A Supramolecular Assembly Formed by Heteroassociation of Ciprofloxacin and Norfloxacin in the Solid State: Co-Crystal Synthesis and Characterization

The interaction in the solid state between ciprofloxacin
(CIP)
and norfloxacin (NOR) was investigated and a new multicomponent molecular
complex (COP) was obtained and characterized. It is composed of equimolar
quantities of both compounds, according to TLC and 1H NMR
data. As single-crystal X-ray analysis showed, COP crystallizes in
the triclinic system, space group P(1̅). The
asymmetric unit comprises three independent zwitterions (A, B, and
C) with contributions of both CIP and NOR, as well as 14.6 water molecules.
At the zwitterion A site, the occurrence is about 37% (CIP), 63% (NOR);
at the B site, there is predominantly CIP (70%); and at the C site
the occurrence is about 43% (CIP) and 57% (NOR), yielding an overall
1:1 CIP-NOR ratio in the crystal. TG data evidenced a one-step loss
of solvent (about 18% per COP zwitterion) below 110 °C, confirming
the presence of water molecules. The propensity for heteroassociation
between CIP and NOR, under the conditions described herein, originated
a co-crystal reported for the first time between FQs, and became one
of the most interesting aspects of this research.