Spectroscopie RMN en phase solide de haute sensibilité pour les surfaces catalytiques

The vast majority of chemical reactions used today in industry relies on heterogenous catalysts. Over the last three decades, many efforts were devoted at preparing well-defined, single-site supported organometallic catalysts that would combine the advantages of molecular complexes in terms of productivity, rates and selectivity, with an ease of separation and recyclability. The rational design of such systems depends on our ability to characterize with a high level of certainty their atomic-scale structure.Among the panoply of analytical techniques that can be applied to get a detailed knowledge of the active center, solid-state Nuclear Magnetic Resonance (NMR) spectroscopy under Magic Angle Spinning (MAS) plays an essential role. This spectroscopy suffers however from one key drawback, its inherently low sensitivity. This sensitivity issue is further exacerbated when the substrate of interest is located on a surface. The recent advent of hyperpolarization techniques and more specifically of Dynamic Nuclear Polarization (DNP) opens new opportunities for a highly detailed structural characterization of active sites in heterogeneous catalysts. The core of my PhD has concerned the implementation of advanced NMR methods of high sensitivity, namely DNP enhanced NMR spectroscopy and proton NMR spectroscopy, to unravel the atomic-scale structure of single-site, well defined heterogeneous catalysts. Three systems were investigated, all involved in key catalytic reactions. First, I elucidated the three-dimensional structure of an Iridium-NHC complex supported onto mesoporous silica. DNP enhanced NMR was applied to determine the structure of the surface species at various steps of the synthetic pathway. I showed that the implementation of conventional and selective REDOR (Rotational Echo Double Resonance) experiments between 29Si, 13C and 15N nuclei enables the measurement of distance constraints corresponding specifically to the Ir-NHC complex despite the presence of unreacted spectator species. The result is a well-defined structure where the Ir atom points to the porous cavity. This unexpected feature was interpreted by the residual presence of a cyclooctadiene (COD) ligand, coordinating the molecular Ir complex which could be confirmed by Extended X-Ray Absorption Fine Structure (EXAFS). I also demonstrated the use of static DNP-enhanced wide-line 195Pt NMR spectroscopy in combination with room-temperature proton-detected experiments to provide unique insight into the surface structure of a Platinum organometallic complex, representative of Pt single-site catalysts. Finally, I implemented proton double- and triple-quantum NMR experiments to unravel the structure of a new family of hetero-bimetallic surface complexes. The approach allowed the unambiguous determination of the number of hydride protons surrounding the metal centers. Different synthetic pathways yielded distinct surface structures that were elucidated by NMR in combination with IR spectroscopy and X-ray methods. Overall, several groundbreaking NMR methodologies were developed that are expected to become more widely implemented to guide the rational design of immobilized catalystLa grande majorité des réactions chimiques utilisées aujourd'hui dans l'industrie repose sur des catalyseurs hétérogènes. Au cours des trois dernières décennies, de nombreux efforts ont été consacrés à la préparation de catalyseurs organométalliques supportés bien définis, qui allient les avantages des complexes moléculaires en termes de rendement et de sélectivité, avec une facilité de séparation et de recyclage. La conception rationnelle de tels systèmes dépend de notre capacité à caractériser avec un haut niveau de certitude leur structure à l'échelle atomique. Parmi la panoplie de techniques analytiques qui peuvent être appliquées pour obtenir une connaissance détaillée du centre actif, la spectroscopie par résonance magnétique nucléaire (RMN) à l'état solide sous rotation à l'angle magique (MAS) joue un rôle essentiel. Cette spectroscopie souffre cependant d'un inconvénient majeur, sa sensibilité intrinsèquement faible. Ce problème de sensibilité est encore exacerbé lorsque le substrat d'intérêt est situé sur une surface. L'avènement récent des techniques d'hyperpolarisation et plus particulièrement de la Polarisation Dynamique Nucléaire (DNP) ouvre de nouvelles opportunités pour une caractérisation structurale très détaillée des sites actifs dans des catalyseurs hétérogènes.Le cœur de ma thèse a porté sur la mise en œuvre de méthodes RMN avancées de haute sensibilité, à savoir la spectroscopie RMN améliorée par DNP et la spectroscopie RMN du proton, pour démêler la structure à l'échelle atomique de catalyseurs hétérogènes bien définis en un seul site. Trois systèmes ont été étudiés, tous impliqués dans des réactions catalytiques clés. Tout d'abord, j'ai élucidé la structure tridimensionnelle d'un complexe Iridium-NHC supporté sur de la silice mésoporeuse. La RMN améliorée par DNP a été appliquée pour déterminer la structure des espèces de surface à différentes étapes de la voie de synthèse. J'ai montré que la mise en œuvre d'expériences REDOR (Rotational Echo Double Resonance) sélectives entre les noyaux 29Si, 13C et 15N permet de mesurer des contraintes de distance correspondant spécifiquement au complexe Ir-NHC malgré la présence d'espèces spectatrices n'ayant pas réagi. Le résultat est une structure bien définie, où l'atome d'iridium pointe vers la cavité poreuse. Cette caractéristique inattendue a été interprété par la présence résiduelle d'un ligand cyclooctadiène (COD), coordonnant le centre métallique, ce qui a été confirmé par les données Extended X-Ray Absorption Fine Structure (EXAFS) et les analyses chimiques. J'ai également démontré l'utilisation de la spectroscopie RMN statique du 195Pt améliorée par DNP qui combinée avec des expériences de détection des protons à température ambiante permet de confirmer la structure de surface d'un complexe organométallique de platine, représentatif de catalyseurs bien défini de Pt. Enfin, j'ai mis en œuvre des expériences de RMN proton double et triple quanta pour démêler la structure d'une nouvelle famille de complexes de surface bimétalliques. L'approche a permis de déterminer sans ambiguïté le nombre de protons hydrure entourant les centres métalliques. Différentes voies de synthèse ont donné des structures de surface distinctes qui ont été élucidées par RMN en combinaison avec de la spectroscopie infrarouge et des méthodes de diffraction des rayons X. Plusieurs méthodologies RMN innovantes ont été développées qui devraient devenir plus largement utilisées pour guider la conception rationnelle de catalyseurs hétérogène

