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Sequence and Surface Confinement Direct Cooperativity in Catalytic Precision Oligomers Prakash Chandra, Alain M. Jonas, and Antony E. Fernandes* Institute of Condensed Matter and Nanosciences, Bio- and Soft Matter, Université Catholique de Louvain, 1348 Louvain-la-Neuve, Belgium S Supporting Information *

ABSTRACT: Confinement and cooperativity are important design principles used by Nature to optimize catalytic activity in enzymes. In these biological systems, the precise sequence of the protein encodes for specific chain folding to preorganize critical amino acid side chains within defined binding pockets, allowing synergistic catalytic activation pathways to be expressed and triggered. Here we show that short synthetic precision oligomers with the optimal sequence of catalytic units, spatially arranged by dense surface grafting to form confined cooperative “pockets”, display an up to 5-fold activity improvement compared to a “mismatched” sequence or free oligomers using the (pyta)Cu/TEMPO/NMI-catalyzed aerobic selective oxidation of alcohols as a model reaction. We thus demonstrate that, in analogy with enzymes, sequence definition combined with surface grafting induce the optimized distribution, both radially (interchain) and axially (intrachain), of a catalytic triad, and that the impressive improvement of catalytic efficiency results predominantly from “matched” interchain interactions in the surfaceconfined system, thereby outperforming the homogeneous system. The concept presented here hence uncovers a new paradigm in the design of multifunctional molecular assemblies to control functions at a level approaching biological precision.

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INTRODUCTION The unique properties of naturally occurring polymers, such as nucleic acids and proteins, are primarily the result of the chemical nature and sequence order of the monomer units distributed along the chain. Such sequential precision encodes for specific tridimensional structures and functions central to the expression and regulation of biological mechanisms. This complex interplay between sequence, structure, and function in the biological world has stimulated research and expectations in synthetic polymer chemistry.1 Within the past decade, diverse synthetic approaches have been proposed to achieve control over the primary structure of polymeric chains. This covers for instance DNA-templated approaches,2 controlled radical copolymerization,3 iterative synthesis,4 or the use of molecular machines,5 resulting in synthetic polymers embracing various levels of control over chain structure, architecture, and functionality, from polydisperse sequence-controlled polymers to monodisperse sequence-defined polymers. These recent advances in synthetic polymer chemistry provide access to polymers of increasing complexity for potential applications in, e.g., information storage,6 self-assembly and folding,7 and catalysis.8 However, the field of precision polymers is still in its infancy and challenges have to be addressed to realize the full potential of single-chain technology beyond fundamental aspects. Multifunctional heterogeneous catalysts based on surfaceconfined cooperative molecular species have recently emerged © 2018 American Chemical Society

as state-of-the-art hybrid materials to tailor and enhance properties far beyond that of the combined soluble parent species.9 In this scenario, the high local concentration of surface-bound molecules creates an array of possible pairing with neighbors−interactions that are entropically disfavored in solution−resulting in significantly magnified synergistic effects.10 This is however only feasible if the correct distribution and intersite distance authorize a concerted catalytic mechanism.11 With the aim to design improved surface-bound multifunctional molecular assemblies for application in catalysis, we reasoned that short sequence-defined oligomers could prove particularly efficient to finely control the composition, distribution, and interaction of the individual components of a cooperative catalytic system. Indeed, traditional monolayer strategies, when applied to the immobilization of multiple functional groups, fundamentally lead to kinetically controlled random distributions on surface, and potentially to phase separation within mixed monolayers, which can significantly hinder the full expression of the catalytic properties of the parent system (Figure 1a). Precision oligomers could allow circumventing this issue when grafted on a surface, by affording a way to control both the radial (interchain) and axial (intrachain) distribution of active centers (Figure 1b). Provided Received: January 23, 2018 Published: March 15, 2018 5179

DOI: 10.1021/jacs.8b00872 J. Am. Chem. Soc. 2018, 140, 5179−5184

Article

Journal of the American Chemical Society

Scheme 1. Grafting of Sequence-Defined Oligomers TIP 1 and ITP 2 on Azide-Functionalized Mesoporous Silica Particles

Figure 1. Schematic view of active site distribution in (a) mixed monolayers and (b) sequence-defined oligomeric monolayers. T = TEMPO, I = imidazole, P = pyta. The statistical distribution in mixed monolayers potentially limits the full exploitation of catalytic cooperativity. Sequence-defined oligomer monolayers provide dense monodisperse structures with perfectly defined axial and radial distribution, increasing significantly the probability of cooperative interactions through controlled intra- and interchain interactions.

