Dendritic polarizing agents for DNP SENS.

Chemical Science View Article Online Open Access Article. Published on 22 August 2016. Downloaded on 17/03/2017 15:41:58. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. EDGE ARTICLE View Journal | View Issue Dendritic polarizing agents for DNP SENS† Cite this: Chem. Sci., 2017, 8, 416 Wei-Chih Liao,a Ta-Chung Ong,a David Gajan,b Florian Bernada,c Claire Sauvée,c Maxim Yulikov,a Margherita Pucino,a Roman Schowner,d Martin Schwarzwälder,a Michael R. Buchmeiser,d Gunnar Jeschke,a Paul Tordo,c Olivier Ouari,c Anne Lesage,b Lyndon Emsleye and Christophe Copéret*a Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) is an effective method to significantly improve solid-state NMR investigation of solid surfaces. The presence of unpaired electrons (polarizing agents) is crucial for DNP, but it has drawbacks such as leading to faster nuclear spin relaxation, or even reaction with the substrate under investigation. The latter can be a particular problem for heterogeneous catalysts. Here, we present a series of carbosilane-based dendritic polarizing agents, Received 15th July 2016 Accepted 19th August 2016 in which the bulky dendrimer can reduce the interaction between the solid surface and the free radical. DOI: 10.1039/c6sc03139k We thereby preserve long nuclear T0 2 of the surface species, and even successfully enhance a reactive www.rsc.org/chemicalscience heterogeneous metathesis catalyst. Introduction Solid-state nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful tools used to probe surface structures at the atomic level.1 However, the application of NMR is impeded by its inherently low sensitivity, which is compounded by the small fraction of sites of interest present on solid surfaces. Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) has recently emerged as an effective method greatly increasing sensitivity for surface species.2–7 It relies on a transfer of the large electron polarization of a paramagnetic dopant to the nearby nuclei.8 A persistent radical, e.g. a nitroxide biradical, is typically used as the polarizing agent (PA). The PA is usually dissolved in a glass-forming solvent and then introduced onto the solid under analysis by incipient wetness impregnation.2 The sample is then usually cooled to 100 K or lower, and microwaves are applied to saturate the electron paramagnetic resonance (EPR) transition, which induces, predominantly through the cross effect mechanism,9–11 the transfer of the electron polarization to the protons of the surrounding solvent molecules. This is typically followed by a cross-polarization12 (CP) step to the heteronuclei on the solid a Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, 8093 Zürich, Switzerland. E-mail: b Centre de RMN à Très Hauts Champs, Institut de Sciences Analytiques (CNRS/ENS Lyon/UCB Lyon 1), Université de Lyon, 69100 Villeurbanne, France c Aix-Marseille Univ, CNRS, ICR UMR 7273, Marseille, 13013, France d Institut für Polymerchemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany e Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland † Electronic supplementary 10.1039/c6sc03139k information 416 | Chem. Sci., 2017, 8, 416–422 (ESI) available. See DOI: surface, resulting in a signicant sensitivity increase of their NMR resonances. With sensitivity gains of one to two orders of magnitude, the signal averaging time needed is dramatically reduced, and the detailed characterization of surface species becomes possible. Notably, the improvement in sensitivity enables advanced two-dimensional (2D) NMR experiments that are not practicable using conventional solid-state NMR spectroscopy.6,13–20 Over the last decade, much effort has been devoted to developing free radicals that yield larger DNP enhancements. In 2004, Hu et al.21 demonstrated that binitroxide radicals were highly efficient for cross effect DNP due to large intramolecular dipolar couplings between the two tethered nitroxide radicals. In 2012, Zagdoun et al.22 showed that binitroxide radicals could be engineered to have longer T1e, based on bulky, rigid skeletons,23 and that this led to unprecedented DNP enhancements. Michaelis et al.,24 Kubicki et al.25 and Sauvée et al.26 have recently reported comprehensive studies, in which many binitroxide radicals were investigated in order to rationalize the effect of biradical structures on DNP performances. Note that hydrophobic radicals have been solubilized successfully in aqueous solutions by incorporating them into cyclic oligosaccharides or micelles.27,28 Using the most recently developed binitroxides, including TEKPol- and AMUPol-like biradicals, DNP enhancements of between 200 and 300 have been achieved in bulk solutions at 9.4 T and 100 K. These PAs have been successfully employed to polarize solutions to characterize the surfaces of a wide range of materials including single-site catalysts,16 silicabased materials,29–31 zeolites,14,32,33 metal oxides,19,20 supported nanoparticles34 and colloidal nanoparticles.35 In this approach, however, free radicals are inevitably in proximity to the surface, which potentially leads to paramagnetic quenching of the NMR signal, to faster transverse This journal is © The Royal Society of Chemistry 2017 View Article Online Open Access Article. Published on 22 August 2016. Downloaded on 17/03/2017 15:41:58. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Edge Article relaxation of the surface nuclei, or most problematically to reaction with the substrate.36,37 Most DNP studies reported so far have focused on relatively stable materials, with only a few targeting the characterization of reactive surface species.16,29,34,38 In particular, in the eld of heterogeneous catalysis, Lewis acidic metal centers can interact with the free radicals.39–43 For instance, bulky PAs are needed to probe the structure of active sites in zeolites because they cannot enter the porous network and interact with the active sites.33 It would thus be desirable to develop polarizing agents that are compatible with non-porous and large-pore-size reactive materials. Here we present the syntheses (Scheme 1) and EPR characterization of dendritic PAs combining a monoradical, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), or biradicals (bTUrea and PyPol)44 with core-functionalized hydrophobic carbosilane dendrimers.45,46 We show in particular that the dendritic PA with the bulkiest and most rigid structure displays the minimum interference with the surface sites, and preserves the coherence life time (T0 2) and has the best DNP SENS performance (313C CP ¼ 128). We also illustrate the advantage of dendritic PAs by characterizing a reactive material, a recently developed cationic heterogeneous alkene metathesi (...truncated)