Polymorphs Related by an Enantiotropic Phase Transition but Having Only Order–Disorder-Level Conformational Differences in a Spiro-Conjugated Imidazolidine Ring Compound

Variable-temperature
single-crystal X-ray diffraction analyses
of 1,3-diethyl-spiro­[imidazolidine-2,2′-[2H]­indene]-1′,3′-dione (1) were performed
at low temperatures. The temperature dependence of the cell parameters
exhibited a discontinuous change between 158 and 153 K, indicating
the existence of a phase transition and hence the occurrence of two
polymorphs, I and II, which was also confirmed
by DSC analysis: reversible exothermic and endothermic peaks with
ΔH = 2.1 kJ mol–1 were observed
in the cooling and heating processes, respectively. The molecular
packing in I and II at 200 and 90 K, respectively,
remained essentially unchanged, retaining the space group Pnma. I and II are conformationally
isomorphic crystals, including two crystallographically independent
molecules, A and B, with different conformations
in a 1:2 ratio. Upon the phase transition from I to II with decreasing temperature, the dynamic disorder of the
imidazolidine ring of A in I, which interconverts
between quasi-half-chair conformations, is frozen to settle in an
envelope conformation, whereas B is retained in a half-chair
conformation in both I and II. The close
intermolecular contacts in I and II were
compared on the basis of Hirshfeld surface analysis. DFT calculations
at the B3LYP/6-311G­(d,p) level show that the half-chair structure
is more stable than the envelope conformer by 2.6 kJ mol–1 in the gas phase. The quasi-half-chair conformation was not obtained
as an energy-minimum geometry on the potential energy surface. Temperature-dependent
IR spectroscopy along with theoretical calculation using the QST method
gave assignments of the carbonyl stretching bands for each conformation.
Compound 1 provides a unique example of the isostructural
polymorphs in which the structural differences are small, being only
at the order–disorder level in the ring conformation; nevertheless,
the existence of the enantiotropic phase transition clearly indicates
that they are polymorphs.