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

Abstract

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.

Extraction of Silver by Glucose

“…Abstract: Unprecedented silver ion leaching, in the range of 0.7 ppm was seen when metallic silver was heated in water at 70 8 8Ci np resence of simple carbohydrates,s uch as glucose, making it ag reen method of silver extraction. Extraction was facilitated by the presence of anions,s uch as carbonate and phosphate.S tudies confirm at wo-step mechanism of silver release,f irst forming silver ions at the metal surface and later complexation of ionic silver with glucose;such complexes have been detected by mass spectrometry.E xtraction leads to microscopic roughening of the surface making it Raman active with an enhancement factor of 510 8 .…”

“…Liu et al have shown that gold ions can complex effectively with cyclodextrin via second sphere co-ordination. [8] They have used different gold salts for this experiment and showed that KAuBr 4 is the most efficient candidate for such complexation. Specificity of extraction of gold by complexation with a-CD was demonstrated by extracting metal ions from an AuAg alloy using amixture of conc.…”

Extraction of Silver by Glucose

“…Similar complex was predicted by Boutreau et al with as eries of monosaccharides,s uch as glucose,s ucrose, fructose,through computational studies. [8] They have used different gold salts for this experiment and showed that KAuBr 4 is the most efficient candidate for such complexation. Among three monosaccharides;g lucose,f ructose,a nd mannose,g lucose was found to be most effective in extracting silver.…”

Syntheses, structures and properties of four second-sphere coordination complexes via metal halide anion and naphthalene-based ligand

“…In general, the construction of second-sphere coordination refers to any intermolecular interactions with the ligands directly bound to the primary coordination sphere of a metal ion. Based on this strategy, some urea(amido)-containing ligands have been applied as extractants for highly selective extraction of [PtCl 6 ] 2− , [ZnCl 4 ] 2− and [CoCl 4 ] 2− , and more recently using α-cyclodextrin for gold recovery [11,12]. Second-sphere coordination was also exploited in anion recognition and binding [13].…”