Metal-Organic Frameworks as Catalysts for Organic Synthesis: A

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Metal-Organic Frameworks as Catalysts for Organic Synthesis: A Critical Perspective Vlad Pascanu, Greco Gonzalez Miera, A. Ken Inge, and Belen Martin-Matute J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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Journal of the American Chemical Society

Metal-Organic Frameworks as Catalysts for Organic Synthesis: A Critical Perspective Vlad Pascanu, ‡a,b Greco González Miera, ‡a† A. Ken Inge,c Belén Martín-Matute*a a

Department of Organic Chemistry, Stockholm University, Stockholm, SE-10691, Sweden. Department of Chemistry, University of Zurich, Zurich, CH-8057, Switzerland. c Current Address: Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, SE-10691, Sweden. b

ABSTRACT: Recent advances in organic chemistry and materials chemistry have enabled the porosity of new materials to be accurately controlled on the nanometer scale. In this context, metal-organic frameworks (MOFs) have rapidly become one of the most attractive classes of solid supports currently under investigation in heterogeneous catalysis. Their unprecedented degree of tunability gives MOFs the chance to succeed where others have failed. The last decade has witnessed an exponential growth in the complexity of new structures. MOFs with a variety of topologies and pore sizes show excellent stability across wide ranges of pH and temperature. Even the controlled insertion of defects, to alter the MOF’s properties in a predictable manner, has become commonplace. However, research on catalysis with MOFs has been sluggish in catching up with modern trends in organic chemistry. Relevant issues such as enantioselective processes, C–H activation, or olefin metathesis are still rarely discussed. In this perspective, we highlight meritorious examples that tackle important issues from contemporary organic synthesis, and that provide a fair comparison with existing catalysts. Some of these MOF catalysts already outcompete state-of-the-art homogeneous solutions. For others, improvements may still be required, but they have merit in aiming for the bigger challenge. Furthermore, we also identify some important areas where MOFs are likely to make a difference, by addressing currently unmet needs in catalysis instead of trying to outcompete homogeneous catalysts in areas where they excel. Finally, we strongly advocate for rational design of MOF catalysts, founded on a deep mechanistic understanding of the events taking place inside the pore.

Introduction

Materials with accurately controllable nanometer-scale porosity have aroused interest for a long time as a result of their ability to selectively accommodate guest molecules that match the properties of their cavities. This is an enticing prospect for heterogeneous catalysis. We may envisage active centers confined within a porous network through which reagents and products could diffuse unimpeded. Metal-organic frameworks (MOFs) have been investigated in research towards this goal, and they have become the most prolific supports in use in heterogeneous catalysis today, judging by the number of publications in the last decade. MOFs are a class of porous materials constructed from metal cations (or clusters of metal cations) which are linked to one another by bridging organic linkers (Figure 1). They offer record-breaking porosity and an unprecedented degree of tunability, and as a result it is possible to control their function through molecular design. Unlike their inorganic relatives (zeolites, aluminophosphates), whose structures are fixed and unalterable, MOFs can be modified using the complete toolbox of organic synthesis, and their pores may be decorated with catalytic sites. At the same time, MOFs are most often crystalline. This allows us to investigate the distribution of active sites within the framework, and also to evaluate the influence of the framework on catalytic activity. In contrast, siliceous mesoporous materials have amorphous pores, and so, unlike MOFs, they cannot be investigated

by crystallographic techniques. Moreover, their pore size typically exceeds 100 Å, which makes it difficult to control guest occupancy. MOFs show a balanced mix of crystallinity, porosity, and tunability, and thus they have the potential to bridge the gap between micro- and mesoporous materials. There is significant potential for the commercial application of such materials in heterogeneous catalysis. In recent years, the complexity of the catalytic processes mediated by MOFs has steadily increased, and it is reasonable to imagine that MOFs will continue to deliver even more efficient solutions to problems in contemporary organic chemistry and modern organometallic catalysis. Unfortunately, from an organic chemist’s perspective, the amount of effort and resources that have been devoted to the development of MOFs with unique catalytic activities and with better performance than homogeneous catalysts has, to date, been rather limited. If MOFs are to occupy their well-deserved position as widely adopted catalysts, there needs to be a clear analysis of the real strengths, weaknesses, and inherent limitations of MOFs as catalysts for organic synthesis, and an accurate identification of the contexts in which their properties could best be exploited. In this perspective, we define the most important challenges in organic chemistry that have been and that could be addressed through the use of MOF catalysis, and differentiate these from areas where MOF catalysis is unlikely to make a significant impact. For example, some catalytic reactions have already been fully solved (in terms of yield and stereocontrol) using catalysts

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that are cheaper than MOFs and simpler in structure. In such cases, there is no logical basis for adopting a catalyst that is more complex and thus more prone to failure. At the other extreme, there are organic reactions that are far from solved, and for which improvements are highly desirable, but that are fundamentally beyond the scope of what can be achieved using MOF catalysts. Nevertheless, between these extremes there is a wealth of opportunities for MOF catalysts to make an impact on a scientific but also on an economic level. We believe that the field as a whole would benefit if more effort was concentrated in this area.

Finally, we believe that it is important to advance the conversation about MOFs in catalysis beyond the argument of recyclability. The recovery of toxic and expensive transition-metal species is hugely important in today’s chemistry.5 MOFs have proved to be excellent supports and/or scavengers that can potentially simplify purification procedures in large-scale processes. However, the reusability of these species requires a more thoughtful discussion. Recyclability becomes relevant in industrial settings where catalyst costs can become prohibitive. However, such processes must be carried out under stringent cGMP (Current Good Manufacturing Practice) standards. For a recyclable MOF catalyst to have realistic adoption prospects, it must show kinetic reproducibility over multiple runs, and not only that full conversion can be achieved repeatedly within a certain cut-off time.

Outstanding Examples of Organic Synthesis Mediated by MOF Catalysts

Figure 1. Catalytically active metal sites in a MOF can include those within the inorganic building units of the framework (dark blue), those that are anchored to the organic linker molecules as metal complexes (pink, purple), and those which reside within the pores (light blue).

