Catalysts Confined in Programmed Framework Pores Enable New

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Catalysts Confined in Programmed Framework Pores Enable New Transformations and Tune Reaction Efficiency and Selectivity Tian-You Zhou, Bernhard Auer, Seok J. Lee, and Shane G. Telfer* MacDiarmid Institute for Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University, Palmerston North, 4442 New Zealand

J. Am. Chem. Soc. Downloaded from pubs.acs.org by TULANE UNIV on 01/15/19. For personal use only.

S Supporting Information *

ABSTRACT: Controlling chemical reactions in porous heterogeneous catalysts is a tremendous challenge because of the difficulty in producing uniform active sites that can be tuned with precision. However, analogous to enzymes, when a catalytic pocket provides complementary close contacts and favorable intermolecular interactions with the reaction participants, the reaction efficiency and selectivity may be tuned. Here, we report an isoreticular family of catalysts based on the multicomponent metal−organic framework MUF-77. The microenvironment around the site of catalysis was successfully programmed by introducing functional groups (modulators) to the organic linkers at sites remote from the catalytic unit. The framework catalysts produced in this way exhibit several unique features, including the simultaneous enhancement of both reactivity and stereochemical selectivity in aldol reactions, the ability to catalyze Henry reactions that cannot be accomplished by homogeneous analogs, and discrimination between different reaction pathways (Henry versus aldol) that compete for a common substrate.

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INTRODUCTION

predictable and well-ordered pore microenvironments can be designed and synthesized. In earlier work,35 we reported the first family of quaternary MOFs constructed from three topologically distinct carboxylate linkers and Zn4O secondary building units (SBUs). These were designated as MUF-7 (MUF = Massey University Framework). Programmed poresmultiple functional groups compartmentalized in a predetermined array in a periodic latticecould be created in a series of isoreticular MUF-7 frameworks by employing the aforementioned strategy. Related MOFs that incorporate tritopic truxene-based ligands (MUF-77) exhibit excellent stability to water and the uptake of guest molecules.36 Recently, we established MUF-77 analogues as a new class of recyclable and stable heterogeneous catalysts37 in which the catalytic units occupy ordered and well-defined single sites38 that are free from disorder and defects. We found that the enantioselectivity innate to homogeneous catalysts could be reversed upon inclusion in a framework pore. In turn, the degree of enantioselectivity could be tuned by modulator groups appended to the ancillary ligands. However, in this system, enhancements to stereochemical selectivity were achieved only at the expense of reactivity. We postulated that further modifications to the active site environment could elevate catalyst performance beyond these initial results. Specifically, we

One powerful strategy that has emerged in heterogeneous catalysis is to transfer active sites from molecular catalysts to solid-state analogs.1,2 This strategy can produce uniform catalytic microenvironments in the solid state that can be tuned in ways not available to molecular catalysts. Analogous to enzymes, a solid-state catalytic pocket can benefit from complementary close contacts and favorable intermolecular interactions with suitably sized substrates. Optimizing these interactions exerts control over the chemical transformation of interest to deliver products with high efficiency and selectivity.3,4 Porosity5−11 is the most important feature of metal−organic frameworks (MOFs), and the diversification of pore spaces encompasses opportunities to optimize functional attributes.12−15 Metal−organic frameworks have found a niche as heterogeneous catalysts.16−26 Research in this domain has been facilitated by modern synthetic organic protocols, which offer routes to functionalized ligands and thus pore environments with tailored characteristics. This is particularly relevant to multicomponent metal−organic frameworks, which are built up from a set of two or three ligands with different geometries.27−34 Isoreticular series of frameworks emerge from certain sets of ligands in which the members bear different functional groups. This constitutes a method for obtaining pores that can be programmed by the size and chemical characteristics of the functional groups installed on the linkers. In this way, diverse yet © XXXX American Chemical Society

