Solvent-Adoptable Pseudohomogeneous Catalysts for Alkyne

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Polymer-Assisted Chain-like Organization of CuNi Alloy Nanoparticles: Solvent-Adoptable Pseudo-Homogeneous Catalysts for Alkyne-Azide Click Reactions with Magnetic Recyclability Mrinmoy Biswas, Anupam Saha, Madhab Dule, and Tarun Kumar Mandal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5071874 • Publication Date (Web): 03 Sep 2014 Downloaded from http://pubs.acs.org on September 9, 2014

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The Journal of Physical Chemistry

Polymer-Assisted Chain-like Organization of CuNi Alloy Nanoparticles: Solvent-Adoptable Pseudo-Homogeneous Catalysts for Alkyne-Azide Click Reactions with Magnetic Recyclability Mrinmoy Biswas, Anupam Saha, Madhab Dule and Tarun K. Mandal* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, ABSTRACT:

A solution-phase reduction method is undertaken to produce polymer

magnetic bimetallic CuNi nanoalloy with chain-like structures, which are formed by the magnetic dipole-directed assembly of spherical alloy nanoparticles as confirmed from TEM analysis. Magnetic property measurement reveals paramagnetic nature of the alloy nanochain. These polymer-capped chain-like alloy nanoparticles are dispersible in water as well as in organic solvents that increase their ease of application as catalyst in both of these environments. The XPS and zeta potential analysis demonstrates the presence of Cu(I) on the alloy particle surface and that justify their catalytic activity towards alkyne-azide click reactions. Consequently, the catalytic activity of the as-synthesized polymer CuNi alloy nanochain is investigated towards a wide variety of alkyne-azide click reactions at room temperature in water and in DMF. Depending upon the nature of the substrate and the surface stabilizing polymer on the nanocatalyst, a moderate to quantitative yield of the click-conjugated product is obtained. Additionally, the advantage of pseudo-homogeneity of CuNi nanoalloy suspension is utilized to modify polymer end group with amino acid and peptide with ionic liquid via click reaction to create new bioconjugates. Moreover, the nanoalloy catalyst is magnetically recoverable and reusable up to three cycles of click reactions without losing much of its original activity. KEYWORDS: polymer, CuNi alloy, magnetic, nanochain, catalyst, click reaction, reusable 1 ACS Paragon Plus Environment


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INTRODUCTION

Magnetic alloy nanoparticles (NPs) have aroused much interest in recent times because of their fascinating size-dependent properties, which find applications in diverse areas such as optical,1 data storage,2 magnetic recording media,3 contrast agent in magnetic resonance imaging (MRI),4 cancer therapy,5 and catalysis.6, 7 Thereby, several synthetic approaches have been developed to prepare bimetallic single crystalline8/polycrystalline9 nanoalloys, such as, FeNi,10 NiPt,11 NiCo9, 12, 13

and CuNi14 with controllable morphology. However, one of the major limitations in using

these magnetic alloy nanoparticles is their susceptibility towards oxidation in air. Therefore, to protect the surfaces of the nanoalloy and also to control their growth,15 various research groups have used capping agents such as long chain acids,15 amines,15 polymers,8,

16, 17

and

biomacromolecules.18 Among all these capping agents, it is expected that the capping with polymers such as poly(ethylene glycol) (PEG),8 poly(vinyl pyrrolidone),16,

19

and polystyrene

based block copolymer20 would sterically protect these magnetic NPs from coalescing, oxidation and simultaneously increase their dispersity in different solvents, which make them useful for heterogeneous catalysis. Recently, tremendous effort has been devoted by organic and material chemists to develop transition metal-based catalyst for various organic cross-coupling reactions using homogeneous transition metal complexes or heterogeneous metal nanoparticle.21,

22

Application of

heterogeneous alloy nanocatalyst give rise to the advantageous catalytic activity due to effective mutual influences of different neighboring atoms in the alloys.19,

21, 23, 24

Till date, researchers

have used FeNi,17 NiPt,11 NiPd25, magnetic nanoalloy in different organic reactions. Recently, our group has also studied the C-O and C-S cross-coupling reactions using CoNi magnetic 2

ACS Paragon Plus Environment

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nanoalloy and established the effect of elemental composition on their catalytic activity.26,

27

Among various magnetic bimetallic alloy, CuNi nanostructure is very important one as Cu is known for its catalytic efficiency, while Ni for magnetic nature.28 However, generally heterogeneous nucleation of Cu and Ni NPs is occurred during its preparation.10, 29, 30 Therefore, CuNi nanoalloy-based catalysis is not well-studied for organic cross-coupling reactions.31-34 The click chemistry denotes a group of robust and green coupling reactions, which give selective conjugation of two different functionalized moieties in high selectivity with 100% atom economy. Perhaps, the most remarkable example is the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of azides and terminal alkynes, which gives generally 1,4-disubstituted 1,2,3triazoles.35 However, Cu ion has a tendency to bind with heteroatoms such as O or N, which led to fatal Cu impurity during the synthesis of bio-relevant click conjugate under homogeneous Cu(I) based click reaction. Recently, heterogeneous supported Cu NPs based catalyst is employed to perform click reaction in organic solvent or in water instead of Cu(I) salt to decrease the Cu impurity in the product and reuse again.36-38 Additionally, to enhance catalyst recovery, the Cu catalyst is typically embedded in magnetic support material. Generally, these reports deal with the catalytic activity study towards model click conjugation in between several alkynes and alkyl azide, and catalyst recovery and their reusability.38, 39 However, the importance of click chemistry also lies in the coupling or modification of macromolecules,40 where the Cu NPs based heterogeneous catalysis is rarely studied, due to lower contact of the active catalyst surface with functional moiety of macromolecules. Hence, a suitable catalyst is required which can be successfully used for click conjugation of different types of substrate, particularly for biomolecules and macromolecules, with effective recovery of the catalyst to reuse further. Polymer3



