Exploring the crystallisation of aspirin in a confined porous material using solid-state nuclear magnetic resonance.

In this study, nuclear magnetic resonance (NMR) is used to investigate the crystallisation behaviour of aspirin within a mesoporous SBA-15 silica material. The potential of dynamic nuclear polarisation (DNP) experiments is also investigated using specifically designed porous materials that incorporate polarising agents within their walls. The formation of the metastable crystalline form II is observed when crystallisation occurs within the pores of the mesoporous structure. Conversely, bulk crystallisation yields the most stable form, namely form I, of aspirin. Remarkably, the metastable form II remains trapped within the pores of mesoporous SBA-15 silica material even 30 days after impregnation, underscoring its persistent stability within this confined environment.

Investigation of UV Light-Promoted Synthesis of α-Sulfonyl Amides from N-Sulfonyl Ynamides.

UV light-promoted synthesis of α-sulfonyl amides from N-sulfonyl ynamides without any additives is reported. The reaction proceeds through a radical chain mechanism involving the photoinduced cleavage of the nitrogen-sulfur bond and addition of an electrophilic sulfonyl radical to the triple bond of the ynamide followed by ß-fragmentation of the sulfonyl group leading to a ketenimine hydrated upon workup. This highly efficient rearrangement leads, after acidic treatment, to a wide range of α-sulfonyl amides in high yields.

Strategies for Accessing cis-1-Amino-2-Indanol.

cis-1-amino-2-indanol is an important building block in many areas of chemistry. Indeed, this molecule is currently used as skeleton in many ligands (BOX, PyBOX…), catalysts and chiral auxiliaries. Moreover, it has been incorporated in numerous bioactive structures. The major issues during its synthesis are the control of cis-selectivity, for which various strategies have been devised, and the enantioselectivity of the reaction. This review highlights the various methodologies implemented over the last few decades to access cis-1-amino-2-indanol in racemic and enantioselective manners. In addition, the various substitution patterns on the aromatic ring and their preparations are listed.

Investigating the efficiency of silica materials with wall-embedded nitroxide radicals for dynamic nuclear polarisation NMR.

Dynamic nuclear polarisation (DNP) can significantly enhance the sensitivity of solid-state nuclear magnetic resonance (SSNMR) experiments by transferring the electron spin polarisation of paramagnetic species to nuclei through microwave irradiation of the sample at cryogenic temperatures. Paramagnetic species required for DNP can be provided in the form of mesoporous silica materials containing nitroxide radicals either located on the porous surface or embedded in the pore walls. The present study focuses specifically on porous materials with wall-embedded radicals that were synthesised using conventional molecular imprinting protocols. More remarkably, by changing the molecular structure of the TEMPO precursor, the theoretical distance between the oxygen atoms in a pair of wall-embedded face-to-face TEMPO radicals was increased stepwise (0.7, 0.9, 1.1, 1.3 and 1.5 nm). The thermal activation of these five series of materials led to 37 TEMPO-functionalised silica materials with different radical concentrations. Their efficiency as DNP polarising agents was subsequently investigated at 9.4 T and ∼110 K under magic-angle spinning conditions (10 kHz) after impregnating them at room temperature with an aqueous solution of isotopically enriched proline. Our results show that the highest DNP efficiency was obtained for the silica materials that exhibited the shortest theoretical oxygen-oxygen distance between the TEMPO rings, suggesting that the design rules accepted for soluble DNP polarising agents may not be transposed to these materials with wall-embedded pairs of nitroxides.

Monitoring Crystallization Processes in Confined Porous Materials by Dynamic Nuclear Polarization Solid-State Nuclear Magnetic Resonance.

Establishing mechanistic understanding of crystallization processes at the molecular level is challenging, as it requires both the detection of transient solid phases and monitoring the evolution of both liquid and solid phases as a function of time. Here, we demonstrate the application of dynamic nuclear polarization (DNP) enhanced NMR spectroscopy to study crystallization under nanoscopic confinement, revealing a viable approach to interrogate different stages of crystallization processes. We focus on crystallization of glycine within the nanometric pores (7-8 nm) of a tailored mesoporous SBA-15 silica material with wall-embedded TEMPO radicals. The results show that the early stages of crystallization, characterized by the transition from the solution phase to the first crystalline phase, are straightforwardly observed using this experimental strategy. Importantly, the NMR sensitivity enhancement provided by DNP allows the detection of intermediate phases that would not be observable using standard solid-state NMR experiments. Our results also show that the metastable ß polymorph of glycine, which has only transient existence under bulk crystallization conditions, remains trapped within the pores of the mesoporous SBA-15 silica material for more than 200 days.

