Fluoride ion sensing and caging by a preformed molecular D4R zinc phosphate heterocubane

Double-4-ring (D4R) zinc phosphate [Zn(dipp)(DMSO)]4 (1, dipp = 2,6-di-iso-propylphenylphosphate, DMSO = dimethyl sulfoxide), on treatment with a free fluoride ion source, exhibited ability to sense and capture fluoride ions from a variety of sources, as evidenced by extensive solution (31)P and (19)F NMR spectral titration studies. The fluoride ion-encapsulated cage [(n)Bu4N][F@{Zn(dipp)(DMSO)}4] (2) was isolated in good yield from an equimolar reaction between 1 and (n)Bu4NF in methanol and characterized by analytical and spectroscopic methods. When 1-methyl-4,4'-bipyridin-1-ium fluoride (MeQ-F) was used as the fluoride ion source a zwitterionic cage [F@{Zn4(dipp)4(MeQ)(DMSO)3}] (3) was isolated. Crystal structure determination for 3 confirmed not only fluoride incorporation inside the D4R cage but also a weak interaction of the central fluoride ion with all four zinc centers of the cubane, resulting in a trigonal bipyramidal geometry around the zinc centers. To establish the selectivity, cubane 1 was treated with 2 equiv of MeQ-X (X = various anions) under similar conditions to isolate [F@{Zn4(dipp)4(MeQ)2(MeOH)2}][X] (X = I 4; BF4 5; PF6 6) in good yields. The crystal structure determination of 4 and 5 showed that the iodide and tetrafluoroborate anions are found outside the cage while fluoride ion has entered the cavity. The final fluoride encapsulated D4R cage is anionic in 2, neutral in 3, and cationic in 4-6, showing the versatility of the cubane framework to stabilize fluoride ions in all three forms. NMR titrations showed that 1 can sense even 1 ppm level of fluoride ions and sequester them from fluoridated water and toothpaste extract.