A Rare Case of Polymorphism in a Three-Component Co-Crystal System, with Each Polymorph Having Ten Independent Molecules in the Asymmetric Unit

We report polymorphism in a co-crystal system comprising
trimesic
acid (benzene-1,3,5-tricarboxylic acid, TMA), tert-butylamine (TBA), and methanol with stoichiometry (TMA)2(TBA)5(methanol)3. The crystal structure of
each polymorph (determined from single-crystal X-ray diffraction)
is shown to have 10 independent molecules in the asymmetric unit.
In each polymorph, all five TBA molecules in the asymmetric unit exist
as protonated cations, whereas, of the two TMA molecules in the asymmetric
unit, one exists as the doubly deprotonated anion and the other exists
as the triply deprotonated anion. In each case, the structure is based
on an extensively hydrogen-bonded two-dimensional network involving
the −N+H3 groups of TBA cations, the
−CO2– and −CO2H groups of TMA anions, and the OH groups of methanol molecules.
Although the two polymorphs share some structural features in common,
there are nevertheless significant differences in several aspects,
including differences in some of the hydrogen-bonding motifs. Among
polymorphic systems reported previously, there are very few examples
for which two polymorphs have 10 or more independent molecules in
the asymmetric unit and very few examples of co-crystals comprising
three or more distinct organic molecules. From both of these perspectives,
the example of polymorphism reported here can be considered very rare.