The Z' = 12 superstructure of [Lambda]-cobalt(III) sepulchrate trinitrate governed by C-H...O hydrogen bonds

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Dey, Somnath ; Schönleber, Andreas ; Mondal, Swastik ; Prathapa, Siriyara Jagannatha ; van Smaalen, Sander ; Larsen, Finn Krebs:
The Z' = 12 superstructure of [Lambda]-cobalt(III) sepulchrate trinitrate governed by C-H...O hydrogen bonds.
In: Acta Crystallographica Section B. Bd. 72 (2016) Heft 3 . - S. 372-380.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S2052520616005503

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Abstract

Λ-Cobalt(III) sepulchrate trinitrate crystallizes in \it P6\sb 322 with \it Z = 2 (\it Z\prime = 1/6) at room temperature. Slabs perpendicular to the hexagonal axis comprise molecules Co(sepulchrate) alternating with nitrate groups \it A and \it B. Coordinated by six sepulchrate molecules, highly disordered nitrate groups \it C are accommodated between the slabs. Here we report the fully ordered, low-temperature crystal structure of Co(sep)(NO\sb 3)\sb 3. It is found to be a high-\it Z\prime structure with \it Z\prime = 12 of the 12-fold 6ah×√3bh× ch superstructure with monoclinic symmetry \it P2\sb 1 (\bf 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 \it C, the latter assuming different orientations for each of the \it Z\prime = 12 independent copies in the superstructure. The Co(sep) moiety exhibits small structural variations over its 12 independent copies, while orientations of nitrate groups \it A and \it B vary less than the orientations of the nitrate group \it C do. Molecular packing in the superstructure is found to be determined by short C—-H⋅sH—-C contacts, with H⋅sH distances of 2.2—2.3Å, and by short C—-H⋅sO contacts, with H⋅sO distances down to 2.2Å. These contacts presumably represent weak C—-H⋅sO hydrogen bonds, but in any case they prevent further densification of the structure and strengthening of weak N—-H⋅sO hydrogen bonds with observed H⋅sO distances of 2.4—2.6Å.