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Lipidic Mesophase-embedded Palladium Nanoparticles: Synthesis and Tunable Catalysts in Suzuki-Miyaura Cross Coupling Reactions Michael Duss, Jijo J. Vallooran, Livia Salvati Manni, Nicole Kieliger, Stephan Handschin, Raffaele Mezzenga, Henning J. Jessen, and Ehud M. Landau Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02905 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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Lipidic Mesophase-embedded Palladium Nanoparticles: Synthesis and Tunable Catalysts in Suzuki-Miyaura Cross Coupling Reactions Michael Duss[1], Jijo J. Vallooran[1], Livia Salvati Manni[1,2], Nicole Kieliger[1], Stephan Handschin[2], Raffaele Mezzenga[2], Henning J. Jessen[3], Ehud M. Landau[1] [1] Department of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zürich, Switzerland. [2] Department of Health Science & Technology, ETH Zurich, Schmelzbergstrasse 9, 8092 Zürich, Switzerland. [3] Institute of Organic Chemistry, Albert-Ludwigs-University of Freiburg, Albertstrasse 21, 79104 Freiburg i. B., Germany.

Abstract Lipidic cubic phases (LCPs) can reduce Pd2+-salts to palladium nanoparticles (PdNPs) of ~ 5 nm size in their confined water channels under mild conditions. The resulting PdNP-containing LCPs were used as nanoreactor scaffolds to catalyze Suzuki-Miyaura cross coupling reactions in the aqueous channels of the mesophase. To turn on catalysis, PdNP-containing LCPs are activated by swelling the aqueous channels of the lipidic framework, thereby enabling diffusion of the water-soluble substrates to the catalysts. The mesophases play a threefold role: they act as the reducing agent for Pd2+, as a limiting template for their growth, and as a support. The system was characterized and investigated by small-angle X-ray scattering (SAXS), cryotransmission electron microscopy (cryo-TEM), dynamic light scattering (DLS) and nuclear magnetic resonance (NMR). Bulk LCPs as well as three dispersed palladium/lipid hybrid nanoparticle types were applied in catalysis. The latter liposomes, hexosomes and cubosomes - can be obtained by design through combination of lipid and additive. The Suzuki-Miyaura cross coupling of 5-iodo-2’deoxyuridine and phenylboronic acid was used as a model reaction to study these systems. Bulk Pd-LCPs deliver the Suzuki-Miyaura product in 24 h in conversions up to 98 % at room temperature, whereas with palladium/lipid dispersions at 40 °C 68 % of the starting material was transformed to the product after 72 h. Key words: LCP, cubosomes, templated PdNP synthesis, Suzuki-Miyaura cross coupling, Pd/lipid hybrid nanoparticles

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Introduction Together with other palladium catalyzed cross coupling reactions,1–3 the Suzuki-Miyaura cross coupling reaction has revolutionized the capabilities of chemists with regard to carboncarbon bond formation.4,5 Further development of these cross coupling reactions broadened their scope, including the coupling of sp2-sp3 and sp3-sp3 carbons.6,7 The 2010 Nobel Prize in chemistry, awarded to the discoverer of the palladium catalyzed cross coupling reactions, underlines the significance of the Suzuki-Miyaura reaction, which is an extensively used method to build-up complex molecules both in industry and academic research. Breslow’s discovery of the positive effect of water on the reaction rate and selectivity of DielsAlder reactions in the 1980s led to a renaissance of water as reaction medium for chemical transformations.8,9 This work has initiated widespread interest in the field of green chemistry, and the repertoire of chemical transformations that can efficiently be carried out in water is ever growing.10 A first example of a water tolerant Suzuki-Miyaura cross coupling catalyst was reported as early as 1990.11 Since then, various examples of water soluble palladium catalysts have been developed12,13 including systems using organic co-solvents14,15, phase transfer catalysts and surfactants.16–18 Commonly used phosphine-based ligands for homogeneous catalysis of these reactions are expensive and their recovery is tedious, which led to the search and development of heterogeneously catalyzed systems. Furthermore, by developing heterogeneous catalytic systems that are more easily recyclable, the life-time is extended, which improves the overall efficiency of the processes.19 However, the heterogenization of established catalysts to different supports is ineffective and many phosphine-free systems are known to leach catalytically active species to the solution.20 Recent discoveries revealed the potential of palladium nanoparticles (PdNPs) in catalysis,20,21 which might provide an alternative approach to overcome the drawbacks mentioned above. Thus, various types of materials have been used for the immobilization of PdNPs and their subsequent application in catalysis: Mesoporous silica or titanium, polymer supports, magnetic nanoparticles carbon nanotubes and metal organic frameworks, to mention only a few.21–24 By tuning morphology, shape and size of PdNPs, tailored heterogeneous catalytic systems can be designed.25,26 Therefore, the method for the formation of the PdNPs is a crucial step for the design of such heterogeneous catalytic systems with desired properties. Not surprisingly, many procedures for the preparation of PdNPs are described in literature, and more often than not they require sophisticated reducing steps, are energy-inefficient, or do not allow precise control over particle size distribution.27 Recently, palladium nanoparticles have attracted attention also due to their use in the nanomedicine field.28 Weiss et al. showed that supported Pd(0) particles can bioorthogonally 2 ACS Paragon Plus Environment

