Structure and Reactivity of Single-Site Vanadium Catalysts Supported on Metal–Organic Frameworks

Metalation of metal-organic frameworks: fundamentals and applications

Metalation of metal-organic frameworks (MOFs) has been developed as a prominent strategy for materials functionalization for pore chemistry modulation and property optimization. By introducing exotic metal ions/complexes/nanoparticles onto/into the parent framework, many metallized MOFs have exhibited significantly improved performance in a wide range of applications. In this review, we focus on the research progress in the metalation of metal-organic frameworks during the last five years, spanning the design principles, synthetic strategies, and potential applications. Based on the crystal engineering principles, a minor change in the MOF composition through metalation would lead to leveraged variation of properties. This review starts from the general strategies established for the incorporation of metal species within MOFs, followed by the design principles to graft the desired functionality while maintaining the porosity of frameworks. Facile metalation has contributed a great number of bespoke materials with excellent performance, and we summarize their applications in gas adsorption and separation, heterogeneous catalysis, detection and sensing, and energy storage and conversion. The underlying mechanisms are also investigated by state-of-the-art techniques and analyzed for gaining insight into the structure-property relationships, which would in turn facilitate the further development of design principles. Finally, the current challenges and opportunities in MOF metalation have been discussed, and the promising future directions for customizing the next-generation advanced materials have been outlined as well.

Precisely Engineering Asymmetric Atomic CoN4 by Electron Donating and Extracting for Oxygen Reduction Reaction

The development of nonpyrolytic catalysts featuring precisely defined active sites represents an effective strategy for investigating the fundamental relationship between the catalytic activity of oxygen reduction reaction (ORR) catalysts and their local coordination environments. In this study, we have synthesized a series of model electrocatalysts with well-defined CoN4 centers and nonplanar symmetric coordination structures. These catalysts were prepared by a sequential process involving the chelation of cobalt salts and 1,10-phenanthroline-based ligands with various substituent groups (phen(X), where X=OH, CH3, H, Br, Cl) onto covalent triazine frameworks (CTFs). By modulating the electron-donating or electron-withdrawing properties of the substituent groups on the phen-based ligands, the electron density surrounding the CoN4 centers was effectively controlled. Our results demonstrated a direct correlation between the catalytic activity of the CoN4 centers and the electron-donating ability of the substituent group on the phenanthroline ligands. Notably, the catalyst denoted as BCTF-Co-phen(OH), featuring the electron-donating OH group, exhibited the highest ORR catalytic activity. This custom-crafted catalyst achieved a remarkable half-wave potential of up to 0.80 V vs. RHE and an impressive turnover frequency (TOF) value of 47.4×10-3 Hz at 0.80 V vs. RHE in an alkaline environment.

Anionic oxyl radical formed on CrVI-oxo anchored on the defect site of the UiO-66 node facilitates methane to methanol conversion

The direct conversion of methane to methanol has attracted increasing interest due to abundant and low-cost natural gas resources. Herein, by anchoring Cr-oxo/-oxyhydroxides on UiO-66 metal-organic frameworks, we demonstrate that reactive anionic oxyl radicals can be formed by controlling the coordination environment based on the results of density functional theory calculations. The anionic oxyl radicals produced at the completely oxidized CrVI site acted as the active species for facile methane activation. The thermodynamically stable CrVI-oxo/-oxyhydroxides with the anionic oxyl radicals catalyze the activation of the methane C-H bond through a homolytic mechanism. An analysis of the results showed that the catalytic performance of the active oxyl species correlates with the reaction energy of methane activation and H adsorption energies. Following methanol formation, N2O can regenerate the active sites on the most stable CrVI oxyhydroxides, i.e., the Cr(O)4Hf species. The present study demonstrated that the anionic oxyl radicals formed on the anchored CrVI oxyhydroxides by tuning the coordination environment enabled facile methane activation and facilitated methanol production.

Mechanistic insights into radical formation and functionalization in copper/N-fluorobenzenesulfonimide radical-relay reactions

Copper-catalysed radical-relay reactions that employ N-fluorobenzenesulfonimide (NFSI) as the oxidant have emerged as highly effective methods for C(sp3)-H functionalization. Herein, computational studies are paired with experimental data to investigate a series of key mechanistic features of these reactions, with a focus on issues related to site-selectivity, enantioselectivity, and C-H substrate scope. (1) The full reaction energetics of enantioselective benzylic C-H cyanation are probed, and an adduct between Cu and the N-sulfonimidyl radical (˙NSI) is implicated as the species that promotes hydrogen-atom transfer (HAT) from the C-H substrate. (2) Benzylic versus 3° C-H site-selectivity is compared with different HAT reagents: Cu/˙NSI, ˙OtBu, and Cl˙, and the data provide insights into the high selectivity for benzylic C-H bonds in Cu/NFSI-catalyzed C-H functionalization reactions. (3) The energetics of three radical functionalization pathways are compared, including radical-polar crossover (RPC) to generate a carbocation intermediate, reductive elimination from a formal CuIII organometallic complex, and radical addition to a Cu-bound ligand. The preferred mechanism is shown to depend on the ligands bound to copper. (4) Finally, the energetics of three different pathways that convert benzylic C-H bonds into benzylic cations are compared, including HAT/ET (ET = electron transfer), relevant to the RPC mechanism with Cu/NFSI; hydride transfer, involved in reactions with high-potential quinones; and sequential ET/PT/ET (PT = proton transfer), involved in catalytic photoredox reactions. Collectively, the results provide mechanistic insights that establish a foundation for further advances in radical-relay C-H functionalization reactions.

