Channel-forming solvate crystals and isostructural solvent-free powder of 5-hydroxy-6-methyl-2-pyridone

Crystals of 5-hydroxy-6-methyl-2-pyridone, (I), grown from a variety of solvents, are invariably trigonal (space group R3̄); these are 5-hydroxy-6-methyl-2-pyridone acetone 0.1667- solvate, C ₆H ₇NO ₂·0.1667C ₃H ₆O, (Ia), and 6-methyl-5-hydroxy- 2-pyridone propan-2-ol 0.1667-solvate, C ₆H ₇NO ₂·0.1667C ₃H ₈O, (Ib), and the forms from methanol, (Ic), water, (Id), benzonitrile, (Ie), and benzyl alcohol, (If). They incorporate channels running the length of the c axis that contain extensively disordered solvent molecules. A solvent-free sublimed powder of 5-hydroxy-6-methyl-2-pyridone microcrystals is essentially isostructural. Inversion-related host molecules interact via pairs of N-H⋯O hydrogen bonds to form R ₂ ²(8) dimers. Six of these dimers form large R ₁₂ ⁶(42) puckered rings, in which the O atom of each N-H⋯O hydrogen bond is also the acceptor in an O-H⋯O hydrogen bond that involves the 5-hydroxy group. The large R ₁₂ ⁶ (42) rings straddle the 3̄ axes and form stacked columns via π-π interactions between inversion-related molecules of (I) [mean interplanar spacing = 3.254 Å and ring centroid-centroid distance = 3.688 (2) Å]. The channels are lined by methyl groups, which all point inwards to the centre of the channels.