Modulated structure of Λ-cobalt(III) sepulchrate trinitrate controled by hydrogen bonds

What is it about?

Lambda-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)NO_(3))(3). It is found to be a high-Z' structure with Z' = 12 of the 12-fold 6a_(h) x sqrt(3) b_(h) x c_(h) superstructure with monoclinic symmetry P2_(1) (c-unique).

Featured Image

Why is it important?

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...H—C contacts, with H...H distances of 2.2–2.3 A, and by short C—H...O contacts, with H...O distances down to 2.2 A. These contacts presumably represent weak C—H...O hydrogen bonds, but in any case they prevent further densification of the structure and strengthening of weak N—H...O hydrogen bonds with observed H...O distances of 2.4–2.6 A.