Supramolecular Paradigm for Capture and Co-Precipitation of Gold(III) Coordination Complexes

Challenges and Applications of Supramolecular Metalate Chemistry

While the supramolecular chemistry of simple anions is ubiquitous, the targeting and exploitation of their metal-containing relatives, the metalates, is less well understood. This mini review highlights the latest advances in this emergent area by discussing the supramolecular chemistry of metalates thematically, with a focus on the exploitation of metalates in a diversity of applications, including medical imaging and therapy, environmental remediation, molecular magnetism, catalysis, perovskite materials, and metal separations. The unifying features of these systems are identified with a view to allow the supramolecular chemist to target the unique material properties of the metalates, even in areas that are currently relatively immature.

A nanofiber with a p-π conjugated structure designed based on the Jahn-Teller effect for the removal of cupric tartrate from wastewater

Copper organic complexes with strong chemical stability and high solubility in water are difficult to eliminate with traditional adsorbents. In this work, a novel amidoxime nanofiber (AO-Nanofiber) with the p-π conjugated structure was fabricated through homogeneous chemical grafting coupled with electrospinning and applied to capture cupric tartrate (Cu-TA) from aqueous solutions. The adsorption capacity of Cu-TA by AO-Nanofiber was 198.4 mg/g at an equilibrium time of 40 min, and the adsorption performance remains basically unchanged after 10 times adsorption-desorption cycles. The capture mechanism of Cu-TA by AO-Nanofiber was jointly validated by experiments and characterization such as Fourier Transform Infrared Spectrometer (FT-IR), X-ray Photoelectron Spectroscopy (XPS), and Density functional theory (DFT) calculations. These results demonstrated that the lone pair of electrons of the N atom from the amino groups and the O atom from hydroxyl groups in the AO-Nanofiber is partially transferred to the 3d orbital of the Cu(II) ions in Cu-TA, leading to the Jahn-Teller distortion of the Cu-TA and the more stable structure of AO-Nanofiber@Cu-TA was generated.

Efficient Recycling of Gold and Copper from Electronic Waste by Selective Precipitation

The recycling of metals from electronic waste (e-waste) using efficient, selective, and sustainable processes is integral to circular economy and net-zero aspirations. Herein, we report a new method for the selective precipitation of metals such as gold and copper that offsets the use of organic solvents that are traditionally employed in solvent extraction processes. We show that gold can be selectively precipitated from a mixture of metals in hydrochloric acid solution using triphenylphosphine oxide (TPPO), as the complex [(TPPO)4 (H5 O2 )][AuCl4 ]. By tuning the acid concentration, controlled precipitation of gold, zinc and iron can be achieved. We also show that copper can be selectively precipitated using 2,3-pyrazinedicarboxylic acid (2,3-PDCA), as the complex [Cu(2,3-PDCA-H)2 ]n  ⋅ 2n(H2 O). The combination of these two precipitation methods resulted in the recovery of 99.5 % of the Au and 98.5 % of the Cu present in the connector pins of an end-of-life computer processing unit. The selectivity of these precipitation processes, combined with their straightforward operation and the ability to recycle and reuse the precipitants, suggests potential industrial uses in the purification of gold and copper from e-waste.

High-efficiency gold recovery by additive-induced supramolecular polymerization of β-cyclodextrin

Developing an eco-friendly, efficient, and highly selective gold-recovery technology is urgently needed in order to maintain sustainable environments and improve the utilization of resources. Here we report an additive-induced gold recovery paradigm based on precisely controlling the reciprocal transformation and instantaneous assembly of the second-sphere coordinated adducts formed between β-cyclodextrin and tetrabromoaurate anions. The additives initiate a rapid assembly process by co-occupying the binding cavity of β-cyclodextrin along with the tetrabromoaurate anions, leading to the formation of supramolecular polymers that precipitate from aqueous solutions as cocrystals. The efficiency of gold recovery reaches 99.8% when dibutyl carbitol is deployed as the additive. This cocrystallization is highly selective for square-planar tetrabromoaurate anions. In a laboratory-scale gold-recovery protocol, over 94% of gold in electronic waste was recovered at gold concentrations as low as 9.3 ppm. This simple protocol constitutes a promising paradigm for the sustainable recovery of gold, featuring reduced energy consumption, low cost inputs, and the avoidance of environmental pollution.