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activate

5-fluoro-1-propargyluracil

and

N4-propargyloxycarbonylgemcitabine,

which

in

combination are cytotoxic to colorectal and pancreatic cancer cells.29,30 Huang et al. used hexagonal palladium nanosheets for photothermal therapy.31 Furthermore, Balbin et al. treated four different human cancer cell lines with mesoporous silica supported PdNPs and reported high cytotoxicity of these PdNPs.32 Additionally, the same particles were successfully applied in Suzuki-Miyaura cross coupling - thus a dual application was reported. Puvvada et al.33 used for the first time monoolein (MO) -based LCPs to synthesize PdNPs. The concept is based on a polyol-type reduction of Pd2+ and allows PdNP synthesis under benign conditions - neither organic solvent, nor high temperature, high pressure or expensive equipment are required. Furthermore, the size of the nanoparticles can be controlled, as the LCP plays a dual role as reducing agent and template with controllable water pore sizes. However, this remains the only such report, and no further investigations with PdNPcontaining LCPs have been carried out. Because of the great potential of this method, we set out to further develop the concept and investigate its applicability within designed mesophases as catalyst scaffolds.

Diffusion off: No reaction 2

1

Pd

in-meso grown Pd-nanoparticles 3

Swelling: Aqueous channel diameter increases

Diffusion on: Reaction takes place

2

1

Pd

3

Figure 1: Schematic representation of PdNPs formed in aqueous channels of LCPs followed by swelling. Top panel: The diffusion of substrates is limited by the size of the aqueous channels, thus no product is formed. Bottom panel: The size of the aqueous channels increases upon adding a swelling agent, resulting in the entry of substrates into the LCP and enabling the reaction to take place.


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We apply this method to directly catalyze cross-coupling reactions in LCPs. Following nanoparticle formation as described by Puvvada et al.33 the aqueous channels were found to be blocked, thereby preventing diffusion and resulting in “switching off” of the reaction. To overcome this drawback, a “switching on” modification of the LCP, effectively brought about by swelling the aqueous channels, is required in order to enable diffusion and allow the reaction to take place (Figure 1). The mesophases thus play a threefold role: they act as a reducing agent for Pd2+, as a template for their growth, and as an immobilizing soft support. The LCP modifications, material characterization and kinetics of the Suzuki-Miyaura cross couplings carried out in bulk LCP, as well as in dispersed palladium lipid hybrid nanoparticles are described herein. Catalyzing Suzuki-Miyaura cross couplings with PdNPs synthesized and stabilized in LCPs is an approach which is distinctly different from our previous work, in which we have developed catalytic LCPs and cubosomes with tunable activity for investigating aldol reactions carried out with improved efficiencies, both chemically34 and enzymatically,35 demonstrating the broad applicability of these bio-nanomaterials in aqueous catalysis.


