Formation and characterization of α·Br

Minimal variations in the building blocks employed in molecular self-assembly processes can lead to totally different superstructures and physical properties29,30,31,32,33, reflecting the subtle interplay between weak non-covalent bonding forces, particularly hydrogen bonding34,35,36,37 in the case of CDs.

Upon mixing any particular aqueous solution (20 mM, 1 ml) of KAuX 4 with any chosen aqueous solution (26.7 mM, 1.5 ml) of α-, β-, or γ-CDs at room temperature, a shiny pale brown suspension forms exclusively within a few minutes (Fig. 2 and Supplementary Movie 1) when KAuBr 4 and α-CD form the 1:2 adduct, α·Br. Centrifugal filtration and drying under vacuum of the suspension permits the isolation of the α·Br complex in bulk as a pale brown powder.

Figure 2: Formation and co-precipitation of α·Br from KAuBr 4 and α-CD. When an aqueous solution (20 mM, 1 ml) of KAuX 4 (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 KAuBr 4 and α-CD within 1–2 min (See Supplementary Movie 1). Full size image

SEM of an air-dried aqueous suspension of the α·Br complex reveals (Fig. 3a) the formation of long, needle-like crystals with extremely high aspect ratios. Examination of a suspension of these nanostructures by TEM reveals (Fig. 3b) that they have diameters of a few hundred nanometres and lengths on the order of micrometres. The nanostructures were stabilized under cryo-TEM conditions and then subjected to selected area electron diffraction (SAED). SAED patterns of the assembly of the α·Br complex show (Fig. 3c) clear and symmetrical diffraction spots, an observation which confirms the crystalline nature of the nanostructures. Although SEM and TEM can provide details of the morphology of α·Br, more detailed atomic-level structural information is required in order to understand the non-covalent bonding forces driving this highly selective molecular self-assembly process.

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. Full size image

Co-crystallisation by slow vapour diffusion of iPrOH into a dilute aqueous solution of KAuBr 4 and α-CD afforded single crystals of α·Br, which were suitable for X-ray crystallography. In the single-crystal superstructure (Fig. 4a–c) of α·Br, two α-CD tori are observed to be held together by means of intermolecular hydrogen bonding between the secondary (2°) hydroxyl faces of adjacent α-CD tori, which adopt a head-to-head packing arrangement, forming a supramolecular dimer. The dimer also serves the role of a second-sphere coordination cavitand occupied by the hexaaqua K+ ion, ([K(OH 2 ) 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). We surmise that this superstructure forms in order for the [K(OH 2 ) 6 ]+ ion to match the confines of the α-CD dimer cavity. 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. 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. The water molecules aligned along the c-axis direction of the octahedral [K(OH 2 ) 6 ]+ ion are located statistically between the two symmetrical sites with 50:50 occupancies, which are related by an ~7° tilt about the c-axis. The square-planar [AuBr 4 ]− ions, centered between the primary (1°) hydroxyl faces of α-CD (A) and the adjacent α-CD (B), are disordered over two orientations with 50:50 occupancies and related by an ~9° rotation about the c-axis. Both α-CD tori A and B of the dimers in α·Br are distorted elliptically and elongated along the [AuBr 4 ]− planes with reference to the glycosidic ring O atoms. Although it was not possible to locate the H atoms associated with the H 2 O molecules on the [K(OH 2 ) 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). All four Br atoms are close to twelve H5 and H6 atoms on the primary (1°) faces of the glucopyranosyl rings (Fig. 1), with [C−H···Br−Au] contacts (Supplementary Table S3) of 2.92−3.19 Å. The [C−H···Br−Au] hydrogen bonds13,49,50,51 favour an equal distribution of orientations of the [AuBr 4 ]− anions around the c-axis. Accordingly, the dimers are stacked along the c-axis with [AuBr 4 ]− anions acting as linkers through multiple [C−H···Br−Au] hydrogen bonds with the α-CDs in the a−b plane and two [O−H···Br−Au] hydrogen bonds with the [K(OH 2 ) 6 ]+ ions oriented in the c-axial direction. These α-CD dimers form parallel channels filled with [K(OH 2 ) 6 ]+ cations and [AuBr 4 ]− anions, which line up in an alternating fashion to generate an infinite inorganic polyionic chain. This infinite cable-like supramolecular polymer can be dissected (Fig. 4d) structurally into head-to-head hydrogen-bonded α-CD dimers oriented tail-to-tail with respect to each other, forming an outer sheath-like organic nanotube with a coaxial, inorganic polyionic, inner chain, core. Bundles of these nanostructures are then tightly packed through hydrogen bonding between columns to form a well-ordered array that constitutes the single crystal.

