Supramolecular zwitterions based on a novel boronic acid-squarate dianion synthon

The reaction of squaric acid (H2SQ) with three pyridine-appended boronic acid derivatives (BA), viz. 4-pyridinylboronic acid (1BA), 3-pyridinylboronic acid (2BA), and (4-(pyridin-4-yl)phenyl)boronic acid (3BA), leads to the formation of supramolecular zwitterions of the general formula [SQ(BAH)2]·xH2O generated by proton transfer from the squaric acid to the pyridine sites. The zwitterion [SQ(1BAH)2]·2H2O can further react with the neutral molecule bipyridine (bpy) leading thus to a ternary system of the general formula [SQ(1BAH)2]·bpy.