Accepted Manuscript Polymer-Coated Palladium Nanoparticle Catalysts for Suzuki Coupling Reactions Tanize Bortolotto, Sara Elisa Facchinetto, Suelen Gauna Trindade, Andreia Ossig, Cesar Liberato Petzhold, Josimar Vargas, Oscar Endrigo Dorneles Rodrigues, Cristiano Giacomelli, Vanessa Schmidt PII: DOI: Reference:

S0021-9797(14)00801-7 http://dx.doi.org/10.1016/j.jcis.2014.10.037 YJCIS 19931

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

26 June 2014 24 October 2014

Please cite this article as: T. Bortolotto, S.E. Facchinetto, S.G. Trindade, A. Ossig, C.L. Petzhold, J. Vargas, O.E.D. Rodrigues, C. Giacomelli, V. Schmidt, Polymer-Coated Palladium Nanoparticle Catalysts for Suzuki Coupling Reactions, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.10.037

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Polymer-Coated Palladium Nanoparticle Catalysts for Suzuki Coupling Reactions

Tanize Bortolottoa, Sara Elisa Facchinettoa, Suelen Gauna Trindadea, Andreia Ossigb, Cesar Liberato Petzholdb, Josimar Vargasa, Oscar Endrigo Dorneles Rodriguesa, Cristiano Giacomellia, Vanessa Schmidta,*

a

Departamento de Química, Universidade Federal de Santa Maria, 97105-900 Santa Maria - RS,

Brazil. b

Instituto de Química, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre -

RS, Brazil.

*Corresponding Author: Vanessa Schmidt ([email protected]); Tel +55 55 3220 9483; Fax +55 55 3220 8031)

Title Running Head: Polymer-Coated PdNP Catalysts for Suzuki Coupling.

1

Abstract A set of seven different palladium nanoparticle (PdNPs) systems stabilized by small amounts (1.0 mg/mL) of structurally related macromolecular capping agents were comparatively tested as catalyst in p-nitrophenol (Nip) reduction and Suzuki crosscoupling reactions. The observed rate constants (kobs) for Nip reduction were in the range of 0.052−3.120×10−2 s−1, and the variation reflected the effects of polymer chain conformation,

ionic

strength

and

palladium-polymer

complex

coordination.

Macromolecules featuring pendant pyridyl moieties or inverse temperature-dependent solubility were found to be unsuitable capping agents for PdNPs catalysts, despite being active. The catalytic activity in Suzuki cross-coupling reactions followed the same behavior; the most active particles in the Nip reaction also mediated the cross-coupling reaction providing the expected products in quantitative yields under relatively mild conditions after only 4 h at 50°C. Experiments involving the successive addition of reactants and catalyst recovery/re-use indicated that the recycling potential was comparable to those of the standards used in this field.

Keywords: Polymers, capping agents, palladium nanoparticles, p-nitrophenol reduction, Suzuki−Miyaura reaction

2

1. Introduction Palladium nanoparticles (PdNPs) are a unique class of heterogeneous catalysts widely used in a variety of organic synthesis procedures, including the well-known Suzuki, Sonogashira and Heck carbon-carbon coupling reactions [1-3]. The distinct catalytic activity of PdNPs in these reaction processes stems from their electronic properties and under-coordination of the surface atoms, which are sensitive to the particle size and shape, inter-particle distance and interaction mechanism. However, the high surface energy of PdNPs requires the use of stabilizers in order to prevent aggregation and loss of activity [1, 4]. The surface chemistry of these nanoparticles is commonly modified and controlled using surfactants, peptides, ionic liquids, dendrimers or copolymers [5]. As a consequence, a myriad of opportunities opens up in terms of the creative design of capping agents, whose physico-chemical properties define the catalytic activity and substrate selectivity of PdNP systems. The use of copolymers, in particular, has long been established as a successful approach not only to improving particle stability in colloid science but also to creating processable functional materials [6]. In general, polymers provide stabilization through steric crowding and weak binding to the particle surface through heteroatoms, playing the role of ligands. In this regard, we can therefore take advantage of modern macromolecular engineering knowledge to synthesize next-generation polymer coatings for PdNPs that feature stimulus-responsiveness (pH, temperature, etc.) and spontaneously form versatile supramolecular structures. Such sophisticated stabilizers modify the interaction between particles, stabilizing layers and the surroundings, inherently transforming the behavior of palladium sols.

