Guanidinium-based receptors for anion separations
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Herein, work on guanidinium-based anion receptors and their anion separation properties are described. First, a novel receptor based on the N,N’-bis(2-pyridyl)guanidinium motif is rationally designed, synthesized, and characterized. In the solid state, X-ray crystallography shows that is has a strong conformational preference for the α,α form of the molecule. This ligand has association constants of 3.78 ± 0.12 and 2.10 ± 0.23 respectively for log K1 and log K2. A lipophilic form of this ligand was synthesized for extraction studies, where it performed better than commercially available Aliquat 336 for the extraction of sulfate into 1,2-DCE from water. Next, a series of bis(imino)guanidinium ligands are created that are capable of forming highly insoluble salts with many oxoanions, the most insoluble being sulfate. This leads to novel methods of separation of highly charged oxoanion species by precipitation followed by simple filtration, and its use was demonstrated on natural seawater. This methodology was adapted to the sequestration of CO2 from ambient air, leading to an effective method for the crystallization of CO2 as its carbonate salt from water as well as its low temperature release. Finally, future work towards the creation of iminoguanidinium-based oxoanion receptors is described.