Langmuir 2007, 23, 9543-9545
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Iron/Iron Oxide Nanoparticle Sequestration of Catalytic Metal Impurities from Aqueous Media and Organic Reaction Products Janet E. Macdonald, Joel A. Kelly, and Jonathan G. C. Veinot* Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada, T6G 2G2 ReceiVed April 23, 2007. In Final Form: July 5, 2007 A host of transition metal ions (Co2+, Cu2+, Ni2+, RuX+, RhX+, Pd2+, Ag+, and Pt4+) were sequestered from aqueous media upon exposure to FexOy@Fe nanoparticles. Concentrations were lowered from approximately 100 ppm to below 40 ppb. This technique was extended to the removal of catalytic ions from the products of a “Click” cycloaddition and a Heck coupling in organic solvents. Residual metal concentrations in organic reaction products matched or exceeded pharmaceutical standards.
Late transition metal homogeneous catalysts are well established as powerful tools in organic synthesis. They allow for otherwise difficult syntheses with rapidity, selectivity, and high yields.1 Unfortunately, even trace catalyst impurities remaining after product purification can interfere with later synthetic steps and eventual material application.2,3 Also, application of metal catalysts is often limited for pharmaceutical syntheses because metal concentrations in the final product exceed health standards (e.g., 5 ppm for Pt, Pd, Ru, or Rh; 10 ppm for Ni; 15 ppm for Cu; 20 ppm for Fe).2-4 To date, techniques for removing transition metal catalysts from organic reaction products are often time-consuming (taking up to 65 h)3 and employ high surface area adsorbents such as carbon black and absorbing polymers. Modern commercial resins, fibers, and powders are commonly modified with metal scavenging thiol, carboxylic acid, or amine functionalities.2,3,5 Each sequestration method has associated benefits and limitations. For example, some are metal specific,3,5 while others absorb less discriminately, sometimes reducing yields;2,5 some achieve only the minimum extraction requirements prescribed by regulatory bodies, while others are expensive or impractical in many laboratory or industrial situations.3 We present a rapid, versatile, inexpensive, and straightforward alternative for removing catalytic metal ions from solutions of organic reaction products in watermiscible solvents (e.g., tetrahydrofuran (THF)). Importantly, the present approach employs common reagents and is practical for both laboratory and industrial application. Herein, oxide-capped metallic iron nanoparticles (FexOy@Fe) are used to sequester catalytic metal ions from organic reaction products. Similar approaches have been applied to the environmental remediation of some metal ions (e.g., Cr(VI), Pb(II), Ni(II), Ag(I), and Cd(II));6-11 however, a direct transposition of * Corresponding author. Tel: 1-780-492-7206. Fax: 1-780-492-8231. E-mail:
[email protected]. (1) Kettler, P. B. Org. Process Res. DeV. 2003, 7, 342-354. (2) McEleney, K.; Allen, D. P.; Holliday, A. E.; Crudden, C. M. Org. Lett. 2006, 8, 2663-2666. (3) Garrett, C. E.; Prasad, K. AdV. Synth. Catal. 2004, 346, 889-900. (4) Committee for Proprietary Medicinal Products. The European Agency for the EValuation of Medicinal Products, 2002. European Medicines Agency Web Site. http://www.emea.europa.eu/pdfs.human/swp/444600en.pdf (accessed April 10, 2007). (5) Thayer, A. Chem. Eng. News 2005, 83, 55-58. (6) Ponder, S. M.; Darab, J. G.; Bucher, J.; Caulder, D.; Craig, I.; Davis, L.; Edelstein, N.; Lukens, W.; Nitsche, H.; Rao, L. F.; Shuh, D. K.; Mallouk, T. E. Chem. Mater. 2001, 13, 479-486. (7) Ponder, S. M.; Darab, J. G.; Mallouk, T. E. EnViron. Sci. Technol. 2000, 34, 2564-2569. (8) Li, X. Q.; Elliott, D. W.; Zhang, W. X. Crit. ReV. Anal. Chem. 2006, 31, 111-122.