3

Copolymer systems that do not passivate the surface of the encapsulated nanostructure, thus maintaining the catalytic activity, are of particular interest. This is the case, for example, of the colloidal palladium particle catalysts stabilized by polystyreneco-poly(ethylene oxide), developed by Beletskaya et al. [7]. The authors observed that the activity of the particles was comparable to that of the low-molecular-weight palladium complexes, while the stability of the colloidal palladium system was much higher. In pursuing this objective, several authors have investigated the use of poly(ethylene glycol)@PdNPs (PEG@PdNPs) prepared simply by heating solutions containing polymers and a salt as a source of Pd(II) ions [1, 8]. PEG@PdNPs can even be generated in situ during catalyzed ultrafast Suzuki-Miyaura cross-coupling reactions at room temperature [9, 10]. In fact, PEG can be used itself as a solvent in such reactions and has been regarded as a potential replacement for volatile organic solvents. However, waterborne systems would be a better option. We have observed that, in general, high polymer concentrations (typically >10 mg/mL) are required to produce stable PEG@PdNPs for use in reactions in aqueous media, with opportunities for recycling. Many other polymer systems have also been used in the preparation of PdNPs, including homopolymers, such as poly(vinyl alcohol), poly(vinyl pyrolidone) and poly(Nisopropylacrylamide), and amphiphilic block copolymers, such as poly(ethylene oxide)b-polystyrene [11], poly(acrylic acid)-b-polystyrene, and poly(4-vinyl pyridine)-bpolystyrene [12]. In most of the above-mentioned cases, the catalytic activity was comparable to that of the low-molecular-weight analogs, but the colloid stability was much higher.

4

In this study, we aimed at evaluating polymer conformation effects on the catalytic activity of nanoparticles synthesized using less amount of capping agents. To this end, a set of structurally related macromolecules, some of them featuring self-assembly and responsiveness properties, were used to prepare PdNP systems, and the catalytic activity of polymer@PdNPs was evaluated comparatively using a model reaction (p-nitrophenol reduction) and typical Suzuki cross-coupling reactions. The findings discussed hereinafter suggest that hybrid polymer@PdNPs with low polymer content (c = 1.0 mg/mL) are stable and show high catalytic activity when the catalyst synthesis is performed under conditions that favor a less compact, more hydrated, and consequently more extended polymer chain conformation. It is also shown that chemical groups known to form coordination complexes with metallic palladium reduce the catalytic activity.

2. Materials and methods 2.1. Materials Palladium acetate, sodium borohydride, and pluronic F-127 (PEO100-b-PPO55-b-PEO100; Mn = 14,300 g/mol; subscripts refer to the mean degree of polymerization of each segment) were purchased from Sigma-Aldrich. α-Carboxy-ω-thiol terminated poly(N-isopropylacrylamide) (HS-PNIPAM113-COOH;

Mn

=

13,000

g/mol)

and

poly(ethylene

oxide)-b-poly(4-

vinylpyridine)(PEO137-b-P4VP45; Mn = 13,500 g/mol) were acquired from Polymer Source. All chemicals were used as received unless otherwise indicated. Solvents were distilled over their respective drying agents under reduced pressure. Glassware was cleaned with aqua regia and rinsed thoroughly with deionized water. Pure water was collected from a Millipore Alpha Q system (conductivity < 0.05 μS/cm) and used to prepare the solutions.

5

Poly(ethylene (diisopropylamino)ethyl

oxide)-b-poly(2,3-dihydroxypropyl methacrylate]

methacrylate)-b-poly[2-

(PEO113-b-PDHPMA30-b-PDPAEMA50)

and

poly(ethylene oxide)-b-poly[2-(diisopropylamino)ethyl methacrylate] (PEO113-b-PDEAEMA50 were synthesized following the procedures described by Liu et al. [13]. with adaptations, as described in detail elsewhere [14]. (OH)2-poly[2-(methacryloyloxy)ethyl phosphorylcholine]40b-PDPAEMA70 (PMPC40-b-PDPAEMA70) was synthesized following a procedure described by Ma et al. [15] with adaptations, using 1-O-(2’-bromo-2’-methylpropionoyl)-2,3-rac-glycerol as the initiator. Poly(N-vinylcaprolactam) (PNVCL) was polymerized using the RAFT process. The macromolecular characteristics of these building blocks and (when applicable) corresponding self-assembled structures in water are summarized in Table 1.

2.2. Nanoparticle synthesis PdNP synthesis in the presence of pH-responsive polymers (1−4) was carried out by mixing 2.48 mL of 0.1 M buffer solution (phosphates at pH 7.4, or sodium tetraborate at pH 9.1) with 2.48 mL of pH ~ 3.5 aqueous solutions containing molecularly dissolved polymer chains (5.0 mg) and Pd(CH3COO)2 (3.09 x 10-5 g; 1.38×10-3 mmol). Soon after, NaBH4 (42 µL of 0.10 M, 0.001 g; 4.10×10-3mmol) was added to the mixture and the solution turned dark gray. The final volume of the solution was 5.0 mL. In the case of non-responsive polymers (5−7), the synthesis was carried out through reactions with the same stoichiometry, but the polymer and palladium salt were directly dissolved in water or buffer solution prior to the addition of the reducing agent. The nanoparticle composition is referred to as polymer@PdNd (e.g.: 1@PdNP) throughout the text.

