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AuNPs-Polymeric Ionic Liquid Composite Multicatalytic Nanoreactors for One-Pot Cascade Reactions Silvia Montolio, Cristian Vicent, Vladimir O. Aseyev, Ignacio Alfonso, M. Isabel Burguete, Heikki Tenhu, Eduardo Garcia-Verdugo, and Santiago V. Luis ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01759 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016

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AuNPs-Polymeric Ionic Liquid Composite Multicatalytic Nanoreactors for One-Pot Cascade Reactions Silvia Montolio,a Cristian Vicent,d Vladimir Aseyev,c Ignacio Alfonso,b M. Isabel Burguete,a Heikki Tenhu,c Eduardo García-Verdugo* a,c and Santiago V. Luis* a a

Departamento de Química Inorgánica y Orgánica, Universitat Jaume I E-12071, Castellón de la

Plana (Spain). E-mail: [email protected], [email protected] b

Departamento de Química Biológica y Modelización Molecular, Instituto de Química

Avanzada de Cataluña (IQAC), Consejo Superior de Investigaciones Científicas (CSIC), Jordi Girona 18-26 E-08034 Barcelona (Spain) c

Laboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, Helsinki,

(Finland). d

Servei Central d'Instrumentació Científica, Universitat Jaume I E-12071, Castellón de la Plana

(Spain).

ABSTRACT: Different AuNPs-PILs nanocomposites have been developed as bifunctional catalysts in which the IL-like units promote the Knoevenagel condensation through a dual electrophilic/nucleophilic activation relay mechanism and metal nanoparticles catalyse the

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reduction of the functional groups of the Knoevenagel product in a one-pot tandem reaction. The characterization of the composites by different techniques suggests that the self-assembly and self-organization of the constituent building blocks during the synthetic procedures define the precise shapes, sizes and catalytic performance of these advanced materials. The resulting catalytic efficiency does not only reflect the presence of the corresponding individual catalytic motifs but their hierarchical organization.

KEYWORDS: Polymeric ionic liquids, gold nanoparticles, catalysis, self-organization, multicatalytic systems

Significant research progress has been achieved over the past decades on the development of one-pot multistep catalytic reactions.1,2,3 However, they are still not of general applicability in chemocatalysis due to the presence of important limitations like the incompatibility of active sites, leaching of metal ions or other catalytic species, and difficulty in the full recovery of all involved catalysts. Hence, catalytic cascade reactions still remain a significant challenge.4 The compartmentalization of the different catalysts, mimicking biological systems, is a key issue to achieve efficient systems.5 Catalytic applications of metal nanoparticles (NPs) continue increasing in number.6 The selection of the right ligand on the NPs surface, containing different structural elements and/or pendant moieties, is important not only for their stabilization, but also to define their architecture and functionality.7,8,9 Furthermore, the NPs themselves can be used not only as functional elements (catalysts) but also as elementary structural building blocks in the bottom-up approach to create more complex and well-defined architectures.10,11


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In this context, ionic liquids (ILs) have emerged as efficient media for the generation and stabilization of metal nanoparticles (MeNPs).12,13,14 ILs are reasonably well-organised solvents,15 whose structure is defined by a complex network of attractive interactions.16,17,18,19 In this way, ILs can be regarded as nanostructured materials, allowing neutral molecules to reside in less polar regions, while ionic or polar species undergo faster diffusion in the more polar regions.20,21,22

Figure 1. Dual catalysis based on AuNPs-PILs composites for the Knoevenagel/hydrogenation single-pot tandem reaction. In this context, advanced polymeric materials having functionalities similar to Ionic Liquids (polymeric ionic liquids: PILs) have been pursued for the development of new classes of advanced materials.23,24 The presence of IL-like units on the polymer provides them with the main features of ILs at molecular level (stability, tuneable polarity, conductivity, etc.) but with a macromolecular architecture, as has been demonstrated even for the case of highly crosslinked Supported Ionic Liquid-Like Phases (SILLPs).25,26 In a similar way to that in bulk ILs, the


