Active Sites in Supported Single-Site Catalysts: An NMR Perspective

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Active Sites in Supported Single-Site Catalysts: An NMR Perspective Christophe Copéret,* Wei-Chih Liao, Christopher P. Gordon, and Ta-Chung Ong† Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, CH-8093 Zürich, Switzerland unprecedented structural information. We will close with possible future directions for the field.

ABSTRACT: Development of well-defined heterogeneous catalysts requires detailed structural characterization of active sites, an essential step toward establishing structure−activity relationships and promoting rational designs of catalysts. Solid-state NMR has emerged as a powerful approach to provide key molecular-level information about active-site structures and dynamics in heterogeneous catalysis. Here, we describe how one can apply solid-state NMR, ranging from 1D chemical shift assignments (and additional parameters, CQ and η, for quadrupolar nuclei, I > 1/2) to 2D correlations, to analysis of chemical shift anisotropy, providing unprecedented structural information about a broad range of materials. We also describe how modern hyperpolarization techniques like dynamic nuclear polarization can be used to improve the sensitivity of NMR, and make various challenging 1D/2D NMR experiments feasible, thus paving the way to determine the structures of surface sites.

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NMR SIGNATURES OF SURFACE SPECIES NMR signatures of surface species is the first step toward the elucidation of surface-site structures, especially because of the large database of NMR chemical shifts obtained in solution and in the solid state over the years. The development of magicangle spinning (MAS) and cross-polarization (CP) at high magnetic fields, combined with 1H decoupling sequences, has enabled recording of 1D NMR spectra with high resolution over a broad range of materials. Structural assignments based on chemical shifts are particularly useful for nuclei that have a large chemical shift window and for which the chemical shifts are very responsive to the environment. For instance, it is usually straightforward to distinguish alkyl, alkylidene, and alkylidyne by 13C NMR5 (Figure 1A) or amido, imido, and nitrido groups by 15N NMR6 in surface complexes, provided these species are isotopically enriched. Also, 27Al with different coordination numbers7 and 29Si in different environments8 can be readily assigned on the basis of their NMR signatures. Similarly, the specific geometries of the metallacycle intermediatestrigonal bipyramidal (TBP) vs square pyramidal (SP)in supported metathesis catalysts can be differentiated from their specific chemical shifts.9 Detailed assignments can even be obtained on Ziegler−Natta precatalyst, e.g., EtOH−TiCl4−MgCl2, where Ti ethoxy complexes have been clearly identified.10 Additionally, in contrast to bulk materials, 1 H NMR can be quite informative in the specific case of surface species. Single-site supported species show a relatively good resolution in 1H NMR spectra, due to the site isolation and the presence of residual dynamics that minimize 1H−1H dipolar couplings responsible for line broadening in bulk solid.11

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INTRODUCTION Supported well-defined single-site catalysis has emerged as a powerful approach toward the understanding and the rational development of heterogeneous catalysts and more efficient chemical transformations.1 A critical step in their development is the structural determination of the surface species with molecular-level precision. While this can only be accomplished by a combination of spectroscopic methods (e.g., NMR, IR, EXAFS, etc.), often supplemented with computational approaches,2 solid-state NMR spectroscopy has emerged as one of the most powerful techniques because of its sensitivity to the local environment and dynamics.3 Unlike in solution-state NMR, the effect of interactions such as chemical shift anisotropy (CSA) and dipolar coupling can be dominant in solid-state NMR. While these interactions contribute to line broadening, they can provide invaluable information to determine the structures of surface sites. Solid-state NMR has already been used extensively in the field of heterogeneous catalysis, for example, probing the acidity of oxide supports and characterizing reaction intermediates in zeolites.4 Here, we discuss how advances in solid-state NMR spectroscopy have allowed researchers to obtain unparalleled information about surface-site structures of well-defined heterogeneous catalysts. We will examine the relationship between specific NMR experiments and structural information on well-defined supported catalysts, using the most illustrative examples from our research group in addition to selective contributions from others. In particular, we will point out the challenges in obtaining these data and how advanced techniques and pulse sequences contribute to obtaining © 2017 American Chemical Society

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2D NMR As in solution NMR, it is often necessary to acquire twodimensional (2D) solid-state NMR spectra in order to ascertain structural assignments of surface species. Heteronuclear correlation (HETCOR) experiments constitute a powerful approach to establish the proximities (and infer connectivities) between nuclei5d,11a,12 by exploiting the dipolar interactions between 1H and heteronuclei, such as 13C (Figure 1B).5a Further refinement of the structure can be achieved using Jresolved NMR spectroscopy.11a,13 For instance, the presence of agostic interactions between the α-CH bond and the electrophilic metal center in supported metathesis catalysts is associated with a decrease of the JC−H coupling constant.14 The JC−H coupling constant is particularly sensitive to the M−C−H Received: December 18, 2016 Published: June 28, 2017 10588

DOI: 10.1021/jacs.6b12981 J. Am. Chem. Soc. 2017, 139, 10588−10596

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Figure 1. (A) 1D 13C CP-MAS NMR spectrum and (B) 2D 1H−13C HETCOR NMR spectrum of silica-supported rhenium alkylidene species. (C) Visualization of the chemical shielding tensor of (a) ethylene, (b) (SiO)(Ta)(CHMe)(CH2Me)2, (c) (SiO)(Re)(CMe)( CHMe)(CH2Me), and (d) actions of angular momentum operators coupling frontier orbitals. Panels A and B are reproduced and modified from ref 5a with permission from the American Chemical Society. Panel C is reproduced and modified from ref 29 with permission from the American Chemical Society.

bond angleJC−H values of 90−130 Hz vs ca. 160 Hz for synand anti-isomer, respectivelyallowing discrimination between them and evaluation of the M−C−H bond angle.13b Single-quantum (SQ)−double quantum (DQ) correlation NMR is also a powerful method to identify nearby nuclei. SQDQ NMR combined with fast MAS allows probing protons in close proximity15 and hence distinguishing, for instance, monohydride and bishydride surface species,16 or mono- and bisphosphine ligated Au surface species, which thus allows probing the various environments of Au(I) species supported on silica.17 Other notable SQ-DQ correlation experiments include the dipolar-interaction-based POST-C718 and SPC519 recoupling, and the J-based 2D INADEQUATE.20 Most aforementioned examples involve NMR-sensitive nucleihigh natural abundance and large gyromagnetic ratio (γ). 2D experiments involving correlations between less NMR sensitive nuclei can also potentially provide important structural information about surface sites (vide inf ra). However, the application of these experiments on heterogeneous catalysts is often impeded by the inherently low sensitivity of NMR.

described by the isotropic chemical shift (δiso), the span (Ω), and the skew (κ) as shown in eqs 2−4, respectively.23 δiso =

1 (δ11 + δ22 + δ33) 3

Ω = δ11 − δ33

κ=

3(δ22 − δiso) Ω

(2) (3) (4)

