Unleashing the Potential of17O NMR Spectroscopy Using Mechanochemistry

Ex Situ Generation of 18O2 and 17O2 from Endoperoxides for *O-Labeling and Mechanistic Studies of Oxidations by Dioxygen

Near-stoichiometric amounts of ¹⁸O₂ and ¹⁷O₂ were generated ex situ from endoperoxides in a two-chamber glassware to oxidize various substrates. This strategy gave [*O₂]endoperoxides, [*O₁]quinones, [*O₁]phenols, and [*O_x]artemisin in moderate to good yields and high isotopic enrichments (up to 84%) at affordable costs. Moreover, mass spectrometry and ¹⁷O NMR of the [*O]products provided valuable information about the chemical mechanisms involved.

Fast and Cost-Efficient 17 O-Isotopic Labeling of Carboxylic Groups in Biomolecules: From Free Amino Acids to Peptide Chains

¹⁷ O NMR spectroscopy is a powerful technique, which can provide unique information regarding the structure and reactivity of biomolecules. However, the low natural abundance of ¹⁷ O (0.04 %) generally requires working with enriched samples, which are not easily accessible. Here, we present simple, fast and cost-efficient ¹⁷ O-enrichment strategies for amino acids and peptides by using mechanochemistry. First, five unprotected amino acids were enriched under ambient conditions, consuming only microliter amounts of costly labeled water, and producing pure molecules with enrichment levels up to ∼40 %, yields ∼60-85 %, and no loss of optical purity. Subsequently, ¹⁷ O-enriched Fmoc/tBu-protected amino acids were produced on a 1 g/day scale with high enrichment levels. Lastly, a site-selective ¹⁷ O-labeling of carboxylic functions in peptide side-chains was achieved for RGD and GRGDS peptides, with ∼28 % enrichment level. For all molecules, ¹⁷ O ssNMR spectra were recorded at 14.1 T in reasonable times, making this an important step forward for future NMR studies of biomolecules.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

In recent years, mechanochemistry has been growing into a widely accepted alternative for chemical synthesis. In addition to their efficiency and practicality, mechanochemical reactions are also recognized for their sustainability. The association between mechanochemistry and Green Chemistry often originates from the solvent-free nature of most mechanochemical protocols, which can reduce waste production. However, mechanochemistry satisfies more than one of the Principles of Green Chemistry. In this Review we will present a series of examples that will clearly illustrate how mechanochemistry can significantly contribute to the fulfillment of Green Chemistry in a more holistic manner.

Unveiling the Structure and Reactivity of Fatty-Acid Based (Nano)materials Thanks to Efficient and Scalable 17O and 18O-Isotopic Labeling Schemes

Fatty acids are ubiquitous in biological systems and widely used in materials science, including for the formulation of drugs and the surface-functionalization of nanoparticles. However, important questions regarding the structure and reactivity of these molecules are still to be elucidated, including their mode of binding to certain metal cations or materials surfaces. In this context, we have developed novel, efficient, user-friendly, and cost-effective synthetic protocols based on ball-milling, for the 17O and 18O isotopic labeling of two key fatty acids which are widely used in (nano)materials science, namely stearic and oleic acid. Labeled molecules were analyzed by 1H and 13C solution NMR, IR spectroscopy, and mass spectrometry (ESI-TOF and LC-MS), as well as 17O solid state NMR (for the 17O labeled species). In both cases, the labeling procedures were scaled-up to produce up to gram quantities of 17O- or 18O-enriched molecules in just half-a-day, with very good synthetic yields (all ≥84%) and enrichment levels (up to an average of 46% per carboxylic oxygen). The 17O-labeled oleic acid was then used for the synthesis of a metal soap (Zn-oleate) and the surface-functionalization of ZnO nanoparticles (NPs), which were characterized for the first time by high-resolution 17O NMR (at 14.1 and 35.2 T). This allowed very detailed insight into (i) the coordination mode of the oleate ligand in Zn-oleate to be achieved (including information on Zn···O distances) and (ii) the mode of attachment of oleic-acid at the surface of ZnO (including novel information on its photoreactivity upon UV-irradiation). Overall, this work demonstrates the high interest of these fatty acid-enrichment protocols for understanding the structure and reactivity of a variety of functional (nano)materials systems using high resolution analyses like 17O NMR.