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stabilized unsupported magnetic nanocatalyst of high solvent dispersity is expected to have better reactant access, where catalyst recovery and reusability is not compromised. Therefore, it would be interesting to explore the highly dispersible magnetic nanoalloy as catalyst for click reactions, which is somehow overlooked, to the best of our knowledge. In this report, we prepare CuNi alloy nanostructures via solution-phase chemical reduction in presence of two polymers, viz., poly(ethylene glycol) (PEG) and poly(4-vinyl phenol) (PVPh). It is observed by TEM that the formed alloy nanostructures have chain-like morphology, which is formed through the self-assembly of spherical alloy nanoparticles. XPS analysis on the CuNi nanoalloy sample shows the presence of Cu(I) in its surface. Furthermore, the formed nanoalloys are dispersible in water and organic solvents. We further explore these assynthesized CuNi nanochains serve as efficient unsupported catalysts to perform alkyne-azide click reaction in water as well as in DMF at room temperature. In addition to the detail study of click reactions with a verity of substrates, we also investigated the catalytic property of nanoalloys to couple polymer with amino acid and peptide with ionic liquid. Finally, the CuNi nanocatalyst is magnetically retrievable and can be reused efficiently thrice in a click reaction without losing its virgin catalytic activity.

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EXPERIMENTAL DETAILS Materials. 1-Bromooctane,

benzyl

chloride,

1-pentyne,

propargyl

acrylate,

phenylacetylene, (4-bromophenyl) acetylene, 4-ethynyltoluene, 4-ethynylanesole, propargyl bromide, propargyl alcohol, poly(ethylene glycol) methyl ether (PEG) (Mw~2000), poly(4-vinyl phenol)

(PVPh)

(Mw~11000),

2-bromoisobutyric

acid

(BIBA),

N,

N,

N´,

N´´,

N´´pentamethyldiethylenetriamine (PMDETA), 2-bromoisobutyryl bromide (BIBB), copper (I) 4


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chloride (CuCl) (purity > 98%) were obtained from Aldrich. L-Valine (Val), L-tyrosine (Tyr), Lphenyl alanine (Phe), dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt) and ditert-butyl-pyrocarbonate (Boc) were purchased from SRL India. Hydrazine (N2H4) (100%), sodium azide (NaN3) were obtained from Merck, India. Nickel acetate {Ni(CH3CO2)2.4H2O}, copper acetate {Cu(CH3CO2)2.H2O} and thionyl chloride (SOCl2) were received from Spectrochem, India. All the above-mentioned chemical were used without further purification. Methyl methacrylate (MMA) (Burgoyne Urbridges and Co.) was washed with aqueous NaOH, dried with calcium chloride and distilled over calcium hydride under reduced pressure prior to the polymerization reaction. Ethyl alcohol (Bengal Chemicals, India), dimethylformamide (DMF), xylene, dichloromethane (DCM) (Merck, India) and water were distilled prior to use. Synthesis of CuNi bimetallic alloy nanostructures.

Copper-nickel

(CuNi)

bimetallic alloy nanostructure was prepared by reduction of nickel acetate and copper acetate by hydrazine hydrate in ethanol in presence of a polymer such as PEG or PVPh as stabilizing-cumstructure directing agent using our earlier protocol.26, 27 Typically, 34 mL of ethanolic solution of PEG (1.4 wt%) was prepared with magnetic stirring in a round bottomed flask under Ar atmosphere. After 45 min, 0.09954 g (0.0004 mol) of nickel acetate and 0.07986 g (0.0004 mol) of copper acetate were added to this stirring solution and was left stirring to homogenize. Subsequently, 3 mL of both ethanolic solution of N2H4 (10 M) and NaOH (2M) were injected to the above mixture and was transferred into a hot oil bath maintained at 70 °C for 2 h. The obtained alloy sample was black in color and was isolated from the reaction mixture using bar magnet. The obtained black product was purified by washing with water-ethanol mixture for several times. Finally, the sample was dried in vacuum at 60 °C, which was denoted as ‘CuNi5



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PEG’. Similarly, ‘CuNi-PVPh’ alloy nanostructures were also prepared using PVPh. For comparison, bare alloy (Bare-CuNi) nanostructure was also synthesized without using any polymer. The reaction recipes for the synthesis of CuNi nanostructures and their characterization were presented in Table 1. Table1. Reaction recipe for the synthesis of CuNi nanostructures with and without polymer and their characterization.