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Results and discussion Investigation of PdNP synthesis Formation of PdNP-containing lipidic cubic phases was accomplished according to a previously reported method: monoolein was hydrated with a 20 mM solution of K2PdCl4, mixed, centrifuged and equilibrated for 24 h.33 Combined data of multiple analytical methods provide evidence for the formation of PdNPs within the LCP. Following the PdNP synthesis, the LCP undergoes color change from orange to black (Figure 2A) and the maximum absorption at 420 nm, characteristic of K2PdCl4, disappears (Figure 2D). The lipid fraction of the LCP was isolated after PdNP synthesis and analyzed by 1H-NMR. A signal at 9.66 ppm appeared after PdNP synthesis and likely originates from an aldehyde proton formed during the redox process (Figure 2B and 2C). Puvvada et al. reported already the corresponding FTIR bands.33 B

A

C

D

Pd2+

O O

OH OH

4

O O

- Pd0

O OH

5

Figure 2: A) Pictures of the initial MO-based LCP hydrated with a K2PdCl4 solution (left), and after 24 h (right). The characteristic brown color of K2PdCl4 disappears and the LCP turns black. B) 1H-NMR spectrum in CDCl3 of the lipid fraction (MO) after PdNP synthesis; a signal with a typical chemical shift (9.67 ppm) of an aldehyde proton, was detected. C) Proposed redox process leading to PdNP formation. D) UV/Vis spectra of K2PdCl4 (blue) and PdNPs (orange).

The kinetics of PdNP formation was analyzed using small angle X-ray scattering (SAXS) and the results are shown in Figure 3. MO was hydrated with 40 % of a 20 mM K2PdCl4 solution and the SAXS measurement was started directly after centrifugation. The slope of the signal at low q value increases in the first 15 h (blue arrow), indicating formation of smaller particles. Since the slope does not increase further after 15 h, the PdNP synthesis likely halts. Interestingly, shortly before the increase of the slope stagnates, the Pn3m LCP undergoes a phase transition to Ia3d cubic phase (Figure 3). It is not clear whether the phase transition is induced directly by the PdNPs since reduction of the Pd-precursor K2PdCl4 is coupled to the oxidation of a hydroxyl moiety in MO’s headgroup to the aldehyde 5 (Figure 2B and 2C), which can alter the lipidic packing. Furthermore, Pn3m-Ia3d transitions have 5 ACS Paragon Plus Environment

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been observed before, during other dynamic events within LCPs, such as in-meso crystallization.36 However, the amount of K2PdCl4 present in the sample can oxidize no more than 0.47 % of MO to aldehyde 5.

Figure 3: SAXS profiles of the time-resolved intensity vs. scattering vector q during PdNP synthesis in MO-based LCPs at room temperature. The Pn3m to Ia3d phase transition occurs after 12 h, the arrow indicates the change of slope at low q value.

PdNPs formed in LCPs were analyzed after lipid removal by transmission electron microscopy (TEM) and DLS. TEM micrographs were taken after adding Pd-LCP on a grid and removing the lipid by rinsing with ethyl acetate (Figure 4A). The diameter of these PdNPs was found to be 5.3 ± 2.5 nm (Figure 4B). For DLS measurements, dodecanethiol was added to the Pd-LCP and mixed. The lipid precipitate was removed by centrifugation, and ethanol was added to the supernatant containing dodecanethiol-coated PdNPs. This procedure was repeated three times before DLS was measured. Nanoparticles with sizes of 3 – 5 nm were detected (Figure 4C), which is in the range of the aqueous channel diameter of LCPs. Larger aggregates of ~ 200 nm are present as well. Bimodal size distribution is often found in such cubosome dispersions, and can be ascribed to the simultaneous presence of unimers and clusters, the latter being formed upon removal of lipids that stabilize the unimer surface.