δiso can be readily observed via 1D MAS NMR, whereas Ω and κ can be evaluated by recording the spectra under static conditions or at low MAS frequency. The Haeberlen convention can also be used to describe the measured CSA using δiso, the anisotropy, and the asymmetry.24 Sideband separation experiments, such as magic-angle turning (MAT)25 or phase-adjusted spinning sidebands (PASS),26 can be employed to distinguish the spinning sidebands stemming from the CSA by their δiso in a 2D spectrum so that the CSA of individual sites can be more readily evaluated. Comparing the experimentally observed CSA in a series of silica-supported alkylidene complexes with static values obtained by DFT calculations revealed the presence of residual dynamics in surface species, as evidenced by partial averaging of both dipolar couplings and CSA. Slower dynamics were observed for a 4d metal complex (Mo) compared to 5d metal complexes (Ta, W, and Re), and the arylimido ligand was found to decrease dynamics, indicating an interaction of the arylimido group with the surface.27 While CSA can be the basis for structural elucidation, the numerical values can be difficult to interpret and rationalize. Analysis of its principal components using computational approaches can now provide useful information on the relationship between NMR parameters and the electronic structures of molecules.28 The chemical shielding tensor (σii) can be calculated quite accurately and analyzed, thus allowing researchers to understand the origin of each measured chemical shift principal component (δii). Indeed, the calculated σii can be decomposed into diamagnetic (σd) and paramagnetic (σp) contributions, which typically lead to shielding and deshielding, respectively (eq 5). Additional contributions arising from spin− orbit (SO) couplings can also be relevant for molecules containing heavy nuclei. While σd results from currents

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BEYOND ISOTROPIC CHEMICAL SHIFTS NMR chemical shielding (σ) arises from the neighboring electronic environment around the detecting nucleus. Experimentally, the chemical shift (δ) is reported as the frequency offset with respect to a reference compound (σref), such as tetramethylsilane, as shown in eq 1.21 σref − σsample ≈ σref − σsample δ= (1) 1 − σref While the chemical shielding interaction is anisotropic, i.e., it is dependent on the orientation of the molecules with respect to the external magnetic field, only the isotropic chemical shift (δiso) is observed in solution because of the fast molecular tumbling that eliminates any orientation dependence. In solidstate NMR of a static sample, which does not experience fast molecular tumbling like in solution, CSA becomes observable. CSA contains important information on the electronic environment of the nuclei and/or the presence of residual dynamics.22 It can be described mathematically as a secondrank tensor with three principal components, denoted as δ11, δ22, and δ33. In the Herzfeld−Berger convention, the CSA is 10589


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Figure 2. (A) Fast MAS (60 kHz) 1D 27Al spectra acquired at (a) 17.6, (b) 20.0, and (c) 23.5 T, with three-component (CQ, η, and δiso) simulations for each 27Al site (green, purple, and orange). (B) MQMAS spectra (blue) acquired at (d) 20.0 and (e) 23.5 T with the simulation superimposed. (C) Graphical representation of NMR parameters, CQ and δiso, of various calculated (cluster models, black, and cristobalite models, blue) and experimental (filled green, purple, and orange diamonds) 27Al sites. The same shapes represent the 27Al sites within the same model; colored boxes indicate the assignment of experimental structures to calculated models. Panels A and B are reproduced and modified from ref 30 with permission from the American Chemical Society.

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QUADRUPOLAR NUCLEI Many NMR-active nuclei have a spin quantum number (I) larger than 1/2, such as 2H, 11B, 17O, and 27Al. These nuclei experience a quadrupolar interaction originating from the interaction between the electric quadrupole moment of the nucleus (eQ) and the surrounding electric field gradient (EFG). For spin-half quadrupoles, the second-order quadrupolar interaction is not fully averaged by MAS, leading to significant line broadening of the NMR resonances that results in a loss of signal-to-noise ratio and spectral resolution. Even so, structural information can be gained by evaluating the quadrupolar interaction, which is characterized by the quadrupolar coupling constant (CQ) and the asymmetry parameter (η). Both parameters are highly sensitive to the coordination environment, and combined with chemical shifts, they can help determining the local structure of surface sites.7a As an example, the reaction of alkylaluminum with silica is extremely complex, and information obtained solely from 13C and 29Si MAS NMR is not useful to determine the structure of aluminum surface sites. Instead, structural information about the aluminum environment was obtained by evaluating sitespecific 27Al CQ and η, as well as δiso, extracted via fast MAS (60 kHz) 27Al NMR at various magnetic fields (Figure 2A).30 Multiple-quantum magic-angle spinning (MQMAS) can further resolve various 27Al resonances compared to 1D spectra and ascertain their assignments (Figure 2B). Combining experimental data with computed NMR parametersδiso and CQ presented in a 2D plot provides a powerful approach to discriminate between possible sites, as illustrated in Figure 2C. By using the aforementioned strategy, two of the Al surface sites are assigned to different bis-grafted dimeric ethylaluminum structures (S1 and S2), and the third site to aluminum inserted in the bulk of the material (S3), thus providing a better understanding of surface restructuration at the grafting step in the reaction of triethylaluminum with SBA-15. 30 This restructuration highly depends on the type of organoaluminum reagent. For example, with R2AlCl, supported Cl-bridged organoaluminum species are formed, which leads to highly

generated in the molecule’s electronic ground state and is usually less anisotropic, σp depends on the admixture of electronically excited states and the ground state and can be highly anisotropic. Paramagnetic deshielding of a nucleus is typically observed whenever associated filled orbitals at the nuclei can be coupled with corresponding empty orbitals by action of the angular momentum operator Li (i = x, y, z), where the x-, y-, and z-axes are the principal axes of σ (eq 6 and Figure 1C-d). It is mostly caused by interactions of the molecular frontier orbitals: a strong deshielding along an axis i is expected if energetically high-lying filled orbitals on a specific nucleus can be coupled with low-lying empty orbitals by a rotation along i.

σii = σiid + σiip + SO σiip ∝

⟨Ψvac|Li|Ψocc⟩ ⟨Ψocc|Li /r 3|Ψvac⟩ ΔEvac−occ

(5)

(6)

CSA (and its corresponding δiso) is extremely sensitive to the local hybridization and electronic structure at the observed nucleus. For instance, it is possible to explain with simple molecular orbital (MO) analysis the origin of the very deshielded 13C chemical shift observed for various metal alkylidenes in comparison with alkenes.29 Despite the nucleophilic character of alkylidene carbons, their 13C δiso is typically found between 200 and 300 ppm, similar to what is observed for carbocations. Additionally, very wide spans (400− 500 ppm) are observed for these compounds. Analogous to alkenes (Figure 1C-a), the most deshielded component σ11 in metal alkylidenes is along the x-axis, which is mainly due to the coupling of σMC to π*MC and of πMC to σ*MC (Figure 1C). This MO view explains the large deshielding in d0 metal−alkylidene complexes such as (SiO)(M)(E)(CHtBu)(X) (E = CtBu or NAr; X= alkyl or pyrrolyl) and the difference observed between metals. Analysis of the chemical shielding tensor further provides an understanding of the effect of agostic interactions in syn-complexes, which leads to a change in orientation of the chemical shielding tensor in the molecular frame (Figure 1C). 10590


