Highly Selective and Efficient Perdeuteration of n-Pentane via H/D Exchange Catalyzed by a Silica-Supported Hafnium-Iridium Bimetallic Complex.

A Surface OrganoMetallic Chemistry (SOMC) approach is used to prepare a novel hafnium-iridium catalyst immobilized on silica, HfIr/SiO2, featuring well-defined [≡SiOHf(CH2 tBu)2(µ-H)3IrCp*] surface sites. Unlike the monometallic analogous materials Hf/SiO2 and Ir/SiO2, which promote n-pentane deuterogenolysis through C-C bond scission, we demonstrate that under the same experimental conditions (1 bar D2, 250 °C, 3 h, 0.5 mol %), the heterobimetallic catalyst HfIr/SiO2 is highly efficient and selective for the perdeuteration of alkanes with D2, exemplified on n-pentane, without substantial deuterogenolysis (<2 % at 95 % conversion). Furthermore this HfIr/SiO2 catalyst is robust and can be re-used several times without evidence of decomposition. This represents substantial advance in catalytic H/D isotope exchange (HIE) reactions of C(sp3)-H bonds.

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

Dynamic 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.

Unravelling the Molecular Structure and Confining Environment of an Organometallic Catalyst Heterogenized within Amorphous Porous Polymers.

The catalytic activity of multifunctional, microporous materials is directly linked to the spatial arrangement of their structural building blocks. Despite great achievements in the design and incorporation of isolated catalytically active metal complexes within such materials, a detailed understanding of their atomic-level structure and the local environment of the active species remains a fundamental challenge, especially when these latter are hosted in non-crystalline organic polymers. Here, we show that by combining computational chemistry with pair distribution function analysis, 129 Xe NMR, and Dynamic Nuclear Polarization enhanced NMR spectroscopy, a very accurate description of the molecular structure and confining surroundings of a catalytically active Rh-based organometallic complex incorporated inside the cavity of amorphous bipyridine-based porous polymers is obtained. Small, but significant, differences in the structural properties of the polymers are highlighted depending on their backbone motifs.

Speciation and Structures in Pt Surface Sites Stabilized by N-Heterocyclic Carbene Ligands Revealed by Dynamic Nuclear Polarization Enhanced Indirectly Detected 195Pt 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 dynamic nuclear polarization (DNP) surface-enhanced solid-state nuclear magnetic resonance (NMR) approach that, in combination with advanced density functional theory (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 13C-195Pt correlation experiments. By exploiting 1J(13C-195Pt) couplings, 2D NMR spectra were acquired, revealing two types of Pt sites. For each of them, 1J(13C-195Pt) value was determined as well as 195Pt 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 195Pt NMR parameters to slight structural changes.

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

The structural characterization of supported molecular catalysts is challenging due to the low density of active 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 an N-heterocyclic carbene based on iridium and supported on silica, at all stages of its synthesis. By combining a suitable isotope labeling strategy with the implementation of multinuclear 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 multistep 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 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, multisite 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.

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 measured 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.

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 NMR 27 Al 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.

TinyPols: a family of water-soluble binitroxides tailored for dynamic nuclear polarization enhanced NMR spectroscopy at 18.8 and 21.1 T.

Dynamic Nuclear Polarization (DNP) has recently emerged as a key method to increase the sensitivity of solid-state NMR spectroscopy under Magic Angle Spinning (MAS). While efficient binitroxide polarizing agents such as AMUPol have been developed for MAS DNP NMR at magnetic fields up to 9.4 T, their performance drops rapidly at higher fields due to the unfavorable field dependence of the cross-effect (CE) mechanism and AMUPol-like radicals were so far disregarded in the context of the development of polarizing agents for very high-field DNP. Here, we introduce a new family of water-soluble binitroxides, dubbed TinyPols, which have a three-bond non-conjugated flexible amine linker allowing sizable couplings between the two unpaired electrons. We show that this adjustment of the linker is crucial and leads to unexpectedly high DNP enhancement factors at 18.8 T and 21.1 T: an improvement of about a factor 2 compared to AMUPol is reported for spinning frequencies ranging from 5 to 40 kHz, with ε H of up to 90 at 18.8 T and 38 at 21.1 T for the best radical in this series, which are the highest MAS DNP enhancements measured so far in aqueous solutions at these magnetic fields. This work not only breathes a new momentum into the design of binitroxides tailored towards high magnetic fields, but also is expected to push the application frontiers of high-resolution DNP MAS NMR, as demonstrated here on a hybrid mesostructured silica material.

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

A novel heterobimetallic tantalum/iridium hydrido complex, [{Ta(CH2tBu)3}{IrH2(Cp*)}] 1, featuring a very short metal-metal bond, has been isolated through an original alkane elimination route from Ta(CHtBu)(CH2tBu)3 and Cp*IrH4. This molecular precursor has been used to synthesize well-defined silica-supported low-coordinate heterobimetallic hydrido species [≡SiOTa(CH2tBu)2{IrH2(Cp*)}], 5, and [≡SiOTa(CH2tBu)H{IrH2(Cp*)}], 6, using a surface organometallic chemistry (SOMC) approach. The SOMC methodology prevents undesired dimerization as encountered in solution and leading to a tetranuclear species [{Ta(CH2tBu)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, sub-atmospheric D2 pressure) without any additives.