Post-synthetic modification of covalent organic frameworks for CO2 electroreduction

To achieve high-efficiency catalysts for CO₂ reduction reaction, various catalytic metal centres and linker molecules have been assembled into covalent organic frameworks. The amine-linkages enhance the binding ability of CO₂ molecules, and the ionic frameworks enable to improve the electronic conductivity and the charge transfer along the frameworks. However, directly synthesis of covalent organic frameworks with amine-linkages and ionic frameworks is hardly achieved due to the electrostatic repulsion and predicament for the strength of the linkage. Herein, we demonstrate covalent organic frameworks for CO₂ reduction reaction by modulating the linkers and linkages of the template covalent organic framework to build the correlation between the catalytic performance and the structures of covalent organic frameworks. Through the double modifications, the CO₂ binding ability and the electronic states are well tuned, resulting in controllable activity and selectivity for CO₂ reduction reaction. Notably, the dual-functional covalent organic framework achieves high selectivity with a maximum CO Faradaic efficiency of 97.32% and the turnover frequencies value of 9922.68 h^-1, which are higher than those of the base covalent organic framework and the single-modified covalent organic frameworks. Moreover, the theoretical calculations further reveal that the higher activity is attributed to the easier formation of immediate *CO from COOH*. This study provides insights into developing covalent organic frameworks for CO₂ reduction reaction.

Recent reports on vanadium based coordination polymers and MOFs

Coordination polymers (CP) and metal-organic frameworks (MOF) have become a topic of immense interest in this century primarily because of the structural diversity that they offer. This structural diversity results in their multifaceted utility in various fields of science and technology such as catalysis, medicine, gas storage or separation, conductivity and magnetism. Their utility inspires a large variety of scientists to engage with them in their scientific pursuit thus creating a buzz around them in the scientific community. Metals capable of forming CPs and MOFs are primarily transition metals. Among them vanadium-based CPs and MOFs demand detailed discussion because of the unique nature of vanadium which makes it stable in many oxidation states and coordination number. Vanadium’s versatility imparts additional structural marvel and usefulness to these CPs and MOFs.

Ultrasmall Cluster Model for Investigating Single Atom Catalysis: Dehydrogenation of 1-Propanamine by Size-Selected Pt1Zr2O7 Clusters Supported on HOPG

The selective dehydrogenation of hydrocarbons and their functionalized derivatives is a promising pathway in the realization of endothermic fuel systems for powering important technologies such as hypersonic aircraft. The recent surge in interest in single atom catalysts (SACs) over the past decade offers the opportunity to achieve the ultimate levels of selectivity through the subnanoscale design tailoring of novel catalysts. Experimental techniques capable of investigating the fundamental nature of the active sites of novel SACs in well-controlled model studies offer the chance to reveal promising insights. We report here an approach to accomplish this through the soft landing of mass-selected, ultrasmall metal oxide cluster ions, in which a single noble metal atom bound to a metal oxide moiety serves as a model SAC active site. This method allows the preparation of model catalysts in which monodispersed neutral SAC model active sites are decorated across an inert electrically conductive support at submonolayer surface coverage, in this case, Pt₁Zr₂O₇ clusters supported on highly oriented pyrolytic graphite (HOPG). The results contained herein show the characterization of the Pt₁Zr₂O₇/HOPG model catalyst by X-ray photoelectron spectroscopy (XPS), along with an investigation of its reactivity toward the functionalized hydrocarbon molecule, 1-propanamine. Through temperature-programmed desorption/reaction (TPD/R) experiments it was shown that Pt₁Zr₂O₇/HOPG decomposes 1-propanamine exclusively into propionitrile and H₂, which desorb at 425 and 550 K, respectively. Conversely, clusters without the single platinum atom, that is, Zr₂O₇/HOPG, exhibited no reactivity toward 1-propanamine. Hence, the single platinum atom in Pt₁Zr₂O₇/HOPG was found to play a critical role in the observed reactivity.

DFT insights into the Ni-catalyzed regioselective hydrocarboxylation of unsaturated alkenes with CO2

The nickel-catalyzed hydrocarboxylation of alkenes using carbon dioxide has recently become an appealing method to prepare functionalized carboxylic acids with high efficiency and regioselectivity. Herein, density functional theory (DFT) calculations were conducted on the Ni-catalyzed hydrocarboxylation of aryl-/alkyl-substituted alkenes with CO₂. The α- and β-carboxylation of aromatic and aliphatic olefins originate from distinct catalytic cycles: H-transfer-carboxylation and carboxylation-H-transfer pathways. The typical hydrometallation-carboxylation mechanism is unlikely because water/carbonic acid (H-resource) are inferior hydride donors.