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The adducts urea:pyrazine (1:1) ( 1 ), thiourea:pyrazine (2:1) ( 2 ), and thiourea:piperazine (2:1) ( 3 ) were prepared and their structures determined. Adduct  1  forms a layer structure, in which urea chains of graph set C(4)[R  2  1${\rm{R}}_{\rm{2}}^{\rm{1}}$   (6)] run parallel to the  b  axis and are crosslinked by N–H···N hydrogen bonding to the pyrazine residues. Adduct  2  is a variant of the well-knownR  2  2${\rm{R}}_{\rm{2}}^{\rm{2}}$   (8) ribbon substructure for urea/thiourea adducts, with the pyrazine molecules attached to the remaining thiourea NH groups via bifurcated hydrogen bonds (N–H···) 2 S; the more distant end of the pyrazine molecules is crosslinked to another symmetry-equivalent but perpendicular ribbon system, thus creating a three-dimensional packing. The packing of adduct  3  involves thiourea layers parallel to the  ab  plane; the piperazine molecules occupy the regions between these layers and are linked to the thiourea molecules by two hydrogen bonds (one as donor, one as acceptor) at each piperazine nitrogen atom.

Structures of the adducts urea:pyrazine (1:1), thiourea:pyrazine (2:1) and thiourea:piperazine (2:1)