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A

B

Frequency / % Frequency /%

25 20 15 10 5 0

1

2

3

4

5

6

7

PdNP diameter PdNP diameter / nm 40

Frequency / %

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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8

9

10

11

/ nm

C

30 20 10 0 0.1

1

10

100

1000

10000

Diameter / nm

Figure 4: A) TEM micrograph of PdNPs after removal of lipid with ethyl acetate. B) Size distribution of the PdNPs in A. C) DLS analysis of PdNPs after removal of lipids with ethanol and PdNP stabilization with dodecanethiol.

Catalysis in Pd-containing LCPs Catalysis in LCPs and cubosomes requires diffusion of water-soluble substrates from bulk water into the aqueous channels of LCPs. To this end, 5-iodo-2’-deoxyuridine (1) and phenylboronic acid (2) were chosen as water-soluble model substrates (Figure 1). The maximal concentration of 1 tolerated by the LCP was found to be ca. 5 mM, as higher concentrations destabilized it. Puvvada et al. suggested that the size of the PdNPs corresponds to the size of the aqueous channels of the LCPs, which were used for their synthesis.33 This proposal is in agreement with our data (Figure 4). Hence, the aqueous channels are blocked by the PdNPs, which strongly reduces diffusion of the substrates into the aqueous compartment, where the catalyst is situated, thereby hindering product formation. When Pd-LCPs were applied for catalysis after PdNP synthesis, only traces (3 %) of the corresponding product were formed (Table 1, entry 5).

To overcome this problem, we applied swelling of the LCP after PdNP synthesis, thereby “switching on” the diffusion and allowing for the formation of the corresponding cross coupling products at greatly increased rates (Figure 1). Swelling of mesophases can be induced via modification of the lipidic compartment or of the aqueous phase. Hence, our system has two potential “switches” to turn on reactivity and to tune it, as has been shown in a previous paper of our group.34 Cholesterol was chosen as a lipidic swelling agent. Following PdNP synthesis, cholesterol was added and the LCP was equilibrated for at least 4 7 ACS Paragon Plus Environment

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days before being used for catalysis. SAXS investigation revealed that the full hydration line with 4 % cholesterol (w/w lipid) was reached at a hydration degree of 47 %. The diameter of this LCP was calculated to be 5.5 nm (see experimental part for equations and references). In comparison, Puvvada et. al. reported an aqueous channel diameter of 5 nm for pure MObased LCPs containing PdNPs.33 DLS measurements showed that the PdNP size does not change significantly after swelling the mesophase (see SI). Polyethylene glycol (PEG 4000), on the other hand, was used to induce swelling via the aqueous phase and was added simultaneously with the substrates. Since PEG 4000 is water-soluble, it can be removed from the mesophase. This approach does not only allow the “on-switching”, but also the “offswitching” of the system. If the PEG/substrate solution was removed and replaced by a new substrate solution without PEG, no product was formed. Subsequently, the catalytic system could be switched on again. However, the reaction progress stagnated after a conversion of ~ 30 %. This process is currently being optimized in our laboratory. Catalysis of Suzuki-Miyaura cross coupling reactions with PdNP-containing LCPs Using this approach, LCPs can be applied for catalysis directly following PdNP synthesis. Both swelling methods led to successful conversion of the starting materials to the desired Suzuki-Miyaura product. When cholesterol was used to modify the LCP, 95 % of 5-iodo-2’deoxyuridine (1) were converted to the product (3) within 24 h (Table 1, entry 1), whereas 87 % of 5-iodo-2’-deoxyuridine were transformed to the product in 32 h when PEG 4000 was used (Table 1, entry 4). Subsequently, the influence of the base on PdNP-LCP catalysis was investigated. Three bases were tested: K2CO3, KOH and NEt3. Besides the impact of the base on the reaction itself, the tolerance of the LCP toward the base needs to be considered. K2CO3 and KOH showed very similar results and allowed for efficient catalysis: after 24 h, 95 % and 98 %, respectively, of 1 were converted (Table 1, entry 1 and 2). When NEt3 was used as a base, the reaction was significantly slower: after 72 h, 84 % of 1 were consumed (Table 1, entry 3). All these experiments were carried out with 3 mol % Pd with respect to 5iodo-2’-deoxyuridine (1), and only entry 5 was carried out with 1 mol % Pd, which resulted in slower conversion (76 % after 48 h). The reaction progress of Pd-LCP with a catalyst loading of 3 mol % and with K2CO3 as base (Table 1, entry 1) is depicted in Figure 5.