In carrying out this analysis, we want to highlight some of the most compelling examples of the use of MOF catalysts to tackle important organic reactions. In some of these examples, though not all, the MOF does perform better than the homogeneous state-of-the-art catalyst. However, other important contributions are clearly facing in the right direction and deserve recognition, even if further development is still required. Furthermore, we advocate for a paradigm shift towards more rational design, which we consider to be imperative if transformative catalysis with MOFs is to be achieved. Some beautiful examples of this have already been published, but these are exceptions. Too often, considerable attention is dedicated to the structural aesthetics of the MOF, and a synthetic application is only identified afterwards to match the existing solution. We limit the scope of our analysis to organic transformations mediated by metal-functionalized MOFs, and also MOFs that act as organocatalysts, that aim to construct complex molecules, and that could possibly lead to future applications in the synthesis of specialty chemicals such as pharmaceuticals or agrochemicals. Other areas of MOF catalysis with undeniable scientific value and economic potential exist, but they are beyond the scope of this perspective. However, it is worth mentioning some outstanding examples: the Dincă1 and Farha2 groups have made outstanding contributions to the use of MOFs for the controlled polymerization of small hydrocarbons. Also, the concept of active-site isolation in MOFs, which we discuss below, has proved to be beneficial in the activation of small molecules for energyrelated applications (water splitting, CO2 fixation, etc.).3 Furthermore, MOF-templated catalysts have been subjected to pyrolysis to yield metallic nanoparticles enclosed in graphitic materials; these materials have great potential for catalysis, as was eloquently shown by Beller and coworkers.4

Over the past three decades, MOFs have become increasingly dominant in the area of functional materials. Their catalytic potential was identified early on. A prominent example is Fujita’s 1994 seminal report of a Cd-4,4-bipyridyl coordination polymer that could promote the cyanosilylation of carbonyl compounds; this is one of the earliest proof-of-concept studies in the field of MOF catalysis (Figure 2).6 For such early studies, catalysis was not a goal in itself, but rather a tool to probe the accessibility of active sites and their distribution throughout the framework. The value of the products resulting from the catalytic reaction was less significant.

Figure 2. A single layer of the {[Cd(4,4’-bpy)2](NO3)2}∞ square network with catalytic activity reported by Fujita et al.

In the years that followed, MOFs became increasingly elaborate as the accuracy of the tools used to functionalize their pores increased. This allowed more complex catalysts to be incorporated in a controlled manner. At the same time, in-situ characterization methods also underwent significant advances.7 These factors created a fertile environment that led to an explosion of interest in the field. Yet it took nearly two decades until MOFs started to tackle current challenges in organic synthesis. The fact that MOFs did eventually start to be considered in this context was, to a large extent, the result of an increased understanding of the power of post-synthetic modifications (PSM). Such modifications enrich the structural and functional diversity of MOFs in a manner that is analogous to the post-translational


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modifications of proteins. PSM opened up the possibility of incorporating complex catalysts that would not survive the harsh conditions typically used in MOF synthesis. The full arsenal of the PSM toolkit has been reviewed extensively,8 and was concisely summarized by Cohen in a recent perspective.9 In the early 2010s, reports of the successful immobilization of complex organometallic catalysts, capable of accommodating multi-electron redox cycles, started to emerge.10 Around that time, investigations into areas such as asymmetric catalysis11,12 and C–H activation13 became more commonplace among the MOF community. The idea of using MOFs to carry out reactions that could not be achieved in solution started to change from a thought experiment into serious research.14 To what extent have these ambitions now become reality? In this section, we attempt to identify reports that come close to providing real solutions to unmet needs in organic chemistry rather than trying to beat homogeneous catalysis at its own game.

1. Unique reactivity enabled by active-site confinement

PdX@MOF (X = TFA, 5 mol% Pd)

O H

HCO2H, HCO2Na H2O/MeOH (1:1)

OH 84% Yield

Figure 3. Top: Topological representation illustrating the underlying net of Zr6O4(OH)4(L-PdX)3 (where [L-PdX]4− = [(2,6(OPAr2)2C6H3)PdX]4−, Ar = p-C6H4CO2−, X = Cl, I, TFA). Blue cuboctahedra and grey squares represent the inorganic building units and the organic building units respectively. Bottom: Reduction of carbonyl compounds by Pd-TFA@MOF using formate as the hydrogen donor.