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RESULTS AND DISCUSSION Alkynes TIP 1 and ITP 2 were synthesized starting from glycidyl propargyl ether using a strategy adapted from the method reported by Johnson and co-workers (Scheme S1).15 Following our copper-catalyzed azide−alkyne cycloaddition (CuAAC)16 protocol,17,12 TIP 1 and ITP 2 (1.2 equiv vs N3 loading) were grafted on azide-modified silica (0.28 mmol/g) to afford supported catalysts sTIP 3 and sITP 4, respectively (Scheme 1). TGA of the demetalated catalysts (Figure S25) gave a grafting yield of ca. 70% from azide silica (Table S1, 0.21 and 0.20 mmol/g for sTIP 3 and sITP 4, respectively) that could also be qualitatively confirmed by FTIR that showed the decreasing of the characteristic azide band (2100 cm−1) in both cases upon CuAAC grafting (Figure S26). The DTG signal moreover showed remaining thermal events corresponding to the azide silane, also indicative of incomplete grafting (Figure S25). The grafting efficiency is more likely here to be limited only by the size of the oligomers, as no difference could be noted in the grafting yield after 1 and 2 days of reaction as determined by TGA (Table S1). The apparition in the FTIR spectra of CO stretching vibrations (1715 cm−1) corresponding to the ester bounds in 1 and 2 provided more evidence for the covalent grafting of the oligomers (Figure S26). The elemental composition of 3 and 4 was confirmed by XPS (Figure S27, Table S2). Detailed analysis of C, N, and Cu high resolution XPS regions did not show significant differences between the two grafted oligomers to account for specific conformations on the surface; yet, Cu 2p1/2 and Cu 2p3/2 peaks at ca. 952.2 and 932.4 eV, respectively, confirmed the presence of CuI species in each catalyst.18 ICP-AES analysis of 3 and 4 gave close Cu contents values of 0.14 and 0.12 mmol/g, respectively, indicating partial but equivalent Cu complexation for both catalysts (Table S3); it is remarkable to notice that the Cu center in 3 and 4 is the copper used for the CuAAC-grafting itself, hence reducing the preparation of the supported hybrid multifunctional catalysts to one single step from azide silica.12

a proper monomer sequencing is selected, the surface confinement of sequenced oligomers could lead to catalytic behavior largely deviating from what can be measured in solution. Herein, we report on the catalytic properties of two trifunctional oligomers that only differ by their monomer sequence, and on the amplification of their differential activity by surface immobilization leading to increased probabilities of cooperative interactions. We show that the high local concentration and proper sequencing in such surface-confined assemblies favor cooperative interchain interactions that are predominantly contributing to catalytic activity. Altogether, we demonstrate that it is possible to rationally design short oligomers to precisely position and control cooperative interactions of pendant functional groups in all directions, and that a small modification in the sequence, i.e., interchanging only two pendant groups, very significantly alters the catalytic properties due to “mismatched” interactions. We recently described12 a robust method for the preparation of supported trifunctional catalysts that incorporate 2,2,6,6tetramethyl-1-piperidinyloxyl (TEMPO), imidazole, and pyridyltriazole (pyta)-Cu complex for the efficient aerobic selective oxidation of alcohols.13 Following molecular engineering of surface composition and intersite distance through the attachment of molecular spacers of various lengths, it is possible to increase the probability of cooperativity in the mixed monolayer. From our previous study, it clearly appeared that the propensity of the imidazole site to reach the copper site is critical for enabling high turnovers, this being probably linked to the formation of a pyta-Cu-imidazole complex with decreased redox potential favoring O2 activation.13a,c,f,14 Hence, trifunctional oligomers TIP 1 and ITP 2 were designed that incorporate all of the required catalytic components TEMPO (T), imidazole (I), and pyta ligand (P), differing only by the position of the imidazole center relative to the pyta site (Scheme 1). 5180

DOI: 10.1021/jacs.8b00872 J. Am. Chem. Soc. 2018, 140, 5179−5184

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Journal of the American Chemical Society Nitrogen physisorption measurements showed a decrease in average pore size and surface area following CuAAC-grafting, which further confirm the efficient functionalization (Table S4). Catalysts sTIP 3 and sITP 4 were tested in the aerobic selective oxidation of benzyl alcohol to benzaldehyde utilizing a Cu loading of 5 mol % (Figure 2a). Impressively, a large