Polarization Amplification in Dynamic Nuclear Polarization Magic-Angle Spinning Solid-State Nuclear Magnetic Resonance by Solubilizing Traditional Ionic Salts

International audienceDynamic nuclear polarization can improve the sensitivity of magic-angle spinning solid-state NMR experiments by 1–2 orders of magnitude. In aqueous media, experiments are usually performed using the so-called DNP juice, a glycerol-d8/D2O/H2O mixture (60/30/10, v/v/v) that can form a homogeneous glass at cryogenic temperatures. This acts as a cryoprotectant and prevents phase separation of the paramagnetic polarizing agents (PAs) that are added to the mixture to provide the source of electron spin polarization required for DNP. Here, we show that relatively high 1H DNP enhancements (∼60) can also be obtained in water without glycerol (or other glass forming agents) simply by dissolving high concentrations of electrolytes (such as NaCl or LiCl), which perturb the otherwise unavoidable ice crystallization observed upon cooling, thereby reducing PA phase separation and restoring DNP efficiency

Metal–Metal Synergy in Well-Defined Surface Tantalum–Iridium Heterobimetallic Catalysts for H/D Exchange Reactions

International audienceA novel heterobimetallic tantalum/iridium hydrido complex, [{Ta(CH2 t Bu)3}{IrH2(Cp*)}] 1, featuring a very short metal-metal bond, has been isolated through an original alkane elimination route from Ta(CH t Bu)(CH2 t Bu)3 and Cp*IrH4. This molecular precursor has been used to synthesize well-defined silica-supported low-coordinate heterobime-tallic hydrido species [≡SiOTa(CH2 t Bu)2{IrH2(Cp*)}] 5 and [≡SiOTa(CH2 t Bu)H{IrH2(Cp*)}] 6 using a surface organometallic chemistry approach (SOMC). The SOMC methodology prevents undesired dimerization as encountered in solution and leading to a tetranuclear species [{Ta(CH2 t Bu)2}(Cp*IrH)]2, 4. This approach therefore allows access to unique low-coordinate species not attainable in solution. These original supported Ta/Ir species exhibit drastically enhanced catalytic performances in H/D exchange reactions with respect to (i) monometallic analogues as well as (ii) homogeneous systems. In particular, material 6 promotes the H/D exchange between fluorobenzene and C6D6 or D2 as deuterium sources with excellent productivity (TON up to 1422; TOF up to 23.3 h-1) under mild conditions (25°C, subatmospheric D2 pressure) without any additives

Speciation and Structures in Pt Surface Sites Stabilized by N-Heterocyclic Carbene Ligands Revealed by Dynamic Nuclear Polarization Enhanced Indirectly Detected 195 Pt NMR Spectroscopic Signatures and Fingerprint Analysis

