Sources de polarisation et méthodologies expérimentales avancées pour la spectroscopie RMN du solide hyperpolarisée

La RMN du solide sous rotation à l’angle magique (MAS) est une spectroscopie puissante permettant l’étude d’une grande variété d’échantillons biologiques et de matériaux avec une précision atomique. Cependant, lorsqu’elle est opérée à très hauts champs magnétiques, elle pâtit d’une résolution et d’une sensibilité limitée pour certains solides complexes. La polarisation dynamique nucléaire (DNP), technique d’hyperpolarisation s’appuyant sur un transfert depuis des électrons non appariés inclus dans ou au voisinage de l’échantillon, est une méthode prometteuse permettant d’accroître considérablement la sensibilité de la RMN sous MAS. Bien qu’établie à champs intermédiaires, elle manque encore de sources de polarisation et de stratégies de formulation efficaces permettant sa mise en œuvre à très hauts champs magnétiques, sous hautes fréquences de rotation et à des températures élevées. Au cours de cette thèse, une nouvelle famille de radicaux binitroxides, les TinyPols, a été introduite, caractérisée par des interactions inter-électroniques adaptées aux champs élevés. Leurs structures ont été optimisées et leurs propriétés étudiées à l’aide des specroscopies RMN et RPE, permettant l’identification des paramètres clefs pilotant leur efficacité. Après plusieurs itérations, un record de performance a été atteint avec un radical de cette famille, atteignant un facteur d’exaltation de150 à 18.8 T et 40 kHz MAS, mettant en lumière le rôle déterminant de l’environnement local des électrons non appariés. D’autres travaux ont porté sur la diversification des sources et matrices de polarisation, notamment dans l’optique de réaliser de la DNP à haute température. Deux systèmes ont été proposés pour parvenir à cet objectif, l’un étant une matrice rigide d’OTP accueillant un radical hybride BDPA-nitroxide, l’autre une matrice d’oxyde de cérium dopée au gadolinium. L’ensemble de ces développements méthodologiques apportés à la MAS DNP à très hauts champs magnétiques permettra, à terme, de rendre cette technique plus efficace, versatile et accessible.Solid-state NMR under magic angle spinning (MAS) is a powerful spectroscopic tool to investigate a wide range of biological samples and materials with atomic-level precision. However, even when performed at high magnetic fields, it often suffers from resolution and sensitivity limitations in the case of the most challenging solids. Dynamic nuclear polarization (DNP), a hyperpolarization technique relying on a transfer from unpaired electrons introduced in or close to the sample, is a promising method to considerably boost the sensitivity of MAS NMR. While well-established at intermediate magnetic fields, this approach still lacks efficient polarizing agents (PAs) and sample formulations strategies to be widely used at high magnetic fields, fast MAS frequencies and elevated temperatures. In this thesis, a new family of binitroxide-based polarizing agents dubbed TinyPols is introduced, with tailored electron-electron interactions for efficient MAS DNP at high magnetic field and under fast MAS. Their structures are fine-tuned and their properties investigated by NMR and EPR spectroscopy to understand which parameters are key to their design. After several iterations, a record 150-fold enhancement factor is achieved with TinyPol-like radicals at 18.8 T and 40 kHz MAS, highlighting the importance of the local environment of the unpaired electrons. Other investigations focus on broadening the applicability of MAS DNP by designing alternative polarizing sources or matrices. Notably, an emphasis is made on enabling high temperature DNP. This is achieved successfully with two different systems, namely a hybrid BDPA-nitroxide biradical in a rigid OTP matrix and gadolinium-doped bulk or nanocomposite ceria. Altogether, all these methodological developments of high field DNP-enhanced MAS NMR will ultimately make this technique more efficient, versatile and accessible

Polarizing agents for efficient high field DNP solid-state NMR spectroscopy under magic-angle spinning: from design principles to formulation strategies

International audienceDynamic Nuclear Polarization (DNP) has recently emerged as a cornerstone approach to enhance the sensitivity of solid-state NMR spectroscopy under Magic Angle Spinning (MAS), opening unprecedented analytical opportunities in chemistry and biology. DNP relies on a polarization transfer from unpaired electrons (present in endogenous or exogenous polarizing agents) to nearby nuclei. Developing and designing new polarizing sources for DNP solid-state NMR spectroscopy is currently an extremely active research field per se, that has recently led to significant breakthroughs and key achievements, in particular at high magnetic fields. This review describes recent developments in this area, highlighting key design principles that have been established over time and led to the introduction of increasingly more efficient polarizing sources. After a short introduction, Section 2 presents a brief history of solid-state DNP, highlighting the main polarization transfer schemes. The third section is devoted to the development of dinitroxide radicals, discussing the guidelines that were progressively established to design the fine-tuned molecular structures in use today. In Section 4, we describe recent efforts in developing hybrid radicals composed of a narrow EPR line radical covalently linked to a nitroxide, highlighting the parameters that modulate the DNP efficiency of these mixed structures. Section 5 reviews recent advances in the design of metal complexes suitable for DNP MAS NMR as exogenous electron sources. In parallel, current strategies that exploit metal ions as endogenous polarization sources are discussed. Section 6 briefly describes the recent introduction of mixed-valence radicals. In the last part, experimental aspects regarding sample formulation are reviewed to make best use of these polarizing agents in a broad panel of application fields