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network of supramolecular interactions, also present in PILs, defines the complex hierarchical architectures adopted by them ranging from spheres to micelles, vesicles, nanotubes, helical nanofibers, mesoporous materials, etc.27,28 Additionally, PILs have been applied for the immobilisation and stabilisation of MeNPs.29,30,31,32,33 Indeed, the combination of MeNPs with polymeric ILs as stabilisers usually exhibits an excellent synergistic effect, enhancing the activity and durability of the resulting composite.34,35,36,37,38 Hence, it is expected that the combination of these two building blocks, MeNPs and PILs, will yield new intricate phases and morphologies which would otherwise be very difficult to create. In this work, we describe how the controlled combination of PILs and MeNPs, in particular gold nanoparticles (AuNPs), leads, by using the adequate synthetic protocols, to well-organized materials with multiple catalytic sites and tuneable functionality. These materials are able to act as bifunctional catalytic nanoreactors (Figure 1). The well-defined hierarchical organisation of the different active sites on the AuNPs-PILs composites demonstrates catalytic-cascade ability for the reduction of the functional groups of the Knoevenagel product in a one-pot tandem reaction. Results and discussion. Synthesis and structural characterisation of AuNPs-PILs. The preparation of polymeric ionic liquids was carried out following the methodology developed by us for the preparation of related chiral polymeric ionic liquids (Figure 1).39,40 Thus, the desired PILs were prepared by chemical modification of the corresponding poly(vinylbenzyl chloride) (PClVB-2) prepared by bulk RAFT polymerization of p-chloromethylstyrene (1).40


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The synthesis of the AuNPs-PILs composites was performed by two different pathways (Scheme 1). The first one relies on the methodology already described for analogous PILs obtained by free radical polymerisation and taking into consideration the ability of the imidazolium units to stabilise MeNPs.29,30,31,32,33 PIL-3 was prepared by reaction of 1butylimidazole with the chloromethyl groups present in PClVB-2. The corresponding composite AuNP-PIL-4 was prepared through the addition of AuCl4- and sodium borohydride to a solution of PIL-3. According to results observed for analogous homogeneous ILs or related polymeric cross-linked systems,33 an initial exchange of the Cl- by the AuCl4-, as the imidazolium counter anion, can take place during the synthesis, before the nucleation and grow of the nanoparticles. In the composite AuNP-PIL-4, the AuNPs are mainly stabilised by the interaction of the ILunits with the gold surface.12,13,14 Although the stabilisation due to the of thiol produced in-situ by the hydrolysis of the polymer terminal group cannot be completely excluded, this should be less relevant as the thiol loading is much lower than that of imidazolium units (ca. 1:68 molar ratio). Besides, the use of sodium borohydride in the reduction provides a negative charge on the periphery of the AuNPs, which should lead to strong electrostatic interactions with the imidazolium fragments of PIL-3 favouring the stabilization by the IL-like units.41 Thus, it is reasonable to assume that the polymeric chains form a random coil structure all over the AuNPs surface.42 A second strategy for the preparation of the composite centers on a two-step synthesis. Firstly, the nanoparticles were produced in the presence of poly-(chlorovinylbenzene) (PClVB-2) following a “grafting-to” methodology to provide AuNP-PClVB-5.43


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Scheme 1. Synthesis of AuNP-PILs composites. i) ClVB, IBN, AIBN, 120 ºC, 24 h; ii) BuIm, 2Me-THF, 40 ºC, 24 h; iii) HAuCl4, NaBH4, H2O, 0 ºC, 4h; iv) HAuCl4, LiBEt3H, 2-Me-THF, r.t., 4 h; v) BuIm, 2-Me-THF, 40 ºC, 48 h. In contrast to the situation with PIL-3, the functional groups present in the main polymeric chain (chloromethy groups) hardly contribute to the stabilisation of the AuNPs. Hence, only the thiol group generated in situ should be involved in the stabilisation of the AuNPs in AuNPPClVB-5. The composite material prepared in this way is likely to spontaneously adopt an extended random coil conformation around the gold nanoparticles forming a well-defined polymeric monolayer, where the polymer chains are tethered in a brush-type configuration on the AuNP surface.44,45 Thus, the composite AuNP-PClVB-5 should present a well-defined highly ordered three dimensional structure around the AuNPs. The chloromethyl groups present in this