N-Heterocyclic carbenes (NHCs) are widely used ligands in transition metal catalysis. Notably, they are increasingly encountered in heterogeneous systems. While a detailed knowledge of the possibly multiple metal environments would be essential to understand the activity of metal-NHC-based heterogeneous catalysts, only a few techniques currently have the ability to describe with atomic-resolution structures dispersed on a solid support. Here, we introduce a new DNP surface enhanced solid-state NMR approach that, in combination with advanced DFT calculations, allows the structure characterization of isolated silica-supported Pt-NHC sites. Notably, we demonstrate that the signal amplification provided by DNP in combination with fast magic angle spinning enables the implementation of sensitive 13 C-195 Pt correlation experiments. By exploiting 1 J(13 C-195 Pt) couplings, 2D NMR spectra were acquired revealing two types of Pt sites. For each of them, 1 J(13 C-195 Pt) values were determined as well as 195 Pt chemical shift tensor parameters. To interpret the NMR data, DFT calculations were performed on an extensive library of molecular Pt-NHC complexes. While one surface site was identified as a bis-NHC compound, the second site most likely contains a bidentate 1,5 cyclooctadiene ligand, pointing to various parallel grafting mechanisms. The methodology described here represents a new step forward in the atomic-level description of catalytically relevant surface metal-NHC complexes. In particular, it opens up innovative avenues for exploiting the spectral signature of platinum, one of the most widely used transition metals in catalysis, but whose use for solid-state NMR remains difficult. Our results also highlight the sensitivity of 195 Pt NMR parameters to slight structural changes

How Halogenation Impacts the Polymer Backbone Conformation: Learning from Combination of Solid-state MAS NMR and X-Ray Scattering

International audienceOver the last decade, halogenated semiconducting polymers have attracted considerable interest due to their outstanding optoelectronic properties. Thus, most today's OPV benchmark organic semiconductors are halogenated materials, either electron donor polymers or non-fullerene acceptor (NFA) small molecules. However, the nature and position of the substituted halogen atoms in halogenated semiconducting polymers impact, through self-assembly modification, on their optoelectronic properties in a way which is 2 difficult to predict. Yet, the solid-state self-assembling of this materials has been shown to be a key parameter towards high charge transport properties and photovoltaic efficiencies. In this context, there is still a need to develop analytical methods that will enable an atomicscale structural characterization of these materials as function of the halogenation. In this manuscript, we explore the solid-state nuclear magnetic resonance (NMR) under magic angle spinning (MAS) as a tool to investigate the local structure and supramolecular organization of a series of conjugated polymers, specially designed for this study. Through a comprehensive study using complementary techniques including MAS-NMR, small and wideangle X-ray scattering (SWAXS) and molecular modelling investigations, we have definitely succeeded in determining the molecular conformation of these polymers in relation to their chemical composition

Stepwise Construction of Silica-supported Tantalum/Iridium Heteropolymetallic Catalysts Using Surface Organometallic Chemistry

International audienceA stepwise surface organometallic chemistry (SOMC) methodology consisting in using a silica-supported tantalum species, [(≡SiO)Ta(CH t Bu)(CH2 t Bu)2], as a reactive center to coordinate an iridium site, was developed to construct tantalum/iridium heterobimetallic edifices. The resulting material, MAT-2, exhibits enhanced catalytic performances-both in H/D isotopic exchange and alkane metathesis reactions-in comparison to MAT-1, which was prepared from the direct grafting on silica of a well-defined heterobimetallic Ta/Ir complex. We projected that the difference in catalytic activity was due to the presence of distinct active sites and we used a combination of advanced spectroscopic methods (IR, solid-state NMR and XAS spectroscopies) as well as modeling (computational studies and molecular models) to identify the structure of these surface species. These investigations point towards the presence of three types of active sites at the surface of MAT-2: the heterobimetallic surface species, [≡SiOTa(CH2 t Bu)2{IrH2(Cp*)}] 2-s, which is also found in MAT-1, as well as an unanticipated heterotrimetallic species, [≡SiOTa(CH2 t Bu)2{IrH2(Cp*)}], 3-s along with some unreacted monometallic Ta sites [(≡SiO)Ta(CH t Bu)(CH2 t Bu)2], 1-s. The well-defined trimetallic surface species 3-s was independently prepared and characterized to support this hypothesis. This study highlights the importance of the synthetic methodology used for the preparation of heterobimetallic species through SOMC, and the difficulty to obtain true single-sites surface species

Atomic-Scale Description of Interfaces between Antigen and Aluminum-Based Adjuvants Used in Vaccines by Dynamic Nuclear Polarization (DNP) Enhanced NMR Spectroscopy

The addition of aluminum-based adjuvants in vaccines enhances the immune response to antigens. The strength of antigen adsorption on adjuvant gels is known to modulate vaccine efficacy. However, a detailed understanding of the mechanisms of interaction between aluminum gels and antigens is still missing. Herein, a new analytical approach based on dynamic nuclear polarization (DNP) enhanced NMR spectroscopy under magic angle spinning (MAS) is implemented to provide a molecular description of the antigen-adjuvant interface. This approach is demonstrated on hepatitis B surface antigen particles in combination with three aluminum gels obtained from different suppliers. Both noncovalent and covalent interactions between the phospholipids of the antigen particles and the surface of the aluminum gels are identified by using MAS DNP (NMRAl)-Al-27 and(31)P correlation experiments. Although covalent interactions were detected for only one of the formulations, dipolar recoupling rotational echo adiabatic passage double resonance (REAPDOR) experiments reveal significant differences in the strength of weak interactions