Sample

Molar ratio of

Wt% of

name

Cu and Ni salts polymer

Target atomic

Composition

Atomic Yield

ratio(Cu:Ni)

(Cu:Ni) (w/w)a

ratio

(%)

CuNi-PEG

1:1

1.2

1:1

52:48

1.04:1

70

CuNi-PVPh

1:1

1.2

1:1

51:49

0.96:1

70

Bare-CuNi

1:1

-

1:1

48:52

0.85:1

70

a

composition is measured by ICP-OES technique Synthesis of different propargyl and azido substituted derivative.

To

perform

click reaction between various types of terminal alkyne and azide, we first synthesized a variety of different propargyl or azido substituted compounds using appropriate chemistry. For example, benzyl azide (PhCH2-N3) and 1-azidooctane (nC8H17N3) were prepared via azide substitution using NaN3 in DMF from the corresponding halide. Azide end-capped poly(methyl methacrylate) (PMMA-N3) was prepared from the as-synthesized azido-conjugated initiator (ABIB), followed by atom transfer radical polymerization (ATRP) of MMA. Azido-conjugated tripeptide [Me2C(N3)-CO-NH-Tyr(1)-Val(2)-Tyr(3)-OMe] [Peptide-N3] was synthesized as follows: At first the tripeptide, [H2N-Tyr(1)-Val(2)-Tyr(3)-OMe] was synthesized by following our previous report (see SI for details).41 In the next step, BIBA was coupled with the tripeptide for preparation of its bromo derivative. Finally, Peptide-N3 was obtained by reacting bromo-peptide with NaN3 in 6


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DMF. Propargyl-functionalized phenylalanine (Phe-OPr) was prepared by the esterification of phenylalanine with propargyl alcohol using dry HCl.42 Propargylated ionic liquid [PrMIM]Br was prepared from propargyl bromide and 1-methylimidazolium. The detailed synthetic strategies, purifications and characterizations of different intermediates of the above-mentioned compounds and their final products were given in the supporting information. Alkyne-azide click reactions using CuNi alloy nanocatalyst.

Typical

reaction

procedure involved the addition of 1.5 mg of alloy nanocatalyst (5 mol%) into 6 mL water or DMF in a round-bottomed flask followed by addition of an azide (0.57 mmol) under N2 atmosphere. After 30 min, an alkyne (0.68 mmol) was added to it and was left for stirring at room temperature for 18 h. After completion of reaction, the catalyst was separated using a bar magnet. In case of reactions performed in DMF, the reaction mixture was diluted with cold water prior to the solvent extraction. The reaction mixture was then extracted in DCM and the DCM extract was dried over anhydrous Na2SO4 and was concentrated in vacuum to obtain crude product. Pure click conjugated product was obtained via column chromatography of the crude product using 200 mesh silica gel and solvent mixture of 5% ethyl acetate and petroleum ether as the eluent. The recovered catalyst was washed, dried and reused. The product yields and the reaction conditions were depicted in Tables 2-4. Alternatively, for preparation of polymer-phenylalanine and peptideionic liquid conjugates, the corresponding terminal alkyne (i.e. PheOPr and [PrMIM]Br respectively) was used in excess (7 mmol) compare to the azido conjugated polymer and peptide. (as shown in Scheme 1 and 2). After the catalyst recovery, the polymer-phenylalanine conjugate was purified by precipitation in methanol. The click conjugates were characterized by 1HNMR and mass spectroscopy as shown in the supporting information. 7



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Me2C(N3)-CO-NH-Tyr(1)-Val (2)-Tyr(3)-OMe CuNi-PVPh

+ R'C CH

O H3C C C N N CH 3 N

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O N CHC H CH2

R'

DMF

O O NH CHC N CHC OMe CHCH3 H CH2 CH3

N R' =

Scheme 1.

OH

/

N Br

OH

Peptide-based bioconjugate

Preparation of peptide-based bioconjugate through CuNi-PVPh nanostructures

assisted click reaction. O H3N CHC O CH2 Cl

O

N3 +

CuNi-PVPh CH3H2 CH3 Br O C n COOCH3 DMF CH3 PMMA-N3 N N O H3N CHC O CH2 Cl

Scheme 2.

N

O

CH3 H CH3 2 Br O C n H3C COOCH3

PMMA-Phe

End group modification of poly(methyl methacrylate) through CuNi-PVPh

nanoalloy assisted click reaction.

Characterization. FTIR spectroscopic study.

The sample pellets were prepared by mixing the samples with KBr

in a 1: 100 (w/w) ratio. The spectra were acquired using this pellet on a Perkin-Elmer Spectrum 400 spectrometer.

8


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X-ray diffraction (XRD) study.

The X-ray diffraction measurement of pure and dried

powdered nanoalloy samples were done on a Bruker AXS D8 Advance diffractometer at an accelerating voltage of 40 kV using a Cukα (λ = 1.5405 Å) as the X-ray radiation source. Transmission electron microscopy (TEM) study.

The dried powder of alloy samples were first

redispersed in ethanol by ultrasonication. A drop of the sample’s suspension was cast onto a carbon-coated Cu grid (in some cases Au grid), dried and imaged at an accelerating voltage of 200 kV under a JEOL high-resolution electron microscope (model JEM 2010E). As the sample is magnetic, the TEM images were acquired by placing a blank grid over the sample grid. Magnetic property measurement (MPMS).

