Cation-Dependent Gold Recovery with α-Cyclodextrin Facilitated by Second-Sphere Coordination

Abstract

Herein, we report an alkali metal cation-dependent approach to gold recovery, facilitated by second-sphere coordination with eco-friendly α-cyclodextrin (α-CD). Upon mixing eight salts composed of Na(+), K(+), Rb(+), or Cs(+) cations and [AuX4](-) (X = Cl/Br) anions with α-, β-, or γ-CD in water, co-precipitates form selectively from the three (out of 24) aqueous solutions containing α-CD with KAuBr4, RbAuBr4, and CsAuBr4, from which the combination of α-CD and KAuBr4 affords the highest yield. Single-crystal X-ray analyses reveal that in 20 of the 24 adducts CD and [AuX4](-) anions form 2:1 sandwich-type second-sphere adducts driven partially by [C-H···X-Au] interactions between [AuX4](-) anions and the primary faces of two neighboring CDs. In the adduct formed between α-CD and KAuBr4, a [K(OH2)6](+) cation is encapsulated inside the cavity between the secondary faces of two α-CDs, leading to highly efficient precipitation owing to the formation of a cation/anion alternating ion wire residing inside a continuous α-CD nanotube. By contrast, in the other 19 adducts, the cations are coordinated by OH groups and glucopyranosyl ring O atoms in CDs. The strong coordination of Rb(+) and Cs(+) cations by these ligands, in conjunction with the stereoelectronically favorable binding of [AuBr4](-) anions with two α-CDs, facilitates the co-precipitation of the two adducts formed between α-CD with RbAuBr4 and CsAuBr4. In order to develop an efficient process for green gold recovery, the co-precipitation yield of α-CD and KAuBr4 has been optimized regarding both the temperature and the molar ratio of α-CD to KAuBr4.

Supramolecular Paradigm for Capture and Co‐Precipitation of Gold(III) Coordination Complexes

“…Common to these potential applications are supramolecular strategies that treat anionic square planar AuX 4 − complexes as guest molecules or building blocks (tectons) [14] with distinctive sizes, shapes and electrostatic surfaces. In recent years, a number of macrocyclic host molecules, such as α‐cyclodextrin, [15, 16] cucurbiturils, [17, 18, 19] and cationic cyclophanes [20] have been reported as additives that can induce co‐precipitation of specific haloaurate salts. X‐ray diffraction analysis of crystals from these co‐precipitates has revealed intricate networks of weakly attractive interactions such as CH⋅⋅⋅XAu hydrogen bonds and ion–dipole interactions, and in some cases hydrogen bonding with coordinated water molecules.…”

Structural properties of the chelating agent 2,6-bis(1-(3-hydroxypropyl)-1,2,3-triazol-4-yl)pyridine: a combined XRD and DFT structural study

“…[1][2][3][4][5] In this regard, combination of metal ions and organic ligands using crystal engineering methods and supramolecular principles is among the most promising approaches towards selective separation of metals. [6][7][8][9][10] These rely on removal of metal ions such as lanthanides and actinides from aqueous or nonaqueous solutions via the formation of metal organic complexes by addition of chelating agents (i.e. extractants).…”

Removal of Au3+ and Ag+ from aqueous media with magnetic nanoparticles functionalized with squaramide derivatives

“…Recently, it has been proposed a more green-friendly method, based on the precipitation of the salts of Au 3+ in water, as KAuBr 4 , with a-cyclodextrins. 7,8 Another smart proposal for recovery of Au + and Au 3+ consists in the formation of a Metal Organic Framework (MOF) functionalized with Lmethionine. 9 The magnetic iron oxide nanoparticles of Fe 3 O 4 (FeNP) are inorganic material, easily modiable and easily to functionalize using simple synthetic processes.…”