N,N′-Bis(5-bromopyridin-2-yl)methanediamine

The V-shaped title compound, C11H10Br2N4, lies on a crystallographic twofold rotation axis which passes through the central C atom. In the crystal, an infinite tape motif, which propagates in the a-axis direction, is formed by inversion-related N—H⋯N hydrogen-bonding interactions. The structure confirmed the identity of the compound as a reaction side product.

The V-shaped title compound, C 11 H 10 Br 2 N 4 , lies on a crystallographic twofold rotation axis which passes through the central C atom. In the crystal, an infinite tape motif, which propagates in the a-axis direction, is formed by inversionrelated N-HÁ Á ÁN hydrogen-bonding interactions. The structure confirmed the identity of the compound as a reaction side product.
The diffractometer was purchased with funding from NSF grant CHE-0741837.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: NG5123).

Comment
The Groebke-Blackburn reaction is the most popular way to prepare imidazo-azines from 2-aminoazines in a single-step. (Groebke et al., 1998;Bienaymé & Bouzid,1998;Blackburn et al., 1998). The reaction involves addition of 2-aminoazine 1 to the aldehyde in the presence of catalytic amounts of acid to generate the respective Schiff base which undergoes a nonconcerted [4 + 1] cycloaddition with an isocyanide to form the imidazoazine 2 (Pathway A, Figure 1). Though imidazoazine 2 remained the major product, it was later found that the reaction also produced the isomeric imidazo[1,2-a]pyrimidine product 3 through an alternative iminium intermediate involving the ring nitrogen of 1 (Pathway B, Figure 1; Parchinsky et al., 2006). Similarly, nucleophilic solvents (for example, methanol) were found to promote interaction of the primary imine intermediate with the second molecule of 2-aminoazine or the solvent itself to give side-products like 4 (Pathway C, Figure 1; Mandair et al., 2002).
The V-shaped structure of 4a is shown in Figure 3. The compound has crystallized with atom C6 on a twofold rotation axis, and has been set in space group I2/a. Molecular dimensions are unexceptional. In the crystal, inversion-related N-H···N hydrogen bonding interactions form an R 2 2 (8) graph set motif (Bernstein et al., 1995). As a result, the crystal forms an infinite hydrogen bonded tape of V-shaped molecules, which propagates in the a-axis direction. The tapes are stacked in the b-axis direction, and the separation between each tape is approximately 3.6 Å. The structure of the related compound N,N'-Di-2-pyridylmethylenediamine exhibits the same V-shaped structure, but with a different crystal packing arrangment (Wu et al., 2004).

Experimental
To a solution of 5-bromopyridin-2-amine 1a (0.58 mmol, 100 mg) in dichloromethane (DCM) (1.5 ml), was added aq. 37% solution of formaldehyde (140 µl, 1.78 mmol) followed by (isocyanomethyl)benzene (75.4 µl, 0.58 mmol) and the solution was stirred for 10 min. DCM was evaporated and the resulting solid was irradiated under microwave at 100° C for 10 min. The crude product was purified through silica gel chromatography to provide 41 mg of 4a in (20% yield). The product was recrystallized from a DCM solution.

Refinement
All H atoms were located in a difference map and are freely refined, with the exception of an N-H distance restaint of 0.88 (1) Å used on H2N. C-H distances lie in the range 0.92 (2) to 1.01 (2) Å.