Structural and Stability Studies of a Series of para-Phenylenediboronic and para-Hydroxyphenylboronic Acid Cocrystals with Selected Aromatic N-Oxides

Abstract

Nine new cocrystals of para-phenylenediboronic or para-hydroxyphenylboronic acids with a series of aromatic N-oxides are reported. All of the complexes form centrosymmetric crystal structures. They are stabilized by an extended net of hydrogen bonds further supported by effective π-stacking interactions. In the studied structures four main synthons were distinguished. Only in the 3,10-phenanthroline-3-N-oxide and 3,10-N,N-dioxide cocrystals from the series, an acid–acid dimeric motif characteristic for arylboronic acid crystals was observed. The majority of the obtained cocrystals exhibit layered architectures, where the N-oxide molecules are separated by boronic acid slabs. Furthermore, most of the cocrystals form hydrates, where water molecules play the role of a “molecular glue”. Water contribution to the lattice stability is significant, and energy gain per water molecule ranges from 66.8 to 117.6 kJ·mol–1. The cocrystal cohesive energies seem to be more favorable when compared to the corresponding values for crystals of their components. However, the presence of various asymmetric unit contents and numerous hydrated crystal structures indicates that the crystallization process and the final products are highly dependent on kinetic and entropy factors. Finally, the calculated energetic features are compared with the experimental thermogravimetric analysis–differential scanning calorimetry results obtained for selected cocrystals.