Conformational Polymorphism and Isomorphism of Molecular Rotors with Fluoroaromatic Rotators and Mestranol Stators

We report the synthesis and characterization
of four molecular
rotors 2, 3, 4, and 5 containing 2-fluoro-, 2,3-difluoro-, 2,5-difluoro-, and 2,3,5,6-tetrafluoro-substituted
1,4-phenylene rotators, respectively, that are axially linked through
the triple bonds of rigid mestranol (3-methoxy-17α-ethynylestradiol)
stators. Crystallization experiments using solvent mixtures of hexanes–ethyl
acetate and acetonitrile–dichloromethane gave rise to polymorphic,
pseudopolymorphic, and isomorphic crystals. Whereas two solids were
obtained for compound 2, four were indentified in the
cases of compounds 3 and 4, and three for
compound 5. The 13 solid forms were characterized by
single-crystal and powder X-ray diffraction, infrared spectroscopy,
differential scanning calorimetry, and thermogravimetric analysis.
Two sets of isomorphous structures were obtained for solvated and
solvent-free structures of compounds 3–5. While polymorphic behavior of these compounds arises from their
conformational freedom in solution, their isomorphism arises from
the closely isosteric relation that exists between hydrogen and fluorine
atoms.