Reversible phase transition in 8,19-dimethyl-2,3,8,19-tetra­aza­penta­cyclo[13.7.0.04,12.06,10.017,21]docosa-1(15),2,4(12),5,10,16,21-hepta­ene-7,9,18,20-tetrone

The title compound undergoes a reversible phase transition upon cooling, during which the space group changes from P2₁/c to P1, which is accompanied by a discontinuous change in the unit-cell volume. In the low-temperature phase, twinning is observed, which vanishes upon reheating to room temperature.

The crystal structure of the title compound, C₂₀H₁₄N₄O₄, was previously reported at 220 K [Businski et al. (2025#). Chem. Eur. J. A71, 3–8], where it crystallizes in the monoclinic space group P2₁/c with one crystallographically independent mol­ecule in a general position. In this structure, disorder of the methyl H atoms is observed. However, upon cooling a continuous splitting of the reflections occurs, that might indicate a phase transition. Therefore, data sets were measured between room temperature and 90 K upon cooling and reheating, which shows that a discontinuity in the unit-cell volume occurs between 170 and 180 K. This process is accompanied by the formation of an additional crystal domain. Both domains can be indexed separately, leading to a unit cell similar to that observed at room temperature, but with both α and β angles revealing significant deviations from 90°. Twin refinements clearly show that the structure becomes triclinic (space group P1) upon cooling, resulting in two crystallographically independent mol­ecules in the unit cell, for which some structural changes especially in the conformation and arrangement of the mol­ecules are observed. In the low-temperature structure, disordering of the methyl H atoms is still observed. If the crystal is reheated, the reflections of the second domain disappear and the structure can be successfully refined again in the monoclinic space group P2₁/c, indicating reversibility of the transition.