon 1), Université de Lyon, Centre de RMN à Très Hauts Champs, 69100 Villeurbanne, France † Electronic supplementary information (ESI) available: Preparation of the samples; nitrogen absorption/desorption isotherms, FT-IR spectroscopy, DNP SENS spectroscopy and DFT calculations. See DOI: 10.1039/c6sc02379g 284 | Chem. Sci., 2017, 8, 284–290 metathesis6 over tantalum hydride, and its subsequent improvement7,8 with [(^Si–O–)W(Me)5] supported on silica, solid-state NMR spectroscopy in particular proved to be an essential technique to characterise surface structures obtained using the SOMC methodology.9–12 However, the low sensitivity of NMR is a major handicap. As an illustration, the identication of surface carbene or carbyne (Ta or W) complexes using 13 C CP MAS NMR spectroscopy is challenging due to a low signal-to-noise ratio, and this despite acquisition times for onedimensional spectra that can be several days.7,8,13,14 To address these issues, the most common route is to introduce carbon-13 enriched complexes, ideally selectively labelled at the a position of the metal center, but the synthesis of such compounds is extremely difficult and time consuming. Dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) has been recently introduced to solve these sensitivity concerns.17–22 In these experiments the material is impregnated with a solution containing stable free radicals (usually binitroxides) and polarization is transferred from the unpaired electrons of the radical to the surrounding nuclei (usually protons) by in situ microwave (mwave) irradiation followed by spin diffusion and cross-polarisation (CP) to the nuclei of interest.22 On commercial instruments, this is performed at temperatures around 100 K and under magic angle spinning (MAS) conditions (typically at 8–12 kHz). This recently introduced method has been very successful for the characterization of a broad panoply of materials ranging from inorganic materials to pharmaceuticals and organic materials.15–30 However, so far the method is problematic when being applied to surface complexes on silica that react with the This journal is © The Royal Society of Chemistry 2017 View Article Online Open Access Article. Published on 15 August 2016. Downloaded on 24/02/2017 15:28:29. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Edge Article radicals. This precludes the characterization of some of the most interesting catalytic species. There is thus a need for a non-destructive strategy for the DNP SENS characterization of SOMC complexes. An important exception has been observed where reactive zirconium amides immobilized on mesoporous silica did not react with the nitroxyl radicals in DNP experiments, although they would normally be expected to do so.31 Here, we propose a new strategy which is based on avoiding direct contact between the active catalytic site and the nitroxide radical by: (a) immobilizing the surface complexes inside mesoporous materials with small windows and, (b) using bulky radicals which presumably will not enter the cavities, but could transfer their polarization through the solvent which could be small enough to penetrate inside the mesopores. Indeed, polarization relayed by spin diffusion has already been observed in porous systems when the radicals were excluded by size effects.29,30,32 As a proof of concept, we investigate here two types of highly sensitive supported tungsten complexes: W(^CtBu)(CH2tBu)3 (A)14 and W(Me)6 (B)7 on different mesoporous materials (SBA15 and MCM-41) using DNP SENS. Fine-tuning of the mesoporous pore diameter (6.0, 3.0 and 2.5 nm) allows us to preserve the integrity of the catalytic sites by sterically excluding the large biradical nitroxide TEKPol.33 The surface complexes are polarized by spin diffusion from the radical through the impregnating solution in the pores, and we obtain signal enha (...truncated)