Magnetization measurement of alloy samples were

carried out in a commercial Quantum Design MPMS XL (EverCool model) in 300 K with the applied magnetic field up to ±10 kOe. Zeta potentials measurement. Zeta potentials of the suspended alloy samples in water and DMF were measured using Malvern Zetasizer NANO ZS 90 (model No. 3690) using HeNe gas laser of 632.8 nm. NMR study.

1

H NMR spectra of all the synthesized organic compounds in CDCl3 (in few cases

in D6-DMSO) were acquired using a Bruker DPX 300 MHz spectrometer. All the spectra are presented in the SI. ESI-MS spectrometry study. The ESI-MS spectra of the organic products after catalysis were recorded from a methanol solution on a quadrupole time-of-fight (Q-Tof) Micro YA263 mass spectrometer. All the mass spectra are presented in the SI. Inductively coupled plasma/optical emission spectrometry (ICP-OES).

The

compositions of the CuNi nanoalloy samples were determined by analyzing the samples in ICP9



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OES (Optima 2100 DV, Perkin-Elmer). The alloy samples were dissolved in nitric acid (5N) before ICP-OES measurement. Elemental analysis.

Elemental analysis of the purified azide samples were carried out by using

a Perkin–Elmer 2400 series II CHN analyzer. X-ray photoelectron spectroscopy (XPS) study.

The dispersed CuNi alloy samples were

casted onto a glass slide (1cm x 1cm) and annealed at 300 °C under vacuum. Subsequently, XPS was studied in Omicron instrument with an Al−Kα radiation source under 15 kV voltage and 5 mA current. Gas chromatography. The yield of the click reaction between phenyl acetylene and benzyl azide was measured using Agilent 7890 GC with FID detector. Using known concentration of the phenylacetylene solution in THF, the amount of unreacted phenylacetylene present in the crude was measured. Consequently, the yield of the reaction was calculated. Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry. The molecular weight measurement of PMMA-phenylalanine (abbreviated as PMMA-Phe) click conjugate was performed in Bruker instrument a 337-nm nitrogen laser. 2,5-Dihydroxybenzoic acid (40 mg/mL in THF) was used as the matrix, and NaI (10 mg/mL in THF) was used as doping salt respectively. The concentration of PMMA-Phe was 2 mg/mL in THF. For measurement, final samples were prepared by mixing 10 µL of polymer solution, and 20 µL of matrix solution, and 5µL of doping salt solution. Typically, 1 µL of the mixture (sample/matrix/doping salt) solution was spotted on the sample plate and allowed to slowly evaporate. Gel permeation chromatography (GPC) study.

Molecular weight distributions of azido

substituted poly(methyl methacrylate) (PMMA-N3) was measured by size exclusion 10


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chromatography (SEC) using a Waters 1515 isocratic HPLC pump connected to three Waters Styragel HR1, HR3, and HR4 columns and a Waters 2414 refractive index detector at room temperature as was reported earlier by our group.41



RESULTS AND DISCUSSION Preparation

and

characterization

of

polymer

CuNi

alloy

nanostructures.

Bimetallic chain-like CuNi alloy nanostructures were prepared by the reduction of nickel acetate and copper acetate salts with feed molar ratio of 1:1 using hydrazine and in presence of two different polymers, PEG and PVPh with nearly 70% of yield (Table 1). The obtained samples prepared with PEG and PVPh were abbreviated as CuNi-PEG and CuNi-PVPh respectively. BareCuNi alloy sample was also prepared without polymer for control experiments. The obtained CuNi-PEG alloy sample was easily dispersible in both water and in some organic solvents (e.g., dioxan, DMF, xylene, acetonitrile and ethanol etc.) as can be seen from the photographs of their dispersions in different solvents shown in Figure 1. However, the dispersity in DMSO and in dichloromethane (DCM) was very poor. Similarly, CuNi-PVPh sample can also be dispersed well in water as well as in DMF. The dispersity of these alloy samples in different solvents can be ascribed to the presence of polymer on the surface of the formed alloy nanostructures. On the other hand, compare to polymer-capped alloy nanoparticle, the dispersity of bare-CuNi in these solvents was not at all good.

11



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Figure 1.

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Photographs of dispersions of different alloy samples in different solvents.

The polymer adsorption on the surface of CuNi nanoalloy was confirmed by FTIR analysis as shown in Figure S15 (see SI). The sample CuNi-PEG showed bands at 2917 and 1050 cm-1 corresponds to the C-H and C–O stretching frequency of PEG, respectively (Figure S15A). Figure S15B represented the spectrum of CuNi-PVPh alloy sample where bands at 2917 cm-1 (CH stretching) and 1485 cm-1 (aromatic C=C stretching) were corresponding to the PVPh polymer. Therefore, the FTIR data collectively conclude that CuNi nanochains were capped with either PVPh or PEG polymer.