Figure 4: Single-crystal X-ray structure of α·Br. The structure has the formula {[K(OH 2 ) 6 ][AuBr 4 ] (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(OH 2 ) 6 ]+ ion with the [AuBr 4 ]− ion. (c) Top view of the arrangement of the [AuBr 4 ]− 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(OH 2 ) 6 ]+ and [AuBr 4 ]− ions. Hydrogen atoms areomitted for clarity. C, black; O, red; Br, brown; Au, yellow; K, purple. Hydrogen bonds are depicted as purple dash lines. Full size image

In order to confirm that the nanostructure of α·Br, obtained as a co-precipitate by solution-phase synthesis, is in agreement with the superstructure present in the single crystal of α·Br, a centrifugally filtrated sample of the as-synthesized suspension of the supramolecular complex was analysed (Supplementary Fig. S9a) by powder XRD (PXRD). The experimental PXRD pattern matches very well with the simulated pattern based on the single-crystal X-ray data, suggesting that the superstructures present in the single crystal of α·Br and the co-precipitated nanostructure are one and the same. In other words, the solution-phase synthesized one-dimensional nanostructures of α·Br are composed of single-crystalline bundles of one-dimensional molecular-level, cable-like, complexes (Fig. 4d) with high aspect ratios.

As solution-phase synthesis affords much smaller crystals, single-molecule imaging studies using AFM can provide dimensional information of the sample, such as its height, with subnanometre precision. In order to investigate the physical dimensions of the self-assembled nanostructure formed between KAuBr 4 and α-CD on surfaces, a sample for AFM measurement was grown directly on the substrate. A droplet of a very dilute aqueous solution of KAuBr 4 (0.5 mM) and α-CD (1 mM) was spin-coated on freshly cleaved mica and dried under ambient conditions. The AFM image reveals (Fig. 5a) that the individual nanoassemblies have lengths on the order of several hundred nanometres and an average height (Fig. 5b) of 1.3±0.2 nm, which is consistent with the external diameter (~1.4 nm) of α-CD (Fig. 5c). These experiments provide insight into the mechanism of the molecular self-assembly process whereby these single-molecule-wide nanostructures are intermediates in the formation of the larger crystals observed by SEM and TEM.

