N-Heterocyclic Carbene-Based Conducting Polymer–Gold

Sep 24, 2014 - Sylvain Roland , Xiang Ling , and Marie-Paule Pileni. Langmuir ... Xiang Ling , Nicolas Schaeffer , Sylvain Roland , and Marie-Paule Pi...
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N‑Heterocyclic Carbene-Based Conducting Polymer−Gold Nanoparticle Hybrids and Their Catalytic Application Sun Gu Song,† Chinnadurai Satheeshkumar,† Jiyoung Park,† Jongho Ahn,† Thathan Premkumar,†,‡ Yunmi Lee,*,§ and Changsik Song*,† †

Department of Chemistry and ‡The University College, Sungkyunkwan University, Suwon, Gyeonggi 440-746, Republic of Korea Department of Chemistry, Kwangwoon University, Seoul 139-701, Republic of Korea

§

S Supporting Information *

ABSTRACT: Hybrid nanocomposites of N-heterocyclic carbene (NHC)-functionalized conducting polymers (CPs) with gold nanoparticles (AuNPs) were prepared by concurrent disproportionation and oxidative coupling. The formation of hybrid nanocomposites, NHC-CP/AuNPs, in the simultaneous process was confirmed by transmission electron microscopy, powder X-ray diffraction, cyclic voltammetry, and 13C solid-state NMR analyses. More importantly, the NHC group played a pivotal role in the dispersion of AuNPs. Further, NHC-CP/AuNPs exhibited good catalytic activity for the reduction of 4-nitrophenol.

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INTRODUCTION During the past several decades, π-conjugated polymers or conducting polymers (CPs) have been extensively studied because of their potential applications in a variety of fields.1,2 Metal nanoparticles have also been receiving much attention owing to their diverse applications in materials science and medicine. 3 Among them, gold nanoparticles (AuNPs), stabilized by a variety of dendrimers and polymers, have been extensively studied.4 In recent years, composites of conducting polymers and nanoparticles have drawn much attention not only because of exhibiting the interesting properties of each component but also synergistic effects of both the components can be expected.5 Inclusion of various nanoparticles tends to drastically affect the electrical and optoelectronic properties of CPs. Such hybrid composites of CPs and nanoparticles can be utilized as effective materials in surface-enhanced Raman scattering,6 sensors,7 heterogeneous catalysis,8 solar cells,9 and memory devices.10 In general, the preparation of hybrid composites of CPs and metal nanoparticles involves the reduction of metal cations in the presence of CPs.11 CPs themselves can reduce the metal cations because their standard reduction potentials are lower than those of certain metal ions.12 This one-pot redox chemistry is also applicable for the polymerization of monomers by metal salts as the oxidant, resulting in metalconducting polymer nanocomposites.13 For instance, HAuCl4 or AuCl3 was used for the oxidative polymerization of aniline,14 pyrrole,15 thiophene,16 and 3,4-ethylenedioxythiophene,17 affording polymer−metal nanocomposites. In the application of such polymer−nanomaterial hybrids, morphology control at the nanoscale is crucial. Depending on the compositions and interactions at the interface, the polymer and nanomaterial may be phase-separated, deteriorating the hybrid’s properties. © 2014 American Chemical Society

In view of the strong interaction between polymers and metal nanoparticles, N-heterocyclic carbene (NHC) moieties have been introduced in the polymer structure.18 Because of the NHC’s strong binding ability to metal ions, we envisioned that NHC-incorporated CPs would have strong interaction with metal nanoparticles, furnishing well-dispersed polymer− nanoparticle hybrids. NHCs are unusually stable carbenes owing to the adjacent heteroatom (i.e., nitrogen) and have a strong σ-donating ability similar to phosphines, the leading class of ligands in organometallic chemistry during the past two decades.19,20 In recent years, NHCs have been utilized as the stabilizing ligands for metal nanoparticles. For example, NHCstabilized AuNPs21 and RuNPs22 were prepared from the corresponding metal complexes by reducing agents. Alternatively, the ligand exchange of thioethers by NHCs was utilized to form AuNPs and PdNPs.23 However, to the best of our knowledge, thiophene-functionalized NHCs have not been utilized in conducting polymer−nanoparticle hybrids. Herein, we report the synthesis of conducting polymer/Au nanoparticle hybrids based on bithiophene-functionalized NHC-Au(I) complexes (BT-NHC-AuCl). The NHC was incorporated into the polythiophene derivative to increase polymer−nanoparticle interactions. Interestingly, AuNPs were synthesized by the disproportionation of Au(I) to Au(0) and Au(III), and during this process, concurrent oxidative coupling of thiophenes by Au(III) occurred, resulting in NHC-CP/ AuNP hybrids (Figure 1). We demonstrated that the NHC played an important role in furnishing well-dispersed NHCCP/AuNPs hybrids. This study shows the concurrent disproportionation and oxidative coupling of BT-NHC-AuCl, Received: September 2, 2014 Revised: September 11, 2014 Published: September 24, 2014 6566

