A Chemical Route to Activation of Open Metal Sites in the Copper-Based Metal–Organic Framework Materials HKUST‑1 and Cu-MOF‑2

Open coordination sites (OCSs) in metal–organic frameworks (MOFs) often function as key factors in the potential applications of MOFs, such as gas separation, gas sorption, and catalysis. For these applications, the activation process to remove the solvent molecules coordinated at the OCSs is an essential step that must be performed prior to use of the MOFs. To date, the thermal method performed by applying heat and vacuum has been the only method for such activation. In this report, we demonstrate that methylene chloride (MC) itself can perform the activation role: this process can serve as an alternative “chemical route” for the activation that does not require applying heat. To the best of our knowledge, no previous study has demonstrated this function of MC, although MC has been popularly used in the pretreatment step prior to the thermal activation process. On the basis of a Raman study, we propose a plausible mechanism for the chemical activation, in which the function of MC is possibly due to its coordination with the Cu²⁺ center and subsequent spontaneous decoordination. Using HKUST-1 film, we further demonstrate that this chemical activation route is highly suitable for activating large-area MOF films

3,5-Diarylimidazo[1,2‑<i>a</i>]pyridines as Color-Tunable Fluorophores

A new protocol for the synthesis of color-tunable fluorescent 3,5-diarylimidazo­[1,2-<i>a</i>]­pyridines has been achieved via palladium-catalyzed C–H amination of pyridinium zwitterions. Based on experimental results and computational analysis, we extracted a high correlation of photophysical properties with the theoretical concept and predicted emission wavelengths of 3,5-diarylimidazo­[1,2-<i>a</i>]­pyridines. The emission wavelengths of imidazo­[1,2-<i>a</i>]­pyridines increase as a function of the electron-withdrawing nature of the substituent on the C5-aryl group of imidazo­[1,2-<i>a</i>]­pyridine as a result of inductive effects on the LUMO levels. Varying the substituent on the C3-aryl group imidazo­[1,2-<i>a</i>]­pyridine changes the HOMO levels. Combining these two sites, the HOMO and LUMO levels can be tuned fairly decoupled from each other. This conceptual trend is demonstrated across a series where the C3 and C5 positions were functionalized independently and then utilizes a combination strategy where both sites are used to prepare fluorophores with a large window of emission wavelengths. In view of the biological properties of imidazo­[1,2-<i>a</i>]­pyridines, the developed method provides an efficient approach for understanding and preparing strongly fluorescent bioprobes

A Double Open-Shelled Au₄₃ Nanocluster with Increased Catalytic Activity and Stability

Atomically precise metal nanoclusters (NCs) are an intriguing class of crystalline solids with unique physicochemical properties derived from tunable structures and compositions. Most atomically precise NCs require closed-shells and coordinatively saturated surface metals in order to be stable. Herein, we report Au43(CCtBu)20 and Au42Ag1(CCtBu)20, which feature open electronic and geometric shells, leading to both paramagnetism (23 valence e–) and enhanced catalytic activity from a single coordinatively unsaturated surface metal. The Au-alkynyl surface motifs of these NCs form five helical stripes around the inner Au12 kernel, imparting chirality and high thermal stability. Density functional theory (DFT) calculations suggest that there are minimal energy differences between the open-shelled NCs and hypothetical closed-shell systems and that the open-shelled electronic configuration gives rise to the largest band gap, which is known to promote cluster stability. Furthermore, we highlight how coordinatively unsaturated surface metals create active sites for the catalytic oxidation of benzyl alcohol to benzaldehyde, leading to high selectivity and increased conversion. This work represents the first example of an atomically precise Au NC with a double open-shelled structure and provides a promising platform for investigating the magnetic and catalytic properties of noble metal nanoparticles