Table 1. Turnover Frequencies catalyst TIP 1 + 5 mol % CuI ITP 2 + 5 mol % CuI sTIP 3 sITP 4 sITP 4 + 10 mol % NMI 6 7 sTIP 3 sITP 4 sTIP 3 sITP 4 sTIP 3 sITP 4 sTIP 3 sITP 4

substrate benzyl alcohol

cinnamyl alcohol trans 2-hexen-1-ol octanol cyclohexanemethanol

initial TOF × 10−2 min−1

sequence effect compared to sITP

8.4

× 1.5

16.5

× 2.9

31.2 5.7 9.9

× 5.5 ×1 × 1.7

8.1 3.8 32.7 7.1 16.0 6.5 2.9 0.8 1.5 0.3

× × × × × × × × × ×

2.2 1 4.6 1 2.5 1 3.5 1 4.7 1

Scheme 2. Preparation of Diluted Catalysts 6 and 7

Figure 2. (a) Catalytic activity of TIP 1, ITP 2, sTIP 3, and sITP 4 in the aerobic oxidation of benzyl alcohol. (b) Effect of surface dilution (catalysts 6 and 7 are dilute versions of 3 and 4). Conditions: BnOH (0.2 mmol) in acetonitrile (0.2 M), O2 bubbling (5.5 mL/min), 60 °C. Reactions were performed using a 5 mol % loading in Cu. Yields were determined by GC analysis using p-xylene as the internal standard.

difference in activity was observed between the two catalysts that only differ by their monomer sequence. Catalyst sTIP 3, having the imidazole site spatially closer to the copper center, is the more active, with an initial turnover frequency (TOF) 5.5 times superior to that of sITP 4 (Table 1). Conversely, free oligomers TIP 1 and ITP 2 showed a lower activity ratio than when immobilized on the surface, with only a 2-fold difference of initial TOF (Figure 2a, Table 1). Interestingly also, the sequence effect in the homogeneous reaction is reversed, with ITP 2 being more active than TIP 1, suggesting that it is more conformationally difficult to form the pyta-Cu-imidazole adduct in the case of TIP 1, whereas ITP 2 has more degrees of freedom to fold and bring imidazole close to the pyta-Cu center with the proper geometrical requirements. Noteworthy, adding free N-methylimidazole (NMI) with supported catalyst sITP 4 gave a slight improvement in activity but did not allow reaching the activity of sTIP 3 (Figure 2a, Table 1), suggesting that the preorganization in sITP 4 is not optimal. In order to evaluate the contribution of interchain interactions, mixed-oligomer catalysts 6 and 7 were prepared from azide silica using an equimolar amount of triethylene glycol methyl ether 5 and TIP 1 or ITP 2 (1.2 equiv. vs N3 loading), respectively (Scheme 2). Specifically, this additional

component is designed to dilute the catalytic chain and thus restrict catalytic activity mainly to intrachain interactions. As previously demonstrated,12,17 the relative composition of the mixed monolayer is quantitatively controlled by the molar ratio of the alkyne components in the grafting solution. TGA and DTG of 6 and 7 revealed efficient grafting (ca. 80%), slightly higher compared to that of 3 and 4 (Figure S25, Table S1) while FTIR showed a decrease of the azide band together with the appearance of the ester CO band (Figure S26). XPS again confirmed the chemical composition of the samples but without being able to differentiate the two solid catalysts (Figure S27, Table S2). ICP-AES gave a Cu content of 0.07 and 0.08 mmol/g for 6 and 7, respectively, which agrees well with the expected dilution by a factor of ca. 2 (Table S3). The diluted catalysts were similarly tested in the aerobic selective oxidation of benzyl alcohol to benzaldehyde (Figure 2b). Remarkably, upon dilution on the surface, the activity of 1 significantly decreased and, more importantly, the activity difference between the two sequences was drastically reduced from a factor of 5.5 to a factor of 2.2 (Table 1), underlining the importance of interchain interactions in such surface-confined systems. 5181

DOI: 10.1021/jacs.8b00872 J. Am. Chem. Soc. 2018, 140, 5179−5184

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Journal of the American Chemical Society From these experiments, it can be inferred that pyta-Cu/ imidazole interactions are critical for catalytic activity, as expected. On this basis, the conformational degrees of freedom and the configuration of the sequence-defined oligomers are governing the kinetics in the liquid phase, with ITP 2 being more active than TIP 1 because the probability of forming an intramolecular imidazole-pyta-Cu complex is higher (Figure 3).