12


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To analyze the actual composition of the constituent elements present in the bimetallic CuNi nanochains, ICP-OES experiment was performed. The weight ratios of Cu/Ni in the samples CuNi-PEG, CuNi-PVPh and bare-CuNi as measured from ICP-OES experiment, were found to be 52:48, 51:49 and 48:52 respectively (Table 1). Thus, the calculated atomic ratios of elemental Cu and Ni in these samples were approximately 1.04:1, 0.96:1, 0.85:1 (atomic weights of Ni and Cu are 58.7 and 63.5) respectively, which are in good agreement with the target atomic ratio (1:1) of the alloy samples. 43.7+ (111) + 51.1 (200)

Bare-CuNI

+ 74.8 (220)

CuNI-PVPh

Intensity

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


CuNI-PEG 43.4+ (111) + 50.6 (200)

Standard Cu

+ 74.6 (220)

+ 44.5 (111)

51.8+ (200)

Standard Ni

40

50

60

76.3+ (220)

70

80

2θ (degree)

Figure 2.

XRD patterns of different bimetallic CuNi alloy samples and standard Cu and Ni

from JCPDS file. 13



The XRD patterns of different alloy samples (Figure 2) showed diffraction peaks at 2θ =43.7°, 51.1°, 74.8°, which corresponds to 111, 200 and 220 plane.4 However, standard Cu and Ni samples have different peak values for face centered cubic lattice structure as obtained from the JCPDS file no 04-0836 and 04-0850 respectively shown in Figure 2.30, 43 The calculated d111 spacing of the CuNi-PEG sample was is 2.08 nm. The absence of bifurcated peaks corresponding to individual lattice planes for neat elemental Cu and Ni metals indicated the elemental homogeneity and the successful synthesis of alloy as shown in Scheme 3.10, 44 8

8 CuNi-PEG

CuNi-PVPh

4 Ms=38 emu g-1 Hc=99.5 Oe 0

-4

Magnetization (emu/g)


-8

4 Ms=34 emu g-1 Hc=95 Oe

0

-4

-8

-60000

-20000

20000

60000

H (Oe)

-60000

-20000

20000

60000

H (Oe) 15 Bare-CuNi


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Page 14 of 34

10 5 Ms=11.5 emu g-1 Hc=89 Oe

0 -5 -10 -15 -60000

-20000

20000

60000

H (Oe)

Figure 3.

Magnetization hysteresis curves, saturation magnetization and coercivity value for

CuNi-PEG, CuNi-PVPh and bare-CuNi samples at 300 K. 14


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We further investigated the magnetic responses of these nanoalloy samples via magnetic property measurement system (MPMS) at 300K using an external magnetic field between ±10.0 kOe. The saturation magnetization (Ms) values (as shown in Figure 3) for the CuNi-PEG, CuNiPVPh were found to be 38 and 34 emu g−1 respectively, whereas the value corresponding to bare CuNi (11.5 emu g−1) was lower, which can be ascribed due to presence of oxide layer deposited on the alloy nanoparticles’ surface. The respective coercivity values for these alloy samples are 99.5, 95, 89 Oe. These MPMS data conclude that these alloy samples are soft ferromagnetic in nature.26, 27 Figure 4 showed the TEM images of different CuNi alloy samples. The CuNi-PEG sample showed the formation of chain-like assembled nanostructures formed by the assembly of smallsized magnetic alloy nanospheres (individual spheres are encircled in figure) of average diameter 8 ± 1.1 nm. In the case of CuNi-PVPh sample, we also observed the nanochain formed by the assembly of spherical alloy particle of sizes in the range of 7.5± 1.8 nm. In comparison to polymer-capped nanoalloy, bare-CuNi was somewhat aggregated instead of forming nanochains, indicating the definite role of polymer capping agent in chain-like nanostructure formation. The selected area electron diffraction (SAED) pattern of CuNi-PEG sample consisted of concentric rings revealing polycrystalline nature of the sample. The d-value obtained from lattice fringe distance calculation (2.12 nm) for CuNi-PEG sample was matched well with the d111 from XRD pattern (2.08 nm).

15



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Page 16 of 34

CuNi-PEG

(111)

CuNi-PVPh

Bare-CuNi

20 nm

Figure 4.

TEM images of CuNi-PEG, CuNi-PVPh and Bare-CuNi alloy samples.

Additionally, EDS mapping was carried out based on HAADF-STEM, which showed the intensity of the signals corresponding to Ni and Cu is almost similar (Figure S16 in the SI). The atomic composition estimated from EDX analysis was 1: 1.02, which matched well with that obtained from the ICP-OES result discussed above. The possible mechanism of the formation of alloy nanochain in this case was explained based on our previous report as schematically shown in Scheme 3. For the synthesis of the alloy nanostructure, the used PEG or PVPh are soluble in ethanol as well as in water. Therefore, the 16


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reaction mixture containing polymer (PEG or PVPh), Cu(II) and Ni(II) acetate, hydrazine hydrate and NaOH in ethanol is a homogeneous mixture. The simultaneous reduction of Cu(II) and Ni(II) produces spherical nanoalloy particles. Due to the presence of (-CH2-O-CH2) group in PEG and (C6H4O-) group in PVPh, these polymers act as stabilizer for the alloy nanoparticle. Additionally, these polymers helped to conveniently disperse the nanostructure in water as well as in organic solvent. On the other hand, the formed alloy nanospheres are soft ferromagnetic in nature and relatively easy to polarize due to low coercivity values. Consequently, they are assembled to form nanochain via magnetic dipole-dipole interaction.27

n /

O

n

OH Cu2+Ni2+ Cu2+ Ni2+ Cu2+ N2H4+NaOH Cu2+ Ni2+ 2+ Reduction Ni2+ Cu2+ Cu Ni2+ Ni2+

Polymer complexed Cu(OAc)2 and Ni(OAc)2

Scheme 3.

