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Magnetically Directed Assembly of Nanocrystals for Catalytic Control of a Three-Component Coupling Reaction Alaa M. Munshi, Vipul Agarwal, Dominic Ho, Colin L. Raston, Martin Saunders, Nicole M. Smith, and K. Swaminathan Iyer Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00582 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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Crystal Growth & Design

Magnetically Directed Assembly of Nanocrystals for Catalytic Control of a Three-Component Coupling Reaction Alaa M. Munshi,a Vipul Agarwal,a Dominic Ho,a Colin L. Raston,b Martin Saunders,c Nicole M. Smith,a* and K. Swaminathan Iyera,* a

School of Chemistry and Biochemistry, The University of Western Australia, Crawley, Western

Australia, Australia b

School of Chemical and Physical Sciences, Flinders University, Bedford Park, South Australia,

Australia c

Centre for Microscopy, Characterization & Analysis, The University of Western Australia,

Australia. KEYWORDS: A3 coupling reaction, Fe3O4@Au nanoparticles, magnetic field.

ABSTRACT: Randomly distributed colloidal magnetic nanoparticles in solution are polarized in the presence of an external magnetic field and the interparticle dipole–dipole attraction drives their assembly into linear chains. In this communication, we report using a model A3-coupling

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reaction, that the field-directed self-assembly of gold-coated magnetite (Fe3O4@Au) nanoparticles can be used to remotely control the rate of reaction by manipulating their colloidal assembly of catalyst in situ.

One-pot multicomponent coupling reactions (MCRs) are an attractive strategy in organic synthesis since they provide easy and rapid access to large libraries of organic compounds with diverse substitution patterns.1 This enables high molecular diversity in a single reaction step, with high atom efficiency in minimum synthetic time.2, 3 Among MCRs, the transition metal catalyzed three-component coupling (A3-coupling) reaction of an aldehyde, amine and alkyne to obtain propargylamine derivatives is one of the most widely explored reactions.4 Propargylamines are versatile synthetic building blocks for various natural products and therapeutic drugs.5, 6 The A3-coupling reaction is regulated via catalytic activation of the C–H bond of terminal alkynes in the presence of transition metal catalysts including Cu,7 Ag,8 Au,9 Ni,10 Ir,11 and Fe.12 Importantly, metal nanoparticles have attracted enormous attention as catalytic substrates owing to their unique physicochemical properties.13,14 The overall performance of these metal nanoparticles is governed by their size, shape, composition, crystal phase and surface properties.15-17 Traditionally, shape and size controlled catalytic nanoparticles are synthesized using a template-assisted fabrication via a porous support matrix, self-assembly on functional substrates and surfactant-assisted synthesis.18-21 Among the various methods used, fabrication of nanoparticles on mesoporous matrices has been attractive for catalysis as these systems offer highly ordered pores with controllable pore architectures, such as pore size, surface area and pore volume.22 Among the various nanoparticle-based catalysts that have been explored


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in the A3-coupling; gold nanoparticles have emerged as front-runners due to their high alkynophilicity, which in turn results in a highly effective C–H alkyne-activation step in the reaction.23-26 Herein, we fabricate gold (Au) coated superparamagnetic iron oxide (Fe3O4) nanoparticles (Fe3O4@Au) that make the catalyst responsive to external magnetic fields, allowing for the control of nanoparticle assembly in situ which in turn affects the rate of reaction and enables for ease of separation of the catalyst.27 The colloidal assembly of the Au-coated superparamagnetic catalyst Fe3O4@Au nanoparticles can be manipulated in a reaction using an external magnetic field to yield linear chains and demonstrate the efficacy of this system in remotely regulating the rate of reaction using a model A3-coupling reaction. Formation of anisotropic colloidal assemblies in suspensions using an external magnetic field have been exploited to generate linear assemblies of magnetic nanoparticles in a liquid matrix.28 The anisotropic magnetic dipole-dipole interaction in the presence of an external magnetic field dominates the isotropic van der Waals forces to induce chain-formation. This strong dipole– dipole interactions can be manipulated by the field strength, which can in turn be utilized to control the morphology of the chains as desired.28, 29 Here, stable 130 ± 2 nm (average ± standard error mean) Fe3O4@Au nanoparticles were synthesized using a multistep assembly process (see Supporting Information for method). Briefly, the Fe3O4 nanoparticle core was synthesized and functionalized with polyethyleneimine (PEI) to yield 88 ± 1.5 nm particles (Figure 1a). PEI coated Fe3O4 nanoparticles were further functionalized with 2 nm Au seeds to facilitate the formation of a uniform coating of Au yielding 130 ± 2 nm colloidal Fe3O4@Au nanoparticles (Figure 1b and c and Supporting Information Figure S1). The formation of the Au shell was confirmed sing transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and powder X-ray diffraction (XRD)