Received: October 18, 2018

A

DOI: 10.1021/jacs.8b11221 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

prolinyl group, was exposed by thermolytic cleavage of the Boc protecting group.40,41 The framework comprises two coligandsa truxene-based tritopic carboxylate and a biphenyldicarboxylatethat occupy unique positions in the lattice without randomness or disorder. Functional groups can be installed on these coligands by organic synthesis prior to framework assembly (Figure 1a). Although they are positioned remotely from the catalytic unit, these functional groups serve as “modulators” that are able to influence the course of the catalyzed reaction by engaging in noncovalent interactions with the reaction participants.37 This parallels the way by which homogeneous catalysts can be tuned by noncovalent interactions.4,42 The influence of the modulators on the catalyzed reaction can be evaluated by measuring the rate constant and enantiomeric excess (ee) of catalyzed reactions. The measured rate constant reflects the reactivity of the catalyst, and the measured ee reports on the stereochemical selectivity of the catalyst. We emphasize that our intention was not to make highperformance catalysts, per se. Many such catalysts exist in the literature. Rather, it is the relative performance of the programmed MOF catalysts that is crucial. This emanates from modifications to the three-dimensional spatial environment of the catalytic site. The single-site nature of these catalysts, with the prolinyl group embedded in a uniform microenvironment, allows framework modifications to be correlated with the catalysis metrics. Our observations start with frameworks derived from bdc-Pro and 4,4′-biphenyl dicarboxylate (bpdc). The third linker in the framework featured methyl, butyl, hexyl, octyl, or decyl alkyl groups (hmtt, hbtt, hhtt, hott, or hdtt). This isoreticular series of MUF-77 catalysts has the formula [Zn4O(bdcPro)1/2(bpdc)1/2(hxtt)4/3]. These catalysts adopt the ith-d topology of the parent framework, as shown by powder XRD, and they are free of missing-linker defects, as established by 1H NMR spectroscopy on dissolved samples. The single-crystal structure of a representative catalyst, [Zn 4 O(bdcPro)1/2(dppdc)1/2(hhtt)4/3], was collected. As expected, the structure of its catalytic site is in accord with the model presented in Figure 1a. Although gas-phase porosity is not strictly necessary for the solution-phase catalysis presented herein, N2 adsorption isotherms and pore size distributions for two catalysts demonstrate the accessibility of small-molecule substrates in these materials (Figures S5 and S6). The bdc-Pro/bpdc/hxtt series of MUF-77 frameworks is effective at catalyzing aldol reactions involving acetone and either p-nitrobenzaldehyde (p-aldol reaction) or m-nitrobenzaldehyde (m-aldol reaction). Introducing alkyl group modulators on the truxene ligands into the catalytic pore has a pronounced impact on the reactivity of the catalysts (Table 1, Figure 2). The catalytic rate constants for the MOFs containing hmtt and hbtt are very similar for both the p-aldol and m-aldol reactions (Table 1, entries 1 and 2). In contrast, we observed a significant jump in the rate constants with hexyl group modulators (Table 1, entry 3). This is indicative of the alkyl chains of the truxene modulator ligand becoming long enough to interact beneficially with the reaction participants. Truxene ligands bearing octyl chains led to a minor decrease in the reactivity, but these groups still accelerate the catalysis with respect to the shorter chains. When hdtt, featuring decyl groups, was introduced, the rate constant dropped dramatically, resulting in the lowest value among this series of catalysts (Table 1, entry 5). Only in this case is deceleration observed, possibly because of filling of the pore volume by the alkyl chains.

sought to address the challenge of simultaneously enhancing both reactivity and stereochemical selectivity. To date, the repertoire of reactions catalyzed by the MUF-77 frameworks is rather narrow; hence, we were also interested in broadening their scope to encompass new types of reactions. We were particularly intrigued by the potential for these catalysts to discriminate between different reaction channels available to a given substrate and by hints that these framework catalysts were able to catalyze reactions that cannot be accomplished by homogeneous analogs.