6

2.3. Nanoparticle Characterization UV-Vis Spectroscopy. UV-vis spectra were recorded using a Shimadzu UV2600 spectrophotometer. For the measurements, 3.0 mL of solution were placed in a 10 mm square quartz cell. All spectra were recorded in the wavelength range of 200-800 nm at a scan rate of 600 nm/min (0.1 s integration per 1.0 nm) for air-equilibrated thermostated solutions under stirring. Dynamic light scattering (DLS). DLS measurements were performed using a Brookhaven laser light scattering system consisting of a 75 mW He-Ne linear polarized laser source operating at a wavelength (λ) of 632.8 nm, a BI-200SM laser goniometer and a BI-APD avalanche photodiode detector. The autocorrelation functions were acquired using a BI-9000AT digital correlator, and recorded using the BI-DLSW Correlator Control software. Samples were placed in 10 mm diameter glass cells and maintained at a constant temperature of 25.0±0.1 °C. The accessible scattering angles ranged from 15° to 150°. The measured intensity correlation functions g2(t) were determined using the algorithm REPES (incorporated in the GENDIST program). The hydrodynamic radius (RH) (or diameter, D = 2RH) was calculated from the StokesEinstein relation, and polydispersity was estimated from the analysis of cumulants. Transmission electron microscopy (TEM). Morphological analysis of the nanoparticles was performed using a Zeiss transmission electron microscope (model Libra 120) at an accelerating voltage of 60 kV. Nanoparticles were dropped onto formvar-coated copper grids. The particle size was measured for each sample using UTHSCSA ImageTool software. Crystallographic details of these NPs were observed using an FEI HRTEM microscope (model Tecnai 20), operating at an accelerating voltage of 200 kV.

7

2.4. Catalytic activity assays Reduction of p-nitrophenol (Nip). The catalytic activity of the polymer-coated PdNPs was first evaluated for the reduction of p-nitrophenol (Nip) to p-aminophenol (Amp) using UV-vis spectroscopy [16-18]. The reaction was carried out in a quartz cell using an aqueous solution (2.5 mL) containing Nip (3.00×10-4 mmol) and NaOH (3.00×10-3 mmol). After the addition of the reducing agent (1.50×10-2 mmol NaBH4), the cell was placed into the cell holder and the solution was kept under stirring. Finally, a solution containing PdNP (7.37×10-5 mmol of Pd) was added, and the reaction was monitored through the decrease in the absorbance of the p-nitrophenolate ions at 400 nm [16, 17]. Catalyst Recycling. Polymer@PdNPs were separated as a dark gum from the reaction medium used in Nip reduction assays by ultracentrifugation (Eppendorf 5418 centrifuge) at 12,000 rpm at 5˚C during 15 min. The recovered material was redispersed in aqueous medium with ultrasound assistance (Branson 3510-DTH ultrasonic cleaning bath) within 5 min.

2.5. Suzuki cross-coupling reactions Suzuki coupling reactions were carried out using PdNPs coated with different macromolecules. In a typical procedure, K2CO3 (0.5 mmol), boronic acid (0.375 mmol) and polymer-coated PdNPs (0.025 mmol of Pd; 10 mol%, unless otherwise indicated) were added to 1.6 mL of a 50% v/v ethyl alcohol/water solution. Next, 4-iodoanisole (0.25 mmol) was added to the solution, the tube was sealed and the reaction was kept under stirring at 50 oC. After 4 h, the reaction mixture was cooled down to room temperature and filtered under vacuum using a Teflon membrane (TE36 0.45 µm, 47 mm). The solvent was removed by rotatory evaporation and the final product was purified by column chromatography using silica gel as the stationary phase and ethyl acetate/hexane (5/95) as the eluent. The reaction progress was monitored by thin layer

8

chromatography and 1H NMR. The isolated product was characterized by 1H NMR (CDCl3, 200 MHz, 298 K) δ (ppm): 7.49−6.88 (9H, m, Ph), 3.77 (3H, s, OCH3)) and mass spectrometry (m/z calcd. for C13H12O [M]+ 184.1; found 184.1).

