American Chemical Society
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Cation-Dependent Gold Recovery with α‑Cyclodextrin Facilitated by Second-Sphere Coordination

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posted on 2016-08-12, 00:00 authored by Zhichang Liu, Avik Samanta, Juying Lei, Junling Sun, Yuping Wang, J. Fraser Stoddart
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.