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monolayer surrounding the AuNPs were accessible and easily converted into ionic liquid-like units by reaction of the composite with butylimidazole in 2-Me-THF to obtain the final composite AuNP-PClVB-6. This modification could be monitored, as for the synthesis of PIL3, using NMR and FTIR-ATR spectroscopy (Figures S1, S2, S5 and S6 in the Supporting Information). The composite showed a complete transformation of the chloromethyl groups into the corresponding butyl imidazolium fragments. Structural characterisation of AuNP-PILs. TGA and TEM studies. TGA analyses allowed the determination of the content of organic polymeric material present in the different composites by studying the corresponding mass losses observed at temperatures below 500-600 °C in an inert atmosphere (Figure S7 in the Supporting Information). The initial weigh losses at temperatures close to 100 °C can be assigned to adsorbed water in the composites. Similar compositions were obtained for the two AuNP-PIL composites. For AuNP-PIL-4, 11% of its weight corresponds to the gold nanoparticle and 85% to the polymeric stabiliser, while the composition for AuNP-PIL-6 is 14% and 82% respectively (Table 1). The nitrogen content determined by elemental analysis of the composites confirmed the results obtained by TGA. Both materials present similar loadings of imidazolium units corresponding to analogous polymer/nanoparticle ratios (2.83 and 2.85 mmol of IL-like units/g of composite for AuNP-PIL-4 and AuNP-PIL-6 respectively). TEM analyses (Figures S8, S9 and S10 in the Supporting Information) revealed that the mean size distribution for AuNP-PIL-4 was 4.0 ± 1.0 nm, while AuNP-PIL-6 showed a slightly larger mean size of 4.5 ± 1.5 nm (Table 1). In the last case, this represents that some aggregation processes took place during the substitution reaction with butylimidazole as the mean size


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distribution for the polymeric nano-composite precursor AuNP-PClVB-5 was significantly smaller: 2.8 ± 1.1 nm. Table 1. Characterisation of AuNP-PILs polymeric composites Compound

AuNPs (nm)a

wt%

SPB b

loading

pol/AuNP/H2O

λmax

IL units

c

dh (nm)d

AuNP-PIL-4

4.0 ± 1.0

85.3/10.7/4.0

514

2.83

18

AuNP-PClVB-5

2.8 ± 1.1

67.3/32.7

519

-

-

AuNP-PIL-6

4.5 ± 1.5

82.4/14.1/3.5

523

2.85

148

a

mean diameter of the gold nanoparticles determined by TEM. determined by elemental analysis as mmol of IL-like units per gram. light scattering in water.

b d

determined by TGA. c determined by dynamic

UV-visible spectroscopy. The UV-visible spectra of both composites showed a transverse surface plasmon band (SPB) in good agreement with AuNPs well separated from each other without any sign of aggregation in solution (2.5 mg/mL).46 The SPB shifted from 514 nm for the AuNP-PIL-4, directly prepared with PIL-3, to 523 nm for the AuNP-PIL-6 (Figure 2 and Figure S11 in the Supporting Information). The very small differences on mean size do not justify this red shift. However, the position of the SPB is highly dependent on the dielectric constant of the local medium and, accordingly, on the nature of the polymeric shell. Indeed, it has been reported that the variation of the grafted polymer-shell volume fraction can induce a red shift of the SPB peak of up to about 15 nm.46 Thus, the difference observed in the maximum wavelengths of AuNP-PIL-4 and AuNP-PIL-6 (514 vs. 523 nm) is likely to be associated to the different nature of the ligand-surface interactions stabilising the AuNPs. In the case of AuNPPIL-6, they are mainly polymer-SHAuNPs, while for AuNP-PIL-4 they involve the imidazoliumAuNPs electrostatic interaction. A smaller red shift from 519 nm to 523 nm is also observed associated to the transformation of AuNP-PClVB-5 into AuNP-PIL-6. In this


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case, as the ligand−surface interaction is approximately the same for both composites, the change on the SPB can be attributed to the different dielectric constant of the polymeric shell.46 1.25 1,25 λmax 523 nm

1,00 1.00

Abs. (u.a.)

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0,75 0.75

0,50 0.50 λmax 514 nm

0,25 0.25

0,00 0.00 400 425 450 475 500 525 550 575 600 625 650 λ (nm)

Figure 2. UV-vis spectra and Surface Plasmon Bands of AuNP-PIL-4 (red) and AuNP-PIL-6 (blue) in MeOH. Dynamic Light Scattering (DLS) studies. DLS studies clearly highlighted the different behaviour of both gold nanocomposites, AuNP-PIL-4 and AuNP-PIL-6, in water (Figure S12 in the Supporting Information). A red incident light of λo=637 nm was used to maximize the intensity of the light scattered by the solutions, as solutions of both particles in water were red. Noteworthy, the AuNPs-PILs-4 scattered the light weaker than the related AuNPs-PILs-6. As a matter of fact, in order to obtain good data, a more concentrated solution (5 fold) had to be used for AuNP-PIL-4. AuNP-PIL-4 displayed a clear uni-modal size distribution with the mean diameter of 18 nm. However, the mean diameter obtained for AuNP-PIL-6 was significantly larger (dh = 148 nm), which certainly must originate from inter-particle association. The correlation functions of the electric field g1(t) of AuNP-PIL-6 can be fit with two exponents, thus suggesting the existence of individual particles and their aggregates (lower inset, Figure S12