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Figure 3. (A) Representative picture of polarization transfer pathway in a DNP experiment. (B) Chemical structures of TEKPol, TEKPol2, and TEKPol3.

supported catalysts (few wt% of the sample) and the many possible distributions of sites, which can lead to additional broadening of the NMR signals. While CP-MAS and isotopic labeling alleviate part of the problems, hours or even days of signal averaging time is still often needed to obtain a spectrum with an adequate signal-to-noise ratio. Increasing the signal intensity by the polarization transfer from high-γ nuclei with high natural abundance, e.g., 1H (42.576 MHz/T, 99.99%), to low-γ heteronuclei has been an active field of research.36 Similarly, dynamic nuclear polarization (DNP) increases nuclear polarization by 1−2 orders of magnitude by transferring polarization from the much higher-γ electrons (28024.952 MHz/T) to the nuclei, and it has emerged as a promising technique at high magnetic fields (5.0− 18.8 T).37 Typically, the sample is prepared by impregnating the analyte with a solution of polarizing agent (PA), usually a persistent organic radical. The polarization transfer from the unpaired electrons to the nearby protons is promoted via the cross effect by microwaves saturating the electron paramagnetic resonance (EPR) transition at low temperature (100 K). Combined with an additional CP step, it is possible to greatly enhance the signal of the targeted heteronuclei, e.g., 13C, 15N, 27 Al, 29Si, 111Cd, etc. (Figure 3A). The cross effect requires dipolar couplings among three spinstwo electrons and one proton.37e,38 To fulfill its condition, the frequency difference between two electrons needs to match the proton Larmor frequency (eq 7). Furthermore, the polarization transfers more efficiently at low temperatures due to slower electron relaxation rates.

efficient supported co-catalysts for the Ni-catalyzed dimerization of ethylene.31 While solid-state NMR experiments of quadrupolar nuclei such as 7Li, 11B, and 27Al are relatively common, experiments involving 17O are rarer, despite the ubiquity of oxygen in many chemical systems, in particular metal oxide supports and supported species. This is because 17O, the only stable NMRactive nucleus of oxygen, has a very low natural abundance (0.0373%) and small γ (5.772 MHz/T), resulting in low NMR sensitivity. While 17O isotopic labeling is often prohibitively expensive and can be difficult, 17O solid-state NMR was used to observe metal−oxo species and metal−surface interactions in silica-supported catalysts.32 With more complex systems like the Ziegler−Natta catalysts, 25 Mg and 35,37Cl solid-state NMR can be used to investigate the structure of the MgCl2 support.33 However, the acquisition of 25 Mg and 35,37Cl solid-state NMR can be challenging due to sensitivity issues since these nuclei have low γ values, combined with the significant line broadening stemming from the quadrupolar interaction. Sensitivity-enhancing techniques such as sideband-selective double-frequency sweeps (ssDFS) and quadrupolar Carr−Purcell−Meiboom−Gill (QCPMG) sequences can be used to reduce the necessary signal averaging time. When the line-broadening effect of the second-order quadrupolar interaction is exceedingly large (hundreds of kHz to tens of MHz), MAS is not as helpful. The incomplete averaging of the quadrupolar interaction below the fast limit results in a line shape that is more complicated to analyze. In such circumstances, acquiring the static powder pattern can be a preferred alternative. Wide-band uniform rate and smooth truncation (WURST) pulses can provide broadband excitation along with frequency-stepped spectral acquisition to acquire data over a wide frequency range, and CPMG can be applied for sensitivity gain.34 This approach was used to study metallocene catalysts and catalyst precursors using 91Zr and 35 Cl NMR.35

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ω1H = ωe1 − ωe2

(7)

The state-of-the-art PAs have been based on dinitroxyl radicals. Adequate inter-radical distance, rigidity, and molecular weight of the biradicals are key parameters to maximize the polarization transfer.39 While originally developed in aqueous conditions, DNP can take place in various organic solvents, in particular 1,1,2,2-tetrachloroethane (TCE), which can be of interest to characterize air-/moisture-sensitive materials.40 While TOTAPOL39b and, in particular, AMUPol39e are the well-performing PAs in water, the TEKPol series shows the best performance in organic media (Figure 3B).39d,f

DNP SENS

PRINCIPLE. One critical limitation of NMR is its intrinsically low sensitivity, often originating from the low natural abundance and/or low γ value of the active nuclei. The limitation is compounded by the low amount of surface sites on 10591


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Figure 4. DNP-enhanced (A) 1D 15N CP-MAS spectra of IrCp*-ppy-PMO and ppy-PMO, (B) 2D 27Al−29Si scalar (black trace/contour) and dipolar (red trace/contour) based refocused INEPT spectra of aluminosilicate with varied Al/Si ratios, (C) 2D 1H−29Si HETCOR spectra of MatImR and Mat-ImF, and (D) 2D 111Cd-13C D-HMQC spectrum of CdSe quantum dots capped with 13C-1-oleate ligands. (E) DFT-calculated T6 Snclosed-site structure, and experimental (black trace) and simulated (red trace) 119Sn CSA. (F) DNP-enhanced 2D refocused INADEQUATE spectrum of silica-supported tungsten TBP metallacyclobutane. Panel A is reproduced and modified from ref 42 with permission from The Royal Society of Chemistry. Panels B−E are reproduced and modified from refs 43a, 44, 46, and 47c, respectively, with permission from the American Chemical Society. Panel F is reproduced and modified from ref 48 with permission from Wiley.

DNP has recently been proven to be a powerful approach to increase the signal intensity of surface species, leading to a characterization method coined DNP surface-enhanced NMR spectroscopy (DNP SENS).37b,41 Recording solid-state NMR at 9.4 T combined with DNP SENS can enhance the 1H nuclear polarization by up to 100−200-fold, translating into a significant reduction of signal averaging time by a factor of 10 000−40 000 (1002−2002). The dramatic decrease of the necessary experimental time allows the implementation of many different 1D/2D NMR experiments to determine the structures of surface species at the molecular level, as discussed below in several examples. APPLICATIONS. DNP SENS was first applied to obtain information on the nature of the organic and silicon moiety in organic−inorganic hybrid materials as a prototype isolated organic fragment on an oxide surface, and it has now been expanded to a broad range of materials. For instance, it allows acquiring 15N NMR spectra at natural isotopic abundance, distinguishing organic ligands (on-the-surface vs in-the-wall) in periodic mesoporous organosilicates (PMOs), and obtaining direct evidence of the coordination of organometallic species on PMOs (Figure 4A).42 In another example, 2D INEPT-type 27 Al−29Si heteronuclear correlation experiments were implemented to investigate the structure at the interface between the silica and the alumina layers in aluminosilicates, in particular

revealing the connectivity between tetrahedral Si and Al atoms and corroborating the existence of pseudo-bridging silanols (Figure 4B).43 In the third example, the surface−ligand interaction in hybrid materials could be probed by 1H−X (29Si or 13C) correlations, therefore allowing the rationalization of the stability of Ru metathesis catalysts44 (Figure 4C) and the characterization of the structure of supported Ir catalysts.45 A similar approach (111Cd−13C correlation) was also applied to probe the interface between the organic capping agent and the surface of CdSe quantum dots, and hence to discriminate Cd atoms on the surface from bulk Cd atoms (Figure 4D).46 DNP SENS was also used to obtain 2D 119Sn CPMAT experiments at natural abundance of Sn-beta zeolites and further extract the CSA of each isotropic site. Combining experimental data and DFT calculations enabled determining the coordinating environment and the position of Sn in the zeolitic framework (Figure 4E).47 It was thus possible to show that the highly active catalysts for the glucose−fructose isomerization have mostly so-called Sn closed sites. Apart from relatively air-/moisture-stable materials, DNP SENS can also be used on highly sensitive surface organometallic species, helping to determine the structure of metallacyclobutane intermediates in supported metathesis catalysts. The increased sensitivity from DNP SENS made the implementation of a Jbased 2D INADEQUATE experiment feasible, which allowed 10592


