Herein, we report the unprecedented and rapid formation of a well-defined one-dimensional single-crystalline material, which features a high-aspect-ratio and coaxial core-shell superstructure. It relies on the self-assembly of KAuBr4 and α-CD in aqueous solution to form (Fig. 1), as a co-precipitate, a 1:2 complex, KAuBr4·(α-CD)2 (α·Br), with an extended {[K(OH2)6][AuBr4] (α-CD)2}n chain superstructure. The formation of this hybrid material is confirmed by scanning (SEM), transmission (TEM) electron and atomic force (AFM) microscopies, as well as by electron diffraction, while the superstructure has been characterized by single-crystal and powder X-ray diffraction (XRD). The rapid co-precipitation of the α·Br complex between KAuBr4 and α-CD is highly specific: it does not materialize, for example, even if KAuCl4 is employed as an alternative gold salt, or if β- or γ-CD is substituted for α-CD. We have also discovered that the co-precipitation of α·Br is selective for gold, even in the presence of other square-planar noble metal complexes, such as those involving palladium and platinum. A laboratory scale gold recovery process has also been developed, based on the selective co-precipitation of α·Br, by employing gold-bearing scraps as raw materials. From a detailed analysis of the single-crystal X-ray superstructures of an extensive range of inclusion complexes between α-, β- or γ-CDs with KAuX4 (X=Cl, Br), we hypothesize that the perfect molecular recognition between the α-CD ring and the square-planar [AuBr4]− anion, which leads to an axial orientation of the anion with respect to the channel of α-CD rings, facilitates a highly specific second-sphere coordination involving, not only the [AuBr4]− anion, but also the [K(OH2)6]+ cation, driving the formation and rapid selective co-precipitation of the α·Br complex. 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.