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in the Supporting Information), although the number of the formed aggregates can be low. The angular dependence of the mean diameter measured for the AuNP-PIL-6 solution is strong, which also stands for association. As most of the imidazolium groups in AuNP-PIL-6 are not interacting with the AuNPs surface, they are available for association with related groups of other chains of the same particle or from a second composite particle. This interaction can lead to the assembly of particles without modification of the size of gold nanoparticles. This is in agreement with TEM data and also with the UV-visible spectra only showing a band at 523 nm (transverse SPB) lacking the adsorption at 630-640 nm (longitudinal SPB) expected for AuNPs aggregation. An interdigitation mechanism can be envisaged for the formation of such aggregates, as observed for the protecting shell in other NPs like QDs,47 though in our case the driving force for interdigitation is likely to involve imidazolium-anion-imidazolium interactions instead of Van-der-Waals forces. NMR studies. The structural features of the different polymeric composites were also studied by NMR spectroscopy. It is expected that morphological differences of AuNP-PIL-4 and AuNP-PIL-6 composites will lead to some dissimilarities on the molecular dynamics of both systems in solution. Thus, the effects of the polymer structure on the mobility of the IL-like units were evaluated by measuring both longitudinal (T1) and transverse (T2) proton relaxation times for PIL-3, AuNP-PIL-4 and AuNP-PIL-6 in D2O for the 296-358 K temperature range. Altogether the analysis of T2 and T1 (see Supporting Information for a detailed discussion) indicates that the motion of the IL-like segments of the polymer, in the case of AuNP-PIL-6, is more restricted and/or locally more anisotropic than those for the free polymer (PIL-3) or for the AuNP-PIL-4 composite. It seems reasonable to suppose that, in solution, for both, the free polymeric ionic liquid (PIL-3) and the gold composite AuNP-PIL-4, the polymer chains present


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a random structure not favouring any particular organization of the functional fragments. Thus, the presence of the gold nanoparticles in AuNP-PIL-4 does not lead to significant changes in the mobility of the polymeric chains, except for the small fraction directly bound to the surface of the NP, as it is reflected by the similar T1 and T2 relaxation times observed for both systems. However, in the case of AuNP-PIL-6, its preparation through the chemical modification of AuNP-PClVB-5 may provide the composite with certain degree of pre-organization. In this regard, the AuNP acts as a template to organize a dense assembly of parallel polymer chains as in polymer brushes. The presence of additional IL-like units can contribute further to the selfassembly of the polymer chains leading to a well-defined molecular nanostructure involving strong coulombic inter-chain interactions but also other attractive forces. In a similar way than in bulk ILs,21 the imidazolium ionic liquid-like units present in the polymer may induce a nanoscale structure arising from the clustering of the alkyl chains and the formation of ionic networks due to the ordering of anion and the imidazolium ring cations modifying the pre-organized surfacetethered polymer chains. This type of assembly at molecular level may explain the enhanced rigidity found for the AuNP-PIL-6 composite and the significant differences observed for the relaxation times for the protons of the butyl residues and the imidazolium rings. These observations suggest that the different methodology used in the preparation of the gold nanocomposites leads to systems with a different degree of nano-structuring AuNP-PIL-6 being the most organized system. Infra-Red Spectroscopy studies. The ATR-FT-IR spectra of the composites also provided an unambiguous verification of the higher self-organization in AuNP-PIL-6. Figure 3 gathers selected regions of the spectra obtained for the two composites. The bands between 3200 and 3070 cm-1 are considered diagnostic for aromatic C-H bonds of the imidazolium cation.48,49


