Channel-forming solvates of 6-chloro-2,5-dihydroxy-pyridine and its solvent-free tautomer 6-chloro-5-hydr-oxy-2-pyridone

On crystallization from CHCl₃, CCl₄, CH ₂ClCH₂Cl and CHCl₂CHCl₂, 6-chloro-5-hydr-oxy-2-pyridone, C₅H₄ClNO₂, (I), under-goes a tautomeric rearrangement to 6-chloro-2,5-di-hy-droxy-pyridine, (II). The resulting crystals, viz. 6-chloro-2,5-di-hy-droxy-pyridine chloro-form 0.125-solvate, C₅H₄ClNO₂·0.125CHCl ₃, (IIa), 6-chloro-2,5-dihydroxy-pyridine carbon tetra-chloride 0.125-solvate, C₅H₄ClNO₂.·0.125CCl ₄, (IIb), 6-chloro-2,5-di-hy-droxy-pyridine 1,2-dichloro-ethane solvate, C₅H₄ClNO₂·C₂H ₄Cl₂, (IIc), and 6-chloro-2,5-dihydroxy-pyri-dine 1,1,2,2-tetra-chloro-ethane solvate, C₅H₄ClNO ₂·C₂H₂Cl₄, (IId), have I41/a symmetry, and incorporate extensively disordered solvent in channels that run the length of the c axis. Upon gentle heating to 378 K in vacuo, these crystals sublime to form solvent-free crystals with P21/n symmetry that are exclusively the pyridone tautomer, (I). In these sublimed pyridone crystals, inversion-related mol-ecules form R ² 2(8) dimers via pairs of N - H⋯O hydrogen bonds. The dimers are linked by O - H⋯O hydrogen bonds into R ⁴ 6(28) motifs, which join to form pleated sheets that stack along the a axis. In the channel-containing pyridine solvate crystals, viz. (IIa)-(IId), two independent host mol-ecules form an R ² 2(8) dimer via a pair of O - H⋯N hydrogen bonds. One mol-ecule is further linked by O - H⋯O hydrogen bonds to two 41 screw-related equivalents to form a helical motif parallel to the c axis. The other independent mol-ecule is O - H⋯O hydrogen bonded to two related equivalents to form tetra-meric R ⁴ 4(28) rings. The dimers are π-π stacked with inversion-related dimers, which in turn stack the R ⁴ 4(28) rings along c to form continuous solvent-accessible channels. CHCl₃, CCl₄, CH₂ClCH₂Cl and CHCl₂CHCl ₂ solvent mol-ecules are able to occupy these channels but are disordered by virtue of the site symmetry within the channels.