1 Nature Communications 2013 Vol: 4():1855-. DOI: 10.1038/ncomms2891

Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin

Gold recovery using environmentally benign chemistry is imperative from an environmental perspective. Here we report the spontaneous assembly of a one-dimensional supramolecular complex with an extended {[K(OH2)6][AuBr4] (α-cyclodextrin)2}n chain superstructure formed during the rapid co-precipitation of α-cyclodextrin and KAuBr4 in water. This phase change is selective for this gold salt, even in the presence of other square-planar palladium and platinum complexes. From single-crystal X-ray analyses of six inclusion complexes between α-, β- and γ-cyclodextrins with KAuBr4 and KAuCl4, we hypothesize that a perfect match in molecular recognition between α-cyclodextrin and [AuBr4]− leads to a near-axial orientation of the ion with respect to the α-cyclodextrin channel, which facilitates a highly specific second-sphere coordination involving [AuBr4]− and [K(OH2)6]+ and drives the co-precipitation of the 1:2 adduct. This discovery heralds a green host–guest procedure for gold recovery from gold-bearing raw materials making use of α-cyclodextrin—an inexpensive and environmentally benign carbohydrate.

Mentions
Figures
Figure 1: Schematic representation of the spontaneous self-assembly of α·Br.Upon mixing KAuBr4 and α-CD in water, a hydrogen-bonded linear superstructure forms spontaneously in <1 min. The cavities of the α-CDs oriented head-to-head, tail-to-tail form a continuous channel, which is filled by an alternating [K(OH2)6]+ and [AuBr4]− polyionic chain to generate a cable-like superstructure that then tightly packs one with another (Supplementary Fig. S1) leading to crystals observable to the naked eye. Figure 2: Formation and co-precipitation of α·Br from KAuBr4 and α-CD.When an aqueous solution (20 mM, 1 ml) of KAuX4 (X=Cl or Br) is added to an aqueous solution (26.7 mM, 1.5 ml) of α-, β-, or γ-CD, a shiny pale brown suspension forms exclusively from the combination of KAuBr4 and α-CD within 1–2 min (See Supplementary Movie 1). Figure 3: Morphology of the nanostructures of α·Br.(a) SEM images of a crystalline sample prepared by spin-coating an aqueous suspension of α·Br onto a silicon substrate, and then air-drying the suspension. (b) TEM images of α·Br prepared by drop-casting an aqueous suspension of α·Br onto a specimen grid covered with a thin carbon support film and air-dried. (c) Cryo-TEM image (left) and SAED pattern (right) of the nanostructures of α·Br. As the selected area includes several crystals with different orientations and the crystals are so small that the diffraction intensities are relatively weak, we can assign the diffraction rings composed of diffraction dots but not the specific angles between different diffraction dots from the same crystal. The scale bars in a and b are 25 (left), 5 (right), 10 (left), 5 μm (right) and in c are 1 μm (left) and 1 nm−1 (right), respectively. Figure 4: Single-crystal X-ray structure of α·Br.The structure has the formula {[K(OH2)6][AuBr4](α-CD)2}n. (a) Side-on view showing the orientation of the primary and secondary faces of the α-CD rings in the extended structure. (b) Side-on view illustrating the second-sphere coordination of the [K(OH2)6]+ ion with the [AuBr4]− ion. (c) Top view of the arrangement of the [AuBr4]− ion inside the cavity of α-CD. (d) Schematic illustration of the one-dimensional nanostructure extending along the c-axis in which the α-CD tori form a continuous channel occupied by alternating [K(OH2)6]+ and [AuBr4]− ions. Hydrogen atoms areomitted for clarity. C, black; O, red; Br, brown; Au, yellow; K, purple. Hydrogen bonds are depicted as purple dash lines. Figure 5: AFM analysis of α·Br on a mica surface.(a) AFM image of a spin-coated sample of α·Br on a freshly cleaved mica surface. (b) The cross-sectional analysis of (a). (c) Dimensions of the cross-section of the one-dimensional α-CD channel in α·Br. Scale bar, 100 nm. Figure 6: Single-crystal superstructures of α·Br, α·Cl, β·Br, β·Cl, γ·Br and γ·Cl.The rotation angle of the [AuX4]− anion viewed from the front is defined as φ, and the inclination angle of the [AuX4]− anion viewed from the side with respect to the central axis of the CD tori is defined as θ. C, black; O, red; Br, brown; Cl, green; Au, yellow; K, purple. Figure 7: Selective precipitation and separation of gold.The separation percentage, from mixtures 1 (red) and 2 (blue), is defined as (Cb−Ca)/Cb, where Cb and Ca are the concentrations of each metal before and after addition of α-CD, respectively. Figure 8: Gold recovery process flow diagram.Red arrows indicate the flow direction of the gold recovery. Na2S2O5: Sodium metabisulfite.
