1,2-Diiodo-4,5-dimethylbenzene

The structure of the title compound, C8H8I2, conforms closely to the mm2 symmetry expected for the free molecule and is the first reported structure of a diiododimethylbenzene. Repulsion by neighboring I atoms and the neighboring methyl groups opposite to them results in a slight elongation of the molecule along the approximate twofold rotation axis that bisects the ring between the two I atoms. In the extended structure, the molecules form inversion-related pairs which are organized in approximately hexagonal close-packed layers and the layers then stacked so that molecules in neighboring layers abut head-to-tail in a manner that optimizes dipole–dipole interactions.

The structure of the title compound, C 8 H 8 I 2 , conforms closely to the mm2 symmetry expected for the free molecule and is the first reported structure of a diiododimethylbenzene. Repulsion by neighboring I atoms and the neighboring methyl groups opposite to them results in a slight elongation of the molecule along the approximate twofold rotation axis that bisects the ring between the two I atoms. In the extended structure, the molecules form inversion-related pairs which are organized in approximately hexagonal close-packed layers and the layers then stacked so that molecules in neighboring layers abut head-to-tail in a manner that optimizes dipoledipole interactions.

Related literature
For the synthesis see: Suzuki (1988). For the structure of 1,2diiodo-4,5-dimethoxybenzene, see: Cukiernik et al. (2008). For methods of iodinating substituted benzenes, see: Hathaway et al. (2007). For related work on diacetylenes, see: Hathaway (1988); Hathaway & Scates (1997 Comment 1,2-Diiodo-4,5-dimethylbenzene, (I), was prepared as part of an investigation of methods of iodination of organic compounds. In previous work we prepared diacetylenes as potential non-linear optical materials (Hathaway, 1988;Hathaway & Scates, 1997). Since iodobenzenes are common starting materials for preparations of phenyl alkynes, we investigated several methods of iodinating substituted benzenes (Hathaway et al., 2007). The title compound was prepared as an extension of this investigation. Ortho-xylene was iodinated using iodine and periodic acid (Suzuki, 1988), to learn if two iodine atoms could be substituted onto the benzene ring in the positions opposite the methyl groups. Since no structures of diiododimethylbenzenes have yet been reported, we undertook the crystal structure determination of (I). The structure of the similar compound 1,2-diiodo-4,5-dimethoxybenzene has recently been reported. (Cukiernik et al., 2008). The structure of (I) offers a distinct contrast due to the lack of key intermolecular interactions.
Although the title molecule does not possess any crystallographic site symmetry, it closely approximates mm2 point symmetry-even by the methyl hydrogen atoms (which were clearly identified on an electron density difference map). The two C-I bonds have lengths [2.097 (3) Å] that are identical to each other and also agree with the sum of the covalent radii within 1 su. Carbon-carbon bond lengths and angles within the aromatic ring and to the methyl groups conform closely to expected values. The average C-C bond length within the ring is 1.387 Å with deviations from the average within 1 su.
Steric repulsion between the iodine atoms ortho to one another on the ring increases the facing C-C-I angles by over 3°f rom the ideal value. A similar repulsive distortion between the methyl groups ortho to one another is also found to give facing C-C-C angles between 121 and 122°. Internal C-C-C angles for the aromatic ring are greater than 120° for the two ring atoms without substituents (C3 and C6) and are less than 120° for the substituted ring atoms. This arises from a slight elongation of the ring along the approximate twofold rotation axis that bisects ring the between like substituent groups. The elongation produces contact distances to the opposite atom in the ring that are longer [2.791 (6) Å] for subsituted ring atoms than for the two unsubstituted ring atoms [2.738 (5) Å]. The I1-C1-C2-I2 and C41-C4-C5-C51 torsion angles are similar in value [2.5 (4) and 2.6 (6) ° respectively] and arise from just a small twist of the molecular plane about its long axis. A thermal ellipsoid plot of the molecule is presented in Figure 1.
The molecules pack in layers coinciding with the (101) family of planes. The layers are constructed from pairs of inversion related molecules which have their iodine atoms pointing in opposite directions outward from the the layer plane.
The perpendicular distance between the molecular planes of the inversion related molecules is 3.752 (12)  Neighboring layers can be related to one another by an a axis translation. The a axis translation does establish a head-totail line of molecules in which the diiodo end of a molecule in one layer abuts the dimethyl end of a molecule in a neighboring layer and vice versa. The shortest I···H contact distances both within and between layers range from 3.32-3.35 Å and are close to the sum of the van der Waals radii. So it is unlikely that significant hydrogen-bonding exists in the stucture. A short I···I interaction, 4.2126 (4) Å [I1···I2 ii ; symmetry code: (ii) -x + 3/2,y -1/2,-z + 1/2], is found between molecules in different layers and related to one another by a 2 1 rotation. These short intra-and inter-layer I···I contact distances are comparable in length to those found in the 1,2-diiodo-4,5-dimethoxy structure and in both structures do play a role in the packing arrangement. However the inversion-pairing within layers and head-to-tail arrangement between layers of molecules in (I) is more simply explained on the basis of optimizing dipole-dipole interactions. In contrast, the presence of I···O contacts in the dimethoxy analogue dictate a more complicated packing arrangement based on linear chains of molecules. The simpler structure of (I) in the absence of any I···O contacts further supports the earlier conclusion that these I···O contacts prevail in establishing the molecular packing in the dimethoxy analogue. A unit cell packing diagram of the structure is shown in Figure 2.

Refinement
All hydrogen atoms were clearly visible on a difference Fourier map but, because of the heavy atoms present, their positions were calculated to give an idealized geometry, with C-H bond distances of 0.96 Å for methyl hydrogen atoms and 0.93 Å for aromatic hydrogen atoms. They were constrained to ride on their parent carbon atoms during refinement with the torsion angle for the methyl hydrogen atoms refined to best match the observed electron density. Fig. 1. View of (I) (50% probability displacement ellipsoids).