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Figure 1. Schematic representation of NHC-CP/AuNPs.

Scheme 1. Synthesis of NHC-Containing Electropolymerizable Monomersa

Reagents and conditions: (i) Pd(PPh3)2Cl2, DMSO, 8 h, 110 °C, 79%; (ii) Ag2O, CH2Cl2, room temperature, 7 days, 74%; (iii) AuCl, CH2Cl2, room temperature, 8 h, BT-NHC-AuCl (79%), Br-NHC-AuCl (85%); (iv) Ag2O, CH2Cl2, room temperature, 10 h, 89%.

a

136.5, 137.7, 139.1, 140.5. MS (FAB-MS): m/z calculated for C35H29ClN2S4 [M − Cl]+: 605; found: 605. Synthesis of BT-NHC-AgCl. A 50 mL flask was charged with 3 (130.3 mg, 0.276 mmol) and 10 mL of dichloromethane. To this solution was added Ag2O (72.1 mg, 0.304 mmol). The resulting solution was stirred for 7 days in the dark and then filtered over Celite, and the filtrate was reduced to 2 mL. The solution was precipitated with pentane (20 mL) and dried under a vacuum. Yield was 153.2 mg (74%). The product was used for next step without further purification and characterization. Synthesis of BT-NHC-AuCl. A flame-dried 20 mL Schlenk tube was charged with BT-NHC-AgCl (17.3 mg, 0.023 mmol) and 2 mL of dichloromethane. To this solution was added AuCl (5.35 mg, 0.023 mmol). The resulting solution was stirred for 12 h and then filtered over Celite, and the filtrate was reduced to 1 mL. The solution was precipitated with pentane (10 mL) and dried under a vacuum. Yield is 15.3 mg (79%). 1H (500 MHz, CDCl3): δ 2.21 (s, 12H); 7.05 (dd, J = 5, 5 Hz, 2H); 7.18 (d, J = 2 Hz, 2H); 7.19 (s, 2H); 7.23−7.25 (m, 4H); 7.28 (d, J = 4 Hz, 2H); 7.43 (s, 4H). 13C (175 MHz, CDCl3): δ 18.0, 122.3, 123.9, 124.6, 124.7, 124.8, 125.9, 127.9, 135.6, 135.7, 136.1, 137.2, 137.7, 141.4, 173.6. MS (HRMS): m/z calculated for C35H29AuClN2S4 [M]+: 837.0568; found: 837.0575. Synthesis of Br-NHC-AuCl. Compound Br-NHC-AuCl was synthesized according to the described for compound BT-NHCAuCl, except the reaction time was 10 h instead of 7 days. The yield was 59 mg (85%). 1H (500 MHz, CDCl3): δ 2.13 (s, 12H); 7.14 (s, 2H); 7.37 (s, 4H). 13C (125 MHz, CDCl3): δ 17.8, 122.2, 123.9, 131.9, 135.9, 137.1, 173.5. MS (HRMS): m/z calculated for C19H19AuBr2ClN2 [M]+: 664.9269; found: 664.9261.

affording hybrid composites (NHC-CP/AuNPs) and their application for the catalytic reduction of 4-nitrophenol.

