Sequence and Surface Confinement Direct Cooperativity in Catalytic

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Sequence and Surface Confinement Direct Cooperativity in Catalytic Precision Oligomers Prakash Chandra, Alain M. Jonas, and Antony E. Fernandes J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00872 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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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-laNeuve, Belgium ABSTRACT: Confinement and cooperativity are important design principles used by Nature to optimize catalytic activ-

ity 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 to express and trigger synergistic catalytic activation pathways. 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 5fold activity improvement compared to a “mismatched” sequence or free oligomers using the (pyta)Cu/TEMPO/NMIcatalyzed aerobic selective oxidation of alcohols as a model reaction. We thus demonstrate that, in analogy with enzymes, sequence definition combined with surface grafting induces 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 surface-confined 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.

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 last 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 synthesis4 or the use of molecular machines,5 resulting in synthetic polymers embracing various level 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 surface-confined cooperative molecular species have recently emerged 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 cen-

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ters (Figure 1b). Provided a proper monomer sequencing is selected, the surface confinement of sequenced oligomers could lead to a catalytic behavior largely deviating from what can be measured in solution.

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,13c,13f,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). 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. Sequencedefined 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.

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 favors 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 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), imidazole and pyridyltriazol (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.

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 azidealkyne cycloaddition (CuAAC)16 protocol,17,12 TIP 1 and ITP 2 (1.2 equiv. vs N3 loading) were grafted on azidemodified 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, 0.20 and 0.21 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). 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

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grafting yield after one and two days 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 another 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 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 catalysts.18 ICP-AES analysis of 3 and 4 gave close Cu contents 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 Nitrogen physisorption measurements showed the decreasing of 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 difference of 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 lower activity ratio than when immobilized on 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, 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 Nmethylimidazole (NMI) with supported catalyst sITP 4 gave slight improvement of activity but did not allowed to reach the activity of sTIP 3 (Figure 2a, Table 1), suggesting that the preorganization in sITP 4 is not optimal. TABLE 1. Turnover frequencies. catalyst

substrate

initial TOF × 10-2 min-

8.4

sequence effect compared to sITP × 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

1

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

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.

benzyl alcohol

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

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 to 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.

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TGA and DTG of 6 and 7 revealed efficient grafting (ca. 80 %), slightly higher to that of 3 and 4 (Figure S25, Table S1) while FTIR showed the decreasing of the azide band together with the appearing 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 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 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. SCHEME 2. Preparation of diluted catalysts 6 and 7.

From these experiments, it can be inferred that pytaCu/imidazole interactions are critical for catalytic activity, as expected. On this basis, the conformational degrees of freedom and the configuration of the sequencedefined oligomers are governing kinetics in the liquid phase; ITP 2 being more active than TIP 1 because the probability of forming an intramolecular imidazole-pytaCu complex is higher (Figure 3). This conformational flexibility is considerably hindered when densely grafted on surface, which explains the stronger impact of the monomer sequence (configuration); sTIP 3 being 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-Cuimidazole adduct, both through intra- or interchain interactions; sITP 4 being 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 modelling 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.

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

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 re-addition of 3 (Figure 4a).

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 re-added 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 de-

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Finally, we tested the generality of the monomer sequence effect in the aerobic selective oxidation of more challenging alcohols (Figure 4b). As expected, 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).

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 chains; interchanging two components dramatically reduced the catalytic activity as a result of the decreased ability of the imidazole site to reach the pytaCu center. Yet, interchain interactions are predominating 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 peptide sequence must be associated to 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. ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedures, characterization data and experimental methods.

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.

ACKNOWLEDGMENT The authors acknowledge the Fonds européen de développement régional et la Wallonie (project Intense4Chem), the Belgian Federal Science Policy (IAP P7/05) and the Fonds de la Recherche Scientifique - FNRS and the Fonds Wetenschappelijk Onderzoek under EOS project n° 30650939 for financial support. Anne Iserentant, Cécile D’Haese, and François Devred are acknowledged for ICP-AES, XPS, and physisorption measurements.

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