A Self-Assembled Iron(II) Metallacage as a Trap for Per- and Polyfluoroalkyl Substances in Water

An anionic iron(II) tetrahedral molecular cage (FeMOP) was studied for its ability to interact with various per- and polyfluoroalkyl substances (PFASs) in aqueous media. Liquid chromatography tandem mass spectrometry revealed that longer-chain-length (more than six carbons) perfluorocarboxylic, -sulfonic, and fluorotelomers were removed from solution. In contrast, the steric bulk of N-ethyl substituted fluorosulfonamido acetic acid PFASs hindered association with the cage. Solution binding studies in D2O using 19F nuclear magnetic resonance (NMR) titrations and a Job plot show a 1:1 binding stoichiometry for perfluorohexanoic acid (PFHxA) and perfluoroheptanoic acid (PFHpA) with an association constant (Ka) of <103 and thus a favorable free energy of association (ΔG°). Perfluorononanoic acid (PFNA), on the other hand, forms an insoluble host–guest complex with FeMOP with a 1:1 host–guest ratio. Variable temperature (VT) NMR was used to determine the thermodynamic parameters of binding for a 1:1 FeMOP/PFHp...