Figure 3. Schematic view of cooperative interactions in soluble and supported sequence-defined catalytic oligomers.

This conformational flexibility is considerably hindered when densely grafted on the surface, which explains the stronger impact of the monomer sequence (configuration); sTIP 3 is thus more active than sITP 4. Moreover, surface-confinement and high local concentration provide an array of additional possible interchain interactions (Figure 3), explaining why sTIP 3 is more active than ITP 2. However, in sITP 4, it appears very difficult to form the pyta-Cu-imidazole adduct, through both intra- or interchain interactions; sITP 4 is thus the least active catalyst. Overall, this explains the observed activity order sTIP 3 > ITP 2 > TIP 1 > sITP 4 and the magnitude of the sequence effect in the homogeneous and grafted systems (Figure 3). Molecular modeling studies are underway to obtain a more precise view of the solution- and solid-phase conformational behavior of the precision oligomers and the link with their distinct catalytic properties. Catalyst sTIP 3 could be filtered and reused without severe loss of activity after 5 recycles (Figure 4a). A hot filtration test also demonstrated the interruption of activity upon removal of the catalyst, activity that could be totally recovered after readdition of 3 (Figure 4a). Finally, we tested the generality of the monomer sequence effect in the aerobic selective oxidation of more challenging alcohols (Figure 4b). As expected, the activity decreases with unactivated aliphatic alcohols (octanol and cyclohexanemethanol), but still, sTIP 3 proved more efficient than sITP 4 in all the tested alcohols, with a TOF improvement factor from ca. 3 to 5 (Table 1).

Figure 4. (a) Recycling and hot filtration experiments with catalyst sTIP 3 in the aerobic oxidation of benzyl alcohol. For the hot filtration test, catalyst is removed at t = 30 min while the reaction is still monitored, and readded at t = 120 min. (b) Aerobic oxidation of various alcohols with sTIP 3 and sITP 4. Conditions: alcohol (0.2 mmol) in acetonitrile (0.2 M), O2 bubbling (5.5 mL/min), 60 °C. Reactions were performed using a 5 mol % loading in Cu. Yields were determined by GC analysis using p-xylene as the internal standard.

chains; interchanging two components dramatically reduced the catalytic activity as a result of the decreased ability of the imidazole site to reach the pyta-Cu center. Yet, interchain interactions predominate in dense surface-confined molecular structures, leading to significantly increased sequence effects compared to when testing the oligomers in solution. The utilization of sequence-defined oligomers for catalysis applications hence affords precise ways to tune and boost cooperative activity provided the spatial organization of the oligomers be also controlled; this is akin to enzymes for which the protein sequence must be associated with a proper ternary structure for efficient catalysis. Aspects regarding the use of longer polymeric chains of defined composition and sequence are now being investigated together with the influence of their stereochemistry and tacticity.15 Defining other ways to connect catalytic units to drive the probability of cooperativity is also an important future development.

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CONCLUSIONS In summary, we demonstrated here the application of sequence-defined oligomers for the precise preorganization of a catalytic triad on mesoporous silica particles. This approach provides tools to circumvent the random distribution associated with more traditional mixed monolayer scaffolds, random distribution that can prevent the full pairing of all the cooperative components, hence the full exploitation of the catalytic potential. Here, the catalytic partners are uniformly distributed both axially and laterally, with controlled stoichiometry, which moreover allows drawing more accurate structure/activity relationships. The ordered positioning of the components, i.e., the correct monomer sequence, is critical for favoring cooperative interactions within and between the

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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.8b00872. Synthetic procedures, characterization data, and experimental methods (PDF) 5182

DOI: 10.1021/jacs.8b00872 J. Am. Chem. Soc. 2018, 140, 5179−5184

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Alain M. Jonas: 0000-0002-4083-0688 Antony E. Fernandes: 0000-0002-7993-2980 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the European Regional Development Fund (ERDF) and Wallonia (Operational Program “Wallonia2020.EU”), the Belgian Federal Science Policy (IAP P7/05), and the Fonds de la Recherche Scientifique - FNRS and the Fonds Wetenschappelijk Onderzoek under EOS Project No. 30650939 for financial support. Anne Iserentant, Cécile D’Haese, and François Devred are acknowledged for ICPAES, XPS, and physisorption measurements.

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DOI: 10.1021/jacs.8b00872 J. Am. Chem. Soc. 2018, 140, 5179−5184

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DOI: 10.1021/jacs.8b00872 J. Am. Chem. Soc. 2018, 140, 5179−5184