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Journal of the American Chemical Society elucidation of bond connectivities between carbons (Cα and Cβ) in the surface metallacyclobutane intermediates (Figure 4F). It also allowed the application of dipolar-based POST-C7 experiments, and thus the measurement of the Cα−Cβ bond distance by simulating the DQ build-up curve to extract the dipolar coupling constant between Cα and Cβ.48 Through the high sensitivity gains achieved by DNP, obtaining a 3D structure of surface species is now in reach.49 This makes solid-state NMR an ideal tool to determine the structure of active sites in heterogeneous catalysts. However, despite its many advantages, challenges are still present in the use of DNP. First, the use of glass-forming solvents is needed to ensure an optimal DNP enhancement, and in some cases the intense signals from solvents can be overwhelming in the region of the chemical shifts of interest. Second, the chemical compatibility between the PA and the catalytic species can be problematic. Adding a dendritic tether to increase the steric hindrance of the PA reduced the interaction between the nitroxide radical and the surface alkylidene species.50 Utilization of porous platforms with small pore sizes coupled with PAs with large steric hindrance was also shown to prevent the reaction between the PA and surface active sites.47b,51 Third, the essential presence of PA can induce faster nuclear transverse relaxations (T2′), which can not only broaden the NMR signals but also negatively impact the application of many 2D NMR techniques. Ultrafast MAS DNP (up to 44 kHz at 100 K) and the reduction of PA concentration were shown to effectively preserve nuclear T2′.48,52 In general, the formulation of the DNP mixture (solvent, PA, and solid under analysis) and the development of complementary new hardware (ultrafast low-temperature DNP MAS probes52,53 and other essential electronics) may play crucial roles in generalizing this method for heterogeneous catalysis.

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The development of computational tools also constitutes a major advance in the field. Besides computing NMR parameters such as chemical shift tensors, it allows unraveling their origin through an orbital decomposition analysis scheme, which helps to relate readily accessible experimental values to the electronic properties of the observed nuclei. Particularly, it is possible to extract the contribution of frontier molecular orbitals (FMO) to the chemical shift. Because FMOs are also directing reactivity, one can foresee the possibility to derive detailed electronic structures and fingerprints of surface reaction intermediates, and in turn to establish relations between NMR chemical shift and reactivity. Finding predictive molecular descriptors of reactivity would constitute a major advance in (heterogeneous) catalysis (see examples in this direction in ref 29c). With DNP and/or other advanced NMR techniques, a broad range of functional materials and catalyst supports can now be studied: metal−organic frameworks (MOFs),55a,57 covalent organic frameworks (COFs),58 colloids,46,59 complex inorganic and/or organic materials,60 and composites.61 In addition, surfaces and interfaces also play an essential role in many other fields where “smart” functional materials are being developed, ranging from glass and ceramics,62 to microelectronic devices,63 as well as imaging and sensing.64 The tremendous gain in NMR sensitivity obtained by DNP opens the possibility to investigate surface and defect sites, which are typically related to high reactivity and catalytic activity in solids, including crystalline systems such as zeolites, MOFs, and related materials. Furthermore, the sensitivity gain provides unique opportunities to answer many open questions in catalysis, such as the relation between low surface area model systems, typically prepared and studied via surface science techniques (STM/AFM),65 and classical catalysts, thus bridging the gap between disciplines. The current state of the art in solid-state NMR has been applied on industrial catalysts like zeolites;47 however, it remains a challenge for supported nanoparticles.66 For this very large class of catalysts, several key questions about the structure of active sites and the role of the interface could be solved by NMR spectroscopy, provided that one can observe the adsorbate/reaction intermediates and even identify their location, i.e., on the metallic nanoparticle, on the solid support, or at the metal−support interface.66c,67 In turn, one can envision the characterization of active/defect sites and of the heterogeneity of surface sites over a broad range of industrial heterogeneous catalysts.1g,68 While many investigations can now be carried out in a short time and provide structural information on surface sites in many instances, solid-state NMR still faces several challenges, such as its use under more relevant conditions (operando) or for materials that contain paramagnetic centers. Additionally, in the case of DNP NMR, the development of alternative polarizing agents or methods compatible with a broader range of materials or having longer relaxation time at higher temperature is critical to broadly apply this technique, considering the chemical compatibility between free radical and solid catalyst surface and the loss of polarization induced by fast nuclear relaxation. Together with other spectroscopic methods, solid-state NMR will provide invaluable information, which will be critical in achieving detailed structure−activity relationships toward a more rational design of heterogeneous catalysts. It is also important to stress that NMR alone will not be able to provide all the necessary information and that combination of NMR with other advanced spectroscopic and

SUMMARY AND FUTURE DIRECTION

Over the years, the frontiers of solid-state NMR toward the characterization of surface sites have moved forward, thanks to advancement in hardware and novel pulse sequences. Combined with hyperpolarization methods like DNP, solidstate NMR can now be used not only to provide high-quality spectra but also to obtain 2D correlation spectra needed to determine the 3D structures of well-defined surface sites.49 This would be nearly impossible by other techniques that either require long-range order or provide only average information. The characterization with atomic-level precision of the structure and local environment of surface sites constitutes a paradigm shift in single-site heterogeneous catalysis; this builds the first step toward molecular-level structural characterization of active sites, and one can hope that it will emulate the progress that has been experienced in molecular biology with the emergence of multi-dimensional solution NMR.54 However, surface sites are, by nature, highly asymmetrical and suffer from local disorder (heterogeneity) that can lead to dramatic line broadening, in particular for quadrupolar nuclei, making them in some cases nearly impossible to be detected. This, together with fast relaxation, makes polarization-transfer techniques inefficient and challenging. Much research effort will have to be directed at improving the detection of such broad signals, such as direct DNP (direct polarization transfer from electrons to the heteronuclei of interest), DNP-enhanced broadband cross-polarization,55 or indirect detection techniques.56 10593