The Molecular and Electronic Structure of Isolated Platinum Sites Enabled by Expedient Measurement of 195Pt Chemical Shift Anisotropy

Techniques that can characterize the molecular structures of dilute surface species are required to facilitate the rational synthesis and improvement of single-site and single-atom, such as the important class of Pt-based systems. In this context, 195Pt solid-state NMR spectroscopy could be an ideal tool for this task because 195Pt NMR spectra and sizeable chemical shift anisotropy (CSA) are highly sensitive probes of the local chemical environment and electronic structure. However, the broadening of 195Pt solid-state NMR spectra by CSA often results in low NMR sensitivity. Furthermore, characterization of Pt sites on surfaces is complicated by the typical low Pt loadings that are between 0.2 to 5 wt.%. Here, we introduce a set of solid-state NMR methods that exploit fast MAS and indirect detection of a sensitive spy nucleus (1H or 31P) to enable rapid acquisition of 195Pt MAS NMR spectra. We demonstrate that high-resolution wideline 195Pt MAS NMR spectra can be in minutes to a few hours for a series of molecular and single-site Pt species grafted on silica with Pt loading of only 3-5 wt.%. Low-power, long-duration, sideband-selective excitation and saturation pulses are incorporated into t1-noise eliminated (TONE) dipolar heteronuclear multiple quantum coherence (D-HMQC), perfect echo resonance echo saturation pulse double resonance (PE RESPDOR) or J-resolved pulse sequences. The complete 195Pt MAS NMR spectrum is then reconstructed by recording a series of 1D NMR spectra where the offset of the 195Pt pulses is varied. Analysis of the 195Pt MAS NMR spectra yields the 195Pt chemical shift tensor parameters. Analysis of the NMR signatures based on relativistic zeroth order approximation (ZORA) DFT calculations enables the rationalization of changes in the observed 195Pt CSA across the series of Pt compounds. Simple and predictive orbital models relate the measured spectral signatures to specific electronic environments and allows the identification of coordination environment by inspection of the CSA (isotropic chemical shift and measured spans). The methodology developed here paves the way for the detailed structural and electronic analysis of dilute platinum sites in single-atom and single-site heterogeneous catalysts

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

Molecular and Electronic Structure of Isolated Platinum Sites Enabled by the Expedient Measurement of 195 Pt Chemical Shift Anisotropy

International audienceTechniques thatcancharacterize themolecular structures ofdilute surfacespeciesarerequired tofacilitate therationalsynthesis andimprovement of Pt-based heterogeneous catalysts. 195Ptsolid-state NMRspectroscopy couldbean idealtoolforthistaskbecause 195Ptisotropic chemical shiftsandchemical shift anisotropy (CSA)arehighlysensitive probesofthelocalchemical environment andelectronic structure. However, thecharacterization ofPtsurface-sites is complicated bythetypicallowPtloadings thatarebetween 0.2and5wt%and broadening of 195Ptsolid-state NMRspectrabyCSA.Here,weintroduce asetof solid-state NMRmethods thatexploitfastMASandindirectdetection usinga sensitive spynucleus( 1Hor 31P)toenabletherapidacquisition of 195PtMAS NMRspectra.Wedemonstrate thathigh-resolution wideline 195PtMASNMRspectracanbeacquired inminutestoafewhoursfor aseriesofmolecular andsingle-site PtspeciesgraftedonsilicawithPtloadingofonly3-5wt%.Low-power, long-duration, sidebandselective excitation, andsaturation pulsesareincorporated into t1-noiseeliminated dipolarheteronuclear multiple quantum coherence, perfectechoresonance echosaturation pulsedoubleresonance, or J-resolved pulsesequences. Thecomplete 195PtMAS NMRspectrum isthenreconstructed byrecording aseriesof1DNMRspectrawheretheoffsetofthe 195Ptpulsesisvariedin increments oftheMASfrequency. Analysisofthe 195PtMASNMRspectrayieldsthe 195Ptchemical shifttensorparameters. Zeroth orderapproximation densityfunctional theorycalculations accurately predict 195PtCStensorparameters. Simpleandpredictive orbitalmodelsrelatetheCStensorparameters tothePtelectronic structure andcoordination environment. Themethodology developed herepavesthewayforthedetailedstructural andelectronic analysisofdiluteplatinum surface-sites