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A

O I H1 O

HO

N

B(OH)2

O

Pd mesophase 3 mol % Pd

NH HO

H 2O Base (2 eq.)

H2

HO

B

O NH

1

HO

2

H1 O

N

O

H2 3

H1 of 1

H1 of 1 H2 of 3 H2 of 3

Figure 5: A) Section of a 1H NMR spectrum showing the chemical shifts of H1 (blue) and H2 (red) when transformed from starting material to product. B) Progress of the Suzuki-Miyaura cross coupling reaction of phenylboronic acid (2) with 5-iodo-2’-deoxyuridine (1) in Pd-LCP containing 3 mol % Pd. Conditions: Reactions were carried out in special, home-built metal holders at room temperature in deionized H2O with 2 eq. of base and phenyl boronic acid (2). Conversions determined by 1H-NMR.

Table 1: Comparison of the catalytic efficiencies of PdNP-containing LCPs with various bases, Pd concentrations and swelling approaches on the model Suzuki-Miyaura reaction of 5-iodo-2’deoxyuridine (1) and phenylboronic acid (2). Entry

Swelling agent

Base

Time [h]

Conv.a [%]

1

Cholesterol, 4 %b

K2CO3

24

95

2

Cholesterol, 8

%b

K2CO3

19

97

3

Cholesterol, 4 %b

KOH

24

98

4

Cholesterol, 4 %b

Et3N

72

84

5

Cholesterol, 4

%c

K2CO3

48

95

6

PEG 4000

K2CO3

32

87

7

-

K2CO3

24

3

8d

-

K2CO3

24

0

9e

Cholesterol, 4 %b

K2CO3

24

88

Conditions: Reactions were carried out in special, home-built metal holders at room temperature in deionized H2O with 2 eq. of base and 2 eq. of phenylboronic acid (2). aDetermined by 1H-NMR. w/w of lipid content.

c

b

%

Reaction carried out with Pd-LCP containing 1 mol % Pd. dControl experiment:

Aqueous phase was separated from PdNP-containing LCP and controlled for catalytic activity by 1HNMR. eRecycled LCP (2nd cycle).


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Overall, the best results were obtained when K2CO3 was used as base with 3 mol % palladium (Table 1, entry 1). This system was therefore used for further investigations such as lowering Pd concentration (Table 1, entry 5), PEG-swelling and recycling of Pd-LCPs. K2CO3 is a weaker base compared KOH and nearly as efficient. Weaker bases are preferred due to the better tolerance towards LCPs (e.g. due to hydrolysis of ester group of monoolein). To expand the scope of this methodology, two additional synthetic applications have been successfully performed. Thus substrates 6 and 8 have been converted into the corresponding Suzuki-products using Pd-LCPs which have been swollen using 4% cholesterol (table 2). Table 2: Structures of substrates 6 and 8 and their corresponding Suzuki- products tested for SuzukiMiyaura cross coupling reactions catalyzed by Pd-LCPs.

Entry

Substrate

Suzuki- product O

O I

1

Na2O3PO

NH

NH N O

Na2O3PO

O

Na2O3PO

N

91 % conversion after 24 h

7 O

O

OH

OH

65 % conversion after 48 h

I

8

O

O

Na2O3PO

6

2

Outcomea

9

Conditions: Reactions were carried out in special, home built metal holders at room temperature in miliQ H2O with 2 eq. K2CO3 and 2 eq. of phenyl boronic acid (2). a Determined by 1H-NMR.

