N-H...O and N-H...N inter­actions in three pyran derivatives

The three pyran structures 6-methyl­amino-5-nitro-2,4-di­phenyl-4H-pyran-3-carbo­nitrile, C₁₉H₁₅N₃O₃, (I), 4-(3-fluoro­phen­yl)-6-methyl­amino-5-nitro-2-phenyl-4H-pyran-3-carbo­nitrile, C₁₉H₁₄FN₃O₃, (II), and 4-(4-chloro­phen­yl)-6-methyl­amino-5-nitro-2-phenyl-4H-pyran-3-carbo­nitrile, C₁₉H₁₄ClN₃O₃, (III), differ in the nature of the aryl group at the 4-position. The heterocyclic ring in all three structures adopts a flattened boat conformation. The dihedral angle between the pseudo-axial phenyl substituent and the flat part of the pyran ring is 89.97 (1)° in (I), 80.11 (1)° in (II) and 87.77 (1)° in (III). In all three crystal structures, a strong intra­molecular N-H...O hydrogen bond links the flat conjugated H-N-C=C-N-O fragment into a six-membered ring. In (II), mol­ecules are linked into dimeric aggregates by N-H... O(nitro) hydrogen bonds, generating an R₂²(12) graph-set motif. In (III), inter­molecular N-H...N and C-H...N hydrogen bonds link the mol­ecules into a linear chain pattern generating C(8) and C(9) graph-set motifs, respectively.