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.