From Competition to Commensuration by Two Major Hydrogen-Bonding Motifs

Carboxylic
acid–acid hydrogen-bonding dimer and acid–pyridine
hydrogen-bonding motif are two competing supramolecular synthons that
a molecule possessing both carboxylic acid and pyridine functional
groups could form in the solid state. Their coexistence has been observed
but for the molecules with the molar ratio of carboxylic acid and
pyridine groups being greater than 1:1. In this crystal engineering
study, 2-[phenyl­(propyl)­amino]­nicotinic acid with a 1:1 molar ratio
of these two functional groups was discovered to have two polymorphs,
in which one consists of unique hydrogen-bonded tetramer units bearing
both acid–acid and acid–pyridine hydrogen-bonding motifs,
while the other is composed of acid–pyridine hydrogen-bonded
chains. Quantum mechanical calculations were employed to unravel the
essence of the coexistence of the two vying counterparts as well as
the origins of the tetramer and chain structures.