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RESULTS AND DISCUSSION The composition of the programmable catalytic sites in MUF-77 frameworks, as deduced from earlier results,37 is illustrated in Figure 1a. The preparation of these frameworks relies on two

Figure 1. (a) Schematic illustration of the catalytic pocket in MUF-77 series, based on single-crystal X-ray diffraction (SCXRD) structures, highlighting the way by which its environment can be programmed by the installation of both a catalytic linker and functional groups on remote linkers. These groups act as modulators to influence the outcome of the catalytic reaction. (b) Aldol and Henry reactions are reported herein.

steps: installation of a catalytic linker using bottom-up framework self-assembly and its subsequent deprotection. First, a Boc-protected (Boc = tert-butoxycarbonyl) prolinyl procatalyst39 is installed by appending it to the benzenedicarboxylate (bdc) ligand. This linker is referred to as bdc-ProBoc. In a subsequent step, the catalytically active unit, an enantiopure B

DOI: 10.1021/jacs.8b11221 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society Table 1. Aldol Reaction of Acetone with pNitrobenzaldehyde (p-Aldol) or m-Nitrobenzaldehyde (mAldol) Catalyzed by [Zn4O(bdc-Pro)1/2(bpdc)1/2(hxtt)4/3]a rate constantb (L mol−1 day−1)

We subsequently turned our attention to a second isoreticular series of MUF-77 derivatives that have the general formula [Zn4O(bdc-Pro)1/2(dppdc)1/2(hxtt)4/3] (Figure 1a, Table 2). Table 2. Aldol Reaction of Acetone and p-Nitrobenzaldehyde Catalyzed by [Zn4O(bdc-Pro)1/2(dppdc)1/2(hxtt)4/3]

eec (%)

no.

catalyst linker set

p-aldold

m-aldole

p-aldol

m-aldol

1 2 3 4 5 6

bdc-Pro/bpdc/hmtt bdc-Pro/bpdc/hbtt bdc-Pro/bpdc/hhtt bdc-Pro/bpdc/hott bdc-Pro/bpdc/hdtt Me2bdc-Prof

0.69 0.68 2.42 2.12 0.54 0.05

0.72 0.73 1.76 1.38 0.54 0.19

−3.5 4.5 3.0 6.1 2.9 9.7

−4.0 1.8 −3.6 4.4 −3.3 21.4

a

Standard reaction conditions: 10 mol % catalyst (prolinyl units relative to total aldehyde), p- or m-nitrobenzaldehyde in acetone (0.035 M) at 20 °C. bRate constant based on the consumption of nitrobenzaldehyde over the initial 12 h of the reaction. cEnantiomeric excess of aldol products, a positive sign indicates the enantiomer of shorter retention time on high-performance liquid chromatography is dominant and vice versa. dp-Aldol is between p-nitrobenzaldehyde and acetone. em-Aldol is between m-nitrobenzaldehyde and acetone. Detailed data are available as Supporting Information. fControl reaction using the dimethyl ester of the catalytic linker (Me2bdc-Pro) under homogeneous conditions.

no.

catalyst linker set

rate constant (L mol−1 day−1)

ee (%)

1 2 3 4 5 6

bdc-Pro/dppdc/hmtt bdc-Pro/dppdc/hbtt bdc-Pro/dppdc/hhtt bdc-Pro/dppdc/hott bdc-Pro/dppdc/hdtt Me2bdc-Proa

0.37 0.66 1.21 1.20 0.18 0.05

−0.3 4.7 19.4 27.7 1.3 9.7

a

Control reaction using the dimethyl ester of the catalytic linker under homogeneous conditions.