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Vibrational bands at higher wavenumbers in this region correspond to the symmetric and asymmetric stretching vibrations of the C(4)H and C(5)H groups of the imidazolium, whereas those at lower wavenumbers can be assigned to C(2)-H stretching modes. It is noteworthy that AuNP-PIL-4 and AuNP-PIL-6 display a clearly visible band below 3150 cm-1 (3133 and 3125 cm-1 respectively), which is not present in PIL-3. A reduction in the wavelength and an increase in intensity are characteristic of the C(4)H and C(5)H stretching vibrations when participating in extensive hydrogen bonding networks.48 A similar trend is observed for the C(2)H stretching vibration appearing at 3057 cm-1 for AuNP-PIL-4 and at 3050 cm-1 for AuNP-PIL-6 with an important increase in intensity of these bands relative to PIL-3. All these data indicate that the PIL-components in the composites AuNP-PIL-4 and AuNP-PIL-6 present a higher level of ordering, relative to PIL-3, in particular for AuNP-PIL-6. This involves intermolecular association of the imidazolium units through stronger C(2)H···X and C(4)/(5)H···X hydrogen bond interactions. a)

b) -1 2957 cm

0,10 0.10

-1 2959 cm

-1 2929 cm

-1 3133 cm

-1

1153 cm -1

1260 cm

-1 2923 cm

Abs (a.u.)

0,05 0.05

-1

1156 cm

0,10 0.10

-1 2931 cm

-1 3050 cm

Abs (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1 3057 cm

0,05 0.05

0,00 0.00

-1 3125 cm

0,00 0.00

3200

3100

3000

2900

2800

1700

1600

-1

1500

1400

1300

1200

1100

-1

Wavenumber (cm )

Wavenumber (cm )

Figure 3. Selected regions for the ATR-FT-IR spectra of PIL-3 (black), AuNP-PIL-4 (red) and AuNP-PIL-6 (blue).


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The symmetric and asymmetric CH2 stretching modes (νs(CH2) and νas(CH2)) at the 30002800 cm-1 region provide direct information on the local structure of the molecules and the degree of conformational order and packing of aliphatic chains.50,51,52,53 Therefore, they can be used as an indicator of their ordering/organization. Appearance of νs(CH2) and νas(CH2) at 2846 and 2915 cm-1, respectively, reflects a fully ordered all-trans structure of the alkyl chains, while the presence of gauche defects is accompanied by a small blue shift.54,55 For AuNP-PIL-4 and AuNP-PIL-6 the observed frequencies for these bands (2959 / 2931 and 2957 / 2929 cm-1 respectively) reveal a predominance of gauche conformations, though a slightly higher degree of conformational order in the packing of the butyl chains seems to be present in AuNP-PIL-6. The intense peak between 1000 and 1300 cm−1 in Figure 3b is attributable to the skeletal symmetric stretching of the imidazolium ring along with components assigned to CH2(N) stretching.56,57 This band has been observed at 1178 cm-1 for [EMIM][Cl] 0.5 M in CH2Cl2,56 and at 1170 cm-1 for pure [BMIM][PF6].57 For the composites this band was found at 1156 cm-1 for AuNP-PIL-4 and at 1153 cm-1 for AuNP-PIL-6. More interestingly, the corresponding band appears at 1260 cm-1 for PIL-3, highlighting the presence of a more structured organization of the imidazolium units in the composites. DSC studies. Xue and coworkers have used the variation in Tg to study the molecular mobility of polystyrene monolayers confined on the surface of gold nanoparticles.58 Accordingly, in order to obtain additional insights into the supramolecular structure and organization of the PIL-chains in the composites, differential scanning calorimetry studies were carried out. The glass transition temperatures (Tg) for PIL-3 and the two gold composites showed the following trend: PIL-3 (128 °C) < AuNP-PIL-4 (134 °C) < AuNP-PIL-6 (145 °C). The larger Tg observed for AuNP-PIL-6 can only be explained by its higher degree of


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organization mediated by strong inter-chain interactions involving the imidazolium groups, in agreement with the results from other techniques. Altogether, DLS, NMR, FT-IR and DSC data suggest that the methodology employed in the preparation of the gold nanocomposites has a strong influence in their properties, organization and structure. The next question to evaluate is how this nanostructure can affect their catalytic performance. Reduction p-nitrophenol AuNPs. The reduction of p-nitrophenol (7) to the corresponding paminophenol (8) by NaBH4 was initially studied as a reliable model reaction to test the catalytic activity of these composites and the effect on this activity of the different nanostructures present in the AuNP-PILs.59,60 The addition of an aqueous solution of AuNPs-citrate (d = 15 nm by TEM) at 20 °C to a mixture of 7 and NaBH4 caused a decrease in the absorbance at 400 nm with time, corresponding to the disappearance of the p-nitrophenolate ion (Figure S17 in the Supporting Information). At the same time, the absorption peak at 300 nm increased in intensity, due to the increasing 8 concentration. The reaction was considerably fast, reaching ca. 70% of conversion in only 20 seconds (Figure 4). The experiment was repeated in the presence of AuNP-PIL-4 and AuNP-PIL-6 to evaluate the effect of the polymeric composite architecture. Both composites were able to reduce quantitatively compound 7 to the corresponding product 8. The amount of BH4- in the system is 787-times higher than that of 7, which eliminates the influence of the donor BH4- on the kinetics of the catalytic reaction, allowing a pseudo-first-order analysis.