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. Full size image

Insight into the spontaneous co-precipitation of α·Br

It would appear that the spontaneous co-precipitation of the one-dimensional nanostructure of α·Br is highly selective as no similar phenomenon was observed from the other five combinations between KAuX 4 salts (X=Cl or Br) with α-, β- and γ-CDs. In order to gain insight into the mechanism behind the formation of the nanostructure of α·Br and the reason for its rapid co-precipitation, single crystals (Supplementary Table S1 and Supplementary Figs S2−S7) of a series of inclusion complexes KAuCl 4 ·(α-CD) 2 (α·Cl), KAuBr 4 ·(β-CD) 2 (β·Br), KAuCl 4 ·(β-CD) 2 (β·Cl), KAuBr 4 ·(γ-CD) 3 (γ·Br), and KAuCl 4 ·(γ-CD) 3 (γ·Cl) were grown employing similar slow vapour diffusion methods and subjected to single-crystal XRD analysis. In contrast to α·Br, which adopts the orthorhombic space group P2 1 2 1 2 (Supplementary Fig. S1), the crystal structure (Supplementary Fig. S2) of α·Cl is in the monoclinic (β=90.041(4)°) space group P2 1 . Both α-CD tori of the dimers in α·Br and α·Cl are elliptically distorted and elongated along the [AuX 4 ]− planes with respect to the glycosidic ring O atoms (Supplementary Fig. S3). A significant difference in the role exhibited by the K+ ion was observed between α·Br and α·Cl. In α·Cl, the K+ ion is located outside the dimer cavity and enters into first-sphere coordination with seven primary hydroxyl groups belonging to two adjacent CD dimers. This observation suggests that the role of the K+ ion is to act as a linker between adjacent CD dimers along the b-axis direction in α·Cl instead of acting as an isolated [K(OH 2 ) 6 ]+ guest inside the CD dimer cavity in α·Br. The bridging of the K+ ions in α·Cl along the b-axis results in the formation of one-dimensional coordination polymer chains composed of alternating α-CD dimers and K+ ions, which then stack alternatively with [AuCl 4 ]− along the a-axis to constitute an extended two-dimensional superstructure (Supplementary Fig. S8a). In order to compare the orientation of the square-planar anions [AuBr 4 ]− and [AuCl 4 ]− in the CD channel, we define (i) the rotation angle of the [AuX 4 ]− anion viewed from the front (Fig. 6) as φ, and (ii) the inclination angle of the [AuX 4 ]− anion viewed from the side with respect to the central axis of the CD tori (Fig. 6) as θ. The [AuBr 4 ]− anion in α·Br has an orientation with φ=9.2° and θ=2.6°, whereas the [AuCl 4 ]− anion in α·Cl has a more tilted orientation with φ=17.1° and θ=2.6°. All the differences in superstructure between α·Br and α·Cl, as well as the unique spontaneous co-precipitation of α·Br, can be ascribed to the subtle size differences between the [AuBr 4 ]− and [AuCl 4 ]− anions. It is crucial to note that the average Au−Br bond length of 2.42 Å in α·Br is only 0.15 Å longer than the average Au−Cl bond length of 2.27 Å in α·Cl (Supplementary Table S2). This observation highlights the fact that the longer bond length in [AuBr 4 ]− facilitates the second-sphere coordination of α-CD to [K(OH 2 ) 6 ]+ and [AuBr 4 ]−, giving rise to the formation of α·Br and its unique superstructure. In addition, this second-sphere coordination results in the encapsulation of [K(OH 2 ) 6 ]+ cations inside the cavities of the α-CD dimers, which we hypothesize restricts solvation of [K(OH 2 ) 6 ]+ cations by water molecules from the bulk—the reason for the observed rapid co-precipitation.

Figure 6: Single-crystal superstructures of α·Br, α·Cl, β·Br, β·Cl, γ·Br and γ·Cl. The rotation angle of the [AuX 4 ]− anion viewed from the front is defined as φ, and the inclination angle of the [AuX 4 ]− 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. Full size image