The catalyst generated from bdc-Pro/dppdc/hmtt (Table 2, entry 1) shows a low ee value, as we observed for bdc-Pro/bpdc/ hmtt (Table 1, entry 1), along with a small rate constant. By replacing the hmtt with hhtt, we induced a significant enhancement in both the rate constant and the ee (Table 2, entry 3). By extending this strategy, we observed that the ee rises with hott, although the rate constant plateaus (Table 2, entry 4). The catalysts produced from bdc-Pro/dppdc/hhtt and bdc-Pro/ dppdc/hott have much better overall performance than those from bdc-pro/dppdc/hmtt and bdc-Pro/bpdc/hmtt, which emanates from the introduction of alkyl group modulators to the truxene units. Moreover, the catalysts generated from bdcPro/dppdc/hhtt and bdc-Pro/dppdc/hott outperform the Me2bdc-Pro homogeneous catalyst (Table 2, entry 6). When hdtt was introduced (Table 2, entry 5), a deleterious pore-filling effect is probably responsible for the rapid drop-off in both the rate constant and ee since the alkyl groups on the truxene ligand are overly long. This is a key series of findings. In the bdc-Pro/bpdc/hxtt isoreticular series of MOF catalysts, the truxene modulator has a significant impact on the reactivity and a negligible effect on stereoselectivity. In this bdc-Pro/dppdc/hxtt series, the replacement of the bpdc linker with the dppdc modulator induces a consistent growth in both the rate constants and ee of the aldol product. This suggests that a cooperative effect between the dppdc and truxene modulators bearing suitable alkyl groups switches on an enhancement of the stereoselectivity. This couples to an underlying rate increase brought about by the alkyl groups alone. These advances draw on the multicomponent nature of the MUF-77 framework since they rely on an organized array of catalytic and modulator linkers, and identify a simple and effective way to simultaneously boost two key characteristics of these catalysts. While aldol reactions are a tremendously important class of carbon−carbon bond-forming reactions, the most powerful synthetic catalysts are competent toward a wide range of transformations.4 In this light, we sought to establish the broad utility of MUF-77 catalysts by exploring their potential to accelerate new types of reaction. We found that the [Zn4O(bdcPro)1/2(dppdc)1/2(hxtt)4/3] family is competent toward the Henry reaction between nitromethane and aromatic aldehydes (Figure 1b). A series of control reactions that confirm [Zn4O(bdc-Pro)1/2(dppdc)1/2(hxtt)4/3] to be the active catalyst are listed in Tables S4 and S5. As part of these controls, we made the striking observation that the dimethyl ester of the catalytic linker, Me2bdc-Pro, does not catalyze the Henry reaction under

Figure 2. Observed rate constants of the p-aldol and m-aldol reactions by MUF-77 analogs from [Zn4O(bdc-Pro)1/2(bpdc)1/2(hxtt)4/3].

In addition, the total amount of nitrobenzaldehyde converted after 24 h falls in the range 38−91%, and as expected, it exhibits the same trend as the rate constants (Table S1, Supporting Information). Interestingly, the rate constants of all the heterogeneous catalysts are all considerably higher than those under homogeneous conditions, measured using Me2bdc-Pro, the dimethyl ester of the catalytic linker (Table 1, entry 6). Not only do the MUF-77 pores provide an environment for tuning the activity of the aldol reaction, but also they accelerate the transformation with respect to this solution-based catalyst. Modulation of the alkyl groups on the truxene linkers does not significantly improve the stereoselectivity of the aldol reaction. The ee fluctuates over a small range between −4.5% and 6.1% for both the p-aldol and m-aldol reactions with the full set of truxene ligands (Table 1). The results allow us to conclude that for this reaction the alkyl modulator groups on truxene linkers are primarily effective for modulating the reactivity of the catalytic site, rather than its stereoselectivity. C

DOI: 10.1021/jacs.8b11221 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 3. Rate constants of m-aldol reaction and m-Henry reactions catalyzed by [Zn4O(bdc-Pro)1/2(dppdc)1/2(hxtt)4/3]. The Henry reaction is between m-nitrobenzaldehyde and nitromethane. Standard reaction conditions for the Henry reaction: 10 mol % catalyst (prolinyl units relative to total aldehyde), 0.035 M m-nitrobenzaldehyde in tetrahydrofuran (THF)/nitromethane (9:1) at 20 °C. Rate constants are based on the consumption of nitrobenzaldehyde over the initial 12 h of the reaction.