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EXPERIMENTAL SECTION

Materials and Characterization. All the chemicals were purchased from Sigma-Aldrich, TCI, and Alfa Aesar and used without further purification. The 1H and 13C NMR spectra were recorded on Bruker 500 and 700 MHz spectrometers. The chemical shifts are reported in ppm (δ) with TMS as an internal standard, and the coupling constants (J) are expressed in hertz. Ultraviolet−visible absorption data were acquired on a UV-1800 (Shimadzu) spectrophotometer. Electrochemical measurements were carried out using Epsilon electrochemical analyzer in a three-electrode cell. The electrolyte solution employed was 0.10 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in freshly dried dichloromethane (MC). The Ag/AgNO3, Pt button (1.6 mm in diameter), and Pt wire (0.5 mm in diameter) electrodes were utilized as reference, working, and counter electrodes, respectively. The scan rate was at 100 mV/s. Transmission electron microscopy (TEM) was conducted using a JEOL 2100F unit. The powder X-ray diffraction (PXRD) pattern was obtained using a Rigaku Ultima IV. The solid-state 13C NMR spectrum was recorded using a Varian Inova 500 MHz NMR spectrometer. Synthesis of Compound 3. Bis(bromophenyl)imidazolium salt, 1 (0.47 g, 1 mmol), tri-n-butylstannylbithiophene, 2 (1.28 mL, 3 mmol), and DMSO (10 mL) were added to a flame-dried Schlenk tube. The reaction container was purged with N2 for 30 min to remove O2. Pd(PPh3)2Cl2 (5 mol %, 35.9 mg) was added, and the reaction mixture was heated to 110 °C and stirred overnight. After cooling, ether (100 mL) was added and the mixture was filtered. The precipitate was collected as a yellow solid. Yield: 0.51 g (79%). 1H (500 MHz, DMSOd6): δ 2.24 (s, 12H); 7.15 (t, J = 4.2 Hz, 2H); 7.42 (d, J = 3.5 Hz, 4H); 7.59 (d, J = 5 Hz, 2H); 7.68 (d, J = 4 Hz, 2H); 7.75 (s, 4H); 8.37 (s, 2H); 9.76 (s, 1H). 13C (125 MHz, DMSO-d6): δ 17.5, 124.9, 125.2, 125.7, 125.8, 126.5, 126.9, 128.2, 129.0, 132.9, 134.3, 135.9, 136.2,

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RESULTS AND DISCUSSION The polymerizable monomer BT-NHC-AuCl was designed and synthesized (Scheme 1). The introduction of bithiophene (BT) units renders the monomer BT-NHC-AuCl polymerizable under oxidative conditions.24,25 Cowley and co-workers 6567

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Figure 2. 1H NMR spectra of the (a) Br-NHC-AuCl, (b) Br-NHC-AuCl with silver triflate (AgOTf), and (c) Br-NHC-AuCl with AgOTf and 2,2′bipyridine. The inset pictures show that Au(I) disproportionation to Au(0) was inhibited by 2,2′-bipyridine. The ∗ indicates the residual solvent peak of CDCl3, and # is from 2,2′-bipyridine (see Supporting Information Figure S9).

Figure 3. (a) Cyclic voltammogram of NHC-CP/AuNPs on a Pt button electrode in CH2Cl2 with 0.1 M Bu4NPF6 as a supporting electrolyte. (b) Powder X-ray diffraction (PXRD) patterns of NHC-CP/AuNPs (∗ indicates AgCl(s) fcc).

NHC-CP/AuNPs hybrid composite materials. To exchange the strongly bound chloride with the weakly bound triflate, BTNHC-AuCl was treated with silver triflate (AgOTf) in dichloromethane. As expected, AgCl was precipitated, indicating the formation of BT-NHC-AuOTf. However, we also observed the color change from yellow to dark green, which normally occurs in the oxidative polymerization of thiophene monomers.30 While the reaction proceeded, a dark-blue precipitate was formed and agglomerated. When the strongly bound chloride was attached to Au(I), no change was observed, and the compound remained stable. We hypothesized that the removal of the chloride ion from BT-NHC-AuCl facilitated the disproportionation of Au(I) to Au(III) and Au(0). Au(III) gave rise to an oxidative polymerization through thiophene moieties, and Au(0) induced Au nanoparticle growth. The disproportionation of Au(I) afforded AuNPs, and metallic Au or Au nanoseeds are known to catalyze the disproportionation reaction.31 Previously, the Das and Raj groups reported that

