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 -ethyl substituted fluorosulfonamido acetic acid PFASs hindered association with the cage. Solution binding studies in DO using F 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 () of <10 and thus a favorable free energy of association (Δ). Perfluorononanoic acid (PFNA), on the other hand, forms an insoluble hostguest complex with FeMOP with a 1:1 hostguest ratio. Variable temperature (VT) NMR was used to determine the thermodynamic parameters of binding for a 1:1 FeMOP/PFHpA complex in water using a Curie-like model for fast-exchange processes. The extracted parameters suggest a low binding interaction ( < 10) driven by an increase in entropy from cage desolvation upon guest binding. The solid-state host-guest complexes formed from solution complexation of PFHxA, PFHpA, and PFNA into the cage were characterized by infrared spectroscopy (FT-IR) and powder X-ray diffraction (PXRD) methods. FT-IR studies suggest an interaction between the fluorocarbon groups of PFASs to the phenylsulfonate functional groups of the ligand. A docking model predicted by computation also indicates this interaction may occur, with the PFASs adsorbing onto the surface of the cage rather than forming a true hostguest complex within the internal cavity. PXRD studies reveal a crystal packing of the complex that is very similar to that of the water-treated FeMOP, with the exception of 1:2 FeMOP/PFNA and 1:1 and 2:1 FeMOP/PFHpA.