In Search of Tetrafluoroborate Anion: ¹⁹F-NMR Chemical Shifts Dependence of Substituents in Tri-Aryl Pyrylium Cations

Pyrylium salts belong to a very important class of cationic organic molecules that have trivalent oxygen in a six membered aromatic ring.[1] Numerous references are available for this kind of compounds such as crystallographic and structural studies[2] and specially their good absorption, fluorescence, and electron acceptor behavior. The pyrylium salts have been exploited to design sensors for anion,[3] amines, amino acids and chameleon labels for quantifying proteins[4] among others. It is known that the anion nature, in particular, BF₄^- has a significant influence on the reactivity of Pyrylium salts. With the aim to demonstrate the presence of this anion by using ¹⁹F-NMR we have observed peculiar results. In this context, we have studied the small but significant variations on chemical shifts from lightly to heavily modified tri-arylpyrylium cations 1-5. We thank the AECID (Projects A/023577/09 and A/030422/10) and the \'Junta de Andalucía\' (FQM 142 and Project P09-AGR-4597) for financial support. [1] Manoj, N.; Ajayakumar, G.; Gopidas, K. R.; Suresh, C. H., J. Phys. Chem. A, 2006, 110 (39), 11338-45. [2] Turowska-Tyrk, I.; Krygowski, T. M. J. Mol. Struct. 1991, 263, 235-45. [3] García-Acosta, B.; García, F.; García, J.M.; Martínez-Máñez, R.; Sancenón, F.; San-José, N.; Soto, J. Org. Lett. 9, 13, 2007, 2429-32. [4] Wetzl, B.K.; Yarmoluk, S.M.; Craig, D.M.; Wolfbeis, O.S., Angew. Chem. Int. Ed. 2004, 43, 5400-02.