ORIGINAL PAPER Protonation of electroneutral p-tert-butylcalix[4]arenetetraacetic acid Jaroslav Kr ˇı ´z ˇ • Jir ˇı ´ Dybal • Emanuel Makrlı ´k • Jan Budka • Petr Van ˇura Received: 4 September 2009 / Accepted: 22 October 2009 / Published online: 20 November 2009 Ó Springer-Verlag 2009 Abstract Using 1 H NMR spectroscopy together with density functional theoretical calculations, it is shown that electroneutral p-tert-butylcalix[4]arenetetraacetic acid forms an equimolar complex with a proton in the form of the H 3 O ? ion in nitrobenzene-d 5 . Protons were offered by hydrogen bis(1,2-dicarbollyl)cobaltate and converted to hydroxonium ions by traces of water. In the resulting complex, the H 3 O ? cation is bound by strong hydrogen bonds to two phenoxy oxygen atoms of the parent calix[4]arene ligand and to one carbonyl oxygen of the corresponding COOH group of this ligand. Keywords Calixarenes  Macrocycles  Protonation  Ab initio calculations  Complex structure Introduction Calix[n]arenes are a well-known family of macrocyclic molecules with many potential applications in various branches of chemistry. Because of their simple one-pot preparation, easy derivatization, and unique complexation abilities, calix[n]arenes are widely used as building blocks for the construction of more sophisticated molecular sys- tems. Their unique three-dimensional pre-organization make them very attractive as the receptors for the com- plexation of cations, anions, and even neutral molecules. Calix[n]arenes find applications as selective binders and carriers, as analytical sensors, as catalysts, and as model structures for biomimetic studies [1, 2]. In the field of host– guest chemistry, many studies have focussed on the bind- ing ability of calixarene derivatives with carbonyl groups at their lower rims toward metal ions, predominantly alkali and alkaline-earth, but also transition and heavy metal cations [3–11], and even toward H 3 O ? [12–19]. Recently, deprotonation of calix[n]arenes in which n = 4, 6, and 8 has been studied using acetonitrile as the solvent [20]. In this communication, on the basis of 1 H NMR spectroscopy and DFT quantum mechanical calcu- lations, we suggest the most probable structure of protonated p-tert-butylcalix[4]arenetetraacetic acid. Results and discussion NMR measurements Similarly to our previous studies on protonation of valinomycin [21, 22], we used hydrogen bis(1,2-di- carbollyl)cobaltate (HDCC) as a versatile proton source [23]. The protons were converted to hydroxonium ions H 3 O ? by traces of water in the system under study. In order to ensure full ionization of HDCC we had to use a solvent with a rather high dielectric constant, e.g. nitrobenzene (e = 35.6), acetonitrile (e = 36.6), or dimethylformamide (e = 38.3). Among these, nitrobenzene-d 5 proved to give least equivocal results in our previous works as it does not form inclusion complexes with calixarenes and does not J. Kr ˇı ´z ˇ  J. Dybal Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic E. Makrlı ´k (&) Faculty of Applied Sciences, University of West Bohemia, Pilsen, Czech Republic e-mail: makrlik@centrum.cz J. Budka  P. Van ˇura Institute of Chemical Technology, Prague, Czech Republic e-mail: petr.vanura@vscht.cz 123 Monatsh Chem (2010) 141:19–22 DOI 10.1007/s00706-009-0218-3