The β-CD complexes β·Br and β·Cl, as well as the γ-CD complexes γ·Br and γ·Cl, are all isomorphous, an observation which indicates that the subtle differences between [AuBr 4 ]− and [AuCl 4 ]− no longer result in significant changes in superstructure. The K+ ions in β·Br and β·Cl have similar bridging roles as they do in α·Cl, while the K+ ions in γ·Br and γ·Cl reside outside the CD channel and are disordered. The β-CD tori in β·Br and β·Cl form head-to-head dimers similar to those in α·Cl, whereas the γ-CDs in γ·Br and γ·Cl form head-to-tail/head-to-head trimeric repeating units. Moreover, β-CD dimers in β·Br and β·Cl form zig-zag two-dimensional layered superstructures (Supplementary Fig. S8b and c). One common feature which describes all six complexes are that the CD rings stack along the longitudinal axes with [AuX 4 ]− anions acting as bridges, which are located inside the CD channels and are supported between the primary faces of the adjacent CD rings by [C−H···X−Au] hydrogen bonds, forming one-dimensional superstructures. With the expansion in size ongoing from α- to γ-CD tori, the angle φ increases from 9.2 to 45° for [AuBr 4 ]− and from 17.1 to 45° for [AuCl 4 ]−, while the angle θ increases from 2.6 to 90° for [AuBr 4 ]− and [AuCl 4 ]− (Fig. 6), respectively. We hypothesize that this trend of [AuX 4 ]− to lie flatter is to shorten the length of the [C−H···X−Au] hydrogen bonds as much as possible and so facilitates the formation of the most stable host–guest superstructures. Except for α·Br in which the CD channels are filled up by [K(OH 2 ) 6 ]+ and [AuBr 4 ]−, the other five complexes demonstrate (Fig. 6) ‘bamboo’-like superstructures with isolated empty capsules formed by CD channels segmented by [AuX 4 ]−, and the K+ cations are not encapsulated by the CD cavities. We suspect that the superstructures for these fives complexes result in the exposure of the K+ cations to water molecules from the bulk, keeping these complexes solvated, and hence restricting precipitation.

Stability and porosity of all complexes

In order to assess the stability of all six complexes after activation using supercritical CO 2 (refs 52, 53) (see Methods), we first of all examined their thermal stabilities using thermogravimetric analysis (TGA). TGA traces (Supplementary Fig. S10) for all complexes start to show significant weight loss at 150 °C, suggesting that the samples are fully evacuated, while thermal decomposition occurs at temperatures over 150 °C.

In order to examine sample structural stability upon activation, PXRD analyses (Supplementary Fig. S9) were carried out on as-synthesized and activated samples. The well-matched PXRD patterns of as-synthesized and activated samples of α·Br confirm that its superstructure remains intact upon activation despite the lack of first-sphere coordination of K+ ions between CD dimers. Comparison of the PXRD patterns of as-synthesized and activated samples of α·Cl, β·Br and β·Cl confirms the stability and crystallinity of their superstructures, which are formed by hydrogen bonding and K+ ion coordination, upon activation. In contrast to these three samples, the activated samples of γ·Br and γ·Cl were found to be amorphous by PXRD, an observation which indicates that the fully hydrogen-bonded structures of γ·Br and γ·Cl are unstable and collapse after activation in the absence of K+ ion coordination.

The surface areas and porosities of all complexes were examined by CO 2 adsorption at 273 K (Supplementary Figs S11 and S12). The isotherm of α·Br reveals a poor uptake and a capacity of only 3.5 cm3 g−1 at 1 bar, corresponding to a Bruhauer-Emmett-Teller (BET) surface area of 35 m2 g−1. This observation stands in sharp contrast to α·Cl, which shows a modest uptake ability of 23 cm3 g−1 at 1 bar with a BET surface area of 210 m2 g−1. This not insignificant difference can be attributed to their structural dissimilarities, in that the isolated capsules formed by segmentation of α-CD channels with [AuX 4 ]− are empty in α·Cl, whereas they are filled up by [K(OH 2 ) 6 ]+ in α·Br (Fig. 6). β·Br and β·Cl have empty capsules, similar to those in α·Cl, and so also show modest uptakes and comparable BET surface areas (224 m2 g−1 for β·Br and 138 m2 g−1 for β·Cl). 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. S9). These results are consistent with our hypothesis that co-precipitation of α·Br is a result of the α-CD dimer cavities being filled—that is, the [K(OH 2 ) 6 ]+ cations are encapsulated and restricted from solvation.