3. Results and discussion 3.1. Nanoparticle characterization PdNPs were synthesized using NaBH4 as the reducing agent and the polymers listed in Table 1 as stabilizers. Although some of these polymers could, in principle, simultaneously play the roles of reductant and stabilizer, we previously observed that in such cases the reaction time and yield have a strong dependence on the chemical structure of the macromolecules [19]. In addition, due to the presence of weak base groups, some of the polymers are involved in an aqueous dissociation equilibrium, which affects the Pd(II) speciation and electrochemical redox potentials. Also, the capping agents used in this study often act as ligands to form complexes with original metal precursors, affecting the reduction kinetics [20-22]. This may alter the reaction characteristics of the PdNP formation, possibly leading to particles of different sizes and shapes. In order to ensure that the reduction of Pd(II) ions to Pd(0) was similar for all of the systems regardless of the nature of the polymer stabilizer, the reaction was conducted in the presence of NaBH4. Furthermore, nanoparticle synthesis was carried out under experimental conditions where the block copolymer chains are self-organized in the form of spherical micelles (i.e.: at pH > pKa in the case of pH-responsive systems). The resulting PdNPs consisted, therefore, of metallic Pd(0) cores of practically the same size, stabilized by macromolecules adsorbed onto the core surface. The synthesis showed typical behavior which has been previously reported [23], and is illustrated here for the case of the HS-

9

PNIPAM113-COOH thermo-responsive homopolymer. The profiles of the UV-visible spectra for the Pd(II) ions and PdNPs are shown in Figure 1. Before the addition of NaBH4, an absorption band at around 300 nm due to the presence of Pd(II) ions in solution is observed [1, 8, 23, 24]. This signal vanishes after these ions are quantitatively reduced to Pd(0) upon addition of NaBH4, yielding the desired PdNPs. The resulting nanoparticles also absorb light to a certain extent within the whole range of 300-800 nm (see background absorption), and their maximum absorbance is at a higher wavelength. The buffer solution used to induce micellization had no effect on the absorption properties of the nanoparticles, since the spectra recorded in PBS and Borax are identical. It was interesting to note that all of the polymers except PEO137-b-P4VP43 exhibited exactly the same behavior. The PdNPs were not stable in the case of PEO137-b-P4VP43 and DLS measurements confirmed that they initially aggregate and ultimately precipitate out of the solution as a black solid (visual inspection) within a couple of minutes after the reaction.

Table 1. Molecular characteristics of polymers and corresponding self-assembled structures in water prepared by nanoprecipitation methods (when applicable).

Entry

Polymera

Mw(g/mol)

Mw/Mn

10

hydrophobic b

Morphology / Conformation in waterc

1

PEO113-b-PDHPMA30-bPDPAEMA50

20,500

1.3

0.52

Spherical micelles

2

PEO113-b-PDEAEMA50

15,700

1.2

0.68

Spherical micelles

3

PMPC40-b-PDPAEMA70

26,700

1.2

0.56

Spherical micelles

4

PEO137-b-P4VP45

13,500

1.1

0.44

Spherical micelles

5

PEO100-b-PPO65-b-PEO100

12,600

1.3

0.30

Spherical micelles

6

HS-PNIPAM113-COOH

15,500

1.2

−

Coil-like

7

PNVCL53

13,100

1.2

−

Coil-like

a

Subscripts refer to the mean degree of polymerization (DP) of each block. Volume fraction of the hydrophobic block, assuming that the polymer density is equal to 1.0 g/mL. c Dominant morphology of self-assemblies or conformation of chains in buffered aqueous solutions (pH > pKa) at room temperature, as confirmed by scattering and imaging techniques. b

The metallic core size of the nanoparticles used in this study was in the range of 5 - 8 nm (as observed by TEM imaging, see Figure 2) and the width of the outer shell was dependent on the adsorbed homo- or copolymer chain characteristics. The overall size was determined by DLS experiments, as illustrated in Figure 2, and the results are presented and discussed below along with those of the catalytic activity assays.

11

Figure 1. UV-vis spectra for Pd(II) ions and 6@PdNPs synthesized in the presence of HSPNIPAM113-COOH homopolymer dissolved in PBS or Borax buffers, as indicated ([Pd(II)] = 0.275 M, Cpolymer = 1.0 mg/mL).

12

(A)

(B)

Figure 2. Representative TEM micrograph (A) and particle size distribution determined by DLS (B) of 6@PdNP hybrid particles. Scale bar = 20 nm.

