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.