Abstract:

Crystals of 4-[3-(trifluoromethyl) phenyl]-3H-1,2,3,5-dithiadiazol-3-yl grown by slow sublimation in a three-zone furnace form in P (1) over bar with Z = 12. The six independent molecules combine into two cis-oid and one twisted dimer, which arrange in a layer structure as two elegant clusters, with non-crystallographic fourfold and two-fold symmetry, respectively. Crystals of 4-(3-cyano-5-tertbutyl-phenyl)-3H-1,2,3,5-diselenadiazol- 3-yl, formed from decomposition of a bis-diselenadiazolyl, grow in two forms under different sublimation conditions using the same furnace. The alpha form crystallizes in P (1) over bar with Z = 6 and the beta in P2(1)/c with Z = 20. The a polymorph consists of a distorted cis-oid dimer linked electrostatically to a monomer in the asymmetric unit. These cluster with a second unit to a centrosymmetric aggregate of six heterocycles. The beta polymorph consists of clusters of four cis-oid dimers that link into chains along the b axis, whereas the fifth molecule links via -C equivalent to N center dot center dot center dot Se supramolecular synthons into an infinite chain of monomers also along the b axis but out-of-register with the dimer chain. A carefully curated compilation of all 83 distinct 1,2,3,5-dithiadiazolyl and of all 24 distinct 1,2,3,5-diselenadiazolyl crystal structures reveals an extraordinarily large number of structures with Z' > 1 (72%) and also those with Z' >= 3 (24%), compared to the average for organic molecules over the whole collection in the Cambridge Structure Database. This high incidence is analyzed and attributed to competing intermolecular synthons.