13

3.2. Catalytic Hydrogenation of Nip The hydrogenation of p-nitrophenol (Nip) has become a tool widely used to evaluate the catalytic activity of nanoparticle systems, primarily because the reaction progress can easily be followed in situ by UV-vis spectroscopy [18]. A typical absorption band centered at 400 nm appears when Nip is in a medium which is sufficiently basic to shift the aqueous dissociation equilibrium towards the p-nitrophenolate ion [18, 25]. The reduction to the p-aminophenolate ion is accompanied by two main absorbance variations: a decrease at 400 nm due to the consumption of the Nip reactant and an increase at 300 nm associated with the formation of the Amf product (Figure 3). Because the amount of PdNPs added is very small, the absorption spectra of Nip are hardly affected by the presence of palladium nanoparticles [4]. An induction period, denoted by t0 in Figure 4, has often been observed after the injection of nanoparticles into the reaction mixture, in agreement with earlier studies involving the same reaction. Interestingly, the induction time detected for PdNP systems is either absent (or too short to be detected using the experimental setup of this study) or it is significantly shorter than that registered for other metallic nanoparticles, such as gold [17], silver [4, 25] and platinum [18]. Also, a slight decrease in the absorbance during the induction period for PdNPs was evident, in contrast to other systems where no such change occurs. This period, which is generally dependent on the capping agent structure, could involve i) nanoparticle surface restructuring induced by the binding of Nip, ii) a reaction with borohydride such as the transfer of a surfacehydrogen species to the metal nanoparticles, or iii) a diffusion-controlled process of substrate adsorption onto the nanoparticle surface, as discussed by Signori et al. [4].

14

The rate of this six-electron transfer process in the presence of a large molar excess of NaBH4 may be increased by several orders of magnitude using a catalyst, and is best described (so far) as a pseudo-first order process [16, 17, 26]. The observed rate constant, kobs (s-1), is determined via the first order integrated rate law by plotting the natural log of the corrected absorbance at 400 nm versus time (Figure 4 - inset).

Figure 3. UV-vis absorption spectra recorded as a function of time during the Nip reduction reaction at 25˚C in the presence of 1@PdNPs ([Nip] = 1.0×10-4 M, [NaBH4] = 5.0×10-3 M, [NaOH] = 1.0×10-3 M; [Pd] = 2.5×10-5 M).

15

Figure 4. UV-vis absorbance variation at 400 nm recorded as a function of time during the Nip reduction reaction at 25˚C in the presence of 6@PdNPs. The inset shows the corresponding natural log plot from which kobs was determined. ([Nip] = 1.0×10-4 M, [NaBH4] = 5.0×10-3 M, [NaOH] = 1.0×10-3 M; [Pd] = 2.5×10-5 M).

Table 2 shows selected properties of the polymer@PdNPs catalysts prepared under different experimental conditions and the corresponding observed rate constant (kobs) for the reduction of Nip to Amp. We addressed the effect of the NP synthesis pH in order to investigate the influence of the block copolymer chain conformation on the resulting properties of the PdNPs. The pH-responsive polymers 1 - 4 feature weak base groups with distinct pKa values, meaning that the extent of deprotonation is dependent on the solution pH, as is the interaction between the palladium species and the polymers. Importantly, the strong interaction between these macromolecules and palladium originates from nitrogen containing groups. An aqueous acid/base equilibrium involving these groups might, therefore, affect the chemical speciation. 16

The NP synthesis pH was adjusted using two different buffer solutions (PBS at pH 7.4 and Borax at pH 9.1), which were added to a mixture of Pd(OAc)2 and molecularly dissolved (not self-assembled) positively-charged polymer chains, prior to the addition of the reducing agent. As shown in Table 2, the weak base groups of all block copolymers are almost fully deprotonated at pH 9.1 (>98%), whereas the equilibrium is slightly shifted toward the protonated species at pH 7.4 (56-80%). For example, 2@PdNPs synthesized in PBS (deprotonation = 80%) exhibited a larger hydrodynamic radius (42 nm) mainly due to interchain repulsion caused by positively-charged groups (20%), which favors a more extended conformation being assumed by the polymer chain. It is known that the presence of a charge on a polymer chain leads to its expansion with respect to the equivalent neutral polymer chain (or highly screened equivalent polyelectrolyte chain) [27]. As expected, the hydrodynamic radius decreases to ca. 12 nm when the solution pH is increased to 9.1 through the addition of a strong concentrated base. The size of the 1−3@PdNPs remained almost the same regardless of the capping polymer used (all containing a pH-responsive poly[2-(dialkylamino)ethyl methacrylate] segment) and was comparable to the size of the corresponding self-assembled block copolymer micelles [14, 28]. When these particles are used as catalysts for the reduction of Nip to Amp, they are transferred to a highly basic reaction media (see experimental section), exceeding the buffering capacity of the as-prepared polymer@PdNPs in PBS or Borax. In all cases, the final pH following catalyst addition ranged between 9.8 and 10.8. This implies that during the hydrogenation of Nip all weak base groups of the capping agents are deprotonated, independently of the nanoparticle synthesis method applied. Nevertheless, the kobs was systematically higher for 1−3@PdNPs prepared using PBS buffers (entries 1−5).

