The Z′=12 superstructure of A-cobalt(III) sepulchrate trinitrate governed by C-H•••O hydrogen bonds

A-Cobalt(III) sepulchrate trinitrate crystallizes in P6(3)22 with Z = 2 (Z' = 1/6) at room temperature. Slabs perpendicular to the hexagonal axis comprise molecules Co(sepulchrate) alternating with nitrate groups A and B. Coordinated by six sepulchrate molecules, highly disordered nitrate groups C are accommodated between the slabs. Here we report the fully ordered, low temperature crystal structure of Co(sep)(NO3)(3). It is found to be a high-Z' structure with Z' = 12 of the 12-fold 6a(h) x root 3b(h) x c(h) superstructure with monoclinic symmetry P2(1) (c unique). Correlations between structural parameters are effectively removed by refinements within the superspace approach. Superstructure formation is governed by a densification of the packing in conjunction with ordering of nitrate group C, the latter assuming different orientations for each of the Z' = 12 independent copies in the superstructure. The Co(sep) moiety exhibits small structural variations over its 12 independent copies, while orientations of nitrate groups A and B vary less than the orientations of the nitrate group C do. Molecular packing in the superstructure is found to be determined by short C-H center dot center dot center dot H-C contacts, with H center dot center dot center dot H distances of 2.2-2.3 angstrom, and by short C H center dot center dot center dot O contacts, with H center dot center dot center dot O distances down to 2.2 angstrom. These contacts presumably represent weak C-H center dot center dot center dot O hydrogen bonds, but in any case they prevent further densification of the structure and strengthening of weak N-H center dot center dot center dot O hydrogen bonds with observed H center dot center dot center dot O distances of 2.4-2.6 angstrom.