Selective isolation of gold from gold-bearing raw materials

The selective co-precipitation of α·Br between α-CD and KAuBr 4 has prompted us to investigate if the high selectivity of α-CD rings towards trapping [AuBr 4 ]− anions is also effective in the presence of other square-planar noble metal complexes, for example, [PtX 4 ]2− and [PdX 4 ]2− (X=Cl, Br). In an attempt to obtain an estimate of the separation efficiency, α-CD (0.2 mmol × 2) was added separately to (i) a solution (3 ml) containing KAuBr 4 (33 mM), K 2 PtBr 4 (26 mM, saturated) and K 2 PdBr 4 (33 mM) (Mixture 1), and also to (ii) another solution (3 ml) of KAuBr 4 (33 mM), K 2 PtCl 4 (24 mM, saturated) and K 2 PdCl 4 (33 mM) (Mixture 2), respectively. Pale brown precipitates formed in both solutions immediately after the addition. Both precipitates were filtered, and the filtrates were diluted and subjected to inductively coupled plasma optical emission spectroscopy (ICP-OES) elemental analysis to determine the residual amounts of Au, Pt and Pd remaining in the mother liquor. The separation percentages for Au, Pt and Pd in both mixtures are defined by comparing contents of Au, Pt and Pd in the two mixtures before and after addition of α-CD. The ICP-OES analysis results show (Fig. 7) that 78.3% of Au in Mixture 1 and 77.8% of Au in Mixture 2 were separated out of solution, whereas <3% of Pt and Pd was removed from both mixtures, values which are within the error limit of the experiment. These results reveal that the capture of [AuBr 4 ]− ions by α-CD to form α·Br in Mixture 1 and Mixture 2 is a highly selective process even in the presence of other noble metals and augurs well for developing a low cost and environmentally benign procedure for the separation of gold from complex mixtures of similar metal salts.

Figure 7: Selective precipitation and separation of gold. The separation percentage, from mixtures 1 (red) and 2 (blue), is defined as (C b −C a )/C b , where C b and C a are the concentrations of each metal before and after addition of α-CD, respectively. Full size image

In order to explore the practical potential of the selective co-precipitation of α·Br for gold recovery, we have employed gold-bearing scrap as the raw material to develop a laboratory scale gold recovery process. Taking into account that the pH of the [AuBr 4 ]−-bearing solution may vary in practice, we first of all studied the effect of pH on the co-precipitation of α-CD with [AuBr 4 ]−. Co-precipitation experiments conducted by adding α-CD into [AuBr 4 ]− solutions (50 mM) of various pH (1.4−5.9) indicate (see Methods and Supplementary Fig. S13) a trend in the residual concentration of [AuBr 4 ]− in the filtrate after filtration to remove the co-precipitates. When the pH is increased from 1.4 to 2.5, the residual concentration of [AuBr 4 ]− decreases to ~6.8 mM—which is consistent with the bulk solubility of α·Br in water—and remains constant at this value until pH 5.9 is attained. This result reveals that the co-precipitation of α-CD with [AuBr 4 ]− is dependent on pH, and that the pH range 2.5–5.9 is the suitable one to initiate this co-precipitation process. We have managed (see Fig. 8 and Methods) to convert two scrap gold-bearing alloys containing 58% wt of Au and 42% wt of other metals (Zn, Cu and Ag) into HAuBr 4 by dissolving them with a mixture of concentrated HBr and HNO 3 as the etchant solution. On the basis of this pH experiment, KOH was used to neutralize both dissolved gold solutions to pH 4–6, and consequently convert HAuBr 4 to KAuBr 4 . When α-CD was added to both solutions, the co-precipitation of α·Br occurred immediately, even in the presence of the significant amounts of Zn and Cu salts. The co-precipitated α·Br complex—namely, recovered gold—was separated from impurities by filtration and then reduced with a reductant, such as Na 2 S 2 O 5 , to afford the recovered gold metal. The residual gold in the filtrate can be recycled while α-CD can be reused by recrystallisation from the filtrate. This laboratory scale process is highly selective for gold, as well as being economic, fast and feasible. As it doesn’t involve the use of toxic inorganic cyanides, this process is much more environmentally benign in comparison with the universally accepted cyanidation process2. Further optimization of process conditions and parameters is ongoing in our laboratories.