Incommensurate phase in Λ-cobalt (III) sepulchrate trinitrate governed by highly competitive N–H···O and C–H···O hydrogen bond networks

Phase transitions in molecular crystals are often determined by intermolecular interactions. The cage complex of [Co(C12H30N8)]3+·3NO3– is reported to undergo a disorder–order phase transition at Tc1 ≈ 133 K upon cooling. Temperature-dependent neutron and synchrotron diffraction experiments revealed satellite reflections in addition to main reflections in the diffraction patterns below Tc1. The modulation wave vector varies as function of temperature and locks in at Tc3 ≈ 98 K. Here, we demonstrate that the crystal symmetry lowers from hexagonal to monoclinic in the incommensurately modulated phases in Tc1 ˂ T ˂ Tc3. Distinctive levels of competitions: trade-off between longer N–H···O and shorter C–H···O hydrogen bonds; steric constraints to dense C-H···O bonds give rise to pronounced modulation of the basic structure. Severely frustrated crystal packing in the incommensurate phase is precursor to optimal balance of intermolecular interactions in the lock-in phase.