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

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] subset of (alpha-cyclodextrin)(2)}(n) chain superstructure formed during the rapid co-precipitation of alpha-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 alpha-, beta- and gamma-cyclodextrins with KAuBr4 and KAuCl4, we hypothesize that a perfect match in molecular recognition between alpha-cyclodextrin and [AuBr4](-) leads to a near-axial orientation of the ion with respect to the alpha-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 alpha-cyclodextrin-an inexpensive and environmentally benign carbohydrate.