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 31P and 19F NMR spectral titration studies. The fluoride ion-encapsulated cage
[nBu4N]­[F@{Zn­(dipp)­(DMSO)}4] (2) was isolated in good yield from an equimolar
reaction between 1 and nBu4NF 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.