MOF catalysts with superior turnover numbers and lifetime The protective effect achieved by encapsulation of the catalyst is one of the most significant advantages of using a MOF. Confining the active species in a pore can offer the catalyst a level of protection from other reactive species that is difficult to The same year, the Lin group showed that a similar strategy achieve in the homogeneous phase by ligand engineering alone. could be used to stabilize Rh and Ir phosphines and prevent detMoreover, immobilization in the framework prevents the catarimental disproportionation reactions (Figure 4).18 Excellent lyst from undergoing deactivation pathways through aggregaTONs were achieved for hydrogenation and hydrosilylation retion and/or self-association. Such deactivation pathways repreactions, while only moderate activity was obtained for more sent a significant problem for first-row transition metals that opchallenging C(sp2)–H borylation reactions of simple substrates. erate through single-electron-transfer mechanisms, and also for The activity of the immobilized catalysts surpasses that of hohydrogen-bond-donating organocatalysts. Engineering the catmogeneous control catalysts and that of the Wilkinson catalyst alyst’s microenvironment can prolong its lifetime, which trans(developed in the 1960s).19 A more extended comparison of the lates into higher turnover numbers (TONs). Research in this catalytic activity of the MOF with modern, state-of-the-art catarea has already demonstrated that MOFs can outcompete hoalysts for hydrogenation and C–H borylation was not carried mogeneous catalysts. out. Early implementations of this approach were published by However, it was not long until the power of active-site isolaHupp and Farha, who showed that urea (2012)15 or squaramide tion was put to good use in what represents one the first com(2015)16 organocatalysts can efficiently catalyze Friedel–Crafts pelling examples of truly unique catalytic performance with alkylation reactions of electron-rich aromatic compounds with MOFs carrying out a reaction that would be unachievable in sonitroolefins, but only upon immobilization. The development of lution.20 The work published by Figueroa, Cohen, and coworknew catalytic protocols for this well-established transformation ers addresses the amination of activated C(sp3)–H bonds using was unlikely to make an impact in terms of applications. Howa nitrene precursor, a highly relevant reaction with a broad apever, these reports represent illustrative proofs-of-concept of plicability in the late-stage functionalization of active pharmathis powerful strategy, thus setting the stage for further develceutical ingredients (APIs). The authors demonstrated that a Mn opments. Soon after this, Wade and coworkers showed that the MOF with exposed unsaturated metal sites could achieve resame concept was also applicable to transition-metal catalysts (Figure 3). They showed that an electrophilic pincer-Pd(TFA) species was stabilized by incorporation into a MOF, and that its lifetime in the transfer hydrogenation of benzaldehyde was increased.17 The catalytic activity of the MOF is better overall than that of the homogeneous model catalyst, which decomposes rapidly in solution. Even though the authors did not discuss the substrate scope, and the MOF Figure 4. Crystal structure of the triarylphosphine MOF, P1-MOF·Rh, as reported by Lin and is far from competing with established coworkers. The coordination polyhedron around Zr cations are colored blue, while Rh, Cl, P, benchmark catalysts for transfer hyand O are shown as orange, green, pink, and red spheres, respectively. drogenation, this clearly represented an important step forward.



functional groups decorating the pore surface. Weller, Rosseinsky, and coworkers reported an elegant demonstration of how the pore microenvironment can influence the reaction pathway and control the selectivity.21 The authors used Crabtree’s wellknown cationic Ir hydrogenation catalyst, and immobilized it into a sulfonated variant of the chromium terephthalate MIL101. Crabtree’s [Ir(cod)(PCy3)(py)][PF6] no longer represents the state of the art, but it remains one of the most widely used commercially available catalysts for hydrogenations. Deactivation through self-association to form inactive polymetallic hydride clusters remains a major concern for this catalyst, but this can be minimized through the design of suitable ligands or the use of an appropriate counterion. In this work, the framework itself behaves like a counterion through its sulfonyl groups; it is through ionic interactions with these groups that the complex is immobilized. Once more, active-site isolation prolongs the lifetime of the catalyst. For the limited set of hydrogenation examples reported in the paper, the TON and TOF values are comparable to those of the homoX X geneous counterpart. What is reI CPF-5(Mn) NHR NTs H markable though, is that the perCH3CN or CHCl3 n n formance of the MOF-supported r.t., 30 min R = Ts catalyst greatly surpasses the hoR=H mogeneous Crabtree catalyst for the hydrogenation of allylic and Figure 5. Top: Crystal structure of CPF-5. Close-up view of the inorganic building units with 2+ homoallylic alcohols, which are Mn sites on the surface with three open coordination sites shown as black spheres. The coordiprone to undergo undesired isomnation polyhedra of other cations that lack open metal sites are colored purple. Bottom: C-H erization with homogeneous Ir amination reaction catalyzed by CPF-5(Mn). systems.22 The authors attribute this beneficial effect to the funcThe use of an abundant first-row transition metal justifies the tionalized pore surface, which can engage the hydroxy group in costs of the MOF; in homogeneous catalyst complexes, such noncovalent interactions that prevent it from coordinating to the metals typically require sophisticated ligands that serve a simiIr center. lar purpose of creating a protective pocket. This is essential A similar heterogenization approach for a Rh hydrogenation given the propensity of metals like Mn or Fe to form high-spin catalyst was developed by Metzger and Sanford.23 They showed species and engage in radical cycles, which makes them prone that MOFs with negative charge either at the node or on the to self-deactivation. The outstanding performance of this MOF linker can be used to immobilize cationic Rh species, without catalyst is indeed attributed to the fact that the active site is emany detrimental effects on their hydrogenation activity. The hetbedded in a rigid framework. Another significant aspect is the erogenized catalysts were recyclable, and showed interesting simplicity of the design, which results in a remarkable robustsize selectivity. ness. The authors supported their findings with mechanistic investigations, both experimental and computational, which led An even more striking effect was reported by Rannochiari and to a thorough understanding of the events taking place inside coworkers, who described a completely unprecedented behavthe pore. These studies are valuable, as they make it possible to ior of an organocatalyst when the catalysis takes place in a conpredict how modifications of the system can influence catalytic fined environment. Similarly to what is seen with enzymes, the performance. Although the substrate scope of this reaction is reaction outcome may be altered not by modifying the active rather limited, this work certainly represents a leap in the right site, but by tuning the surrounding microenvironment. The audirection for the use of MOFs in catalysis. thors investigated the effect of MOF additives on the Morita– Baylis–Hillman reaction, which is commonly used by total synthesis chemists for C–C bond formation. Note that the triMOF catalysts with improved / alternative selectivity phenylphosphine (PPh3) catalyst is not a priori immobilized Examples where catalyst encapsulation does not necessarily imwithin the framework, but it diffuses into the MOF together prove the TON but rather helps to tune the selectivity in a way with the reactants. Whether engagement of the target enone by that is difficult to achieve in solution are more common but PPh3 occurs before or after diffusion inside the MOF was not equally valuable. In such instances, the added value stems from clarified. However, experimental and theoretical mechanistic additional (secondary) interactions between the reactants and investigations unambiguously identify the expected triphenylphosphonium intermediate trapped inside the pore. A markable TONs (up to 10^5) and TOFs (above 10^4/h) that surpass those of benchmark homogeneous catalysts. Importantly, the authors carried out a fair comparison with the state-of-theart homogeneous catalytic systems for the same reaction. Furthermore, for some substrates containing a competitive double bond that could also react with the nitrene reagent to form undesired aziridines, the MOF catalyst showed superior selectivity towards C–H amination (Figure 5).


