Hydrogen-bonded supra­molecular motifs in 2-amino-4,6-dimethoxy­pyrimidinium 4-hydroxy­benzoate monohydrate, 2-amino-4,6-dimeth­oxy­pyrimidinium 6-carboxy­pyridine-2-carboxyl­ate monohydrate and 2-amino-4,6-dimethoxy­pyrimidinium hydrogen (2R,3R)-tartrate 2-amino-4,6-dimethoxy­pyrimidine

In the crystal structures of the title compounds, C₆H₁₀N₃O₂⁺·C₇H₅O₃^-·H₂O, (I), C₆H₁₀N₃O₂⁺·C₇H₄NO₄^-·H₂O, (II), and C₆H₁₀N₃O₂⁺·C₄H₅O₆^-·C₆H₉N₃O₂, (III), the 2-amino-4,6-di­meth­oxy­pyrimidinium cation [abbreviated as (MeO)₂-Hampy⁺] inter­acts with the carboxyl­ate group of the corresponding anion through a pair of nearly parallel N-H...O hydrogen bonds to form R₂²(8) ring motifs. In (I), the (MeO)₂-Hampy⁺ cation is centrosymmetrically paired through a pair of N-H...N hydrogen bonds involving the 2-amino group and a ring N atom forming an R₂²(8) motif. In (II), inversion-related R₂²(8) motifs (amino-pyrimidine-carboxyl­ate motifs) are further bridged by N-H...O hydrogen bonds on either side forming a DDAA array of quadruple hydrogen bonds. This array is extended further on either side by O_water-H...O_meth­oxy hydrogen bonds, resulting in an array of six hydrogen bonds (ADDAAD). The water mol­ecule plays a pivotal role, and five hydrogen-bonded fused rings are formed around the water mol­ecule. In (III), the carb­oxy group of the tartrate anion inter­acts with the ring N atom and 2-amino group of the neutral (MeO)₂-ampy mol­ecule through N-H...O and O-H...N hydrogen bonds. There is also an intra­molecular O-H...O hydrogen bond in the tartrate anion. In all three crystal structures, C-H...O hydrogen bonds are observed.