17

The origin of this behavior is probably related to the fact that high pH values during Pd(II)/Pd(0) reduction leads to more compact capping layers due to a higher degree of deprotonation (99% at pH 9.1 and 80% at pH 7.4), which ultimately renders the nitrogencontaining block more hydrophobic and compact. As a consequence, the capping layer acts as a stronger physical barrier, restricting the free access of reactants to catalytic nanoparticles [29]. Recently, Niu and Li [29] noted that the way in which different conformations influence the access of reactants to a catalyst still remains unclear, but the capping layer is certainly detrimental to this process.

Table 2. Selected properties of PdNP catalysts and rate constant (kobs) observed for the reduction of Nip to Amp. Entry Capping agent

NP synthesis pHa

Polymer pKa / % deprotonated

RH (nm)b

Rate constant (kobs×102) (s-1)

1

1

9.1

6.8 / 99

17

0.849 ±0.007

2

1

7.4

6.8 / 80

15

2.279 ±0.612

3

2

9.1

7.3 / 98

12

1.319 ±0.144

4

2

7.4

7.3 / 56

42

2.980 ±0.771

5

3

7.4

6.8 / 80

21

3.078 ±0.122

6

4

9.1

5.8 / 99

34

0.052 ± 0.001

7

4

7.4

5.8 / 95

43

0.050 ± 0.003

8

5

9.1

−

20

0.690 ±0.054

9

5

7.4

−

30

0.664 ±0.019

10

5

5.9c

−

48

1.763 ±0.022

11

6

9.1

−

40

0.590 ±0.070

12

6

7.4

−

27

1.521 ±0.104

13

6

5.9c

−

21

2.430 ±0.031

18

14

7

9.1

−

21

0.695 ±0.500

15

7

7.4

−

92

0.670 ±0.164

16

7

5.9c

−

16

3.120 ±0.500

a

Solution pH during PdNP synthesis mediated by NaBH4. Hydrodynamic radius of polymer@PdNPs determined by dynamic light scattering (DLS) 24 h after the synthesis. c Pure water; non-buffered conditions. b

The use of polymeric capping agents containing pyridine groups should be avoided in the case of PdNP catalysts. The data shown in Table 2 reveal that 4@PdNPs (palladium nanoparticles stabilized by PEO45-b-P4VP137), although active [30], were not efficient as a catalyst system since the kobs values were considerably lower than those for the 1−3@PdNPs analogs. This drawback in the case of 4@PdNPs stems from the fact that pyridyl group moieties have a strong affinity for the Pd(II) and Pd(0) centers [30], leading to slower adsorption and desorption from the catalyst surface with a considerable loss of catalytic activity. Furthermore, P4VP at pH ≥ 9.8 is strongly hydrophobic and less compatible with the substrate, which also contributes to a reduction in the activity. Ideally, the substrate should match the capping polymer in terms of chemical compatibility (i.e.: Flory-Huggins interaction parameter close to zero) in order to achieve high catalytic activity. It is also clear in Table 2 that the catalytic activity of nanoparticles synthesized in the presence of polymers 5−7 is affected by the ionic strength. The rate constant values determined using 5−7@PdNPs synthesized in pure water were notably higher than those obtained using the same particles prepared in buffer solution. This effect is probably related to the well-known “salting out” effect caused by the additional buffer salts, which alter the polymer solubility and conformation in water by disrupting the hydration structure surrounding the polymer chains [31].

19

This effect leads to slower adsorption and desorption (due to lower solubility) from the catalyst surface with consequent inhibition of the catalytic activity. The overall finding, based on the results given in Table 2, is that the highest catalytic activity is achieved when polymer@PdNPs synthesis is carried out under experimental conditions where the polymer chains are allowed to assume a more extended (partial deprotonation of pH-responsive systems) and/or hydrated (in case of hydrophilic polymers) conformation. As a result, kobs for Nip reduction did not correlate with the hydrodynamic radii of hybrid polymer@PdNPs. In fact, kobs depended on the chemical composition, molar mass, and conformation of macromolecules adsorbed onto the core surface rather than the surface area of the metallic core alone. In addition, macromolecules featuring pendant pyridyl moieties are not suitable capping agents for PdNPs applied in catalysis, even though they exhibit some activity. Comparatively, the kobs values determined in this study are up to one order of magnitude higher than those previously observed for PdNPs prepared in the presence of poly(amidoamine) and poly(propyleneimine) dendrimers [32], and poly(ethylene oxide) functionalized with triazoyl rings [33].

