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

Abstract

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.