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Taoufik, M.; Copéret, C.; Lefebvre, F.; Basset, J.-M.; Solans-Monfort, X.; Eisenstein, O.; Lukens, W. W.; Lopez, L. P. H.; Sinha, A.; Schrock, R. R. Organometallics 2006, 25, 3554. (c) Blanc, F.; Copéret, C.; Thivolle-Cazat, J.; Basset, J.-M.; Lesage, A.; Emsley, L.; Sinha, A.; Schrock, R. R. Angew. Chem., Int. Ed. 2006, 45, 1216. (d) Blanc, F.; Copéret, C.; Lesage, A.; Emsley, L. Chem. Soc. Rev. 2008, 37, 518. (6) (a) Avenier, P.; Lesage, A.; Taoufik, M.; Baudouin, A.; De Mallmann, A.; Fiddy, S.; Vautier, M.; Veyre, L.; Basset, J. M.; Emsley, L.; Quadrelli, E. A. J. Am. Chem. Soc. 2007, 129, 176. (b) Avenier, P.; Taoufik, M.; Lesage, A.; Solans-Monfort, X.; Baudouin, A.; de Mallmann, A.; Veyre, L.; Basset, J. M.; Eisenstein, O.; Emsley, L.; Quadrelli, E. A. Science 2007, 317, 1056. (7) (a) Smith, M. E. Appl. Magn. Reson. 1993, 4, 1. (b) Wischert, R.; Florian, P.; Copéret, C.; Massiot, D.; Sautet, P. J. Phys. Chem. C 2014, 118, 15292. (8) (a) Lippmaa, E.; Maegi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R. J. Am. Chem. Soc. 1980, 102, 4889. (b) Leonardelli, S.; Facchini, L.; Fretigny, C.; Tougne, P.; Legrand, A. P. J. Am. Chem. Soc. 1992, 114, 6412. (9) (a) Feldman, J.; Davis, W. M.; Thomas, J. K.; Schrock, R. R. Organometallics 1990, 9, 2535. (b) Blanc, F.; Berthoud, R.; Copéret, C.; Lesage, A.; Emsley, L.; Singh, R.; Kreickmann, T.; Schrock, R. R. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 12123. (c) Mougel, V.; Copéret, C. Chem. Sci. 2014, 5, 2475. (d) Solans-Monfort, X.; Copéret, C.; Eisenstein, O. Organometallics 2015, 34, 1668. (e) Valla, M.; Wischert, R.; Comas-Vives, A.; Conley, M. P.; Verel, R.; Copéret, C.; Sautet, P. J. Am. Chem. Soc. 2016, 138, 6774. (10) (a) Grau, E.; Lesage, A.; Norsic, S.; Copéret, C.; Monteil, V.; Sautet, P. ACS Catal. 2013, 3, 52. (b) D’Anna, V.; Norsic, S.; Gajan, D.; Sanders, K.; Pell, A. J.; Lesage, A.; Monteil, V.; Copéret, C.; Pintacuda, G.; Sautet, P. J. Phys. Chem. C 2016, 120, 18075. (11) (a) Blanc, F.; Copéret, C.; Thivolle-Cazat, J.; Basset, J.-M.; Lesage, A.; Emsley, L.; Sinha, A.; Schrock, R. R. Inorg. Chem. 2006, 45, 9587. (b) Lesage, A. Phys. Chem. Chem. Phys. 2009, 11, 6876. (12) (a) Petroff Saint-Arroman, R.; Chabanas, M.; Baudouin, A.; Copéret, C.; Basset, J.-M.; Lesage, A.; Emsley, L. J. Am. Chem. Soc. 2001, 123, 3820. (b) Lesage, A.; Emsley, L. J. Magn. Reson. 2001, 148, 449. (13) (a) Lesage, A.; Steuernagel, S.; Emsley, L. J. Am. Chem. Soc. 1998, 120, 7095. (b) Lesage, A.; Emsley, L.; Chabanas, M.; Copéret, C.; Basset, J.-M. Angew. Chem., Int. Ed. 2002, 41, 4535. (14) (a) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds: The Chemistry of Transition Metal Complexes Containing Oxo, Nitrido, Imido, Alkylidene, or Alkylidyne Ligands; Wiley: Weinheim, 1988. (b) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 6908. (15) (a) Geen, H.; Titman, J. J.; Gottwald, J.; Spiess, H. W. Chem. Phys. Lett. 1994, 227, 79. (b) Brown, S. P.; Spiess, H. W. Chem. Rev. 2001, 101, 4125. (16) Rataboul, F.; Baudouin, A.; Thieuleux, C.; Veyre, L.; Copéret, C.; Thivolle-Cazat, J.; Basset, J. M.; Lesage, A.; Emsley, L. J. Am. Chem. Soc. 2004, 126, 12541. (17) Gajan, D.; Levine, D.; Zocher, E.; Copéret, C.; Lesage, A.; Emsley, L. Chem. Sci. 2011, 2, 928. (18) Hohwy, M.; Jakobsen, H. J.; Eden, M.; Levitt, M. H.; Nielsen, N. C. J. Chem. Phys. 1998, 108, 2686. (19) Hohwy, M.; Rienstra, C. M.; Jaroniec, C. P.; Griffin, R. G. J. Chem. Phys. 1999, 110, 7983. (20) Lesage, A.; Bardet, M.; Emsley, L. J. Am. Chem. Soc. 1999, 121, 10987. (21) Duer, M. J. Introduction to solid-state NMR spectroscopy; Blackwell: Oxford, UK, 2004. (22) (a) Widdifield, C. M.; Schurko, R. W. Concepts Magn. Reson., Part A 2009, 34A, 91. (b) Fry, E. A.; Sengupta, S.; Phan, V. C.; Kuang, S.; Zilm, K. W. J. Am. Chem. Soc. 2011, 133, 1156. (23) Herzfeld, J.; Berger, A. E. J. Chem. Phys. 1980, 73, 6021. (24) Haeberlen, U. High Resolution NMR in Solids: Selective Averaging; Academic Press: New York, 1976.

computational techniques will be essential in the characterization of the structure and the dynamics of surface species. Work in these directions are currently on going in our group.

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Christophe Copéret: 0000-0001-9660-3890 Wei-Chih Liao: 0000-0002-4656-6291 Christopher P. Gordon: 0000-0002-2199-8995 Ta-Chung Ong: 0000-0001-9463-4765 Present Address †

T.-C.O.: Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge ETH Zürich and the Swiss National Science Foundation (SNSF) for continuous funding. We are also grateful to our long-standing collaborators Prof. L. Emsley, Dr. A. Lesage, and Dr. G. Pintacuda for discussions and joint research programs during the past 15 years, and to the Ph.D. students and postdoctoral fellows who accepted to work between groups with different expertise. We are also indebted to many collaborators and their group members, Prof. O. Eisenstein, Prof. P. Sautet, Dr. C. Chizallet, and Dr. P. Raybaud in computational chemistry; Dr. P. Florian, Dr. D. Massiot, Prof. A. Rossini, Prof. G. Bodenhausen, and Dr. R. Verel for NMR spectroscopy; and Prof. M. Kovalenko for the development of DNP on the characterization of nanocrystals.

