All good things come in threes: first example of a trimorphic, ternary molecular salt complex

The ternary molecular salt complex (2-aminopyridinium)·(9-anthracenecarboxylate)·(trinitrobenzene) occurs in three polymorphic forms. This makes it the first ternary complex that has more than two structural forms. The three forms occur in concomitant pairs when crystallized from specific solvents. FormIis most commonly encountered and crystallizes as orange needles in a variety of solvents. In acetonitrile, red block-shaped formIIcrystallizes out with formI, and in ethylacetate, a minority of yellow needles of formIIIcrystallizes out with formI. In all three forms, the trinitrobenzene interacts with the 9-anthracenecarboxylate in a charge-transfer interaction, and the 9-anthracenecarboxylate using charge-assisted hydrogen bonding to 2-aminopyridinium. Differential scanning calorimetry shows that formIIis the thermodynamically most stable form and formIthe kinetically favoured form.