4,5-Diiodo-2-phenyl-1H-imidazole

The structure of the title compound, C9H6I2N2, contains two symmetry-independent molecules. The interplanar angles between the imidazole and phenyl ring planes are 16.35 (3) and 17.48 (6)°. Molecules are connected via N—H⋯N hydrogen bonds to form zigzag chains along the b axis. The title compound is the first example of a structurally characterized 4,5-diiodoimidazole with an organic substituent in the 2-position and without protection on the N—H group of imidazole.

The structure of the title compound, C 9 H 6 I 2 N 2 , contains two symmetry-independent molecules. The interplanar angles between the imidazole and phenyl ring planes are 16.35 (3) and 17.48 (6) . Molecules are connected via N-HÁ Á ÁN hydrogen bonds to form zigzag chains along the b axis. The title compound is the first example of a structurally characterized 4,5-diiodoimidazole with an organic substituent in the 2-position and without protection on the N-H group of imidazole.

Comment
Halogenated derivatives of heterocycles are important starting compounds for building new heterocyclic chiral ligands via cross-coupling reactions. In recent times 4,5-diiodoimidazole and its derivatives were used for the preparation of various disubstituted 4,5-dicarbaimidazoles (Zhang et al., 2006;Ito & Uedaira, 2004;Kim et al., 1999;Haruki et al., 1965). To the best of our knowledge, there are a couple of structures of dihalogenated imidazoles determined by X-ray diffraction methods, but the 4,5-diido-1H-imidazole structure as the most versatile and simpliest member of the series is missing. Compound I (Scheme 1) was separated as a byproduct from a reaction described by Ishihara (Ishihara & Togo, 2006) but is usually prepared by the method of Garden (Garden et al., 2001). The title compound crystallizes ( In contrast to typical C-I bond lenghts of ca. 2.095 Å (Allen et al., 1987) which is the same for monoiodoimidazoles, the C-I separations in diiodoimidazoles significantly differ between ca. 2.05 and 2.09 Å (Panday et al., 2000;Terinek & Vasella, 2003). This phenomenon is also seen in one of the molecules of I, where the C11-I3 bond length is 2.075 (5) Å and the C12-I4 bond located next to the N-H function is shortened to 2.058 (5) Å. On the other hand, in the first molecule both C-I bond lenghts (2.051 (5) and 2.058 (5) Å) are almost identical and thus comparable to the same parameters found in compounds containing the 4,5-diidoimidazolium ion (Mukai & Nishikawa, 2010a,b). Only one iodine atom I1 is located in the same plane defined by the imidazole ring, while the others show deviations of 0.07-0.148 Å. Bond lenghts between the atoms forming the imidazole rings are comparable to literature values (Allen et al., 1987) except of C1-N2 which is elongated by 0.02 Å and C10-N3 which is slightly shortened.

Experimental
The title compound I was isolated in 30% yield as a byproduct from the reaction mixture of 2-phenyl-1H-imidazole-4-carbaldehyde with ethan-1,2-diamine, iodine and potassium carbonate in tert-butylalcohol, according to Ishihara (Ishihara & Togo, 2006) in order to prepare 2'-phenyl-4,5-dihydro-1H,1'H-2,4'-biimidazole. The reaction mixture was separated by the help of column chromatography on silicagel (ethyl acetate, hexane and dichloromethane -1:3:10). Further crystallization from dichloromethane gave pure I. The identity and purity of I was confirmed by the same melting point, 1 H NMR and mass spectra patterns as published elsewhere (Garden et al., 2001). Single crystals of I were obtained by slow vapour diffussion of hexane into a solution of I in dichloromethane.
supplementary materials sup-2 Refinement All hydrogen atoms were discernible in the difference electron density map. However, all hydrogen atoms were placed into idealized positions and refined riding on their parent C or N atoms, with N-H = 0.86 Å, C-H = 0.93 Å for aromatic H atoms, with U(H) = 1.2U eq (C/N) for the NH group and U(H) = 1.5U eq (C/N) for other H atoms, respectively. Fig. 1. View of one of the independent molecules of the title compound with the displacement ellipsoids shown at the 50% probability level. H atoms are shown with arbitrary radii.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
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 > 2sigma(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.