Coniferyl Alcohol Radical Detection by the Dirigent Protein AtDIR6 Monitored by EPR.

Plant dirigent proteins (DIRs) control the stereoselectivity of the monolignol coniferyl alcohol radical coupling. The main mechanistic hypothesis on this chemo- and stereoselective reaction invokes a binding of coniferyl alcohol radical substrates in the dirigent protein active site so that only one enantiomeric form can be produced. We have studied the influence of the Arabidopsis thaliana AtDIR6 protein on the transient coniferyl alcohol radical by EPR. Herein, we show that AtDIR6 stabilizes coniferyl alcohol radicals prior to directing their coupling towards the formation of (-)-pinoresinol.

Modulating lifetimes and relaxation times of phenoxyl radicals through their incorporation into different hybrid nanostructures.

The development of new open shell systems is essential for advances in spin science. In this work, we report the synthesis and characterization of three nanostructured materials, namely SBA-15 silicas, periodic mesoporous organosilicas (PMOs) and lamellar polysilsesquioxanes, all functionalized with the same diazene-based phenoxyl radical precursor. The impact of the nature of the material, i.e. loading of radical precursor and structure, on half-lifetimes (t1/2) and relaxation times of phenoxyl radicals was investigated. Although phenoxyl radicals are transient in solution, their t1/2 range from hours to years at room temperature (RT) when they are embedded in nanostructured materials. The above mentioned functionalized materials were used to generate the corresponding phenoxyl radicals and their relaxation times were measured (〈T1e〉 and Tm) from 50 K to RT. The results were rationalized in terms of limited mobility of the radical as a result of supramolecular interactions and structure rigidity. All these data show that it is possible to design functionalized nanostructured material with radicals possessing specific electronic relaxation properties which can be of interest in fields like DNP, organic magnetism or spin qubit.

Embedding cyclic nitrone in mesoporous silica particles for EPR spin trapping of superoxide and other radicals.

The generation of superoxide radical anion in biological systems is one of the major initiating events in the redox biology of NADPH oxidases and mitochondrial redox signalling. However, the pallette of chemical tools for superoxide detection is very limited, hampering progress in understanding the chemical biology of superoxide. Although EPR spin trapping is regarded as the most rigorous technique for superoxide detection, rapid reduction of the EPR-active superoxide spin adducts to EPR-silent hydroxylamines, or to hydroxyl radical adducts by bioreductants, significantly limits the applicability of this technique in biological systems. To overcome these limitations, in this work, we report the synthesis and characterization of a new mesoporous silica functionalized with a phosphorylated cyclic spin trap (DIPPMPO nitrone). The DIPPMPO-grafted silica is a versatile spin-trap agent enabling the identification of a wide range of carbon or oxygen-centered transient radicals in organic and in aqueous media. Moreover, superoxide was efficiently trapped under in vitro conditions in both cell-free and cellular systems. The generated superoxide adduct exhibited an exceptional half-life of 3.5 h and a resistance toward bioreductant agents such as glutathione for several hours.

Investigating Unusual Organic Functional Groups to Engineer the Surface Chemistry of Mesoporous Silica to Tune CO2-Surface Interactions.

As the search for functionalized materials for CO2 capture continues, the role of theoretical chemistry is becoming more and more central. In this work, a strategy is proposed where ab initio calculations are compared and validated by adsorption microcalorimetry experiments for a series of, so far unexplored, functionalized SBA-15 silicas with different spacers (aryl, alkyl) and terminal functions (N3, NO2). This validation then permitted to propose the use of a nitro-indole surface functionality. After synthesis of such a material the predictions were confirmed by experiment. This confirms that it is possible to fine-tune CO2-functional interactions at energies much lower than those observed with amine species.

Diazene-Functionalized Lamellar Materials as Nanobuilding Blocks: Application as Light-Sensitive Fillers to Initiate Radical Photopolymerizations.

New polysilsesquioxane-based lamellar materials, functionalized with radical precursors, were synthesized. They play a double role in the preparation of composite materials: first, as filler homogeneously dispersed in the monomer after delamination, second as radical initiator in photopolymerization. These polysilsesquioxanes enable fast and efficient photopolymerization upon UV light for thick samples. High conversions in monomers as well as the formation of hybrid polymers covalently linked to the filler are observed. This strategy, based on a double bottom-up approach, avoids the solubility/dispersion problem encountered in the classical preparation of composite polymers from preformed organic polymers.