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As expected, linear relationships of ln(At/A0) versus reaction time were obtained, where At represents the absorbance at different time intervals and A0 initial absorbance after the induction time (t0). In a control experiment, PIL-3 was added to the reaction mixture. As expected, the absorbance at 400 nm did not change with time as the reduction of the nitro group did not proceed in the absence of AuNPs (Figure 4).

100

80

60 Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

20

0 0

2

4

6

8

10

12

14

16

18

20

Time (min.)

Figure 4. Yield of 8 (%) vs. time profile for the reduction of 7 at room temperature catalysed by AuNPs-citrate (green diamonds), AuNP-PIL-4 (red circles), AuNP-PIL-6 (blue squares), PIL-3 (black triangles). Reaction mixture: 1190 µL of a 0.2 mM of 7 aqueous solution, 320 µL of MiliQ water and 1690 µL of 0.11 M NaBH4 aqueous solution (V = 3200 µL, initial concentration 7 = 0.074 mM).


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Table 2 summarises the kinetic constants (kapp) found for the different catalytic systems assayed. The presence of an induction time (t0) was observed for the composites. This phenomenon, not noticeable for the AuNPs stabilised with a citric acid monolayer, can be ascribed to the initial reorganization of the polymeric shell to allow the diffusion of the reagents towards the catalyst surface. Table 2. Kinetic parameters for the reduction of p-nitrophenol (7) by different AuNPs.

a

Entry

Compound

kapp (min-1)

Induction time t0 (s)

1

AuNPs (citrate)

0.877 ± 0.113

-

2

AuNPs-PIL-4

0.736 ± 0.076

50

3

AuNPs-PIL-6

0.289 ± 0.0014

360

4

AuNP-PIL-6a

0.0378 ± 0.015

960

in the presence of 0.3 M NaCl

Noteworthy, the induction period observed for the nanocomposites was significantly different (Table 2, Figure 4). In fact, the catalyst prepared in a single step using PIL-3 as the stabiliser (AuNP-PIL-4) led to a shorter induction time than the AuNPs-PIL-6 composite (50 vs. 360 s). The reaction rate, excluding the induction time, was also affected by the structure of the AuNPs protecting layer. The kapp for AuNPs-PIL-6 is ca. 2.5 times slower than the one for AuNP-PIL4. The AuNPs stabilised with citric acid, although presented the larger mean particle size (15 nm), were the more active system (entry 1, Table 2). The observed differences must be associated to the different morphologies of the organic shell, taking into account that the nanoparticle size is almost the same for the two nanocomposites (ca. 5 nm). It has been proposed that the catalytic reduction of 7 proceeds in two steps: (1) diffusion and adsorption of 7 on the catalyst surface and (2) electron transfer mediated by the catalyst surface from BH4- to 7.59,60 As there was practically the same amount of polymer in both composites (ca.


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ACS Catalysis

80% by weight), the differences observed can only be due to the different assembly of the PILslayer around the gold nanoparticle. According with DLS and TEM data, the average hydrodynamic diameter (Dh) of the AuNP-PIL-4 particles is approximately 18 nm and the gold size distribution range is ca. 5 nm, revealing that the mean width of the shell around the surface of the AuNP is approximately 6-7 nm. The situation is very different for AuNP-PIL-6. In spite of involving AuNPs of similar size, the measured Dh is significant larger, ca. 148 nm. Along with NMR and FT-IR data, this reveals the existence of strong chain-chain interactions, within the same particle and between chains of different particles, which lead to the formation of aggregates displaying highly organized PIL-chains. This highly organised nanostructure justifies the slow diffusion of the reagents to the surface of the AuNPs.61 In agreement with this, when the reduction reaction was catalysed by AuNP-PIL-6 in the presence of NaCl (0.3 M, 0.1 mg/mL of AuNP-PIL-6) a significantly longer induction time (ca. 16 minutes, entry 4 in Table 2) was observed along with a reduction in the kinetic constant. The increased ion concentration in the medium must enhance the interchain interactions and favour aggregation. This was confirmed when T1 and T2 values were measured for AuNP-PIL-6 from 1

