Catechol-Ligated Transition Metals: A Quantum Chemical Study on a Promising System for Gas Separation

Metal-organic frameworks (MOFs) have received a great deal of attention for their potential in atmospheric filtering, and recent work has shown that catecholate linkers can bind metals, creating MOFs with monocatecholate metal centers and abundant open coordination sites. In this study, M-catecholate systems (with M = Mg²⁺, Sc²⁺, Ti²⁺, V²⁺, Cr²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) were used as computational models of metalated catecholate linkers in MOFs. Nitric oxide (NO) is a radical molecule that is considered an environmental pollutant and is toxic if inhaled in large quantities. Binding NO is of interest in creating atmospheric filters, at both the industrial and personal scale. The binding energies of NO to the metal-catecholate systems were calculated using density functional theory (DFT) and complete active space self-consistent field (CASSCF) followed by second-order perturbation theory (CASPT2). Selectivity was studied by calculating the binding energies of additional guests (CO, NH₃, H₂O, N₂, and CO₂). The toxic guests have stronger binding than the benign guests for all metals studied, and NO has significantly stronger binding than other guests for most of the metals studied, suggesting that metal-catecholates are worthy of further study for NO filtration. Certain metal-catecholates also show potential for separation of N₂ and CO₂ via N₂ activation, which could be relevant for carbon capture or ammonia synthesis.