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(Figure 2 c, d, e and f and Supporting Information Figure S2). The purified and resuspended Fe3O4@Au nanoparticles formed a stable colloidal solution in toluene and their catalytic efficacy was investigated using a model A3-coupling reaction between benzaldehyde, piperidine and phenylacetylene for the preparation of propargylamine (see Supporting Information Scheme 1). Next, we explored the effect of an external magnetic field in inducing linear assemblies of the Fe3O4@Au nanoparticles in toluene at 100 °C (temperature for the A3-coupling reaction). Fe3O4@Au nanoparticles (15 mg) were suspended in toluene (10 ml) and placed in a magnetic field (65 mT with a field gradient of 2.3 T m−1) for 1 hour at room temperature as described previously to allow the formation of linear assemblies of nanoparticles, following which the temperature was increased to 100 °C (see Supporting Information for method).30 TEM analysis revealed the formation of linear chains of Fe3O4@Au nanoparticles ranging from 6 µm to 60 µm in length (Figure 2a and b). Having established that Fe3O4@Au nanoparticles can be maneuvered reversibly using an external magnetic field from linear chains to a colloidal dispersion in the presence or absence of an external magnetic field respectively; we explored its effect in controlling the rate of conversion of the A3-coupling reaction between benzaldehyde, piperidine and phenylacetylene. Fe3O4@Au nanoparticles (4 mol %) were suspended in toluene (3 mL) under a nitrogen atmosphere in the presence of benzaldehyde (1 mmol), piperidine (1 mmol), and phenylacetylene (1 mmol). The reaction was carried out both in the presence and absence of an external magnetic field. The conversion of benzaldehyde to propargylamine was monitored at various time points over 48 hours at 100 °C using NMR (see Supporting Information for method). It was determined that the rate of conversion of propargylamine can be regulated remotely using a magnetic field by manipulating the colloidal assembly of the Fe3O4@Au nanoparticles. In the present case, the rate of conversion of benzaldehyde to propargylamine in


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the A3-coupling reaction is dependent on the effective C–H alkyne-activation by the Au surface. In the presence of a magnetic field, the formation of an anisotropic linear assembly reduces the catalytic surface area available for C–H alkyne-activation which in turn lowers the rate of reaction, where linear chain formation results in a 40% decrease in conversion at 24h (Figure 3). It is noteworthy that Fe3O4@Au catalyst is reversibly controlled by regulating the external magnetic field; the linear alignment of nanoparticles was disintegrated to a stable colloidal suspension via Brownian motion at elevated temperature. Furthermore, Fe3O4@Au nanoparticles showed superior catalytic activity in the A3-coupling reaction in comparison with the Fe3O4 nanoparticles and Au nanoparticles under similar reaction conditions (Supporting Information Figure S3). In summary, the size, shape and surface area of colloidal nanoparticles are important characteristics that dictate their performance as catalysts. We have demonstrated that the fielddirected assembly of colloidal catalysts in solution provides an additional level of control to reversibly manipulate their assembly and active surface enabling remote control over the catalytic process.


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Figure 1. TEM image of a) PEI coated Fe3O4 (Fe3O4-PEI) nanoparticles; b) Au seed functionalized Fe3O4-PEI nanoparticles; c) Au-coated Fe3O4-PEI (Fe3O4@Au) nanoparticles. Scale bars a) and c) 100 nm; b) 50 nm.


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Figure 1. TEM image of the Fe3O4@Au chains a) low magnification; b) high magnification; c) dark field image of the nanoparticle chain cluster; d) Energy dispersive X-ray microanalysis map of the specified region highlighting Au (yellow) coating around Fe (blue) nanoparticles; e) Elemental analysis showcasing the presence of Au and Fe in the nanoparticles constituting the chains; f) Powder XRD spectrum displaying the presence of Fe3O4 (purple square) and Au (green circle) in the nanoparticles. Scale bars: a) 1 µm; b) 200 nm; c) and d) 200 nm.