3.3. Successive Addition of Reactants and Catalyst Recycling The recycling of nanoparticle catalyst systems is an exciting prospect from the economic, environmental, and health and safety standpoints. We assessed the possibility of reusing the PdNPs listed in Table 2 by carrying out reactions with the successive addition of the reactant (Nip) to the reaction mixture after its near complete consumption, which was verified by a decrease in the absorbance to values close to zero. The results shown in Figure 5 clearly indicate that 1@PdNPs remains active, since Nip is promptly reduced to Amp after successive additions

20

provided that a high molar excess of the reducing agent NaBH4 is present in the medium. The slopes of each curve are very similar, suggesting that successive reductions take place with essentially the same rate constant. The same comments can be extended to any of the other polymer@PdNP systems listed in Table 2. The concept of catalyst recycling differs from that of the successive additions, as it involves the recovery of the catalyst for subsequent re-use in any other catalytic process. The polymer@PdNPs can be separated as a dark gum from the reaction medium used in Nip reduction assays by ultracentrifugation at 12,000 rpm at 5˚C. The recovered material can then be re-dispersed in water by vigorous ultrasound-assisted stirring.

Figure 5. Dilution-corrected absorbance variation at 400 nm recorded as a function of time during the Nip reduction reaction at 25˚C in the presence of 1@PdNPs after successive additions of the substrate (5 aliquots of Nip were added after the initial reaction). ([Nip]initial = 1.0×10-4 M, [NaBH4] = 5.0×10-3 M, [NaOH] = 1.0×10-3 M; [Pd] = 2.5×10-5 M).

21

3.4. Suzuki Coupling Reactions Having verified the catalytic activity of the palladium nanoparticles stabilized in micelles formed with the polymers listed in Table 1 in a model reduction reaction, the particles were then investigated as a catalyst for the Suzuki coupling reaction shown in Scheme 1. Suzuki reactions involve the cross-coupling of organoboronic acids with aryl halides to produce biaryls which are of interest in many fields, including drug development [10, 34]. The data shown in Table 3 also indicate that the catalysts developed in this study were, in general, active in mediating the reaction towards the formation of the expected product, in almost quantitative yields, under relatively mild conditions, after only 4 h (note: 100% yields can never be obtained when thin layer chromatography is used to purify the products due to the inherent loss of material).The catalysis occurs on the surface of the Pd nanoparticles [34]. There was clear evidence, however, that capping agents which interact strongly with palladium by coordination (4@PdNPs; PEO-b-P4VP as capping agent, Table 3 entry 4) or hinder mass transport across the stabilizing layer barrier (6@PdNPs; HS-PNIPAM-COOH as capping agent, Table 3 entry 6) reduce the catalytic activity. In the case of 4@PdNPs the reaction did not proceed, in agreement with the results obtained for the reduction of Nip to Amp where this catalyst was also not active (see discussion above). The replacement of the pendant 4-vinylpyridine groups in P4VP by benzene groups, such as those in PS, should lead to a recovery of the catalytic activity. Beletskaya and co-workers found that PdNPs stabilized in PEO-b-PS micelles were as active as low-molecular weight palladium complexes [7]. On the other hand, 6@PdNPs were reasonably active in the reduction of Nip, but not as

active in the Suzuki coupling (yield was slightly lower than for other systems). We attribute this

22

behavior to the reaction temperature (25˚C during Nip reduction and 50˚C in Suzuki reactions), which might affect the chain conformation of the thermo-responsive polymer HS-PNIPAMCOOH [35]. Below the lower critical solution temperature (LCST; 32˚C for PNIPAM), the chains are well-solvated, hydrophilic and assume an extended conformation. When the temperature is raised above the LCST, the PNIPAM phase separates out, originating hydrophobic and hydrophilic domains that hinder mass transport across the stabilizing layer [23]. It should be noted that PNIPAM is not a hydrophobic polymer above the LCST, since the polymer phase remains wet (52% of water) [35]. The differences in chain conformation below and above the LCST explain, therefore, the reduced activity at high temperature. Indeed, the rate constant decreased to 0.383 ± 0.694×10-2 s-1 when the Nip reduction reaction was performed at 50˚C, confirming the aforementioned hypothesis. The reaction conditions initially selected to assess the catalytic activity of polymer@PdNP could be modified to significantly improve the reaction yield. We found that the catalyst concentration can be considerably reduced since near quantitative yield was observed for reactions carried out using 3@PdNPs when the concentration was at least 1 mol% Pd, which is very low (Table 3, entries 8–2) [36]. Regarding the reaction time, we observed that a 4 h period is required to obtain an appropriate product yield, since shorter periods of time clearly resulted in lower conversion (Table 3, entries 13–15). The well-known effect of carbon-halogen bond dissociation energies was also evident in this study, with aryl iodides being more reactive than the corresponding bromides. No reaction was observed at low catalyst concentration using 4-bromoanisole, and the desired product could only be obtained upon increasing the catalyst amount (Table 3, entries 16–17). It is worth noting that although the nanoparticles used in this study were synthesized applying significantly less capping agent than in other studies (1/5 of the amount in some cases) they 23

remained stable during the reaction and performed similarly to those previously used by other authors [7, 9, 10, 24, 34, 37].