Table 3. Competition Reactiona between m-Aldol and mHenry Reaction Catalyzed by MUF-77 Analogs

homogeneous conditions (Table S4, Figure S38). The reaction proceeds only when the bdc-Pro linker is embedded in the MUF-77 framework. This result highlights a unique feature of MOF catalysts: the framework microenvironment created around the prolinyl unit switches on its catalytic activity. With both the aldol and Henry reactions in hand, we set out to investigate how modifying the active site environment in the catalysts might tune the way they discriminate between the two reaction pathways. Our initial experiments involved independently conducting m-aldol reactions (between m-nitrobenzaldehyde and acetone) and m-Henry reactions (between mnitrobenzaldehyde and nitromethane) using the [Zn4O(bdcPro)1/2(dppdc)1/2(hxtt)4/3] catalysts. In accord with expectations based on the p-aldol reactions, the rate constant of the maldol reaction increases when hhtt replaces hmtt and then decreases with truxene modulators that bear longer alkyl chains (hott and hdtt, Figure 3). For the m-Henry reaction, we noted that the reaction did not proceed if nitromethane was used as both a reactant and the solvent. However, when nitromethane was diluted with solvents such as THF, the reaction progressed smoothly. As uniformly observed for MUF-77 catalysts, the stability and crystallinity of the framework are well-maintained throughout this process (Figures S39 and S40 and Figures S42− S49). As illustrated in Figure 3, the rate constant of the m-Henry reaction monotonously decreases upon increasing the length of the alkyl groups on the truxene linker. It is thus evident that introducing alkyl substituents to the truxene linkers in bdc-Pro/ dppdc/hxtt MUF-77 catalysts correlates with distinct trends in the rates of m-aldol and m-Henry reactions. These results signify a new concept for multicomponent MOF catalysts: pores that are programmed in a suitable way can discriminate between different types of reactions that involve the same substrate. Building on these results, we investigated reactions where both aldol and Henry reaction pathways were simultaneously available to the catalysts. These experiments involved combining the MOF catalyst, m-nitrobenzaldehyde, acetone, and nitromethane in the same vessel. m-Nitrobenzaldehyde serves as a common reactant for both the m-aldol and m-Henry reactions (Figure 3). Initially, six representative MOF catalysts were chosen from the [Zn4O(bdc-Pro)1/2(dppdc)1/2(hxtt)4/3] and [Zn4O(bdc-Pro)1/2(bpdc)1/2(hxtt)4/3] families (Table 3). After

conversion after 24 h no.

catalyst linker set

m-aldol (%)

1 2 3 4 5 6

bdc-Pro/dppdc/hmtt bdc-Pro/dppdc/hhtt bdc-Pro/dppdc/hdtt bdc-Pro/bpdc/hmtt bdc-Pro/bpdc/hhtt bdc-Pro/bpdc/hdtt

28.0 41.1 29.0 32.7 47.5 15.8

m-Henry (%)

total (%)

ratio

14.2 8.9 4.6 17.9 11.5 5.4

42.2 50.0 33.6 50.6 59.0 21.2

1.99 4.66 6.43 1.84 4.19 2.97

a

Standard reaction conditions for the competition reaction: 10 mol % catalyst (prolinyl units relative to total aldehyde), 0.035 M mnitrobenzaldehyde in a mixed solvent (acetone/nitromethane = 95/5) at 20 °C.