Direct 17O Isotopic Labeling of Oxides Using Mechanochemistry

While ¹⁷O NMR is increasingly being used for elucidating the structure and reactivity of complex molecular and materials systems, much effort is still required for it to become a routine analytical technique. One of the main difficulties for its development comes from the very low natural abundance of ¹⁷O (0.04%), which implies that isotopic labeling is generally needed prior to NMR analyses. However, ¹⁷O-enrichment protocols are often unattractive in terms of cost, safety, and/or practicality, even for compounds as simple as metal oxides. Here, we demonstrate how mechanochemistry can be used in a highly efficient way for the direct ¹⁷O isotopic labeling of a variety of s-, p-, and d-block oxides, which are of major interest for the preparation of functional ceramics and glasses: Li₂O, CaO, Al₂O₃, SiO₂, TiO₂, and ZrO₂. For each oxide, the enrichment step was performed under ambient conditions in less than 1 h and at low cost, which makes these synthetic approaches highly appealing in comparison to the existing literature. Using high-resolution solid-state ¹⁷O NMR and dynamic nuclear polarization, atomic-level insight into the enrichment process is achieved, especially for titania and alumina. Indeed, it was possible to demonstrate that enriched oxygen sites are present not only at the surface but also within the oxide particles. Moreover, information on the actual reactions occurring during the milling step could be obtained by ¹⁷O NMR, in terms of both their kinetics and the nature of the reactive species. Finally, it was demonstrated how high-resolution ¹⁷O NMR can be used for studying the reactivity at the interfaces between different oxide particles during ball-milling, especially in cases when X-ray diffraction techniques are uninformative. More generally, such investigations will be useful not only for producing ¹⁷O-enriched precursors efficiently but also for understanding better mechanisms of mechanochemical processes themselves.

The direct ¹⁷O enrichment of s-, p-, and d-block metal oxides is achieved with high efficiency using mechanochemistry. Atomic-level insight into the enrichment process is obtained using high-resolution solid-state ¹⁷O NMR and dynamic nuclear polarization analyses, which demonstrate that enriched oxygen sites are present both at the surface and within the oxide particles. Moreover, it is demonstrated how these labeling schemes allow the study of unique aspects of mechanochemical reactions between oxides by ¹⁷O NMR.

Facile, Room-Temperature 17O Enrichment of Zeolite Frameworks Revealed by Solid-State NMR Spectroscopy

A new approach for room-temperature ¹⁷O enrichment of zeolites reveals a surprisingly dynamic and labile framework, where rapid and reversible bond breaking takes place. ¹⁷O NMR spectroscopy shows that although O sites in both framework Si-O-Al and Si-O-Si linkages are enriched simply on exposure to H₂¹⁷O(l), the enrichment of Si-O-Al species is more rapid, with a more uniform framework enrichment observed at longer durations. We demonstrate that this unexpected enrichment can be observed for two different framework topologies and for Na-exchanged (i.e., nonacidic) zeolites, as well as their protonic forms, confirming that the Brønsted acid proton is not necessary for isotopic exchange into the framework. This work not only offers new opportunities for structural characterization of these chemically and industrially important materials using NMR spectroscopy but suggests that further investigation of the rate and position of enrichment in zeolite frameworks could provide new insight into their chemical reactivity and their stability in aqueous-based applications such as ion exchange and catalysis.