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REFERENCES

(1) (a) Copéret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, J. M. Angew. Chem., Int. Ed. 2003, 42, 156. (b) Wegener, S. L.; Marks, T. J.; Stair, P. C. Acc. Chem. Res. 2012, 45, 206. (c) Stalzer, M. M.; Delferro, M.; Marks, T. J. Catal. Lett. 2015, 145, 3. (d) Pelletier, J. D. A.; Basset, J.-M. Acc. Chem. Res. 2016, 49, 664. (e) Copéret, C.; Comas-Vives, A.; Conley, M. P.; Estes, D. P.; Fedorov, A.; Mougel, V.; Nagae, H.; Núñez-Zarur, F.; Zhizhko, P. A. Chem. Rev. 2016, 116, 323. (f) Copéret, C.; Fedorov, A.; Zhizhko, P. A. Catal. Lett. 2017, DOI: 10.1007/s10562-017-2107-4. (g) Copéret, C.; Allouche, F.; Chang, K. C.; Conley, M.; Delley, M. F.; Fedorov, A.; Moroz, I.; Mougel, V.; Pucino, M.; Searles, K.; Yamamoto, K.; Zhiziko, P. Angew. Chem., Int. Ed. 2017, DOI: 10.1002/anie.201702387. (2) (a) Sautet, P.; Delbecq, F. Chem. Rev. 2010, 110, 1788. (b) Comas-Vives, A.; Larmier, K.; Copéret, C. Chem. Commun. 2017, 53, 4296. (3) (a) Zhang, W.; Xu, S.; Han, X.; Bao, X. Chem. Soc. Rev. 2012, 41, 192. (b) Gutmann, T.; Grünberg, A.; Rothermel, N.; Werner, M.; Srour, M.; Abdulhussain, S.; Tan, S.; Xu, Y.; Breitzke, H.; Buntkowsky, G. Solid State Nucl. Magn. Reson. 2013, 55−56, 1. (c) Kobayashi, T.; Perras, F. A.; Slowing, I. I.; Sadow, A. D.; Pruski, M. ACS Catal. 2015, 5, 7055. (4) (a) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. W.; Ferguson, D. B. Acc. Chem. Res. 1996, 29, 259. (b) Hunger, M.; Weitkamp, J. Angew. Chem., Int. Ed. 2001, 40, 2954. (c) Hunger, M.; Wang, W. Solid-State NMR Spectroscopy. In Handbook of Heterogeneous Catalysis; WileyVCH Verlag GmbH & Co. KGaA: Weinheim, 2008. (d) Dědeček, J.; Sobalík, Z.; Wichterlová, B. Catal. Rev.: Sci. Eng. 2012, 54, 135. (5) (a) Chabanas, M.; Baudouin, A.; Copéret, C.; Basset, J. M.; Lukens, W.; Lesage, A.; Hediger, S.; Emsley, L. J. Am. Chem. Soc. 2003, 125, 492. (b) Rhers, B.; Salameh, A.; Baudouin, A.; Quadrelli, E. A.; 10594


Perspective

Journal of the American Chemical Society (25) (a) Bax, A.; Szeverenyi, N. M.; Maciel, G. E. J. Magn. Reson. 1983, 52, 147. (b) Hu, J. Z.; Wang, W.; Liu, F.; Solum, M. S.; Alderman, D. W.; Pugmire, R. J.; Grant, D. M. J. Magn. Reson., Ser. A 1995, 113, 210. (26) Dixon, W. T. J. Chem. Phys. 1982, 77, 1800. (27) Blanc, F.; Basset, J. M.; Copéret, C.; Sinha, A.; Tonzetich, Z. J.; Schrock, R. R.; Solans-Monfort, X.; Clot, E.; Eisenstein, O.; Lesage, A.; Emsley, L. J. Am. Chem. Soc. 2008, 130, 5886. (28) (a) Jokisaari, J.; Järvinen, S.; Autschbach, J.; Ziegler, T. J. Phys. Chem. A 2002, 106, 9313. (b) Autschbach, J.; Zheng, S. Magn. Reson. Chem. 2008, 46, S45. (c) Facelli, J. C. Prog. Nucl. Magn. Reson. Spectrosc. 2011, 58, 176. (29) (a) Halbert, S.; Copéret, C.; Raynaud, C.; Eisenstein, O. J. Am. Chem. Soc. 2016, 138, 2261. (b) Yamamoto, K.; Gordon, C. P.; Liao, W.-C.; Copéret, C.; Raynaud, C.; Eisenstein, O. Angew. Chem., Int. Ed. 2017, DOI: 10.1002/ange.201701537. (c) Gordon, C. P.; Yamamoto, K.; Liao, W.-C.; Allouche, F.; Andersen, R. A.; Copéret, C.; Raynaud, C.; Eisenstein, O. ACS Cent. Sci. 2017, DOI: 10.1021/acscentsci.7b00174. (30) Kerber, R. N.; Kermagoret, A.; Callens, E.; Florian, P.; Massiot, D.; Lesage, A.; Copéret, C.; Delbecq, F.; Rozanska, X.; Sautet, P. J. Am. Chem. Soc. 2012, 134, 6767. (31) Kermagoret, A.; Kerber, R. N.; Conley, M. P.; Callens, E.; Florian, P.; Massiot, D.; Delbecq, F.; Rozanska, X.; Copéret, C.; Sautet, P. J. Catal. 2014, 313, 46. (32) Merle, N.; Trebosc, J.; Baudouin, A.; Del Rosal, I.; Maron, L.; Szeto, K.; Genelot, M.; Mortreux, A.; Taoufik, M.; Delevoye, L.; Gauvin, R. M. J. Am. Chem. Soc. 2012, 134, 9263. (33) Blaakmeer, E. S.; Antinucci, G.; Busico, V.; van Eck, E. R. H.; Kentgens, A. P. M. J. Phys. Chem. C 2016, 120, 6063. (34) O’dell, L. A.; Rossini, A. J.; Schurko, R. W. Chem. Phys. Lett. 2009, 468, 330. (35) Rossini, A. J.; Hung, I.; Johnson, S. A.; Slebodnick, C.; Mensch, M.; Deck, P. A.; Schurko, R. W. J. Am. Chem. Soc. 2010, 132, 18301. (36) (a) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (b) Harris, K. J.; Lupulescu, A.; Lucier, B. E. G.; Frydman, L.; Schurko, R. W. J. Magn. Reson. 2012, 224, 38. (c) Perras, F. A.; Kobayashi, T.; Pruski, M. Phys. Chem. Chem. Phys. 2015, 17, 22616. (37) (a) Barnes, B. A.; De Paëpe, G.; van der Wel, A. P. C.; Hu, K. N.; Joo, C. G.; Bajaj, S. V.; Mak-Jurkauskas, L. M.; Sirigiri, R. J.; Herzfeld, J.; Temkin, J. R.; Griffin, G. R. Appl. Magn. Reson. 2008, 34, 237. (b) Rossini, A. J.; Zagdoun, A.; Lelli, M.; Lesage, A.; Copéret, C.; Emsley, L. Acc. Chem. Res. 2013, 46, 1942. (c) Ni, Q. Z.; Daviso, E.; Can, T. V.; Markhasin, E.; Jawla, S. K.; Swager, T. M.; Temkin, R. J.; Herzfeld, J.; Griffin, R. G. Acc. Chem. Res. 2013, 46, 1933. (d) Lelli, M.; Chaudhari, S. R.; Gajan, D.; Casano, G.; Rossini, A. J.; Ouari, O.; Tordo, P.; Lesage, A.; Emsley, L. J. Am. Chem. Soc. 2015, 137, 14558. (e) Can, T. V.; Ni, Q. Z.; Griffin, R. G. J. Magn. Reson. 2015, 253, 23. (38) (a) Hwang, C. F.; Hill, D. A. Phys. Rev. Lett. 1967, 18, 110. (b) Hwang, C. F.; Hill, D. A. Phys. Rev. Lett. 1967, 19, 1011. (c) Hovav, Y.; Feintuch, A.; Vega, S. J. Magn. Reson. 2012, 214, 29. (39) (a) Hu, K. N.; Yu, H. H.; Swager, T. M.; Griffin, R. G. J. Am. Chem. Soc. 2004, 126, 10844. (b) Song, C. S.; Hu, K. N.; Joo, C. G.; Swager, T. M.; Griffin, R. G. J. Am. Chem. Soc. 2006, 128, 11385. (c) Matsuki, Y.; Maly, T.; Ouari, O.; Karoui, H.; Le Moigne, F.; Rizzato, E.; Lyubenova, S.; Herzfeld, J.; Prisner, T.; Tordo, P.; Griffin, R. G. Angew. Chem., Int. Ed. 2009, 48, 4996. (d) Zagdoun, A.; Casano, G.; Ouari, O.; Schwarzwälder, M.; Rossini, A. J.; Aussenac, F.; Yulikov, M.; Jeschke, G.; Copéret, C.; Lesage, A.; Tordo, P.; Emsley, L. J. Am. Chem. Soc. 2013, 135, 12790. (e) Sauvée, C.; Rosay, M.; Casano, G.; Aussenac, F.; Weber, R. T.; Ouari, O.; Tordo, P. Angew. Chem., Int. Ed. 2013, 52, 10858. (f) Kubicki, D. J.; Casano, G.; Schwarzwälder, M.; Abel, S.; Sauvee, C.; Ganesan, K.; Yulikov, M.; Rossini, A. J.; Jeschke, G.; Copéret, C.; Lesage, A.; Tordo, P.; Ouari, O.; Emsley, L. Chem. Sci. 2016, 7, 550. (40) Zagdoun, A.; Rossini, A. J.; Gajan, D.; Bourdolle, A.; Ouari, O.; Rosay, M.; Maas, W. E.; Tordo, P.; Lelli, M.; Emsley, L.; Lesage, A.; Copéret, C. Chem. Commun. 2012, 48, 654.

