Thermal Defect Engineering of Precious Group Metal-Organic Frameworks: A Case Study on Ru/Rh-HKUST-1 Analogues

A methodology is introduced for controlled postsynthetic thermal defect engineering (TDE) of precious group metal-organic frameworks (PGM-MOFs). The case study is based on the Ru/Rh analogues of the archetypical structure [Cu₃(BTC)₂] (HKUST-1; BTC = 1,3,5-benzenetricarboxylate). Quantitative monitoring of the TDE process and extensive characterization of the samples employing a complementary set of analytical and spectroscopic techniques reveal that the compositionally very complex TDE-MOF materials result from the elimination and/or fragmentation of ancillary ligands and/or linkers. TDE involves the preferential secession of acetate ligands, intrinsically introduced via coordination modulation during synthesis, and the gradual decarboxylation of ligator sites of the framework linker BTC. Both processes lead to modified Ru/Rh paddlewheel nodes. These nodes exhibit a lowered average oxidation state and more accessible open metal centers, as deduced from surface-ligand IR spectroscopy using CO as a probe and supported by density functional theory (DFT)-based computations. The monometallic and the mixed-metal PGM-MOFs systematically differ in their TDE properties and, in particular in the hydride generation ability (HGA). This latter property is an important indicator for the catalytic activity of PGM-MOFs, as demonstrated by the ethylene dimerization reaction to 1-butene.

High-Efficiency Gold Recovery Using Cucurbit[6]uril

Developing an extremely efficient and highly selective process for gold recovery is urgently desired for maintaining a sustainable ecological environment. Herein, we report a highly efficient gold-recovery protocol on the basis of the instantaneous assembly between cucurbit[6]uril (CB[6]) and [AuX₄]^- (X = Cl/Br) anions. Upon mixing CB[6] with the four gold-bearing salts MAuX₄ (M = H/K, X = Cl/Br) in aqueous solutions, yellow or brown coprecipitates form immediately, as a result of multiple weak [Au-X···H-C] (X = Cl/Br) hydrogen-bonding and [Au-X···C=O] (X = Cl/Br) ion-dipole interactions. The gold-recovery efficiency, based on CB[6]·HAuCl₄ coprecipitation, reaches 99.2% under optimized conditions. In the X-ray crystal superstructures, [AuCl₄]^- anions and CB[6] molecules adopt an alternating arrangement to form doubly connected supramolecular polymers, while [AuBr₄]^- anions are accommodated in the lattice between two-dimensional layered nanostructures composed of CB[6] molecules. DFT calculations have revealed that the binding energy (34.8 kcal mol^-1) between CB[6] molecules and [AuCl₄]^- anions is higher than that (11.3-31.3 kcal mol^-1) between CB[6] molecules and [AuBr₄]^- anions, leading to improved crystallinity and higher yields of CB[6]·MAuCl₄ (M = H/K) coprecipitates. Additionally, a laboratory-scale gold-recovery protocol, aligned with an attractive strategy for the practical recovery of gold, was established based on the highly efficient coprecipitation of CB[6]·HAuCl₄. The use of CB[6] as a gold extractant provides us with a new opportunity to develop more efficient processes for gold recovery.