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concerted mechanism then unfolds, where again the active intermediate interacts with the functional groups that decorate the pore surface (i.e., amino groups), forming stable noncovalent interactions. Concomitantly, the three phenyl groups of the trapped intermediate cause significant steric hindrance, while conformational freedom is limited by the MOF. The tetrahedral configuration of the phosphonium moiety is distorted, opening the door for a completely unprecedented electrophilic attack leading to an unexpected aldol-Tischenko product (Figure 6).

O

PPh3

O Ph3P

UMCM1-NH2 or MOF5-NH2

OR1 C 2H 5

OR2 C 2H 5

No MOF OH

O

R1 = H, R2 = C3H7CO2 or R1 = C3H7CO2, R2 = H

C 2H 5

Figure 6. Top: Topological representation of UMCM1-NH2 (left) and MOF5-NH2 (right). Bottom: Diverging reaction pathways from a common triphenylphosphonium intermediate in the presence and absence of MOF.

All relevant control experiments show that this result cannot be achieved with homogeneous catalysts. Perhaps the most interesting aspect of this work is that the selectivity switch by confinement is observed only with some MOF topologies — those that can impose the necessary geometric constraints but also bear the chemical functionalities necessary to stabilize the reactive intermediate. This work represents a splendid example of correlation between pore environment (geometry and functional groups) and reaction pathway. An interesting potential continuation of this work could involve the use of MOFs as chiral auxiliaries to develop a heterogeneous, asymmetric version of this reaction. A very similar concept of selectivity modulation by pore engineering was also demonstrated for a proline organocatalyst.24 The authors carried out systematic point modifications around the proline group, and then measured the stereoselectivity of the aldol reaction catalyzed by the resulting MOFs to establish a structure–activity relationship. This allowed them to design a system in which the innate enantioselectivity of the catalyst was overridden by certain perturbations in its environment or reinforced by others. It is important to note that the authors did not aim to outcompete homogeneous catalysts in terms of TON (which would be a futile endeavor for an aldol reaction), and also that the diastereomeric ratios achieved are not excellent. However, this type of stereocontrol by pore engineering is extremely desirable for organic chemists, since numerous organo-

catalysts derived from natural products are only readily available as one enantiomer. Therefore, further developments of this method could have a significant impact on organic chemistry and eventually add to the toolkit for the construction of complex molecules. Stabilized intermediates for mechanistic studies The idea of active-site isolation typically focuses on achieving superior catalytic performance, but the same concept could also be useful when it comes to the stabilization of reaction intermediates. The characterization of transient intermediates during the development of new catalytic processes can be rather tedious. In the case of transition-metal catalysis, it involves probing the coordination sphere around the metal at different stages to test mechanistic hypotheses. Demanding crystallization experiments are frequently required for each intermediate, often under glovebox conditions. Using MOF-encapsulated catalysts, careful experimental design allows transformations to proceed at the metal center with retention of crystallinity, in a single-crystalto-single-crystal fashion. X-ray diffraction (XRD) snapshots of the active center can be taken at every stage of the catalytic cycle, with no need for additional crystallization work. In this context, MOFs present a terrific opportunity to speed up reaction development. This was illustrated beautifully by Doonan and Sumby25, who observed changes in the coordination geometry of a Co center upon heating or exposure to acetonitrile. The same paper gives snapshots of the oxidative addition of an alkyl halide to Rh, and also of the insertion of CO into the metal– carbon bond at a Rh center (Figure 7). Remarkably, Rh(I) is oxidized to Rh(III) without affecting the long-range order in the crystal. A much anticipated follow-up study was published three years later, where the entire catalytic cycle of the Rh-catalyzed carbonylation of methyl halides was described in great detail using single-crystal XRD.26 This allowed the authors to elucidate the reasons why only MeBr and not MeI could be catalytically converted into the desired acetyl halide, which is a key intermediate in the industrial production of acetic acid. As this technology becomes more general and user-friendly, we predict that it will be adopted by many research groups working on the development of organometallic methods.

2. Advanced Pore engineering Section 1 dealt with the simple yet powerful strategy of stabilizing a fragile catalyst by enclosing it in a protective confined environment. However, we believe that MOF catalysts show tremendous potential beyond simply offering protection. By taking full advantage of the tunability of MOFs, it should be possible to achieve an unprecedented level of control over the microenvironment of a catalytic reaction, reminiscent of enzymatic processes. Research aiming to push the boundaries of controlled structural complexity within the pores of MOFs would not immediately lead to catalytic processes applicable on an industrial scale. However, this is a niche of undeniable fundamental interest because it could open the door to unprecedented catalytic phenomena. The idea of using the rigid MOF network to incorporate several different functionalities27 and achieve heterogeneity within order also gained popularity in the early 2010s with the seminal reports of Yaghi and coworkers, who coined the term “multivariate” or MTV-MOFs.28 Elegant structures were achieved, and some limited control over the distribution of functionalities