Scheme 1: Suzuki coupling reaction between phenylboronic acid and aryl halides used to test the catalytic activity of selected polymer@PdNPs.

B(OH)2

+ X

OCH3

polymer@PdNPs

OCH3

K2CO3, EtOH:H2O (1:1), 50 ºC

Table 3. Results of Suzuki coupling reaction between phenylboronic acid and aryl halides using polymer@PdNPs as the catalyst.a Entry

Capping agent

mol% Pd

X

Time (h)

Yield (%)b

Effect of capping agent 1

PEO113-b-PDHPMA30-b-PDPAEMA50 (1)

10

I

4

96

2

PEO113-b-PDEAEMA50 (2)

10

I

4

96

3

PMPC40-b-PDPAEMA70 (3)

10

I

4

98

4

PEO137-b-P4VP43 (4)

10

I

4

−c

5

PEO100-b-PPO55-b-PEO100 (5)

10

I

4

96

6

HS-PNIPAM113-COOH (6)

10

I

4

92

7

PVCL94 (7)

10

I

4

95

Effect of catalyst concentration 8

PMPC40-b-PDPAEMA70 (3)

10

I

4

98

9

PMPC40-b-PDPAEMA70 (3)

5

I

4

96

10

PMPC40-b-PDPAEMA70 (3)

1

I

4

98

11

PMPC40-b-PDPAEMA70 (3)

0.5

I

4

88

12

PMPC40-b-PDPAEMA70 (3)

0.1

I

4

8

Effect of reaction time

24

13

PMPC40-b-PDPAEMA70 (3)

1

I

4

98

14

PMPC40-b-PDPAEMA70 (3)

1

I

2.6

90

15

PMPC40-b-PDPAEMA70 (3)

1

I

2

77

Effect of the halogen 16

PMPC40-b-PDPAEMA70 (3)

1

Br

4

−c

17

PMPC40-b-PDPAEMA70 (3)

10

Br

4

80

a

Reaction conditions: B(OH)2 (0.38 mmol), K2CO3 (0.50 mmol), EtOH:H2O (0.80 mL), XC6H4OCH3 (0.25 mmol). b Isolated yield. c No reaction was observed.

4. Conclusions Functional polymers and block copolymers that interact with palladium nanoparticles mainly through tertiary nitrogen atoms are suitable capping agents and can be used in relatively small amounts. The low polymer concentration is an interesting advantage in terms of obtaining a catalyst with a larger active surface area and a more effective mass transport of reactants and products across the stabilizing layer. The polymer chain conformation during nanoparticle synthesis has an effect on the final catalytic activity. The best results (higher activity) are obtained whenever the catalyst synthesis is performed under conditions that favor a less compact, more hydrated and consequently more extended polymer chain conformation. Chemical groups known to form coordination complexes with metallic palladium (e.g.: pyridyl moieties) slow down the adsorption and desorption from the catalyst surface, reducing the catalytic activity. The most active catalysts in the Nip reaction are also effective in the Suzuki−Miyaura cross-coupling reaction between 4-iodoanisole and phenylboronic acid, leading to the expected product in quantitative yields under relatively mild conditions.

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The polymer@PdNP catalysts maintain their activity upon successive addition of reactants. Despite the low concentration of stabilizer used, compared to previous studies, the catalysts can still be recycled (recovered/re-used) using standard protocols applied in this field.

Acknowledgements The authors acknowledge financial support from CNPq (Grants Ns. 590070/2010-0 and 472945/2012-2) and FAPERGS (Grants Ns. 11/0842-8 and 11/2080-9). We also thank A. G. O de Freitas and P. I. R. Muraro for their assistance with some of the polymer samples.

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Graphical Abstract

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Highlights

• We report the use of polymer-coated palladium nanoparticles in catalysis • The effect of polymer structure on the catalytic activity was established • A low concentration of stabilizer was used, compared to previous studies • Yields of Suzuki couplings correlated with rate constants of a model reaction • Macromolecules with pendant pyridyl moieties are unsuitable capping agents

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