Single crystals that spontaneously spawn other single crystals: a ternary and a binary adduct of thiourea and 2,5-dimethylpyrazine??

CrystEngComm Pub Date: 2016-02-09 DOI: 10.1039/C6CE00100A

Abstract

Liquid diffusion of n-pentane into a solution of thiourea in 2,5-dimethylpyrazine led to a crystalline 4?:?3 adduct (1), in which corrugated thiourea layers are crosslinked with pyrazines. Attempts to obtain adducts with other stoichiometries, by crystallizing thiourea from a mixture of 2,5-dimethylpyrazine and methanol, formed the ternary 1?:?1?:?1 adduct 2 instead. Adduct 2 displays a layer structure in which parallel thiourea ribbons are linked on the one side by pyrazines and on the other side by methanol and pyrazines, leading to repeating crosslink sequences (?thiourea?methanol?pyrazine?methanol?thiourea?pyrazine?); the ribbons within a layer are thus unequally spaced. In inert oil, individual single crystals of 2 spontaneously convert to several smaller crystals, some single, of 1. The process may be regarded as a single-crystal to single-crystal transformation, although not in the usual sense. The 4?:?3 adduct (3) of thiourea with 2-methylpyrazine is isotypic to 1. In all three structures, the preponderant secondary interactions are classical hydrogen bonds.

Graphical abstract: Single crystals that spontaneously spawn other single crystals: a ternary and a binary adduct of thiourea and 2,5-dimethylpyrazine
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