reported an electropolymerizable NHC-Au complex, where two BTs were directly attached to the backside carbons of the heterocycle.26 Our design was to affix BTs at the periphery of N-aryl groups of the NHC owing to its synthetic modularity. In other words, based on the rich chemistry and structural diversity of NHCs,20,27 bithiophene can be introduced without substantial modification. The Stille cross-coupling reaction of bis(bromophenyl)imidazolium salt (1)28 and tri-n-butylstannylbithiophene (2)29 afforded BT-functionalized imidazolium salt (3) in good yields. The treatment of 3 with Ag2O furnished BT-NHC-AgCl, which was subsequently transmetalated with AuCl to afford desired BT-NHC-AuCl in good yields. Diagnostic 13C NMR signal of BT-NHC-AuCl (δ = 173.6 ppm in CDCl3) confirmed that the carbene was bound to Au(I). Interestingly, we found that when the chloride in BT-NHCAuCl was removed, simultaneous disproportionation of Au(I) and oxidative polymerization of thiophenes occurred, affording 6568

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Figure 4. TEM images of the (a) NHC-CP/AuNPs (N = 60, average = 3.6 ± 1.1 nm), (b) Br-NHC-AuCl and bithiophene nanoparticles (N = 46, average = 5.3 ± 1.4 nm), where NHC-AuCl and bithiophene were separate, and (c) DMS-AuCl and bithiophene nanoparticles (N = 13, average = 10 ± 4.2 nm), where there was no NHC-Au insertion.

oxidatively coupled to form segmented polythiophene-like materials, probably because of the Au(III) complex. The formation of AuNPs in the hybrid materials (NHC-CP/ AuNPs) by the disproportionation of Au(I) and the reduction of Au(III) (during the oxidative polymerization) was confirmed by powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM) analyses. The XRD profile of hybrid materials showed peaks at 38.2°, 44.2°, 64.9°, and 77.6°, corresponding to the (111), (200), (220), and (311) planes, respectively, of the face-centered cubic (fcc) lattice of Au (Figure 3b). The result is in well agreement with the previously reported AuNPs.35 The average size of AuNPs was estimated to be 3.7 nm as calculated by the Scherrer equation from the Au(111) peak. The XRD pattern corresponding to AgCl was also observed during the removal of chloride ion. Figure 4a shows the TEM images of the synthesized NHCCP/AuNP hybrids, indicating that spherical AuNPs were welldispersed with narrow size distributions. The histogram of the size distribution shown in the inset of Figure 4a was obtained directly from an enlarged TEM image by counting 60 particles. The average size of the particles was ∼3.6 nm. We suspected that NHCs played an important role in dispersing AuNPs in the polymer matrix. To test the hypothesis, we conducted two control experiments of the concurrent disproportionation and oxidative polymerization with a separate electroactive monomer (bithiophene, BT): (i) in the presence of NHC−Au interaction (i.e., Br-NHC-AuCl with BT) and (ii) in the absence of NHC− Au interaction (i.e., Me2S-AuCl with BT). In the case of BrNHC-AuCl and monomer BT, we observed well-dispersed AuNPs with an average size of ∼5.3 nm (Figure 4b) in the polymers. In this experiment, the presence of NHC groups contributed to the dispersion of synthesized AuNPs. In contrast, without such NHC interactions, AuNPs formed significantly large size particles (∼10 nm) and also aggregated in clusters (Figure 4c). Thus, we concluded that NHC groups play an important role in dispersing AuNPs in NHC-CP/ AuNPs hybrid nanocomposites by metal−ligand interactions. The isolated NHC-CP/AuNPs were not soluble in organic solvents. Hence, their analysis was performed by 13C solid-state NMR spectroscopy (see Supporting Information, Figure S1), which showed a signal in the region appropriate for a carbene C−Au bonds (185.5 ppm), supporting our hypothesis that