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Figure 3. The correlation between the rate of conversiona of the A3-coupling reaction and the morphology of the Fe3O4@Au catalyst. Random morphology of the nanoparticles (red line) demonstrated a higher conversion rate compared to the linear anisotropic morphology of the nanoparticles (green line) as adopted in the absence and presence of an external magnetic field respectively. Data are presented as mean ± standard error mean. aConversion was determined by 1

H NMR analysis of the crude reaction mixtures based on benzaldehyde conversion.

ASSOCIATED CONTENT Supporting Information. Nanoparticle synthesis, nanowire fabrication, characterization details, electron diffraction data of the catalyst and TEM analysis of the catalyst intermediate. AUTHOR INFORMATION Corresponding Author * E-mail: NMS: [email protected], KSI: [email protected]. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors acknowledge the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterization & Analysis and The University of Western Australia, funded by the University, State and Commonwealth Governments. Dr Aaron Dodd is acknowledged by the authors for his assistance in XRD measurements. A. M. Munshi would like to thank Umm Al-Qura University, Makkah, Kingdom of Saudi Arabia for her scholarship. Support of the work from the Australian Research Council, The Perth Mint, and the Government of South Australia is also acknowledged. ABBREVIATIONS Fe3O4@Au nanoparticles, gold-coated magnetite nanoparticles; PEI, polyethyleneimine; A3coupling, three-component coupling; powder XRD, powder X-ray diffraction; TEM, transmission electron microscopy; EDS, energy-dispersive X-ray spectroscopy. REFERENCES (1) Cioc, R. C.; Ruijter, E.; Orru, R. V. A. Green Chem. 2014, 16, 2958-2975. (2) Tejedor, D.; Garcia-Tellado, F. Chem. Soc. Rev. 2007, 36, 484-491. (3) Dömling, A.; Ugi, I. Angew. Chem. Int. Ed. 2000, 39, 3168-3210. (4) Yan, B.; Liu, Y. Org. Lett. 2007, 9, 4323-4326. (5) Boulton, A. A.; Davis, B. A.; Durden, D. A.; Dyck, L. E.; Juorio, A. V.; Li, X.-M.; Paterson, I. A.; Yu, P. H. Drug Dev. Res. 1997, 42, 150-156. (6) Miura, M.; Enna, M.; Okuro, K.; Nomura, M. J. Org. Chem. 1995, 60, 4999-5004. (7) Luz, I.; Llabrés i Xamena, F. X.; Corma, A. J. Catal. 2012, 285, 285-291. (8) Jeganathan, M.; Dhakshinamoorthy, A.; Pitchumani, K. ACS Sustainable Chem. Eng 2014, 2, 781-787. (9) Villaverde, G.; Corma, A.; Iglesias, M.; Sánchez, F. ACS Catal. 2012, 2, 399-406. (10) Samai, S.; Nandi, G. C.; Singh, M. S. Tetrahedron Lett. 2010, 51, 5555-5558. (11) Sakaguchi, S.; Kubo, T.; Ishii, Y. Angew. Chem. 2001, 113, 2602-2604. (12) Zeng, T.; Chen, W.-W.; Cirtiu, C. M.; Moores, A.; Song, G.; Li, C.-J. Green Chem. 2010, 12, 570-573. (13) Somorjai, G. A.; Frei, H.; Park, J. Y. J. Am. Chem. Soc. 2009, 131, 16589-16605. (14) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem. Int. Ed. 2005, 44, 7852-7872. (15) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024-6036.


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For Table of Contents Use Only Magnetically Directed Assembly of Nanocrystals for Catalytic Control of a Three-Component Coupling Reaction Alaa M. Munshi,a Vipul Agarwal,a Dominic Ho,a Colin L. Raston,b Martin Saunders,c Nicole M. Smith,a* and K. Swaminathan Iyera,*

TOC graphic

Synopsis: The catalytic efficacy of Fe3O4@Au nanoparticles was investigated for an A3 coupling reaction. The paper demonstrates that the colloidal assembly of these nanocrystals can be manipulated in solution using an external magnetic field and this consequentially dramatically influences the rate of the reaction.


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Magnetically Directed Assembly of Nanocrystals ... - ACS Publications

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