Magnetic dipole directed assembly

Polymer CuNi nanosphere

Polymer CuNi alloy nanochain

The synthetic scheme for the preparation of polymer-coated CuNi alloy nanochain

To examine the chemical state of the Cu in different alloy samples, XPS analysis was performed on alloy samples. Figure 5 showed the expanded portion of the full spectrum of CuNiPEG and bare-CuNi in the characteristic region for Cu between 920 eV to 960 eV. For CuNi-PEG sample, Cu 2p3/2 peak at 932.7 eV as well as Cu 2p1/2 at 952.6 eV were the characteristic for Cu(I).45, 46 On the other hand, bare-CuNi exhibited 2p3/2 and 2p1/2 peaks at 934.4 and 954.6 eV respectively, suggesting higher surface oxidation of this sample. Further, the spectrum also 17



showed shake up peak at 948 ev, corresponding to Cu(II) indicating the presence of Cu(II) oxide over bare-CuNi surface.28 But, we are unable to identify the presence of copper oxide from XRD analysis. The presence of copper oxide is probably the reason why the saturation magnetization (Ms) value of the bare-CuNi sambe is lower than the other two polymer alloy samples (Figure 3). In the case of CuNi-PEG sample, the presence of polymer prevents extensive surface oxidation. However, Cu(I) was still remained in the sample. CuNi-PEG

2p3/2

Bare CuNi

Cu 2p

2p3/2

932.7

Cu 2p 934.4 954.5

925

Intensity (a.u.)

2p1/2

Intensity (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

Page 18 of 34

2p1/2 952.6

935

945

955

965

925

Figure 5.

935

945

955

965

Binding energy (ev)

Binding energy(ev)

XPS spectra of CuNi-PEG and bare-CuNi samples.

Magnetic CuNi nanoalloy catalyzed alkyne-azide click reactions.

Finally,

we

explored the use of these as-synthesized magnetic CuNi nanoalloy samples as catalyst in different alkyne-azide click reaction. It is known that the Cu NPs is capable of catalyzing the click reaction owing to the presence of Cu(I) at the particle surface.47-49 From the XPS investigation, we detected the presence of Cu(I) on the alloy surface. This result prompted us to study in detail the scope of as-synthesized CuNi nanoalloy as catalyst for click reactions both in organic solvent and in water. To optimize catalytic process, we conducted the click reaction between phenyl acetylene 18


Page 19 of 34

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and benzyl azide using CuNi-PEG nanoalloy sample at room temperature in water. The reaction was quenched after 2h, 4h, 6h, 12h or 18h and the yields of the obtained product, 1-benzyl, 4phenyl 1, 2, 3-triazole, abbreviated as 1a (for characterization see SI) were found to be 35%, 38%, 46%, 65% and 96% respectively (Table 2). Thus, all the click reactions (with the variation of substrates, solvent and nanocatalyst) in this study were performed for 18 h. It was also observed that the yields of 1a in water (96%) and in DMF (92%) after 18h are nearly same when CuNiPEG nanoalloy sample is used as catalyst (see Table 2). It should be noted that the yield of 1a for CuNi-PEG nanoalloy was higher (96%) than that of CuNi-PVPh (41%) and bare-CuNi (31%) in water. In the case of bare-CuNi sample, the low product yield is probably due its agglomerated nature and its low dispersity in water. However, compared to CuNi-PEG sample, the poor yield of the reaction using CuNi-PVPh sample is not clear to us at this point. To resolve this issue, we further performed the same click reaction with CuNi-PVPh sample in DMF, surprisingly, the product yield was high (83%), which suggested its higher activity in DMF compared to water. Thus, it may be concluded that the CuNi-PVPh catalyst is not accessible to the reactant in water and remains passive, which is manifested in a lower catalytic activity. This is obvious, as PVPh is insoluble in water, but is soluble in DMF. Therefore, DMF is appeared to be the better solvent for CuNi-PVPh catalyst to carry out the click reaction.

19



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Table 2.

Page 20 of 34

Different reaction condition of click coupling reaction between phenylacetylene

and benzyl azide at room temperature by varying reaction time, nanocatalyst and solvent and corresponding product yield (measured by GC) of 1-benzyl, 4-phenyl 1, 2, 3-triazole (1a). N N PhC

CH + PhCH2N3 PhH2C

Catalyst

CuNi-PEG

Ph

Time (h) Solvent %Yield of 1a 2

35

4

38

6

Water

46

12

65

18

96

18

DMF

92

Water

41

DMF

83

Water

32

CuNi-PVPh 18 Bare-CuNi

N

Before detail investigation of catalytic activities of the nanoalloy samples, we also measured the zeta potentials of these samples in water as well as in DMF to reveal the role of surface stabilizing polymer on their catalytic activities (see Figure S19 in the SI). It was observed that the zeta potential values of CuNi-PEG and CuNi-PVPh samples in water was +0.555 and +15.5 mV. The positive zeta potential indicated positive surface charges of alloy nanostructure’s surface caused by the adsorption of precursor metal ions such as Ni2+ or Cu+/Cu2+.27 However, very small potential value of CuNi-PEG sample can be ascribed to strong solvent coordination 20