sis, using mixtures of chiral Salen catalysts to facilitate the development of molecular complexity.31 From a library of six different metal-functionalized Salen dicarboxylate linkers that spanned almost the entire first row of the d-block, up to three were incorporated simultaneously to give ternary MTV-MOFs. Interpenetration brought the active centers into close proximity, which allowed them to operate in sinergy. These advanced materials catalyzed the epoxidation of olefins and simultaneous nucleophilic ring opening with a wide variety of N, O, and S nucleophiles, creating two adjacent chiral centers in a single operation, in good yields and with good to excellent stereocontrol (Figure 8). The ternary MOFs showed improved efficiency over the sum of their individual parts, even when compared to binary MOFs. Figure 7. Single-crystal-to-single-crystal transformation of Rh-functionalized Co-MOF reFinally, Yaghi and coworkers took the ported by Bloch et al. MOF·[Rh(CO)2][Rh(CO)2Cl2] is transformed into enzyme analogy even further in an exMOF·[Rh(CO)(CH3CN)(COMe)I]I upon oxidative addition and CO insertion. Coordination emplary demonstration of accurate pore polyhedra around Co are shown as purple octahedra, while the Rh center is shown as an orfunctionalization.32 A sequence of seven ange sphere. post-synthetic covalent modifications was carried out with surgical precision Nu O to graft various tripeptide sequences OH MTV-MOF, [O] MTV-MOF, [Nu] with catalytic activity in the pores. This R3 R1 R3 R3 R1 R1 DCM, 0 °C DCM, 0 °C unprecedented level of complexity was O O O R2 R2 R2 achieved without compromising the poO I rosity of the material. The resulting [Nu] = RO , RS , N3 O [O] = functionalized MOF showed asymmetS ric induction arising from steric conO straints, and a kind of catalytic activity that had previously only been observed with enzymes (Figure 9). In preliminary N results, it was found that this material N M O O catalyzed a sequence-specific peptide Zn O O Zn cleavage. This cleavage does not occur O O with the unfunctionalized MOF, nor with the molecular analogue of the immobilized tripeptide. If this proof of M = V, Cr, Mn, Fe, Co, Cu concept is followed by an optimized system, as the authors suggest, this may Figure 8. Olefin epoxidation followed by stereocontrolled nucleophilic ring opening, cataindeed fulfill the ultimate goal outlined lyzed by Salen MTV-MOFs. in the beginning of this perspective: to harness the tunability of MOFs to Bifunctional MOFs that combine the Lewis acidity/basicity of achieve enzyme-like complexity and catalyze reactions that are a structural element with the more refined catalytic properties beyond the reach of molecular homogeneous catalysts. The deof a second metallic species, post-synthetically immobilized in velopment of similar peptide-decorated MOFs for enantiomeric the pores, have received a lot of attention as tandem catalysts. resolution would then be an even more appealing prospect. A representative contribution from Huang and coworkers within the framework was demonstrated. However, the implementation of this strategy in a functional catalytic setting would still require a great amount of work. The same concept (two or more distinct functionalities embedded in the same framework, acting in a concerted or sequential manner), reemerged a few years later, albeit in a different guise.

showed that benzyl alcohol could be efficiently oxidized and the resulting benzaldehyde immediately protected to form the corresponding acetal within a single pass through the pores of a Pd-functionalized UiO-66-NH2(Zr) framework.29 Numerous similar reaction cascades have been developed with different metal@MOF architectures,30 facilitating access to some organic products. These early reports of multiple catalytic entities sharing the same microenvironment represent an important stepping stone on the route towards more elaborate cooperative systems. Meanwhile, MTV-MOFs bearing multiple active centers within a single material continued to be refined, and recent reports have started to reveal their true catalytic potential. Cui and coworkers published an elegant example of cooperative cataly-

3. Remarks on future directions It is also important to highlight some admirable recent efforts to expand the applicability of MOFs into unexplored territory. These examples represent pioneering work in directions that are likely to gain traction in the future. While it is not yet clear whether these endeavors will be successful, their merit lies in the fact that they are aiming to catalyze reactions that represent some of the most formidable challenges in organic chemistry.


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tions with a limited substrate scope. We do not mean to diminish the significant and continued role of serendipity in the discovery of new catalysts, but today we are in a position to rationally design MOF catalysts targeted at challenging organic reactions. The abundant supply of topologies, pore sizes, and stability profiles should allow us to create MOFs that meet the expectations and rigors of contemporary catalysis. To improve the design of future catalysts, we must strive to achieve a better understanding of the events that take place in-

One such case is Dincă’s metathesis catalyst immobilized developed on the Zr MOF NU-1000. The Lewis acidity of dehydrated Zr nodes is leveraged to immobilize and activate a rhenium catalyst.33 Olefin metathesis has witnessed tremendous developments since it began to revolutionize organic chemistry in the early 1990s.34 However, severe shortcomings persist and prevent metathesis from being more useful in complex settings. The main aspects that require attention encompass issues of kinetic stereocontrol, the reactivity of polarized or “biased” oleHO

O

HO

HO2C H N O HN

HO O

HO H N O

O N H

OH O

O

H N O

N H

SH

O

H N

N H

NH2 O

N

OH

NH2

HO O

HO H N

O NH2

O

HO

O

H N O

N H

NH2

Figure 9. Selective cleavage of an amide bond in a peptide sequence catalyzed by an enzyme-like biomimetic MOF.

fins such as alkenyl halides, nitroolefins, enol triflates, ketene acetals, etc., and importantly, tolerance of high degrees of substitution.35 Although the performance of the MOF catalyst does not measure up to the current state-of-the-art homogeneous catalysts, the design is remarkable, and the plan to produce an improved catalyst for this reaction is laudable. Another example of a process with still significant unmet needs is the C–H functionalization of completely unactivated C(sp3)– H bonds. The Zhou group has suggested that this reaction could be addressed using their popular “PCN” porphyrin-based MOF series,36 which use a catalyst design resembling the notorious Groves systems.37 It remains to be seen whether the incorporation of the metallated porphyrin into the PCN framework will give the catalyst a competitive advantage. Perhaps this could be achieved through the MTV strategy. However, the importance of this reaction makes any contribution significant.

