A Self-Assembled Iron(II) Metallacage as a Trap for Per- and Polyfluoroalkyl Substances in Water
Cressa Ria P. Fulong, Mary Grace E. Guardian, Diana S. Aga, Timothy R. Cook
Abstract
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/PFHpA complex in water using a Curie-like model for fast-exchange processes. The extracted parameters suggest a low binding interaction (Ka < 103) 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 host–guest 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.