Page 21 of 34

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through surface capped polymer (as PEG is water friendly) which leads to the reduction of its surface charge. Alternatively, the lack of coordination of water molecule with CuNi-PVPh sample exhibited moderate positive surface charge. This clearly revealed that the CuNi-PEG could act as a better catalyst in water than the CuNi-PVPh sample, as the reactant molecule get better access to the catalyst surface. In case of bare-CuNi, the zeta potential value was negative (-12mV), probably due to the presence of oxide layer on the surface. On the other hand, the zeta potential value of CuNi-PVPh nanoalloy in DMF was +3.85 mV, which also make the catalyst suitable for catalyzing click reaction in DMF.

Table 3.

Click coupling reaction between several terminal alkynes and alkyl/aryl azides and

corresponding product yields using CuNi-PEG nanocatalyst in water at room temperature.

H +

R1

R1

R2-N3

R2

N N N

CuNi-PEG Water, RT, 18 h

R1

R2

Product Abbreviation

Yield (%)

1b

82

1c

76

1d

59

1e

56

1f

56

2a

58

2b

72

H3 C

H3CO Br

C3H7O O

C8H17C3H7-

21



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Page 22 of 34

Consequently, as-synthesized CuNi-PEG sample was then used as catalysts in 1,3-dipolar cycloaddition reactions between different terminal alkynes and alkyl azides in water as presented in Table 3 under the optimized condition. Apart from phenyl acetylene, we used three different para substituted aromatic terminal alkynes (4-bromophenyl acetylene, 4-ethynyltoluene, 4ethynylanesole) and two aliphatic alkynes (1-pentyne and propargyl acrylate). Benzyl azide was the prototypical azide in these reactions. For further extension, we also used octyl azide (aliphatic azide). The reaction procedure, product purification and characterization were described in the SI. High product yield of these reactions suggested that the alloy nanocatalyst is tolerable for various substrates. Apart from the conventional substrate variation study, we were also intended to examine the activity of this nanocatalyst in presence of biological moiety for the interest of bioconjugate synthesis. Generally, in conventional homogeneous catalyst with Cu(I) salt to prepare peptide based bioconjugate via click reaction resulted in fatal Cu impurity in the bio-conjugate product. This is because the peptide contain amide group, which form strong complex with Cu ion during alkyne-azide click reaction. This Cu impurity is very hard to remove and consequently dangerous while the bioconjugates are applied for medicinal purposes. To overcome this, we try to use the polymer-capped magnetic CuNi nanoalloy, which is pseudo-homogeneous in nature and Cu ion is not available in the solution. Thus, the chance Cu impurity is almost negligible. In order to check this, we synthesized an azido functionalized peptide [Me2C(N3)-CO-NH-Tyr(1)-Val(2)-Tyr(3)OMe]

[Peptide-N3](see

SI

for

synthesis)

and

perform

the

click

reaction

with

phenylacetylene/propargylated imidazolium ionic liquid ([PrMIM]Br) using the CuNi-PVPh alloy nanocatalyst (see Scheme 1) in DMF. The obtained peptide-phenyl acetylene and peptide-ionic 22


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liquid click bioconjugates were characterized by NMR and Mass spectroscopy (see Figure S34S37 of SI). The recovery of magnetic nanocatalyst and synthesis of Cu-free pure bioconjugates justified the improvement of the method over homogeneous catalysis. This type of peptide-based click conjugate is new in literature, which may have potential in the field of supramolecular chemistry and medicinal chemistry.50 It is known that the end group modification of polymer is a difficult task due to lack of availability of the end functional group for reaction. Click chemistry with Cu(I) salt is generally performed in many occasions to functionalize polymer, which is subsequently useful for modification to block/graft copolymers. However, no attempt of click reaction has been pursued with nanocatalyst to modify polymer end functionality, probably due to unavailability of active catalytic site towards large polymer. On the other hand, unsupported magnetic alloy nanocatalyst could be a better option thanks to their pseudo-homogeneous nature. Accordingly, we prepared poly(methyl methacrylate) polymer with azide end functionality (abbreviated as PMMA-N3) (reaction scheme, detail procedure and characterization was described in the SI). The click reaction was performed in DMF with CuNi-PVPh catalyst using propergyl functionalized phenylalanine (see Scheme 2). The resultant polymer-amino acid conjugate (termed as PMMAPhe) was characterized using FTIR, NMR and MALDI-MS analysis. The azide group has the characteristic band at 2108 cm-1, which was disappeared in the final click bioconjugate confirming the successful click reaction (see Figure S38 of SI). For PMMA-Phe conjugate, the NMR signal at 8 ppm appeared due to the presence of (N-H) proton of terminal amino group and (C-H) of the triazole ring, which confirmed the successful click conjugation between azido substituted polymer and propergyl ester of phenylalanine (see Figure S39 of SI). The enlarged 23



