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

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.