H NMR experiments under these conditions (0.3 M NaCl). Significant decreases for all T2

values, including those for the protons of the butylenic chain, were observed relative to the experiments in pure water (Figure S18 in the Supporting Information). Indeed, the use of a larger concentration of NaCl (1 M) induced the collapse and precipitation of the polyelectrolytic composite. Application of the AuNP-PIL composites as nanoreactors for single-pot cascade reactions. The presence of well differentiated structural components in these AuNP-PIL composites provides the opportunity to develop multicatalytic systems containing diverse active


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sites with specific catalytic functions. It is known that ILs can be used to catalyse different reactions by an electrophilic/nucleophilic dual activation.62 In this context, the PILs shell around the AuNPs could also act catalysing C-C bond formation through a related mechanism.39 On the other hand, AuNPs can act catalytically and they are widely used for the reduction of a variety of functional groups. Finally, both the nanometric IL-like layer around the NPs might act as an equivalent of the crosslinked borohydride exchange resin (BER), a well-known immobilised reagent for reduction reactions.63 In particular, the BER, in the presence of a metal (nickel, copper, zinc, palladium, etc.) can be an efficient reagent for the reduction of an α-β-unsaturation.

Table 3. Knoevenagel reaction between ethyl cyanoacetate (9) and p-nitrobenzaldehyde (10) in water.a

Entry

catalyst

Yield (%)b

1

-

16

2

[BnBIM][Cl] 13

95 %

6

AuNP-PIL-6

74

a

25 oC, 15 min, 9:10 1:1 molar ratio, 25 % mol cat. b determined by 1H-NMR of the reaction crude. c polymer insoluble under the reaction conditions.


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ACS Catalysis

Thus, to evaluate these composites as multicatalytic nanoreactors, the Knoevenagel condensation in water between ethyl cyanoacetate (9) and p-nitrobenzaldehyde (10) was first assayed (Table 3).64 The initial tests showed that the composite AuNP-PIL-4 was an efficient catalyst for the reaction. Indeed, a > 95% yield was achieved after only 15 min. The AuNP-PIL6 composite was also active, although giving a slightly lower yield (74%) for the same reaction time. PIL-3 was also active for this process, showing a 74% yield in 15 min. The reaction in the absence of any catalyst provided a low conversion (16% of yield) probably associated with a favourable hydrophobic effect. Noteworthy, the monomeric homogenous equivalent of the structures present in PIL-3 and in the two composites (benzyl butyl imidazolium chloride: [BnBIM][Cl], 13) did not show any catalytic activity. The large cation of [BnBIM][Cl] is likely to act as an antihydrophobic additive decreasing the hydrophobic effect and therefore suppressing the condensation reaction.65 Accordingly, the catalytic activity found for the ionic liquid-like units in PIL-3 and PIL-AuNPs must be associated with their polymeric nature. Most likely, adjacent imidazolium groups can participate in the catalytic process in a cooperative way. In this regard, the C(2)H of the imidazolium could hydrogen bond with the carbonyl oxygen of the benzaldehyde, increasing its electrophilic character. Meanwhile, an adjacent chloride could act as a Lewis base activating, via hydrogen bonding, one electron deficient hydrogen atom at the methylene of the ethyl cyanoactete.66 In the polymeric systems the cooperative action of anions and cations corresponding to different, but spatially proximal, imidazolium units would increase the catalytic efficiency in comparison with the monomeric counterpart. This cooperative assistance can be considered similar to the proton relay systems observed in enzymatic catalysis.67 The additional ionic liquid-like units present in the main polymeric chain can also be


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involved in the activation process, forming hydrogen bonds with the aldehyde or with water molecules. This will be similar to the dual “electrophilic/ nucleophilic” activation mechanism described for some Lewis basic ILs. Once the catalytic activity of the composites for the C-C bond formation was demonstrated, the possibility of their behaviour as bifunctional tandem catalytic systems to induce one-pot cascade reactions was also explored. For this purpose, the former Knoevenagel reaction was carried out for 1 h in the presence of AuNP-PIL-4 or AuNP-PIL-6 and the resulting mixture was then treated with NaBH4 in a one-pot procedure. Table 4. Single-pot consecutive Knoevenagel reaction between p-nitrobenzaldehyde and ethyl cyanoacetate followed by the reduction with NaBH4.a

Conv. a Entry

(%) 11

Molar ratio 11/14/15/16

Cat.

