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(C 12 H 30 N 8 )] 3+  ⋅ 3 NO 3 - is reported to undergo a disorder-order phase transition at T c1 ≈133 K upon cooling. Temperature-dependent neutron and synchrotron diffraction experiments revealed satellite reflections in addition to main reflections in the diffraction patterns below T c1 . The modulation wave vector varies as function of temperature and locks in at T c3 ≈98 K. Here, we demonstrate that the crystal symmetry lowers from hexagonal to monoclinic in the incommensurately modulated phases in T c1 <T<T c3 . 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.