6-Methyl-2-pyridone pentahydrate

# 2004 International Union of Crystallography Printed in Great Britain ± all rights reserved Crystals of the title compound, C6H7NO 5H2O, were grown over a period of several weeks from an aqueous solution of the commercial compound. The molecule crystallizes in space group P1 and there are two independent 6-methyl-2-pyridone (Hmhp) molecules in the asymmetric unit, together with ten molecules of water. Packing diagrams reveal stacks of hydrogen-bonded Hmhp dimers surrounded by channels of water molecules. The Hmhp molecules pack with face-to-face ± stacking, a common feature of pyridone crystal structures. Each water molecule serves twice as hydrogen-bond donor and twice as acceptor, and is thus pseudo-tetrahedral. The water molecules are arranged in hydrogen-bonded ®veand six-membered rings and the rings are fused together, with the ®ve-membered rings adopting an envelope conformation and the six-membered rings adopting either a chair or boat conformation. This structure is further evidence that Hmhp exists in the solid state as the pyridone tautomer and not the pyridinol tautomer.

Principal factors in¯uencing the position of this equilibrium include solvent effects (polarity and pH) and substituent effects (position on the ring and electron-donating/withdrawing in¯uence). Substituents at position 6 have the greatest effect; electron-withdrawing substituents are seen to drive the equilibrium towards the pyridinol tautomer, whereas electrondonating substituents favour the pyridone tautomer. The rationale behind this has already been explained in terms of resonance stabilization and destabilization by the substituent (King et al., 1972).
Naturally, X-ray crystallography has played a key role in determining the preferred tautomers in the solid state. The structures of 6-chloro-2-hydroxypyridine (Kvick & Olovsson, 1969) and 6-bromo-2-hydroxypyridine (Kvick, 1976) are already known. Both have electron-withdrawing substituents and both crystallize as the pyridinol form, con®rming the conclusions derived from spectroscopic evidence which predicted that they would be observed as the pyridinol tautomer. If there is no substituent, the molecule crystallizes as the pyridone form (Penfold, 1953).
However, crystallographic proof that electron-donating substituents generate a preference for the pyridone form has so far only been obtained via crystal structures of coordination compounds (Rawson & Winpenny, 1995, and references therein) or co-crystallized with (S)-malic acid (Aakero È y et al., 2000). Unfortunately this is far from conclusive; a search of the Cambridge Structural Database (Version 5.25 plus two updates; Allen, 2002) for 6-methyl-2-pyridone, allowing all bonds to be of any type, returned 105 hits. Of these no fewer than 80 reported the ligand as the pyridinol form, rather than the pyridone form, and a handful of hits even contained mixtures of the two. In most cases, this is probably because the CÐO bond was too long to be considered as a genuine double bond. In addition, the ligand has been deprotonated in its complexes; there is no longer the possibility of determining the presence of an OÐH or NÐH bond and hence decide upon pyridone or pyridinol structure; these are now resonance forms rather than discrete tautomers.
We have determined the crystal structure of 6-methyl-2pyridone (Hmhp) as its pentahydrate, (I), crystallized from water ( Fig. 1). There are two independent Hmhp molecules in the asymmetric unit and also ten molecules of water. Face-toface %±% stacking, a common feature of pyridone crystal structures, is observed here. One Hmhp molecule lies above the other, with the methyl groups oriented in opposite directions, as shown in Fig. 2. Both molecules are essentially planar, except for the methyl H atoms. Their mean planes are approximately parallel and 3.30 A Ê apart, just less than the sum of the van der Waals radii for two C atoms, which is 3.4 A Ê .
Packing diagrams of the structure show that Hmhp packs as hydrogen-bonded dimers; these then form stacks, separated by  The asymmetric unit of (I), with displacement ellipsoids drawn at the 50% probability level. H atoms are represented as small spheres of arbitrary size.

Figure 2
Projection along the a axis, with hydrogen bonds in light blue, showing also the %±% stacking.

Figure 3
Projection along the b axis, showing the hydrogen-bonding network involving the water molecules and the 6-methyl-2-pyridone dimer. channels of hydrogen-bonded water molecules (Fig. 2). Each water molecule serves twice as hydrogen-bond donor and twice as acceptor, making four hydrogen bonds in total. This pseudo-tetrahedral arrangement means that the O atoms form ®ve-and six-membered rings. These rings are fused together; the ®ve-membered rings adopt an envelope conformation and the six-membered rings adopt either a chair or boat conformation. The hydrogen-bonding network involving the water molecules, and also the hydrogen bonding between the Hmhp molecules, is shown in Fig. 3.
The CÐO bond lengths are 1.272 (3) and 1.268 (3) A Ê for the two molecules and are in good agreement with those reported by Aakero È y et al. (2000) of 1.275 and 1.284 A Ê . These are a little on the long side for a C O double bond. However, the data clearly show the presence of an H atom bonded to each N atom, and these have been re®ned freely. From this result, together with the lack of signi®cant residual electron density next to the O atoms, the molecule is unambiguously in the pyridone form.

Experimental
Commercially available 2-hydroxy-6-methylpyridine was obtained as a white powder. A sample was dissolved in distilled water with gentle heating and the sample vial stoppered. Storage in a cool cupboard resulted in large plate crystals growing over a period of several weeks.
Methyl H atoms were positioned geometrically (CÐH = 0.98 A Ê ) and re®ned as riding, with free rotation about the CÐC bond, and with U iso (H) = 1.5U eq (C). Aromatic H atoms were also positioned geometrically (CÐH = 0.95 A Ê ) and re®ned as riding, with U iso (H) = 1.2U eq (C). H atoms bonded to N and O atoms were found in a difference map and their positions were re®ned, with U iso (H) = 1.2U eq (N,O). Water OÐH distances were restrained to 0.82 (1) A Ê and HÁ Á ÁH distances restrained to 1.35 (2) A Ê , but NÐH distances were not restrained. We thank the EPSRC for ®nancial support. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.