the Au(I) complex generated by iodide-mediated reduction of AuCl4− underwent disproportionation at room temperature to single crystalline AuNPs, and the reaction was facilitated in the presence of either silver ions or naked Au nanoseeds.32 To support our hypothesis that the removal of the chloride promotes Au(I) disproportionation, we conducted 1H NMR experiments using a chloride-bound Au(I) complex (Br-NHCAuCl) with silver ions and 2,2′-bipyridine (Figure 2). When BrNHC-AuCl was treated with AgOTf in deuterated chloroform, white precipitations formed immediately, indicating the removal of the chloride ion in the form of AgCl. We also found that the peaks of the N-heterocycle (Ha) and bromodimethylphenyl group (Hb) were shifted from δ 7.13 to 7.25 ppm and δ 7.37 to 7.33 ppm, respectively (Figure 2b). There chemical shifts may correspond to the formation of bis(NHC)Au+, as reported in the literature.33,34 Since the chemical shifts were accompanied by coating of thin layer of gold (Figure 2b inset), we attributed it to the disproportionation of Au(I) to Au(0) and Au(III), and the latter was reduced back to Au(I) with the formation of bis(NHC)Au+. However, the disproportionation reaction appeared to be inhibited adding a coordinating ligand (2,2′-bipyridine) to the above-mentioned experimental conditions (Figure 2c). The ligand 2,2′-bipyridine was known to coordinate Au(I) complexes, and its coordination seemed to suppress disproportionation of Au(I), resulting in almost no shift in 1H NMR and no color change in the solution. Such kind of observation has been seen clearly while preparing the NMR samples (Figure 2b and 2c, NMR tube view). Thus, we conclude that the removal of chloride from BT-NHC-AuCl facilitated the disproportionation to Au(III) and Au(0), which eventually afforded NHC-CP/AuNPs hybrid composites in Figure 1. The oxidative polymerization of thiophene moieties was further supported by cyclic voltammetry (Figure 3a). The precipitated hybrid material was coated on a Pt-button electrode and subjected to a swept potential condition in a three-electrode configuration. The resulting CV profiles resembled the typical CVs of conducting thiophene derivatives.25 Furthermore, the oxidative onset of the hybrid material was 0.32 V (vs Fc/Fc+), which was much lower than that of BT-NHC-AuCl (0.51 V vs Fc/Fc+). This result showed that bithiophene units in the BT-NHC-AuCl complex were 6569

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Notes

NHC groups help the dispersion of AuNPs in the polymer matrix through metal−ligand interaction. The catalytic activity of the synthesized NHC-CP/AuNPs hybrids was investigated at room temperature toward the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) (Figure 5). After adding NaBH4 (∼15 mM) into the aqueous

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Global Development Research Center (GDRC) for program of the National Research Foundation of Korea (NRF, Grant No. 2012-0006672), which is being funded by the Ministry of Education, Science and Technology (MEST). This research was also partially supported by a grant from KIMS. The authors also thank Brain Korea 21 plus (BK21+).

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Figure 5. UV−vis absorption spectra of the reduction of 4-nitrophenol by NHC-CP/AuNPs.

solution of 4-NP (∼20 mM), the color of the solution changed from light yellow to dark yellow because of the formation of 4nitrophenolate ion. Then, the color of the 4-nitrophenolate ions faded with time after the addition of AuNPs (∼2 mg/mL). The progress of the reduction was monitored by UV−vis spectroscopy. The intensity of the characteristic peak at 400 nm decreased, and the formation of new peak at 290 nm corresponded to 4-AP. The reduction was completed within 510 s, with a reaction rate of 6.29 s−1 (see Supporting Information, Figure S2). Hence, NHC-CP/AuNPs exhibited considerable catalytic activity for the reduction of 4-NP to 4AP.36

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CONCLUSION In this study, we successfully synthesized NHC-CP/AuNP hybrids by a concurrent disproportionation and oxidative polymerization. The formation of NHC-CP/AuNP hybrids was confirmed by TEM, powder XRD, CV, and 13C solid-state NMR spectroscopy. Further, in well agreement with the controlled TEM analysis, NHC appears to play the main role for dispersing AuNPs in the polymer matrix. The NHC-CP/ AuNP hybrid nanocomposites exhibited high activity for the catalytic reduction of 4-NP to 4-AP. Further studies to delineate the biological applications are in progress.

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ASSOCIATED CONTENT

S Supporting Information *

NMR characterizations for the newly synthesized compounds, solid-state NMR for NHC-CP/AuNPs, and the plot of ln(At/ A0) versus time for the reduction of 4-nitrophenol using NHCCP/AuNPs hybrids. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Y.L.). *E-mail [email protected] (C.S.). 6570

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