How Halogenation Impacts the Polymer Backbone Conformation: Learning from Combination of Solid-state MAS NMR and X-Ray Scattering

International audienceOver the last decade, halogenated semiconducting polymers have attracted considerable interest due to their outstanding optoelectronic properties. Thus, most today's OPV benchmark organic semiconductors are halogenated materials, either electron donor polymers or non-fullerene acceptor (NFA) small molecules. However, the nature and position of the substituted halogen atoms in halogenated semiconducting polymers impact, through self-assembly modification, on their optoelectronic properties in a way which is 2 difficult to predict. Yet, the solid-state self-assembling of this materials has been shown to be a key parameter towards high charge transport properties and photovoltaic efficiencies. In this context, there is still a need to develop analytical methods that will enable an atomicscale structural characterization of these materials as function of the halogenation. In this manuscript, we explore the solid-state nuclear magnetic resonance (NMR) under magic angle spinning (MAS) as a tool to investigate the local structure and supramolecular organization of a series of conjugated polymers, specially designed for this study. Through a comprehensive study using complementary techniques including MAS-NMR, small and wideangle X-ray scattering (SWAXS) and molecular modelling investigations, we have definitely succeeded in determining the molecular conformation of these polymers in relation to their chemical composition

The Structure of Molecular and Surface Platinum Sites Determined by DNP-SENS and Fast MAS 195Pt Solid-State NMR Spectroscopy

The molecular level characterization of heterogeneous catalysts is challenging due to the low concentration of surface sites and the lack of techniques that can selectively probe the surface of a heterogeneous material. Here, we report the joint application of room temperature proton-detected NMR spectroscopy under fast magic angle spinning (MAS) and dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS), to obtain the 195Pt solid-state NMR spectra of a prototypical example of highly dispersed Pt sites (single site or single atom), here prepared via surface organometallic chemistry, by grafting [(COD)Pt(OSi(OtBu)3)2] (1, COD = 1,5-cyclooctadiene) on partially dehydroxylated silica (1@SiO2). Compound 1@SiO2 has a Pt loading of 3.7 wt %, a surface area of 200 m2/g, and a surface Pt density of around 0.6 Pt site/nm2. Fast MAS 1H{195Pt} dipolar-HMQC and S-REDOR experiments were implemented on both the molecular precursor 1 and on the surface complex 1@SiO2, providing access to 195Pt isotropic shifts and Pt–H distances, respectively. For 1@SiO2, the measu red isotropic shift and width of the shift distribution constrain fits of the static wide-line DNP-enhanced 195Pt spectrum, allowing the 195Pt chemical shift tensor parameters to be determined. Overall the NMR data provide evidence for a well-defined, single-site structure of the isolated Pt sites. © 2020 American Chemical Society.ISSN:0002-7863ISSN:1520-512

Multiple Surface Site Three-Dimensional Structure Determination of a Supported Molecular Catalyst

International audienceThe structural characterization of supported molecular catalysts is challenging due to the low concentration of surface sites and the presence of several organic/organometallic surface groups resulting from the often complex surface chemistry associated with support functionalization. Here, we provide a complete atomic-scale description of all surface sites in a silica-supported iridium-N-heterocyclic carbene (Ir-NHC) catalytic material, at all stages of its synthesis. By combining a suitable isotope labelling strategy with the implementation of multi-nuclear dipolar recoupling DNP enhanced NMR experiments, the 3D structure of the Ir-NHC sites, as well as that of the synthesis intermediates were determined. As a significant fraction of parent surface fragments does not react during the multi-step synthesis, site-selective experiments were implemented to specifically probe proximities between the organometallic groups and the solid support. The NMR-derived structure of the iridium sites points to a well-defined conformation. By interpreting extended x-ray absorption fine structure (EXAFS) spectroscopy and chemical analysis data augmented by computational studies, the presence of two coordination geometries is demonstrated: Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand, as well as, more surprisingly, a fragment coordinated by two NHC and two Cl ligands. This study demonstrates a unique methodology to disclose individual surface structures in complex, multi-site environments, a long-standing challenge in the field of heterogeneous/supported catalysts, while revealing new, unexpected structural features of metallo-NHC supported substrates. It also highlights the potentially large diversity of surface sites present in functional materials prepared by surface chemistry, an essential knowledge to design materials with improved performances