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, C(6)H(10)N(3)O(2)+.C(7)H(5)O(3)-.H2O, (I), C(6)H(10)N(3)O(2)+.C(7)H(4)NO(4)-.H2O, (II), and C(6)H(10)N(3)O(2)+.C(4)H(5)O(6)-.C(6)H(9)N(3)O(2), (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 R(2)(2)(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 R(2)(2)(8) motif. In (II), inversion-related R(2)(2)(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.