Co-crystals of tetrahaloauric acid and 1,3,5-(methylacetamide)benzene-based tectons: consistent trapping of high energy molecular conformation

Co-crystal engineering is a promising method to create new classes of advanced materials. Co-crystal structure prediction is more challenging when one or more of the lattice constituents (tectons) are flexible...

Tuneable separation of gold by selective precipitation using a simple and recyclable diamide

The efficient separation of metals from ores and secondary sources such as electronic waste is necessary to realising circularity in metal supply. Precipitation processes are increasingly popular and are reliant on designing and understanding chemical recognition to achieve selectivity. Here we show that a simple tertiary diamide precipitates gold selectively from aqueous acidic solutions, including from aqua regia solutions of electronic waste. The X-ray crystal structure of the precipitate displays an infinite chain of diamide cations interleaved with tetrachloridoaurate. Gold is released from the precipitate on contact with water, enabling ligand recycling. The diamide is highly selective, with its addition to 29 metals in 2 M HCl resulting in 70% gold uptake and minimal removal of other metals. At 6 M HCl, complete collection of gold, iron, tin, and platinum occurs, demonstrating that adaptable selective metal precipitation is controlled by just one variable. This discovery could be exploited in metal refining and recycling processes due to its tuneable selectivity under different leaching conditions, the avoidance of organic solvents inherent to biphasic extraction, and the straightforward recycling of the precipitant.

PCage: Fluorescent Molecular Temples for Binding Sugars in Water

The development of synthetic receptors that recognize carbohydrates in water with high selectivity and specificity is challenging on account of their structural complexity and strong hydrophilicity. Here, we report on the design and synthesis of two pyrene-based, temple-shaped receptors for the recognition of a range of common sugars in water. These receptors rely on the use of two parallel pyrene panels, which serve as roofs and floors, capable of forming multiple [C-H···π] interactions with the axially oriented C-H bonds on glycopyranosyl rings in the carbohydrate-based substrates. In addition, eight polarized pyridinium C-H bonds, projecting from the roofs and floors of the temple receptors toward the binding cavities, form [C-H···O] hydrogen bonds, with the equatorially oriented OH groups on the sugars located inside the hydrophobic cavities. Four para-xylylene pillars play a crucial role in controlling the distance between the roof and floor. These temple receptors are highly selective for the binding of glucose and its derivatives. Furthermore, they show enhanced fluorescence upon binding with glucose in water, a property which is useful for glucose-sensing in aqueous solution.

Supramolecular Gold Stripping from Activated Carbon Using α-Cyclodextrin

We report the molecular recognition of the Au(CN)₂^- anion, a crucial intermediate in today's gold mining industry, by α-cyclodextrin. Three X-ray single-crystal superstructures-KAu(CN)₂⊂α-cyclodextrin, KAu(CN)₂⊂(α-cyclodextrin)₂, and KAg(CN)₂⊂(α-cyclodextrin)₂-demonstrate that the binding cavity of α-cyclodextrin is a good fit for metal-coordination complexes, such as Au(CN)₂^- and Ag(CN)₂^- with linear geometries, while the K⁺ ions fulfill the role of linking α-cyclodextrin tori together as a result of [K⁺···O] ion-dipole interactions. A 1:1 binding stoichiometry between Au(CN)₂^- and α-cyclodextrin in aqueous solution, revealed by ¹H NMR titrations, has produced binding constants in the order of 10⁴ M^-1. Isothermal calorimetry titrations indicate that this molecular recognition is driven by a favorable enthalpy change overcoming a small entropic penalty. The adduct formation of KAu(CN)₂⊂α-cyclodextrin in aqueous solution is sustained by multiple [C-H···π] and [C-H···anion] interactions in addition to hydrophobic effects. The molecular recognition has also been investigated by DFT calculations, which suggest that the 2:1 binding stoichiometry between α-cyclodextrin and Au(CN)₂^- is favored in the presence of ethanol. We have demonstrated that this molecular recognition process between α-cyclodextrin and KAu(CN)₂ can be applied to the stripping of gold from the surface of activated carbon at room temperature. Moreover, this stripping process is selective for Au(CN)₂^- in the presence of Ag(CN)₂^-, which has a lower binding affinity toward α-cyclodextrin. This molecular recognition process could, in principle, be integrated into commercial gold-mining protocols and lead to significantly reduced costs, energy consumption, and environmental impact.

Macrocyclic and acyclic supramolecular elements for co-precipitation of square-planar gold(iii) tetrahalide complexes

Non-covalent interactions control the solid-state packing of AuX₄⁻ anions (yellow circles) co-precipitated with different supramolecular elements.