To prove that the reaction takes place in the aqueous compartment of the LCP rather than in the aqueous overlay that is in contact with the LCP, the aqueous overlay was checked for its catalytic activity by 1H-NMR. However, no product was observed, indicating that PdNPs remain immobilized in the channels traversing the bilayer of the mesophase. ICP-MS analyses supported this finding, as only a small fraction of the Pd (concentration of 75.2 ng/mL, corresponding to less than 0.05 % of total Pd) were found in the aqueous phase. In principle, LCPs can easily be reused by just removing the crude product and by decanting the aqueous phase.34,35,37 After rinsing with water, a new substrate solution can be added to the LCP as an overlay. Two cycles were performed with a slight decrease of the conversion from 95 % to 88 % in the second cycle. This is in contrast to comparable, previous reports on in-meso enzymatic and organocatalytic reactions that are recyclable multiple times.34,35,37 One reason for the slight loss of activity might be the assembly of PdNPs to bigger aggregates. A second reason might be the sinking of PdNPs towards the bottom of the 10 ACS Paragon Plus Environment

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sample by gravity. The observation that the upper part of Pd-LCPs turns colorless after two weeks of storage supports this scenario. Preparation and characterization of PdNP/lipid nanoparticles Since PdNPs need to be grown in bulk LCP, only a top down approach for the formation of lipidic dispersions can be applied. Attempts to synthesize PdNPs in cubosomes after their preparation failed. MO- and PT-based LCPs have been prepared and their aqueous channels were swollen using cholesterol or 1,2-distearoyl-sn-glycero-3-phospho-rac-(1glycerol) (DGPG).38 Interestingly, by varying lipid composition, different Pd/lipidic hybrid nanoparticles can be obtained (Table 3).

Table 3: Formation of various Pd/lipidic hybrid nanoparticles as a function of lipid composition and their catalytic performance in the model reaction of 5-iodo-2’-deoxyuridine (1) and phenylboronic acid (2). (Cf. Table 1). Entry

Lipid

Additive

Nanoparticlea

Conversion [%]b

1

MO

Cholesterol

Liposome

68

2

PT

Cholesterol

Hexosome

51

3

PT

DGPG

Cubosomec

35 54

4

PT

DGPG

Cubosomed

5e

-

-

-

-

6f

MO

Cholesterol

Liposome

59

7g

MO

Cholesterol

Liposome

45

Conditions: Reactions were carried out in special, home-built metal holders (see experimental part) at 40°C with 2 eq. of base and 2 eq. of phenylboronic acid (2), and stopped after 72 h. a Determined by SAXS.

b

Determined by 1H-NMR after 24 h.

c

Cubosomes underwent phase transition to hexosomes

during the reaction. dSubstrate concentration: 1.1 mM (5-iodo-2’-deoxyuridine), Pd loading: 12 mol %. e

Control experiment: Aqueous phase was separated from the lipid dispersion and controlled for

catalytic activity by 1H-NMR. fRecycling, 2nd cycle. gRecycling 3rd cycle.

Dispersion of monoolein/cholesterol LCP, which form cubosomes in the absence of Pd,34 led to formation of liposomes (Figure 6A), probably due to interactions of the double bond of MO with PdNPs. These samples did not scatter when analyzed by SAXS, but the liposomal structure can be visualized by cryo-TEM (Figure 6A). The Pd-liposomes are catalytically active and recyclable (3 cycles, see table 3, entries 6 and 7). The dispersion of phytantriol/cholesterol LCP led to hexosomes, which are catalytically active (slightly less than liposomes, Table 3, entry 2). Cubosomes were obtained by dispersion of PT/DGPG LCPs (Figure 6B and 6C). Interestingly, a phase transition from the cubic Pn3m to HII occurred 11 ACS Paragon Plus Environment

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(Figure 6C) during Suzuki-Miyaura coupling of phenylboronic acid (2) with 5-iodo-2’deoxyuridine (1) using our standard substrate concentrations (4.3 mM of 1, 8.6 mM of 2) (Table 3, entry 3). SAXS spectra of the resulting hexosomes were obtained at 22 °C, 40 °C, 50 °C and 60 °C, demonstrating their stability up to 50 °C. As expected, the lattice parameter shifts to higher q values during heating. The sample loses its desired structure at 60 °C (Figure 6D). The cubic structure could only be retained if the substrate concentration (5-iododeoxyuridine) was substantially reduced from 4.3 to 1.1 mM.

