Pyridine 1:1 adducts of urea (Z' = 1) and thiourea (Z' = 8)

During our studies of urea and thiourea adducts, we noticed that no adducts with unsubstituted pyridine had been structurally investigated. The 1:1 adduct of pyridine and urea, C₅H₅N·CH₄N₂O, crystallizes in the P2₁/c space group with Z = 4. The structure is of a standard type for urea adducts, whereby the urea molecules form a ribbon, parallel to the a axis, consisting of linked R₂²(8) rings, and the pyridine molecules are attached to the periphery of the ribbon by bifurcated (N-H...)₂N hydrogen bonds. The 1:1 adduct of pyridine and thiourea, C₅H₅N·CH₄N₂S, crystallizes in the P2₁/n space group, with Z = 32 (Z' = 8). The structure displays similar ribbons to those of the urea adduct. There are two independent ribbons parallel to the b axis at z ≃ 0 and 1/2, and three at z ≃ 1/4 and 3/4; the latter are crosslinked to form a layer structure by additional long N-H...S interactions, which each formally replace one branch of a bifurcated hydrogen-bond system.