Dantrolene represents yet another interesting example of abundant molecular crystal polymorphism existing in at least six different neat polymorphs, three of which can be obtained via crystallization (I–III) and an additional three (IV– VI) via solid-state dehydration from three different monohydrates (MH-I–MH-III). The reasons for polymorph formation were rationalized by analyzing the crystal structures of the polymorphs and hydrates used in their preparation. The thermodynamic relations among the polymorphs were established from calorimetric data, solubility measurements, and lattice energy calculations.

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The combination of the active pharmaceutical ingredients furosemide [4-chloro-2-(furan-2-ylmethylamino)-5-sulfamoylbenzoic acid] and pentoxifylline [3,7-dimethyl-1-(5-oxohexyl)-3,7-dihydro-1H-purine-2,6-dione] produces a 1:1 cocrystal, C(12)H(11)ClN(2)O(5)S·C(13)H(18)N(4)O(3), (I), a 1:1 cocrystal hydrate, C(12)H(11)ClN(2)O(5)S·C(13)H(18)N(4)O(3)·H(2)O, (II), and a 1:1 cocrystal acetone solvate, C(12)H(11)ClN(2)O(5)S·C(13)H(18)N(4)O(3)·C(2)H(6)O, (III). These structures exhibit the presence of a rarely encountered synthon with the graph set R(2)(2)(7). All potential hydrogen-bond donors of furosemide participate in hydrogen-bond formation in (I)-(III). However, only two hydrogen-bond acceptors of furosemide are active in (I) and (II), and only one is active in (III). Four hydrogen-bond acceptors of pentoxifylline are active in (II), three in (I) and two in (III). These observations are in good agreement with the calculated packing indexes of 69.5, 69.6 and 68.8% for (II), (I) and (III), respectively.