The importance of transitioning to rational catalyst design

The chemistry of MOFs has developed exceptionally over the past two decades. It is now possible to choose from designs that are stable across the entire breadth of the pH scale, and that tolerate moisture and air, and we understand how these properties are related to the structure of the MOF. Bearing in mind our focus on highly functionalized organic molecules, it is notable that today’s MOFs are robust enough to outlive common functional groups with increasing reaction temperature. Therefore, MOFs are no longer the component of the reaction mixture that is most prone to decomposition. We have also acquired a deep understanding of framework defects; their location and density can be controlled, and to a large extent we can predict the influence they will have on the properties of a MOF.38 Earlier, when this wealth of information was unavailable, serendipity played a major contribution in the identification of viable MOF catalysts. However, the catalytic applications of these materials were generally limited to trivial organic reac-

side the pore by carrying out meticulous mechanistic studies on existing MOF catalysts. This will enable us to learn more about i) MOF–substrate secondary interactions (where exactly the catalysis occurs, and how the framework interacts with the relevant species); ii) site accessibility (how these species travel through the framework, and whether they can access all the active sites); and finally, iii) catalyst deactivation (when the catalysis stops, why does this happen, and which element of the design requires further improvements). MOF–substrate secondary interactions Significant efforts have been dedicated to distinguishing whether catalysis indeed takes place inside the pores or through fast leaching-redeposition (boomerang) mechanisms.39,40 It is essential that the reaction should take place inside the pores when the desired catalytic process is only feasible in confinement. When confinement is not strictly required, an alternative could be to use two-dimensional metal-organic layers or metalorganic nanosheets.41 This emerging class of exciting ultra-thin, grapheme-like materials can be accessed through a bottom-up approach and also trough the exfoliation of 3D MOFs. Significant synthetic challenges still remain to obtain layers with controlled thickness in high yields, and the materials achieved thus far have been studied more thoroughly for their charge transport properties.42,43 However, considering they lack any diffusion-related constraints typically associated with MOFs but still achieve catalyst stabilization through active-site isolation, they present interesting potential solutions for catalysis and several interesting application are beginning to emerge.44 Nevertheless, when confinement remains necessary, we must take the conversation a step further, from inside or outside the framework to precisely where and how inside the framework. An excellent example of how relevant this question can be is provided by Speybröck’s mechanistic inquiry into a cross-aldol reaction to produce jasminaldehyde, catalyzed by Zr MOFs UiO-66 and UiO-66-NH2 (Figure 10).45 This is a trivial reaction for which the use of a MOF is not necessarily justified. However, this work represents a prime example of how mechanistic



understanding can improve catalyst design. Previous experimental studies have shown that the rate of jasminaldehyde synthesis was enhanced by functionalization of the terephthalate linkers with amino groups. It was speculated that the introduction of a basic functionality in the vicinity of the Lewis acidic Zr cluster creates a bifunctional catalyst that outperforms the original.46 Using advanced DFT techniques supported by experimental validation, the authors showed that the NH2 group does not participate in the catalytic cycle in any way. Instead, it promotes the adsorption of the substrate onto/into the MOF, which accounts for the accelerated initial rate. However, the reaction

and they found that diffusion through the microporous network is slow compared the amination reaction itself. This knowledge can provide critical guidance for future design. The presented KIE analysis represents a useful new tool to understand the impact of substrate diffusion and it would be interesting to see applications in comparative studies on different MOF topologies.

Furthermore, the Long group has shown that not only the reaction rate but also the selectivity can be manipulated by changing the pore size and hydrophobicity to increase van der Waals interactions (Figure 11).48 This is an interesting report of the modulation of selectivity through confinement that could R H R H fit well under the section disH H cussing outstanding examples H H HO O H O O of catalytic MOFs for selectivR O O H ity control (vide supra). More R H Zr Zr Zr Zr importantly, it also describes H O H OH H O O O H O state-of-the-art mechanistic O H Zr Zr O tools used to understand preO Zr Zr cisely the events taking place O inside the pores. Systematic perturbations were applied to Zr the system by careful variation R R of linker substitution patterns, H H UiO-66-NH2 H and host–guest interactions of H H H O O O the resulting MOFs were monNH2 H OH O O itored spectroscopically. It was Zr Zr Zr Zr demonstrated that strategic inHOH O O O O No influence from -NH2 group sertion of carefully chosen nonZr Zr Zr Zr covalent interactions around O O O the active center could increase O O the activity, selectivity, and staH H H H bility of the MOF catalyst. InH deed, KIE and gas-adsorption Figure 10. Revised mechanism of jasminealdehyde synthesis catalyzed by UiO-66 / UiO-66-NH2. experiments supported a strong correlation between the observed variations in reactivity proceeds by an identical mechanism, and the reaction rates are on the one hand, and the diffusion and adsorption of the reacsimilar at later stages, irrespective of whether or not the NH2 tants within the framework on the other. This thorough mechagroup is present. The authors identified a proton transfer from nistic study shows how predictable catalysis with MOFs can be, the Zr-oxo cluster to the carbonyl oxygen of the adsorbed aldol once the necessary knowledge has been acquired, and how the product to be the rate determining step; hydrogen bonding with catalytic activity can be altered by structural modification of a the amino groups had no significant role in the reaction mechasite remote from the active site. nism. This is indeed a case of bifunctionality inside the pore. The metallic cluster displays Lewis acidic sites to activate the It is also important to note that substrate diffusion can be substrate, and also basic oxo species that serve to relay the proheavily influenced not only by the nature but also the degree of tons. However, the grafted NH2 groups on the linkers have little pore functionalization. Our group, in collaboration with the Ott beneficial effect, and actually promote the formation of undegroup, has shown that an H2-evolution catalyst can benefit sired imine species from the reactants. A precise understanding greatly from active-site isolation in a MOF. The functionalizaof the events taking place inside the pore and the way the reaction level could be forced up to 24%, which is the equivalent of tants interact with the framework is crucial to designing better 1 in 4 linkers bearing a catalytic group. Yet even under extreme catalysts, and more studies like this should be carried out. conditions, a maximum of 80–85% of the active sites could be Kinetics of substrate diffusion and adsorption accessed by a reporter probe.49 Earlier, we observed a similar A better understanding of how to facilitate access to the active pattern for a nano-Pd-loaded MOF, where the highest catalytic sites has direct implications for overcoming substrate-scope activity was observed at intermediate loading.50 This is a genlimitations. Powers and coworkers studied diffusion rates using eral phenomenon with implications in catalyst design but also a customized kinetic-isotope-effect (KIE) approach, adapted for TON calculation. When it comes to catalyst loading inside the MOF catalysis.47 The focus of this work was a Ru-catalyzed C– MOF, it cannot be said that less is more, but rather that interH amination reaction. The performance of the reaction was immediate is best to achieve an optimal balance between using the proved by encapsulation of the Ru active site inside a MOF, pore space efficiently and making sure that all active sites are which resulted in the suppression of side reactions. The authors accessible. gained valuable insight into the details of the catalytic cycle,