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Page 24 of 34

view of MALDI-MS spectrum of PMMA-Phe (linear mode) showed peaks at m/z value of 4569.166, 7741.833, 9448.666, 10875 and 12250 (Figure S40). Subtracting the PMMA chain from the polymer-amino acid conjugate, the rest of the segment (i.e. phenylalanine substituted triazole) has mass value of 440.304. Therefore, the molecular ion peaks can be assigned to the PMMA-Phe conjugate containing 41, 73, 90, 104, 118 MMA units (the detail calculation of peak assignment was given in Figure S40 of the SI). Thus, the analysis of MALDI spectrum also confirmed the successful conjugation of amino acid segment at the end of PMMA chain. Therefore, the adopted methodology is synthetically superior compare to typical heterogeneous catalysis. Reusability test of this magnetically retrievable nanocatalyst was also performed for three cycles in the reaction between phenylacetylene and benzyl azide (see Table 4). After each cycle, catalyst was magnetically separated, isolated and reused. The product yield was measured by gas chromatography. Nevertheless, substantial product yield was obtained in all three cycles with very small gradual decrease. To check the fate of the nanoalloy after catalysis, the recovered CuNi-PEG nanoalloy after the 1st cycle of reaction was collected magnetically and the morphology and oxidation state was evaluated. TEM image of this recovered CuNi-PEG sample was (Figure S41 of SI) appeared to be a bit aggregated compared to original CuNi-PEG sample. Distinct nanochain was unable to identify in the sample. On the other hand, XPS analysis showed that peaks corresponding to 2p3/2 (933.1 ev) and 2p1/2(952.6 ev) were larger than that of original CuNi-PEG sample, which suggested minute surface oxidation on the particle surface (see Figure S42 of SI). However, the satellite peak around 945 ev for Cu(ll) was not very prominent. This

24


Page 25 of 34

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suggested that alloy nanocatalyst were very stable and could be reused several times in the click conjugation without much losing in their virgin catalytic activity

Table 4.

Reusability investigation (upto 3rd cycle) and corresponding product yield of the

click reactions between phenylacetylene and benzyl azide using CuNi-PEG as nanocatalyst. No. of cycle Product Yield% st

96

1

nd

93

2

rd

87

3



CONCLUSIONS In summary, the chain-like bimetallic magnetic CuNi nanoalloy was successfully prepared

in ethanol by chemical reduction route in presence of poly(ethylene glycol) or poly(vinyl phenol) as stabilizing/structure directing agent. The formed nanoalloy showed good dispersity in water and in organic solvents. The formation of chain-like alloy nanostructures was confirmed from TEM analysis. Along with the composition and magnetic property investigation, the presence of Cu(I) in the nanoalloy surface was determined via XPS analysis. Consequently, these polymer CuNi nanoalloys were used as efficient unsupported catalyst for alkyne-azide click reaction in water and in DMF at room temperature. The significant effect of solvophilicity of surface stabilizing polymer on the catalytic activity of alloy nanocatalyst was observed and was explained by zeta potential measurement. The CuNi alloy nanocatalyst has been proven tolerable to different 25



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Page 26 of 34

click reactions involving various alkynes and azides and capable of producing the corresponding click product in good to excellent yield. Additionally, the benefit of good solvent adoptability of the unsupported nanoalloys was utilized to synthesize new peptide-ionic liquid conjugates and the polymer end functionalized with amino acid. The catalyst was unsupported but magnetically recoverable, and can be reused successfully in click reaction without losing its catalytic property. Consequently, we can conclude the engineering of a novel Cu based nanocatalyst, which contains the advantages of homogeneous and heterogeneous catalysis avoiding their drawback and thereby termed as pseudo-homogeneous catalyst.



ASSOCIATED CONTENT

Supporting Information (SI) Available:

Synthetic details and 1HNMR, ESI-MS, MALDI-MS

characterizations of different terminal alkynes and azides and final click-conjugate products, FTIR, EDS mapping and zeta potential data of alloy samples, XPS and TEM data of recovered alloy nanocatalyst. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Notes: The authors declare no competing financial interest



ACKNOWLEDGEMENTS

A.S. and M.D. thank the CSIR, Government of India for providing financial assistance. This research is supported by the grants from CSIR, New Delhi, India and BRNS, Govt. of India.

26


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


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Page 34 of 34

Graphical contents entry

Polymer-Assisted Chain-like Organization of CuNi Alloy Nanoparticles: Solvent-Adoptable Pseudo-Homogeneous Catalysts for Alkyne-Azide Click Reactions with Magnetic Recyclability Mrinmoy Biswas, Anupam Saha, Madhab Dule and Tarun K. Mandal*

Cu(OAc)2 Ni(OAc)2

Polymer CuNi magnetic alloy nanochain

N2H4, NaOH PEG/ PVPh in ethanol

R1C

CH +

R2

R2

N N

N

Click reaction N3

+ R1 Magnetically Recycle recovered nanoalloy R1= alkyl, aryl, ionic liquid, amino acid R2 = alkyl, aryl, peptide, polymer

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Solvent-Adoptable Pseudohomogeneous Catalysts for Alkyne

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