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References
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    • . . . This bulk process, which is reminiscent25 of the second-sphere coordination of transition-metal ammines with [18]crown-6, wherein [Cu(NH3)4(H2O)][PF6]2 can be separated28 as a crystalline co-precipitate from [Co(NH3)6][PF6]3 in aqueous solution, represents a promising strategy that relies on second-sphere coordination, providing a very attractive host–guest procedure for gold recovery in the form of KAuBr4, starting from gold-bearing raw materials and making use of α-CD, an inexpensive and environmentally benign carbohydrate, as the host. . . .
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    • . . . The dimer also serves the role of a second-sphere coordination cavitand occupied by the hexaaqua K+ ion, ([K(OH2)6]+) which adopts an equatorially distorted octahedral geometry with very short K–O distances38, 39 ranging from 2.37(1) to 2.44(1) Å (average 2.39 Å) . . .
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    • . . . It has been shown8, 14, 16, 17, 18, 38, 39 previously that a few metal complexes, such as [12]crown-4·KCl (ref. 39) and metallocenium salts16, 17, 18, can form second-sphere coordination adducts25, 26, 27 with CDs . . .
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    • . . . Although, generally speaking, naked K+ ions are incorporated interstitially between CD columns and are directly first-sphere coordinated with the hydroxyl groups of the CDs9, 40, 41, 42, 43, 44, 45, 46, 47, examples of hydrophilic fully hydrated K+ ions encapsulated in the hydrophobic CD channels by means of second-sphere coordination are rare . . .
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    • . . . Although, generally speaking, naked K+ ions are incorporated interstitially between CD columns and are directly first-sphere coordinated with the hydroxyl groups of the CDs9, 40, 41, 42, 43, 44, 45, 46, 47, examples of hydrophilic fully hydrated K+ ions encapsulated in the hydrophobic CD channels by means of second-sphere coordination are rare . . .
  42. Fu, Y.; Liu, L.; Guo, Q. -X. A theoretical study on the inclusion complexation of cyclodextrins with inorganic cations and anions J. Incl. Phenom. Macrocycl. Chem. 43, 223-229 (2002) .
    • . . . Although, generally speaking, naked K+ ions are incorporated interstitially between CD columns and are directly first-sphere coordinated with the hydroxyl groups of the CDs9, 40, 41, 42, 43, 44, 45, 46, 47, examples of hydrophilic fully hydrated K+ ions encapsulated in the hydrophobic CD channels by means of second-sphere coordination are rare . . .
  43. Smaldone, R. A. Metal–organic frameworks from edible natural products Angew. Chem. Int. Ed. 49, 8630-8634 (2010) .
    • . . . Although, generally speaking, naked K+ ions are incorporated interstitially between CD columns and are directly first-sphere coordinated with the hydroxyl groups of the CDs9, 40, 41, 42, 43, 44, 45, 46, 47, examples of hydrophilic fully hydrated K+ ions encapsulated in the hydrophobic CD channels by means of second-sphere coordination are rare . . .
  44. Klüfers, P.; Piotrowski, H.; Uhlendorf, J. Homoleptic cuprates(II) with multiply deprotonated α-cyclodextrin ligands Chem. Eur. J. 3, 601-608 (1997) .
    • . . . Although, generally speaking, naked K+ ions are incorporated interstitially between CD columns and are directly first-sphere coordinated with the hydroxyl groups of the CDs9, 40, 41, 42, 43, 44, 45, 46, 47, examples of hydrophilic fully hydrated K+ ions encapsulated in the hydrophobic CD channels by means of second-sphere coordination are rare . . .
  45. Klüfers, P.; Schuhmacher, J. Sixteenfold deprotonated γ-cyclodextrin tori as anions in a hexadecanuclear lead(II) alkoxide Angew. Chem. Int. Ed. Engl. 33, 1863-1865 (1994) .
