Charge-Separated and Lewis Paired Metal–Organic Framework for Anion Exchange and CO<inf>2</inf> Chemical Fixation

Charge-separated metal–organic frameworks (MOFs) are a unique class of MOFs that can possess added properties originating from the exposed ionic species. A new charge-separated MOF, namely, UNM-6 synthesized from a tetrahedral borate ligand and Co2+ cation is reported herein. UNM-6 crystalizes into the highly symmetric P43n space group with fourfold interpenetration, despite the stoichiometric imbalance between the B and Co atoms, which also leads to loosely bound NO3− anions within the crystal structure. These NO3− ions can be quantitatively exchanged with various other anions, leading to Lewis acid (Co2+) and Lewis base (anions) pairs within the pores and potentially cooperative catalytic activities. For example, UNM-6-Br, the MOF after anion exchange with Br− anions, displays high catalytic activity and stability in reactions of CO2 chemical fixation into cyclic carbonates.