engleski

Exploring protonation properties of calix[4]arenes bearing urea moieties and their affinities towards anions in acetonitrile

The field of anion receptor chemistry is nowadays spread over many areas such as sensing, extraction, self-assembly, catalysis, and transport. Most classical anion receptors employ hydrogen bonding to bind anions. Calixarenes are a fascinating family of compounds that can, by proper functionalisations of upper and/or lower rim, bind cationic, anionic, and neutral species. (Thio)urea-derivatives of calixarenes were designed as anion binders already in the 1990s. However, the full potential of their exploitation in ion-selective electrodes or for selective binding and transport of biologically important species is yet to be unraveled. In the scope of this work two calix[4]arenes bearing urea moieties were prepared. 1H-NMR titrations of these compounds were performed with the appropriate organic bases of known pKa values, which enabled determination of their pKa values in acetonitrile. Complexation of several anions (chloride, hydrogen sulfate, dihydrogen phosphate, hydrogen pyrophosphate, and benzoate) with M1 and M2 was thermodynamically investigated by 1H-NMR, UV/Vis and ITC titrations. Both receptors form 1:1 complexes with all examined anions except H2PO4 – and hydrogen pyrophosphate which bind with the receptors also as dimers. The affinity trend of both M1 and M2 towards anions matches the anion basicity-order with the exception of H2PO4 – for which M1 has a peak affinity. The anion binding equilibrium constants in the case of M2 are significantly larger than those for M1 which can be correlated with a higher number of urea groups in the former macrocycle. Reaction entropies for complexation of anions with two calixarenes point to significant differences in their conformational freedom in solution.

Anion receptors ; calix[4]arene ; coordination chemistry ; urea

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