A low-temperature redetermination of metaldehyde

# 2005 International Union of Crystallography Printed in Great Britain – all rights reserved A low-temperature redetermination of metaldehyde (systematic name: 2,4,6,8-tetramethyl-1,3,5,7-tetroxocane), (CH3–CHO)4 or C8H16O4, is reported, 69 years after the original determination [Pauling & Carpenter (1936). J. Am. Chem. Soc. 58, 1274–1278]. Metaldehyde crystallizes in the space group I4. The asymmetric unit contains one quarter of a molecule and the complete molecule is generated by the fourfold rotation axis.


Comment
The structure of metaldehyde, or 2,4,6,8-tetramethyl-1,3,5,7tetroxocane, (I), was first reported by Pauling & Carpenter (1936), using photographic methods. In the intervening 69 years, no further single-crystal determination of this compound has been deposited with the Cambridge Structural Database (Version of February 2005; Allen, 2002). We now report a low-temperature redetermination of this structure using a modern area-detector-equipped diffractometer, with all H-atom positions determined from the electron-density map.
Crystals of metaldehyde were grown by chance while attempting a recrystallization of 5-fluorocytosine from acetaldehyde. Under the conditions of the recrystallization experiment, four acetaldehyde molecules cyclized to form metaldehyde and this subsequently crystallized from solution.
The crystals grew as long needles. Attempts to cut a crystal perpendicular to the axis of the needle led to the shattering of the entire crystal. Therefore, a large complete needle was mounted, with no attempt made to reduce its size. This crystal measured approximately 1.25 mm in the direction of the long axis of the needle. Metaldehyde (Fig. 1) comprises a tetramer of CH 3 CHO units, with only one unit present in the asymmetric unit. The fourfold rotation axis generates the complete molecule and two molecules are present in the unit cell. There are no conventional strong hydrogen bonds in the structure, due to the lack of a hydrogen-bond donor. One weak C2-H2Á Á ÁO1 iv hydrogen-bond interaction is present within the accepted range for C-HÁ Á ÁO bonds (Desiraju, 1996) [C2Á Á ÁO1 = 3.631 (2) Å and C2-H2Á Á ÁO1 iv = 164 (2) ; symmetry code: (iv) 3 2 À y, 1 2 + x, z À 1 2 ]. The molecule in the body-centred position of the unit cell forms four C-HÁ Á ÁO bonds, one to each of the four molecules located at the unit-cell vertices with z = 0. It also forms four OÁ Á ÁH-C bonds, one to each of the four molecules at the vertices of the unit cell with z = 1 (Fig. 2). The overall motif is a three-dimensional hydrogen-bonded network (Fig. 3). The molecules stack directly upon one another to form columns, parallel to the c axis. As described in the original paper (Pauling & Carpenter, 1936), there may be electrostatic interactions between the adjacent members of the column, as each molecule has a partially negatively charged face (comprising the four O atoms in the ring), and a partially positively charged face (comprising the four H atoms bonded to the C atoms in the ring). The distance between the plane of the four O1 atoms in one molecule and the plane of the four C1 atoms in the adjacent molecule in the column is 3.51 Å .
The largest geometrical difference between this redetermination and the original structure is in the bond angles about C1. In the original determination, the angles about C1 were constrained to the tetrahedral bond angle, whereas in the structure reported here, these angles deviate by up to 2.5 from the tetrahedral angle [C2-C1-O1 = 106.9 (1) , C2-C1-O1 iii = 106.7 (1) and O1-C1-O1 iii = 112.0 (1) ; symmetry code: (iii) y À 1, 1 À x, z]. These deviations may be due to changes in the molecular conformation with temperature, rather than the use of constraints in the original report.

Experimental
Metaldehyde crystals were produced from an attempt to recrystallize 5-fluorocytosine from acetaldehyde by solvent evaporation at 278 K.

Figure 2
A view inclined to the c axis, showing the C-HÁ Á ÁO hydrogen bonding as dotted lines. The grey molecules are one unit cell lower than the coloured molecules. The central molecule bonds to the four molecules in the upper plane and the four in the lower plane.

sup-1
Acta Cryst. 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.