Zwitterionic Systems Obtained by Condensation of Heteroaryl-Boronic Acids and Rhodizonic Acid

The boronic acid derivatives 4-pyridinylboronic acid (4-pyBA) and pyrimidine-5-boronic acid (pyrBA) have been reacted with the oxocarbon rhodizonic acid or with sodium rhodizonate both in solution and in the solid state. Four novel condensation adducts have been isolated as hydrates, namely C16H16B2N2O12•xH2O (1a•xH2O; x = 2 or 4), C16H14B2N2O10•3H2O (2a•3H2O), C14H14B2N4O12•2.5H2O (1b•2.5H2O), and C14H12B2N4O10•3H2O (2b•3H2O), and fully characterized by X-ray single crystal and powder diffraction. Their thermal stability and behavior upon dehydration have also been investigated by TGA and single-crystal to single-crystal X-ray diffraction experiments. It has been shown that the boronic acid derivatives 4-pyridinylboronic acid and pyrimidine-5-boronic acid react with rhodizonic acid in a similar way producing two distinct products, the so-called “axle” compounds (1a and 1b) and “tweezer” compounds (2a and 2b), depending on the experimental conditions and preparation method. With both boronic acids, the “tweezer” compound is the only product of the mechanical mixing of the reactants. Both “axle” and “tweezer” compounds are in the zwitterionic form with the boronate ion being neutralized via protonation on the pyridinyl and pyrimidine N-terminus.