    • . . . Although, generally speaking, naked K+ ions are incorporated interstitially between CD columns and are directly first-sphere coordinated with the hydroxyl groups of the CDs9, 40, 41, 42, 43, 44, 45, 46, 47, examples of hydrophilic fully hydrated K+ ions encapsulated in the hydrophobic CD channels by means of second-sphere coordination are rare . . .
  46. Norkus, E. Metal ion complexes with native cyclodextrins. An overview J. Incl. Phenom. Macrocycl. Chem. 65, 237-248 (2009) .
    • . . . Although, generally speaking, naked K+ ions are incorporated interstitially between CD columns and are directly first-sphere coordinated with the hydroxyl groups of the CDs9, 40, 41, 42, 43, 44, 45, 46, 47, examples of hydrophilic fully hydrated K+ ions encapsulated in the hydrophobic CD channels by means of second-sphere coordination are rare . . .
  47. Geißelmann, A.; Klüfers, P.; Kropfgans, C.; Mayer, P.; Piotrowski, H. Carbohydrate–metal interactions shaped by supramolecular assembling Angew. Chem. Int. Ed. 44, 924-927 (2005) .
    • . . . Although, generally speaking, naked K+ ions are incorporated interstitially between CD columns and are directly first-sphere coordinated with the hydroxyl groups of the CDs9, 40, 41, 42, 43, 44, 45, 46, 47, examples of hydrophilic fully hydrated K+ ions encapsulated in the hydrophobic CD channels by means of second-sphere coordination are rare . . .
  48. Steiner, T. Hydrogen-bond distances to halide ions in organic and organometallic crystal structures: up-to-date database study Acta Crystallogr. B 54, 456-463 (1998) .
    • . . . Although it was not possible to locate the H atoms associated with the H2O molecules on the [K(OH2)6]+ ion, the distances between the c-axial Br and O atoms, which are 3.35(1) and 3.39(1) Å, are comparable with the mean value of 3.339(7) Å reported by Steiner48, an observation which suggests the presence of the significant c-axial [O−H···Br−Au] hydrogen bonding interactions (Supplementary Table S4) . . .
  49. Taylor, R.; Kennard, O. Crystallographic evidence for the existence of C−H···O, C−H···N and C−H···Cl hydrogen bonds J. Am. Chem. Soc. 104, 5063-5070 (1982) .
    • . . . The [C−H···Br−Au] hydrogen bonds13, 49, 50, 51 favour an equal distribution of orientations of the [AuBr4]− anions around the c-axis . . .
  50. Aakeröy, C. B.; Evans, T. A.; Seddon, K. R.; Pálinkó, I. The C−H···Cl hydrogen bond: does it exist? New J. Chem. 23, 145-152 (1999) .
    • . . . The [C−H···Br−Au] hydrogen bonds13, 49, 50, 51 favour an equal distribution of orientations of the [AuBr4]− anions around the c-axis . . .
  51. Aullon, G.; Bellamy, D.; Orpen, A. G.; Brammer, L.; Eric, A. B. Metal-bound chlorine often accepts hydrogen bonds Chem. Commun. , 653-654 (1998) .
    • . . . The [C−H···Br−Au] hydrogen bonds13, 49, 50, 51 favour an equal distribution of orientations of the [AuBr4]− anions around the c-axis . . .
  52. Nelson, A. P.; Farha, O. K.; Mulfort, K. L.; Hupp, J. T. Supercritical processing as a route to high internal surface areas and permanent microporosity in metal−organic framework materials J. Am. Chem. Soc. 131, 458-460 (2009) .
    • . . . In order to assess the stability of all six complexes after activation using supercritical CO2 (refs 52, 53) (see Methods), we first of all examined their thermal stabilities using thermogravimetric analysis (TGA) . . .
  53. Farha, O. K.; Hupp, J. T. Rational design, synthesis, purification, and activation of metal-organic framework materials Acc. Chem. Res. 43, 1166-1175 (2010) .
    • . . . In order to assess the stability of all six complexes after activation using supercritical CO2 (refs 52, 53) (see Methods), we first of all examined their thermal stabilities using thermogravimetric analysis (TGA) . . .
  54. Gassensmith, J. J. Strong and reversible binding of carbon dioxide in a green metal-organic framework J. Am. Chem. Soc. 133, 15312-15315 (2011) .
    • . . . Both γ·Br and γ·Cl exhibit very low uptakes and BET surface areas, as the activation turned them into amorphous powders54, as indicated by PXRD (Supplementary Fig . . .
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