A

B

C

D

Figure 6: A) Cryo-TEM micrograph of Pd-liposomes B) Cryo-TEM micrograph of Pd-cubosomes C) SAXS pattern of Pd-cubosomes before and after reaction at 40 °C D) SAXS pattern of Pd-hexosomes at 22 °C, 40 °C, 50 °C and 60 °C, where the sample starts to lose its hexagonal structure.

Suzuki-Miyaura cross coupling reactions with PdNP/lipid hybrid-nanoparticles The catalytic activities of Pd-liposomes, Pd-hexosomes and Pd-cubosomes hybridnanoparticles were compared using the model coupling reaction of 5-iodo-2’-deoxyuridine (1) and phenylboronic acid (2). In contrast to bulk Pd-LCPs, which are catalytically active at 12 ACS Paragon Plus Environment

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ambient temperature, the lipid dispersions require elevated temperatures (40 °C) to transform the substrates into products. Pd-liposomes (Table 3, entry 1) exhibited the best catalytic activity, and after 48 h 68 % of the starting material was converted to the product (87 % after 78 h, see SI). Using Pd-hexosomes, 51 % of 5-iodo-2’-deoxyuridine (1) was consumed (Table 3, entry 2). The lowest catalytic activity was observed with Pd-cubosomes and only 35 % of 5-iodo-2’-deoxyuridine (1) were transformed to the product after 48 h (Table 3, entry 3). These cubosomes underwent a phase transition to hexosomes during the coupling reaction. To overcome this problem, the substrate concentration was lowered fourfold (from 4.3 to 1.1 mM of 1). Since Pd concentration in the cubosomes was kept constant, this led to an increase of the relative amount of Pd up to 12 mol % with respect to the substrate 1. Under these conditions, 54 % of substrate were transformed to product after 72 h (Table 3, entry 4). In summary, Pd-liposomes are the most efficient Pd/lipid hybrid nanoparticles, followed by Pd-cubosomes and hexosomes (Figure 7).

Increasing catalytic activity

Pd-liposomes

Pd-cubosomes

Pd-hexosomes

Figure 7: Schematic representation of the Pd/lipid hybrid nanoparticles with increasing catalytic activity from the right to the left.

To control whether the PdNPs remain inside the lipidic particles, the catalytic activity of the aqueous phase was examined by mixing with the substrates 5-iodo-2’-deoxyuridine (1) and phenylboronic acid (95) (including K2CO3). No product was formed after 24 h or after 48 h. Moreover, ICP-MS analysis of the aqueous phase revealed that the Pd-concentration was 5.2 ng/mL, which corresponds to 0.003 % of total Pd in the system, demonstrating efficient retention by the LCP. To recycle Pd-dispersions, the lipid was separated from water by filtration, a new stabilizer solution was added, and the sonication process was repeated. Pdliposomes were recycled, albeit with some loss of catalytic activity after every cycle. Whereas in cycle 1 the conversion was 68 %, it dropped to 59 % in cycle 2 and to 45 % in cycle 3 13 ACS Paragon Plus Environment

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(Table 3). In general, coupling reactions catalyzed by Pd/lipid dispersion were found to be slower as compared to bulk Pd-LCPs and had to be performed at 40 °C.