(41) (a) Lesage, A.; Lelli, M.; Gajan, D.; Caporini, M. A.; Vitzthum, V.; Mieville, P.; Alauzun, J.; Roussey, A.; Thieuleux, C.; Mehdi, A.; Bodenhausen, G.; Copéret, C.; Emsley, L. J. Am. Chem. Soc. 2010, 132, 15459. (b) Lelli, M.; Gajan, D.; Lesage, A.; Caporini, M. A.; Vitzthum, V.; Miéville, P.; Héroguel, F.; Rascón, F.; Roussey, A.; Thieuleux, C.; Boualleg, M.; Veyre, L.; Bodenhausen, G.; Copéret, C.; Emsley, L. J. Am. Chem. Soc. 2011, 133, 2104. (42) Grüning, W. R.; Rossini, A. J.; Zagdoun, A.; Gajan, D.; Lesage, A.; Emsley, L.; Copéret, C. Phys. Chem. Chem. Phys. 2013, 15, 13270. (43) (a) Valla, M.; Rossini, A. J.; Caillot, M.; Chizallet, C.; Raybaud, P.; Digne, M.; Chaumonnot, A.; Lesage, A.; Emsley, L.; van Bokhoven, J. A.; Copéret, C. J. Am. Chem. Soc. 2015, 137, 10710. (b) Mouat, A. R.; George, C.; Kobayashi, T.; Pruski, M.; van Duyne, R. P.; Marks, T. J.; Stair, P. C. Angew. Chem., Int. Ed. 2015, 54, 13346. (44) Samantaray, M. K.; Alauzun, J.; Gajan, D.; Kavitake, S.; Mehdi, A.; Veyre, L.; Lelli, M.; Lesage, A.; Emsley, L.; Copéret, C.; Thieuleux, C. J. Am. Chem. Soc. 2013, 135, 3193. (45) Romanenko, I.; Gajan, D.; Sayah, R.; Crozet, D.; Jeanneau, E.; Lucas, C.; Leroux, L.; Veyre, L.; Lesage, A.; Emsley, L.; Lacôte, E.; Thieuleux, C. Angew. Chem., Int. Ed. 2015, 54, 12937. (46) Piveteau, L.; Ong, T.-C.; Rossini, A. J.; Emsley, L.; Copéret, C.; Kovalenko, M. V. J. Am. Chem. Soc. 2015, 137, 13964. (47) (a) Wolf, P.; Valla, M.; Rossini, A. J.; Comas-Vives, A.; NunezZarur, F.; Malaman, B.; Lesage, A.; Emsley, L.; Copéret, C.; Hermans, I. Angew. Chem., Int. Ed. 2014, 53, 10179. (b) Gunther, W. R.; Michaelis, V. K.; Caporini, M. A.; Griffin, R. G.; Román-Leshkov, Y. J. Am. Chem. Soc. 2014, 136, 6219. (c) Wolf, P.; Valla, M.; Núñez-Zarur, F.; Comas-Vives, A.; Rossini, A. J.; Firth, C.; Kallas, H.; Lesage, A.; Emsley, L.; Copéret, C.; Hermans, I. ACS Catal. 2016, 6, 4047. (d) Wolf, P.; Liao, W.-C.; Ong, T.-C.; Valla, M.; Harris, J. W.; Gounder, R.; van der Graaff, W. N. P.; Pidko, E. A.; Hensen, E. J. M.; Ferrini, P.; Dijkmans, J.; Sels, B.; Hermans, I.; Copéret, C. Helv. Chim. Acta 2016, 99, 916. (48) Ong, T.-C.; Liao, W.-C.; Mougel, V.; Gajan, D.; Lesage, A.; Emsley, L.; Copéret, C. Angew. Chem., Int. Ed. 2016, 55, 4743. (49) Berruyer, P.; Lelli, M.; Conley, M. P.; Silverio, D. L.; Widdifield, C. M.; Siddiqi, G.; Gajan, D.; Lesage, A.; Copéret, C.; Emsley, L. J. Am. Chem. Soc. 2017, 139, 849. (50) Liao, W.-C.; Ong, T.-C.; Gajan, D.; Bernada, F.; Sauvee, C.; Yulikov, M.; Pucino, M.; Schowner, R.; Schwarzwälder, M.; Buchmeiser, M. R.; Jeschke, G.; Tordo, P.; Ouari, O.; Lesage, A.; Emsley, L.; Copéret, C. Chem. Sci. 2017, 8, 416. (51) Pump, E.; Viger-Gravel, J.; Abou-Hamad, E.; Samantaray, M. K.; Hamzaoui, B.; Gurinov, A.; Anjum, D. H.; Gajan, D.; Lesage, A.; Bendjeriou-Sedjerari, A.; Emsley, L.; Basset, J.-M. Chem. Sci. 2017, 8, 284. (52) Chaudhari, S. R.; Berruyer, P.; Gajan, D.; Reiter, C.; Engelke, F.; Silverio, D. L.; Coperet, C.; Lelli, M.; Lesage, A.; Emsley, L. Phys. Chem. Chem. Phys. 2016, 18, 10616. (53) Lee, D.; Bouleau, E.; Saint-Bonnet, P.; Hediger, S.; De Paëpe, G. J. Magn. Reson. 2016, 264, 116. (54) (a) Ernst, R. R. Angew. Chem., Int. Ed. Engl. 1992, 31, 805. (b) Wüthrich, K. Angew. Chem., Int. Ed. 2003, 42, 3340. (55) (a) Kobayashi, T.; Perras, F. A.; Goh, T. W.; Metz, T. L.; Huang, W.; Pruski, M. J. Phys. Chem. Lett. 2016, 7, 2322. (b) Kobayashi, T.; Perras, F. A.; Murphy, A.; Yao, Y.; Catalano, J.; Centeno, S. A.; Dybowski, C.; Zumbulyadis, N.; Pruski, M. Dalton Trans. 2017, 46, 3535. (56) Perras, F. A.; Venkatesh, A.; Hanrahan, M. P.; Goh, T. W.; Huang, W.; Rossini, A. J.; Pruski, M. J. Magn. Reson. 2017, 276, 95. (57) (a) Rossini, A. J.; Zagdoun, A.; Lelli, M.; Canivet, J.; Aguado, S.; Ouari, O.; Tordo, P.; Rosay, M.; Maas, W. E.; Copéret, C.; Farrusseng, D.; Emsley, L.; Lesage, A. Angew. Chem., Int. Ed. 2012, 51, 123. (b) Brozek, C. K.; Michaelis, V. K.; Ong, T.-C.; Bellarosa, L.; López, N.; Griffin, R. G.; Dincă, M. ACS Cent. Sci. 2015, 1, 252. (58) Bertrand, G. H. V.; Michaelis, V. K.; Ong, T.-C.; Griffin, R. G.; Dincă, M. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 4923. (59) (a) Baquero, E. A.; Ojo, W.-S.; Coppel, Y.; Chaudret, B.; Urbaszek, B.; Nayral, C.; Delpech, F. Phys. Chem. Chem. Phys. 2016, 10595


