Selective Recovery and Detection of Gold with Cucurbit[n]urils (n = 5–7)

Environmentally benign methods for gold recovery and detection are highly desirable for a sustainable future. Herein, we demonstrate a selective recovery and detection strategy of gold with cucurbit­[n]­urils (Q­[n = 5–7]) by means of outer-surface interactions. Tetrachloroaurate anion ([AuCl₄]⁻) is able to be rapidly precipitated with Q­[n] in forms of supramolecular adducts. X-ray crystallography of four Q­[n = 5–7]·[AuCl₄]⁻ complexes reveal that strong outer-surface interactions between Q­[n = 5–7] and [AuCl₄]⁻ are the major driving force for the formation of Q­[n = 5–7]·[AuCl₄]⁻ complexes. Impressively, each Q[6] macrocycle is surrounded by 12 [AuCl₄]⁻ anions. Each of these 12 [AuCl₄]⁻ anions is connected to four adjacent [AuCl₄]⁻ anions, leading to the formation of a three-dimensional supramolecular framework with tubular channels. In addition, we found an interesting inclusion complex [Au­(OH₂)₄]³⁺⊂Q­[7] in the Q[7]·[AuCl₄]⁻ complex, which is the first example of hydrated metal cation encapsulated inside the cavity of Q­[n]. Spectroscopic data suggest that Q­[n = 5–7] possess a high affinity and selectivity for [AuCl₄]⁻ even in the presence of other transition-metal ions. Q­[n] modified electrodes are found to be an effective material for the detection of trace gold in dilute solutions.