Reversible phase transition in 8,19-dimethyl-2,3,8,19-tetraazapentacyclo[13.7.0.0^4,12.0^6,10.0^17,21]docosa-1(15),2,4(12),5,10,16,21-heptaene-7,9,18,20-tetrone

The crystal structure of the title compound, C₂₀H₁₄N₄O₄, was previously reported at 220 K [Businski <i>et al.</i> (2025 ▸). <i>Chem. Eur. J.</i> A<b>71</b>, 3-8], where it crystallizes in the monoclinic space group <i>P</i>2₁/<i>c</i> with one crystallographically independent mol-ecule 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 <i>P</i>1) upon cooling, resulting in two crystallographically independent mol-ecules in the unit cell, for which some structural changes especially in the conformation and arrangement of the mol-ecules 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 <i>P</i>2₁/<i>c</i>, indicating reversibility of the transition.