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encapsulated catalyst, in order to avoid the presence of poisonCatalyst deactivation Besides understanding how catalysis works inside the MOF, it is equally Fe-MOF important when it fails to understand OH O t-BuSO2PhIO why this happens. Recent advances in CD3CN, 25 °C operando spectroscopic techniques offer us the opportunity to observe deactivation pathways and learn important lessons. In a previous project, HO we were perplexed by the conflicting HO HO O O O behavior shown by a nano-Pd catalyst HO OH HO OH HO OH supported in Cr terephthalates MILO O O OH OH 101-NH2 and MIL-88B-NH2. These OH materials readily lost structural integrity under mild Suzuki coupling conditions (with Cs2CO3 as a base), yet they remained highly active and recyclable. This was attributed to the excellent scavenging properties of the MOFs, which, even upon loss of long-range order, were able to recapture Pd faster that it could agglomerate into large, inactive aggregates. In contrast, the same materials showed excellent stability under the much harsher conditions of the related Heck coupling. Yet despite the remarkable stability of the composite, the catalytic reaction was sluggish, and the catalyst was deactivated after one run. It became clear to us that one of the most important challenges for HO HO HO O F F O O the MOF catalysis community today HO OH HO OH HO OH is to develop better investigative tools O F F O O for observing catalysis within MOFs. OH OH OH In collaboration with researchers at Figure 11. Top: Fe2(dobdc) and isoreticular and functionalized analogues (dobdc = 2,5-dioxChristian-Albrechts University of ido-1,4-benzenedicarboxylate) used in the catalytic oxidation of cyclohexane. Bottom: cycloKiel and beamline scientists at hexane oxidation catalyzed by Fe-MOF. By increasing the pore size and introducing nonpolar PETRA III, DESY in Hamburg, Gerfunctional groups, a three-fold increase in the alcohol/ketone ratio was observed, along with an many, a custom reactor was develincrease in turnover number by an order of magnitude. oped for the simultaneous acquisition ing species altogether, or alternatively to carry out the operation of operando X-ray absorption and diffraction data (XAS and under continuous-flow conditions, to wash them away. MoreoPXRD).51 This set-up allows information about the catalytically ver, upon changing the reaction conditions, we identified an unactive species to be correlated with changes in the structural inexpected second mechanism by which the catalyst can operate tegrity of the crystalline support. The reactor was designed to in the Heck reaction. Thus, the MOF catalyst shows impressive be used at synchrotron beamlines. It included an inbuilt miniaadaptive behavior. These types of in-situ studies could be apturized stirring plate, and parameters like temperature and presplied to any MOF-supported catalytic systems, where tradisure, as well as the addition of reagents, could be controlled retional spectroscopic techniques are not sufficient. motely. This customized set-up enabled us to scrutinize any small variations in the coordination environment of Pd, and to This example and numerous others demonstrate that often catplot these on a timeline against reaction conditions and framealysts under development will fail completely, even when they work stability when studying the Heck reaction. Further support are one minuscule improvement away from becoming a sucwas drawn from NMR spectroscopy and electron microscopy cessful design. As numerous factors have to align for catalysis analysis (TEM and EDS). Combining all the collected inforwithin MOFs to be successful, this can easily happen. Carrying mation, we were able to map out the entire lifetime of the cataout mechanistic studies on systems that fail to deliver the delyst. The root of the problem of catalyst deactivation was resired result can offer valuable information, and can bring catavealed to be chemical in nature. The quick death of this catalyst lysts to life that would otherwise be abandoned. was not due to loss of crystallinity and porosity, nor to Pd leaching. Instead, catalyst poisoning by an apparently innocent speConclusions cies present in the reaction mixture led to blocking of the active sites. Once this information was acquired, potential redesign soThe exploratory approach to materials syntheses based on highlutions became immediately obvious. The simplest of these was throughput screening has led to an abundance of exciting porous to change the Pd precursor used for the preparation of the MOFstructures. These are of great fundamental interest, yet not all of them can be used in catalytic applications in organic chemistry.



The heterogenization costs can only be justified when the MOF catalyst provides excellent selectivity control and returns yields that outcompete other available catalytic systems. As long as there is no performance or cost margin, the simpler solution will always be preferred. Nevertheless, there is extensive room for innovation, and a candid conversation about the strengths and weaknesses of MOFs will help us focus resources onto areas where MOFs can address unmet catalytic needs. Once a pertinent problem has been identified, catalyst design can begin, harnessing the power of reticular chemistry. Mechanistic studies are essential if we are to understand why catalysis fails when it does, and to understand secondary interactions between substrates and the framework that could lead to superior or different reactivity. Certainly, the vast area of late-stage functionalization of fine chemicals could benefit greatly from the input of the MOF community. Today’s methods are only capable of distinguishing between innate (substrate-controlled) and guided (catalyst-controlled) reactivity. MOFs could reach beyond this, to develop microenvironment-controlled reactivity and enable a greater control over the outcome of catalytic reactions involving elaborate substrates. Transformational catalysis with MOFs will require understanding and manipulating highly complex systems. Working towards this goal, we have all embarked on a journey that is naturally slow, and progress is incremental. But through the identification of areas where catalyst development is most relevant, we can accelerate this process significantly and reach our common destination sooner. It has already been demonstrated that with appropriate catalyst design and in-depth mechanistic investigations, MOF catalysts can outcompete homogeneous catalysts in certain reactions. We see no insurmountable obstacles to continue achieving even more ambitious catalysis in the future.

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Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / V. Pascanu and G. González Miera contributed equally.

Funding Sources The Swedish Research Council Vetenskapsrådet (VR), the Swedish Research Council Formas, the Knut and Alice Wallenberg Foundation and the Göran Gustafsson Foundation are gratefully acknowledged.