these methods to organic reaction products may be complicated by adverse side reactions, leaching of Fe, and so forth (vide infra). Furthermore, while comparable iron nanomaterials have effectively lowered some metal contaminant concentrations in the environment, it remains unclear whether the stringent pharmaceutical regulatory requirements for standard catalytic metal concentrations in organic materials can be met.4 The generally accepted mechanism by which low valent iron nanomaterials remove ionic metal species involves the surface adsorption of ions onto the oxide shell. Subsequently, ions are reduced to their elemental form by electrons supplied from the iron core. While this process increases the sequestration capacity far beyond what is afforded by surface adsorption alone,8,9 it relies heavily upon surface interaction with metal ions. In this regard, surface wettability and the coordination of organic reagents could hinder the application of FexOy@Fe in these environments. Furthermore, while the present method can conceivably remove any metal ion with a suitable reduction potential, it is necessary that we demonstrate that typical catalytic metal concentrations in aqueous media can be lowered from typical catalytic to sub40 ppb levels as a benchmark for the present work. These low concentrations are prerequisites to any further extension of this approach to organic synthetic applications; parts per billion metal levels in solution translate to parts per million concentrations in the organic product upon removal of solvent during workup. We subsequently extend our methodology to two important synthetic reactions for which metal toxicity could limit utility: namely, a Cu(I)-catalyzed “Click” reaction12 and a Pd-catalyzed Heck coupling.13,14 FexOy@Fe was prepared by sodium borohydride reduction of iron (III) chloride in aqueous solution.9,15 Bright-field transmission electron micrographs (TEMs) show that the particles are pseudospherical, have dimensions of 20-80 nm, and tend to aggregate into chains (Figure 1) as a result of their magnetic properties.16,17 Selected area electron diffraction (SAED) shows (9) Li, X. Q.; Zhang, W. X. Langmuir 2006, 22, 4638-4642. (10) Commonly referred to as “zero valent iron”; see references 6 and 7. (11) Li, X. Q.; Zhang, W. X. J. Phys. Chem. C 2007, 111, 6939-6946. (12) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596-2599. (13) Cui, X.; Li, Z.; Tao, C. Z.; Xu, Y.; Li, J.; Liu, L.; Guo, Q. X. Org. Lett. 2006, 8, 2467-2470. (14) Agard, N. J.; Prescher, J. A.; Bertozzi, C. R. J. Am. Chem. Soc. 2004, 126, 15046-15047. (15) Sun, Y. P.; Li, X. Q.; Cao, J. S.; Zhang, W. X.; Wang, H. P. AdV. Colloid Interface Sci. 2006, 120, 47-56. (16) Ngo, A. T.; Pileni, M. P. J. Phys. Chem. B 2001, 105, 53-58. (17) Korth, B. D.; Keng, P.; Shim, I.; Bowles, S. E.; Tang, C.; Kowalewski, T.; Nebesny, K. W.; Pyun, J. J. Am. Chem. Soc. 2006, 128, 6562-6563.
10.1021/la7011827 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/18/2007
9544 Langmuir, Vol. 23, No. 19, 2007
Letters Scheme 1. Application of FexOy@Fe Sequestering to a “Click” Copper-Catalyzed Cycloadditiona,b
a Reference 12. b Copper concentrations are provided in ppm of the organic product.
Scheme 2. Application of FexOy@Fe Sequestering to a Pd-Catalyzed Heck Couplinga,b
Figure 1. A bright-field TEM image of FexOy@Fe, bar ) 200 nm. Inset: SAED image showing characteristic reflections indexed to Fe3O4.15 Table 1. Sequestration of Metal Ions from Basic Aqueous Solutions Using FexOy@Fe
M
metal source
Co2+ Ni2+ Cu2+ Rux+ b,c Rhx+ b,c Pd2+ c Ag+ c Pt4+ Fe
Co(NO3)2‚xH2O Ni(NO3)2‚6H2O Cu(NO3)2‚2.5H2O RuCl3‚xH2O RhCl3‚xH2O Pd(OAc)2 AgNO3 PtCl4 blank particles
a
initial [Mx+] (ppm)d
final [Mx+] (ppb)d
% removed
[Fe]a (ppm)d
117 131 129 101 228 138 123 67.3
16.7 37.1 28.7 4.7 0.6 5.8 0.2 2.4
99.986 99.97 99.98 99.995 99.9997 99.996 99.9998 99.996
0.100 0.143 0.0894 0.869 0.110 4
-
Cl interferes with the inductively coupled plasma mass spectrometry (ICP-MS) analysis for [Fe].18 b RuCl3 and RhCl3 are known to produce multiple oxidation states in aqueous media.1 c These metals are known to form stable oxides in basic conditions.19 As a consequence, analysis was performed in neutral conditions. d Concentrations given have