From Synthesis of Amino Acids and Peptides to Enzymatic Catalysis: A Bottom-Up Approach in Mechanochemistry

Recently, chemical reactions induced or facilitated by mechanical energy have gained recognition in diverse areas of chemical synthesis. In particular, mechanosyntheses of amino acids and short peptides, along with their applications in catalysis, have revealed the high degree of stability of peptide bonds in environments of harsh mechanical stress. These observations quickly led to the recent interest in developing mechanochemical enzymatic reactions. Experimentally, manual grinding, ball-milling techniques, and twin-screw extrusion technology have proven valuable to convey mechanical forces into a chemical synthesis. These practices have enabled the establishment of more sustainable alternatives for chemical synthesis by reducing the use of organic solvents and waste production, thereby having a direct impact on the E-factor of the chemical process. In this Minireview, the series of events that allowed the development of mechanochemical enzymatic reactions are described from a bottom-up perspective.

17O solid-state NMR spectroscopy of A2B2O7 oxides: quantitative isotopic enrichment and spectral acquisition?

The potential of ¹⁷O NMR spectroscopy for the investigation of A₂B₂O₇ ceramic oxides important in the encapsulation of radioactive waste is demonstrated, with post-synthetic enrichment by exchange with ¹⁷O₂ gas. For Y₂Sn₂O₇, Y₂Ti₂O₇ and La₂Sn₂O₇ pyrochlores, enrichment of the two distinct O species is clearly non quantitative at lower temperatures (∼700 °C and below) and at shorter times, despite these being used in prior work, with preferential enrichment of OA₂B₂ favoured over that of OA₄. At higher temperatures, the ¹⁷O NMR spectra suggest that quantitative enrichment has been achieved, but the integrated signal intensities do not reflect the crystallographic 1 : 6 (O1 : O2) ratio until corrected for differences in T₁ relaxation rates and, more importantly, the contribution of the satellite transitions. ¹⁷O NMR spectra of Y₂Zr₂O₇ and Y₂Hf₂O₇ defect fluorites showed little difference with any variation in enrichment temperature or time, although an increase in the absolute level of enrichment (up to ∼7.5%) was observed at higher temperature. DFT calculations show that the six distinct resonances observed cannot be assigned unambiguously, as each has contributions from more than one of the five possible next nearest neighbour environments. For La₂Ti₂O₇, which adopts a layered perovskite-like structure, little difference in the spectral intensities is observed with enrichment time or temperature, although the highest absolute levels of enrichment (∼13%) were obtained at higher temperature. This work demonstrates that ¹⁷O NMR has the potential to be a powerful probe of local structure and disorder in oxides, but that considerable care must be taken both in choosing the conditions for ¹⁷O enrichment and the experimental acquisition parameters if the necessary quantitative measurements are to be obtained for more complex systems.

Investigation of the conditions required for quantitative isotopic enrichment and the acquisition of quantitative ¹⁷O NMR spectra of ceramic oxides.

Cost-effective 17O enrichment and NMR spectroscopy of mixed-metal terephthalate metal-organic frameworks

17O solid-state NMR spectroscopy is employed to investigate the cation disorder in metal-organic frameworks containing two different types of metal cations. Although NMR offers exquisite sensitivity to the local, atomic-scale structure, making it an ideal tool for the characterisation of disordered materials, the low natural abundance of 17O (0.037%) necessitates expensive isotopic enrichment to acquire spectra on a reasonable timescale. Using dry gel conversion and a novel steaming method we show that cost-effective and atom-efficient enrichment of MOFs is possible, and that high-resolution 17O NMR spectra are sensitive both to the structural forms of the MOF and the presence of guest molecules. For mixed-metal forms of MIL-53, NMR can also provide information on the final composition of the materials (notably different to that of the initial starting material) and the preference for cation clustering/ordering within the MOFs. For Al, Ga MIL-53, the distribution of cations results in a mixed-pore form upon exposure to water, unlike the different structures seen for the corresponding end members. This work shows that as good levels of enrichment can be achieved at reasonable cost, 17O NMR spectroscopy should be an invaluable tool for the study of these important functional materials.