these MOF catalysts were subjected to a mixture of mnitrobenzaldehyde, nitromethane, and acetone for 24 h, conversions to the alternative aldol and Henry products were calculated from high-performance liquid chromatography chromatograms. As summarized in Table 3, entry 1, the catalyst built up from bdc-Pro/dppdc/hmtt converts 28.0% of the mnitrobenzaldehyde to the aldol product and 14.2% of this reactant to the Henry product. The selectivity ratio of aldol to Henry is 1.99. When hhtt replaces hmtt (Table 3, entry 2), the aldol conversion increases to 41.1% while the Henry conversion decreases to 8.9%. The ratio of aldol to Henry products is thereby raised to 4.66. When hdtt replaces hhtt, the selectivity further increases to 6.43 (Table 3, entry 3). The results are in accord with predictions based on the aldol and Henry reactions that were conducted independently. They demonstrate a new concept in MOF catalysis by illustrating how selectivity between reaction pathways that are in direct competition can be optimized by introducing framework modifications at sites remote from the catalytic unit. In parallel, the isoreticular [Zn4O(bdc-Pro)1/2(bpdc)1/2(hxtt)4/3] catalysts exhibit similar selectivity trends (Table 3, entries 4−6). Here, the hdtt linker suppresses the aldol reaction, which means that the selectivity for the aldol reaction over the Henry reaction reaches a maximum with hhtt (entry 5). D

DOI: 10.1021/jacs.8b11221 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

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CONCLUSION Intuitively, modifying framework pores by introducing large substituents to the ligands will reduce the void volume. In turn, this would be expected to counteract properties that rely on high pore volumes, such as the accommodation of guest molecules and the diffusion of catalysis reactants and products. Contrary to these expectations, we have shown that introducing bulky substituents into framework pores can increase both the reactivity and selectivity of important chemical transformations. The observation of these effects relies on the multicomponent nature of the framework, which define a complex yet wellordered three-dimensional spatial environment that can be rationally modified. We reiterate here that our focus was not on the absolute performance of the catalysts (other catalysts that produce higher ee’s may be found in the literature), but rather was on identifying how the catalyst reactivity and selectivity can be correlated with the pore architecture. It is now also evident that catalytic activity can be switched on by embedding catalytic units in a heterogeneous framework. Specifically, MUF-77 analogs can catalyze a Henry reaction, which cannot be accomplished by its free linker. Elsewhere, installing a catalytic unit in a framework pore significantly enhances an aldol reaction compared to homogeneous analogs. Changes to the active site environment in these frameworks impacts in specific ways different transformations. This information was gleaned from a comparison of aldol and Henry reactions that use the same substrate. We found that systematic pore modifications accelerate the aldol reaction at the expense of a competing Henry reaction. These results guided our exploration of the way that the catalytic microenvironment can discriminate between two reactions that compete for the same substrate. Selectivity of this kind is reminiscent of enzymes, which typically effect a specific transformation out of many possibilities involving the pool of available substrates. The ability to precisely correlate structural features with selectivity of this kind is extremely rare in abiological systems, and it provides an avenue to design next-generation heterogeneous catalysts. Multicomponent MOF catalysts exhibit some of the attractive features of their biological counterparts, which is inspiring us to further extend the concepts of pore programming in framework catalysts.

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ACKNOWLEDGMENTS We are grateful to the RSNZ Marsden Fund for supporting this study (Contract 14-MAU-024).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b11221. Ligand and MOF synthesis, characterization data, catalysis and control experiments (CCDC 1885842) (PDF) CheckCif file for CCDC 1885842 (PDF) SCXRD structure data for [Zn4O(bdcPro)1/2(dppdc)1/2(hhtt)4/3] (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Seok J. Lee: 0000-0002-8061-6002 Shane G. Telfer: 0000-0003-1596-6652 Notes

The authors declare no competing financial interest. E

DOI: 10.1021/jacs.8b11221 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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DOI: 10.1021/jacs.8b11221 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX