1,4-Bis(iodomethyl)benzene

The centrosymmetric title compound, C8H8I2, was prepared by metathesis from the dibromo analogue. In the crystal structure, weak C—H⋯I interactions link the molecules into stacks down the b axis. The structure is further stabilized by short I⋯I contacts [3.8433 (2) Å], forming undulating sheets in the (101) plane.

The centrosymmetric title compound, C 8 H 8 I 2 , was prepared by metathesis from the dibromo analogue. In the crystal structure, weak C-HÁ Á ÁI interactions link the molecules into stacks down the b axis. The structure is further stabilized by short IÁ Á ÁI contacts [3.8433 (2) Å ], forming undulating sheets in the (101) plane.

Experimental
The molecule lies about an inversion centre located at the centroid of the benzene ring. The C1···C4 atoms lie in a plane (r.m.s. deviation 0.01 Å) and the C-C and C-I distances in the molecule are unremarkable (Allen et al., 1987). This structure is the fourth in a series of XCH 2 C 6 H 4 CH 2 X molecules; X = F, II (Fun et al., 2009); Cl, III (Basaran et al., (1992); Br, IV (Jones & Kus, 2007;Zhang et al., 2007). All four molecules are closely isostructural with only the C-halogen bond distance distinguishing them. Indeed the CH 2 C 6 H 4 CH 2 fragment of the title compound overlays with corresponding portions of the related molecules with r.m.s. deviations, 0.032 Å for II, 0.013 Å for III and 0.007 Å for IV respectively (Macrae et al., 2006).
In the crystal structure, weak C-H···I interactions and short I···I contacts, 3.8433 (2) Å, form undulating sheets in the 101 plane, Fig. 2. Each I atom interacts with two adjacent iodine atoms (symmetry operations 2 -x, -1/2 + y, 1/2 -z and 2 -x, 1/2 + y, 1/2 -z). The I···I contacts observed here fit with the type II designation of halogen···halogen interactions proposed previously (Pedireddi et al. 1994). The packing arrangement for I is closely similar to that observed for IV, (Jones & Kus, 2007;Zhang et al., 2007). I, III and IV all crystallize in the space group P21/c with unit cells that differ only in a small but significant increase in volume as the size of the halogen increases.
The solution was allowed to cool overnight, and the resulting yellow plates of (I) that developed were rinsed gently with water to remove sodium bromide and air dried. Confirmation of the metathesized (iodo) product was by microanalysis, mass spectroscopy and diagnostic tests. 1 H and 13 C NMR spectra are distinct from those of the dibromo precursor.

Refinement
All H-atoms were located in a difference Fourier map and refined freely. Fig. 1. The structure of (I) with displacement ellipsoids for the non-hydrogen atoms drawn at the 50% probability level. Unlabelled atoms are generated by the symmetry operation (1-x, -y, 1-z).