Perspective

Journal of the American Chemical Society 18, 17330. (b) Cros-Gagneux, A.; Delpech, F.; Nayral, C.; Cornejo, A.; Coppel, Y.; Chaudret, B. J. Am. Chem. Soc. 2010, 132, 18147. (60) (a) Liu, J.; Plog, A.; Groszewicz, P.; Zhao, L.; Xu, Y.; Breitzke, H.; Stark, A.; Hoffmann, R.; Gutmann, T.; Zhang, K.; Buntkowsky, G. Chem. - Eur. J. 2015, 21, 12414. (b) Zhao, L.; Smolarkiewicz, I.; Limbach, H.-H.; Breitzke, H.; Pogorzelec-Glaser, K.; Pankiewicz, R.; Tritt-Goc, J.; Gutmann, T.; Buntkowsky, G. J. Phys. Chem. C 2016, 120, 19574. (61) (a) Akbey, U.; Altin, B.; Linden, A.; Ozcelik, S.; Gradzielski, M.; Oschkinat, H. Phys. Chem. Chem. Phys. 2013, 15, 20706. (b) Lee, D.; Monin, G.; Duong, N. T.; Lopez, I. Z.; Bardet, M.; Mareau, V.; Gonon, L.; De Paëpe, G. J. Am. Chem. Soc. 2014, 136, 13781. (c) Geiger, Y.; Gottlieb, H. E.; Akbey, Ü .; Oschkinat, H.; Goobes, G. J. Am. Chem. Soc. 2016, 138, 5561. (62) Sangodkar, R. P.; Smith, B. J.; Gajan, D.; Rossini, A. J.; Roberts, L. R.; Funkhouser, G. P.; Lesage, A.; Emsley, L.; Chmelka, B. F. J. Am. Chem. Soc. 2015, 137, 8096. (63) Alphazan, T.; Mathey, L.; Schwarzwälder, M.; Lin, T.-H.; Rossini, A. J.; Wischert, R.; Enyedi, V.; Fontaine, H.; Veillerot, M.; Lesage, A.; Emsley, L.; Veyre, L.; Martin, F.; Thieuleux, C.; Copéret, C. Chem. Mater. 2016, 28, 3634. (64) Gajan, D.; Bornet, A.; Vuichoud, B.; Milani, J.; Melzi, R.; van Kalkeren, H. A.; Veyre, L.; Thieuleux, C.; Conley, M. P.; Grüning, W. R.; Schwarzwälder, M.; Lesage, A.; Copéret, C.; Bodenhausen, G.; Emsley, L.; Jannin, S. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 14693. (65) Freund, H.-J. J. Am. Chem. Soc. 2016, 138, 8985. (66) (a) Comas-Vives, A.; Furman, K.; Gajan, D.; Akatay, M. C.; Lesage, A.; Ribeiro, F. H.; Copéret, C. Phys. Chem. Chem. Phys. 2016, 18, 1969. (b) Johnson, R. L.; Perras, F. A.; Kobayashi, T.; Schwartz, T. J.; Dumesic, J. A.; Shanks, B. H.; Pruski, M. Chem. Commun. 2016, 52, 1859. (c) Larmier, K.; Liao, W.-C.; Tada, S.; Lam, E.; Verel, R.; Bansode, A.; Urakawa, A.; Comas-Vives, A.; Copéret, C. Angew. Chem., Int. Ed. 2017, 56, 2318. (67) Perras, F. A.; Padmos, J. D.; Johnson, R. L.; Wang, L.-L.; Schwartz, T. J.; Kobayashi, T.; Horton, J. H.; Dumesic, J. A.; Shanks, B. H.; Johnson, D. D.; Pruski, M. J. Am. Chem. Soc. 2017, 139, 2702. (68) Delley, M. F.; Lapadula, G.; Núñez-Zarur, F.; Comas-Vives, A.; Kalendra, V.; Jeschke, G.; Baabe, D.; Walter, M. D.; Rossini, A. J.; Lesage, A.; Emsley, L.; Maury, O.; Copéret, C. J. Am. Chem. Soc. 2017, 139, 8855.

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