15 Yieldb (%) 16 Yieldb (%)

1

-

34

1/0/0/0

-

-

2

AuNP-PIL-4

100

0/0/1/1.2

45

55

3

AuNP-PIL-6

100

0/0/1/2.12

32

68

4

AuNP-PIL-6

100

0/0/0/1c

0

100

a

i) 25 oC, 60 mim, 1:1 9:10 molar ratio, 25 % mol cat., H2O; ii) r.t., 30 min., 5 eq. NaBH4, 25 % mol cat., H2O; b determined by 1H-NMR on the reaction crude. c 24 h., 20 eq. NaBH4.


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ACS Catalysis

Table 4 summarises the results found. As was expected both composites were able to catalyse the Knoevenagel reaction with >99% conversion of the initial aldehyde to the corresponding alkene. Moreover, for both AuNPs composites, the addition of NaBH4 after 30 minutes led to a full catalytic reduction of the NO2 groups, as confirmed by the NMR of the reaction crude. Noteworthy, along with the NO2 reduction the composites also catalysed the reduction of the alkene and the ester groups present in the initial Knoevenagel product. Thus, PILs and AuNPs can act synergistically, in presence of NaBH4, to catalytically reduce these additional functional groups in a similar fashion than the one reported for BER crosslinked solid polymeric reagents.68,69 It is also important to note that the structural differences discussed here for both catalytic composites have a clear influence on the outcome of the reaction. Indeed, both composites provide the complete reduction of the C=C double bond, but the composite AuNP-PIL-6 facilitates the reduction of the ester group to afford compound 16. Thus, the selectivity for the production of this compound over compound 15 (68:32) is appreciably higher in comparison with AuNP-PIL-4 (55:45). This can be related with the more structurally defined organisation provided by the polymeric layer for the composite AuNP-PIL-6. As observed in the previous example, a slower diffusion of the organic compounds is likely to take place in the case of AuNP-PIL-6 in the strongly associated polymeric layer (to and from the AuNP surface). This larger contact time of compound 11 (also of application for 14 and 15) with the active reduction sites of the composite facilitates the consecutive reduction of the NO2, C=C and CO2Et groups. In excellent agreement with this analysis, the product 16 was obtained as the only product (100 % selectivity) for longer reaction times or with the use of a larger excess of NaBH4. Conclusions


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The methodology here reported demonstrates how the controlled self-assembly and selforganisation of a reduced number of building blocks (AuNPs and PILs) can lead to the development of simple and controllable multicatalytic nanoreactors. The diverse catalytic sites of these multifunctional composites containing Lewis base sites, polycations and AuNPs, enable efficient catalytic cascade processes in which a Knoevenagel condensation is followed by the reduction of several functional groups in the condensation product, in a one-pot tandem reaction involving four consecutives chemical transformations. The composites were characterised by different techniques suggesting that appropriately controlling the self-assembly and selforganisation of the constitutive building blocks during their synthesis can provide optimal microenvironments for each reaction. Indeed, the architecture of the composite determines the diffusion of the intermediate(s) from the shell to the core and out from the core to the reaction media. Thus, the catalytic efficiency found is not only due to the presence of different catalytic motifs but a result of their hierarchical organization. In this regard, a proper selection of the corresponding preparation approach for these composites, that determines their hierarchical organization, allows achieving the four transformations synthetic procedure with 100 % conversion and 100 % selectivity. We are currently evaluating this strategy for more complex systems to explore tandem reactions that could involve otherwise incompatible catalytic transformations. ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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* E-mail: [email protected] (Eduardo García-Verdugo). * E-mail: [email protected] (Santiago V. Luis). SUPPORTING INFORMATION It contents experimental procedures together with data and discussion regarding the characterization of the materials prepared by TEM, NMR and DSC. ACKNOWLEDGMENT Financial support by GV (PROMETEO/2016/071) MINECO (CTQ2015-68429-R) and Pla de Promoció de la Investigació de la Universitat Jaume I (P1•1B2013-37) are acknowledged. S.M. (FPU AP2012-0667) thanks MINECO for personal funding. Cooperation of the SCIC of the UJI for instrumental analyses is acknowledged. REFERENCES.

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AuNP–Polymeric Ionic Liquid Composite ... - ACS Publications

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