The Molecular and Electronic Structure of Isolated Platinum Sites Enabled by Expedient Measurement of 195Pt Chemical Shift Anisotropy

Techniques that can characterize the molecular structures of dilute surface species are required to facilitate the rational synthesis and improvement of single-site and single-atom, such as the important class of Pt-based systems. In this context, 195Pt solid-state NMR spectroscopy could be an ideal tool for this task because 195Pt NMR spectra and sizeable chemical shift anisotropy (CSA) are highly sensitive probes of the local chemical environment and electronic structure. However, the broadening of 195Pt solid-state NMR spectra by CSA often results in low NMR sensitivity. Furthermore, characterization of Pt sites on surfaces is complicated by the typical low Pt loadings that are between 0.2 to 5 wt.%. Here, we introduce a set of solid-state NMR methods that exploit fast MAS and indirect detection of a sensitive spy nucleus (1H or 31P) to enable rapid acquisition of 195Pt MAS NMR spectra. We demonstrate that high-resolution wideline 195Pt MAS NMR spectra can be in minutes to a few hours for a series of molecular and single-site Pt species grafted on silica with Pt loading of only 3-5 wt.%. Low-power, long-duration, sideband-selective excitation and saturation pulses are incorporated into t1-noise eliminated (TONE) dipolar heteronuclear multiple quantum coherence (D-HMQC), perfect echo resonance echo saturation pulse double resonance (PE RESPDOR) or J-resolved pulse sequences. The complete 195Pt MAS NMR spectrum is then reconstructed by recording a series of 1D NMR spectra where the offset of the 195Pt pulses is varied. Analysis of the 195Pt MAS NMR spectra yields the 195Pt chemical shift tensor parameters. Analysis of the NMR signatures based on relativistic zeroth order approximation (ZORA) DFT calculations enables the rationalization of changes in the observed 195Pt CSA across the series of Pt compounds. Simple and predictive orbital models relate the measured spectral signatures to specific electronic environments and allows the identification of coordination environment by inspection of the CSA (isotropic chemical shift and measured spans). The methodology developed here paves the way for the detailed structural and electronic analysis of dilute platinum sites in single-atom and single-site heterogeneous catalysts.This is a pre-print of the article Venkatesh, Amrit, Domenico Gioffrè, Benjamin Atterberry, Lukas Rochlitz, Scott Carnahan, Zhuoran Wang, Georges Menzildjian, Anne Lesage, Christophe Coperét, and Aaron Rossini. "The Molecular and Electronic Structure of Isolated Platinum Sites Enabled by Expedient Measurement of 195Pt Chemical Shift Anisotropy." (2022). DOI: 10.26434/chemrxiv-2022-j2f5h. Copyright 2022 The Authors. Attribution-NonCommercial 4.0 International (CC BY-NC 4.0). Posted with permission

Molecular and Electronic Structure of Isolated Platinum Sites Enabled by the Expedient Measurement of 195Pt Chemical Shift Anisotropy

Techniques that can characterize the molecular structures of dilute surface species are required to facilitate the rational synthesis and improvement of Pt-based heterogeneous catalysts. 195Pt solid-state NMR spectroscopy could be an ideal tool for this task because 195Pt isotropic chemical shifts and chemical shift anisotropy (CSA) are highly sensitive probes of the local chemical environment and electronic structure. However, the characterization of Pt surface-sites is complicated by the typical low Pt loadings that are between 0.2 and 5 wt% and broadening of 195Pt solid-state NMR spectra by CSA. Here, we introduce a set of solid-state NMR methods that exploit fast MAS and indirect detection using a sensitive spy nucleus (1H or 31P) to enable the rapid acquisition of 195Pt MAS NMR spectra. We demonstrate that high-resolution wideline 195Pt MAS NMR spectra can be acquired in minutes to a few hours for a series of molecular and single-site Pt species grafted on silica with Pt loading of only 3-5 wt%. Low-power, long-duration, sideband-selective excitation, and saturation pulses are incorporated into t1-noise eliminated dipolar heteronuclear multiple quantum coherence, perfect echo resonance echo saturation pulse double resonance, or J-resolved pulse sequences. The complete 195Pt MAS NMR spectrum is then reconstructed by recording a series of 1D NMR spectra where the offset of the 195Pt pulses is varied in increments of the MAS frequency. Analysis of the 195Pt MAS NMR spectra yields the 195Pt chemical shift tensor parameters. Zeroth order approximation density functional theory calculations accurately predict 195Pt CS tensor parameters. Simple and predictive orbital models relate the CS tensor parameters to the Pt electronic structure and coordination environment. The methodology developed here paves the way for the detailed structural and electronic analysis of dilute platinum surface-sites.ISSN:0002-7863ISSN:1520-512