Crystal structures of pure 3-(4-bromo-2-chlorophenyl)-1-(pyridin-4-yl)benzo[4,5]imidazo-[1,2-d][1,2,4]triazin-4(3H)-one and contaminated with 3-(4-bromophenyl)-1-(pyridin-4-yl)benzo-[4,5]imidazo[1,2-d][1,2,4]triazin-4(3H)-one

The side product of the cyclo­condensation reaction between ethyl benzimidazole-2-carboxyl­ate and the nitrile imine of the corresponding hydrazonyl chloride, C20H11BrClN5O, crystallized in two crystal forms. Form (1) is a co-crystal of the target compound (without any chlorine substituent) and a side product containing a Cl atom in position 2 of the bromo­phenyl group, C20H12BrN5O·0.143C20H11BrClN5O. (2) contains the pure side product. The slightly different conformation of the ring systems leads to a different packing of (1) and (2), but both crystal structures are dominated by p-p inter­actions.