ABBREVIATIONS API: Active Pharmaceutical Ingredient bpy: 2,2’-Bipyridine cGMP: Current good manufacturing practice CPF: Coordination Porous Framework Cod: cyclooctadiene. Dobdc: 4,6-Dioxido-1,3-benzenedicarboxylate EDS: Energy-dispersive X-ray spectroscopy KIE: Kinetic isotope effect MOF: metal-organic frameworks TFA: Trifluoroacetate

NMR: Nuclear magnetic resonance PXRD: Powder X-ray diffraction TEM: Transmission electron microscopy MTV: multivariate NU: Northwestern University PCN: Porous coordination network PSM: post-synthetic modification Py: Pyridine TOF: Turnover frequency TON: Turnover number UiO: Universitetet i Oslo UMCM: University of Michigan crystalline material XAS: X-Ray absorption spectroscopy XRD: X-Ray diffraction

AUTHOR INFORMATION Corresponding Author

ACKNOWLEDGMENTS

* [email protected]

Present Addresses †Institute of Transformative Bio-Molecules (WPI-ITbM) and Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan.

The Swedish Research Council through Ventenskapsrådet and Formas, the Knut and Alice Wallenberg Foundation, the Göran Gustafssons Foundation, and NordForsk are gratefully acknowledged.

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43 For selected representative examples see: a) Skorupskii, G.; Trump, B. A.; Kasel, T. W.; Brown, C. M.; Hendon, C. H.; Dincă, M. Efficient and Tunable One-Dimensional Charge Transport in Layered Lanthanide Metal-Organic Frameworks. ChemRxiv. Preprint. 2018. doi: 10.26434/chemrxiv.7253192.v1; b) Dong, R; Han, P.; Arora, H.; Ballabio, M.; Karakus, M.; Zhang, Z.; Shekhar, C.; Adler, P.; St. Petkov, P.; Erbe, A.; Mannsfeld, S. C. B.; Felser, C.; Heine, T.; Bonn, M.; Feng, X.; Canovas, E. High-mobility band-like charge transport in a semiconducting two-dimensional metal–organic framework. Nat. Mater. 2018, 17, 1027–1032; c) Campbell, M. G.; Liu, S.; Swager, T. M.; Dincă, M. Chemiresistive Sensor Arrays from Conductive 2D Metal−Organic Frameworks. J. Am. Chem. Soc. 2015, 137, 13780–13783. 44 a) Lin, Z.; Thacker, N. C.; Sawano, T.; Drake, T.; Ji, P.; Lan, G.; Cao, L.; Liu, S.; Wang, C.; Lin, W. Metal–organic layers stabilize earth-abundant metal–terpyridine diradical complexes for catalytic C–H activation. Chem. Sci. 2018, 9, 143–151; b) Ding, Y.; Chen, Y.-P.; Zhang, X.; Chen, L.; Dong, Z.; Jiang, H.-L.; Xu, H.; Zhou, H.-C. Controlled Intercalation and Chemical Exfoliation of Layered Metal−Organic Frameworks Using a Chemically Labile Intercalating Agent. J. Am. Chem. Soc. 2017, 139, 9136– 9139; c) Shi, W.; Cao, L.; Zhang, H.; Zhou, X.; An, B.; Lin, Z.; Dai, R.; Li, J.; Wang, C.; Lin, W. Surface Modification of Two-Dimensional Metal–Organic Layers Creates Biomimetic Catalytic Microenvironments for Selective Oxidation. Angew. Chem. Int. Ed. 2017, 56, 9704–9709. 45 Hajek, J.; Vandichel, M.; Van de Voorde, B.; Bueken, B.; De Vos, D.; Waroquier, M.; Van Speybroeck, V. Mechanistic studies of aldol condensations in UiO-66 and UiO-66NH2 metal organic frameworks. J. Catal. 2015, 331, 1–12. 46 Vermoortele, F.; Ameloot, R.; Vimont, A.; Serre, C.; De Vos, D. An amino-modified Zr-terephthalate metal–organic framework as an acid–base catalyst for cross-aldol condensation. Chem. Commun. 2011, 47, 1521–1523. 47 Wang, C.-H.; Das, A.; Gao, W.-Y.; Powers, D. C. Probing Substrate Diffusion in Interstitial MOF Chemistry with Kinetic Isotope Effects. Angew. Chem. Int. Ed. 2018, 57, 3676–3681. 48 Xiao, D. J.; Oktawiec, J.; Milner, P. J.; Long, J. R. Pore Environment Effects on Catalytic Cyclohexane Oxidation in Expanded Fe 2 (dobdc) Analogues. J. Am. Chem. Soc. 2016, 138, 14371–14379. 49 Roy, S.; Pascanu, V.; Pullen, S.; González-Miera, G.; Martín-Matute, B.; Ott, S. Catalyst Accessibility to Chemical Reductants in Metal-Organic Frameworks. Chem. Commun. 2017, 53, 3257–3260. 50 Pascanu, V.; Yao, Q.; Bermejo Gómez, A.; Gustafsson, M.; Yun, Y.; Wan, W.; Samain, L.; Zou, X.; Martín-Matute, B. Sustainable catalysis: Rational Pd loading on MIL-101NH2 for more efficient and recyclable Suzuki-Miyaura reactions. Chem Eur. J. 2013, 19, 17483–17493. 51 Yuan, N.; Pascanu, V.; Huang, Z.; Valiente, A.; Heidenreich, N.; Leubner, S.; Inge, A. K.; Gaar, J.; Stock, N.; Persson, I.; Martín-Matute, B.; Zou, X. Probing the Evolution of Palladium Species in Pd@MOF Catalysts during the Heck Coupling Reaction: An Operando X-ray Absorption Spectroscopy Study. J. Am. Chem. Soc. 2018, 140, 8206–8217.


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Metal-Organic Frameworks as Catalysts for Organic Synthesis: A

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