Conclusions

Lipidic cubic phases are structured biomaterials composed of spatially well-defined, curved lipidic bilayers that are interpenetrated by aqueous channels. The geometric parameters of such materials, i.e. bilayer curvature and aqueous channel diameter are tunable, thus rendering these materials interesting for various chemical and biochemical applications. In this investigation we set out to establish the feasibility of PdNP-containing LCPs as nanoreactor scaffolds to catalyze Suzuki-Miyaura cross coupling reactions in the aqueous channels of the mesophase. Combination of small-angle X-ray scattering (SAXS), cryotransmission electron microscopy (cryo-TEM), dynamic light scattering (DLS) and nuclear magnetic resonance (NMR) were used to characterize the structure-function relationship of the catalytic mesophases. Synthesis of PdNPs with size of ~ 5 nm in LCPs was completed after 15 h at room temperature, and induced a phase transition of the supporting mesophase from the cubic Pn3m to the cubic Ia3d form. Significantly, efficient catalysis can be turned on by swelling of the aqueous channels of the lipidic framework, thereby enabling diffusion of the water-soluble substrates to the catalysts. Swelling was affected with either a lipidic additive (cholesterol or DGPG) or with PEG 4000, thereby providing two different switches to initiate the reaction. The model cross coupling substrates 5-iodo-2’-deoxyuridine and phenylboronic acid were transformed to the product with yields as high as 98 % within 24 h, with Pd loading of 3 mol % in the mesophase. To expand the scope of this methodology, two additional synthetic applications have been performed with good yields. Catalytic studies were additionally performed on three dispersed palladium/lipid hybrid nanoparticle types, liposomes, hexosomes and cubosomes, which can be obtained by design through combination of lipid and additive. Analogous to the parent bulk phases, these dispersed nanoparticles were found to be catalytically active, albeit with reduced efficiency compared to bulk Pd-LCPs. Reactions in these media therefore had to be performed at 40 °C and established that Pd-liposomes are the most efficient Pd/lipid hybrid nanoparticles, followed by Pd-cubosomes and hexosomes. The designed mesophases play a threefold role: they act as the reducing agent for Pd2+, as a limiting template for their growth, and as a support. Future studies will address the substrate scope using small peptides and different nucleosides and nucleotides. Furthermore, these Pd/lipid hybrid nanoparticles have potential medical applications, as PdNPs have been successfully tested against cancer cell lines as discussed in the introduction.29,30 Cubosomes are considered as next-generation smart lipid nanoparticles,39 and Pd-cubosomes and Pd14 ACS Paragon Plus Environment

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hexosomes are potential delivery vehicles for active PdNPs. This approach thus enables the in-situ synthesis of the cargo-PdNPs by its own drug delivery vehicle. Supporting Information Procedures for the preparation of Pd-LCPs and Pd-dispersions, synthetic details, cryo-TEM and characterization of the Suzuki-Miyaura product. DLS, SAXS and kinetic data of dispersions, mesophase identification and calculation of aqueous channel diameter and any associated references. Acknowledgements The authors thank Dr. Salvatore Assenza for help in figure design, Dr. Chiara Speziale for SAXS measurements, and the Scientific Center for Optical and Electron Microscopy ScopeM of ETH. This work was partially supported by the Forschungskredit (FK-15-083) of the University of Zurich to M.D. Notes The authors declare no conflicts of interest. References (1)

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Diffusion off: No reaction 2

1

Pd

in-meso grown Pd-nanoparticles 3

Swelling: Aqueous channel diameter increases

Diffusion on: Reaction takes place

2

1

Pd

3

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B

A

C

D

Pd2+

O O

OH OH

4

O O

- Pd0

O OH

5


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B

Frequency / % Frequency /%

25 20 15 10 5 0

1

2

3

4

5

6

7

PdNP diameter PdNP diameter / nm 40

Frequency / %

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8

9

10

11

/ nm

C

30 20 10 0 0.1

1

10

100

Diameter / nm


1000

10000

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B

C

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Increasing catalytic activity

Pd-liposomes

Pd-cubosomes


Pd-hexosomes

1 2 No3reaction 4 5 6 n off: No reaction 7 8 9 n Pd-nanoparticles 10 11 12 grown Pd-nanoparticles 13 14 15 16 Reaction takes place 17 18 19 n on: 20 Reaction takes place 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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Diffusion off: No reaction Pd

Pd

Swelling

Pd

Pd Diffusion on: Reaction takes place


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Lipidic Mesophase-Embedded Palladium ... - ACS Publications

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