The mol-ecule of the title compound, C(4)H(4)IN(3), has crystallographic mirror plane symmetry. In the crystal, the mol-ecules are connected through N-H⋯N hydrogen bonds into polymeric tapes extended along the a axis, which are typical of 2-amino-pyrimidines. Each mol-ecule acts as a double donor and a double acceptor in the hydrogen bonding.

In the crystal structure of the title compound, C(5)H(5)BrN(2), mol-ecules assemble via pairs of N-H⋯N hydrogen bonds into inversion dimers using only the syn H atom on the amine group. These dimers then assemble further into two-dimensional layers via type I C-Br⋯Br [Br⋯Br = 3.693 (s6) Å] halogen bonding along the (102) plane.

In the crystal of the title compound, C(12)H(12)N(2), inter-molecular N-H⋯N hydrogen bonds form rings of graph-set motif R(2) (2)(8) and C-H⋯π inter-actions further consolidate the dimers. Neighbouring dimers are further connected into a three-dimensional network by C-H⋯π inter-actions. The benzyl and pyridyl rings form a dihedral angle of 67.2 (1)°

In the crystal structure of the title compound, C(7)H(6)N(2)S, mol-ecules related by an inversion center are linked via N-H⋯N hydrogen bonds involving the amino groups, forming dimers. In turn, these dimers are linked via a second N-H⋯N hydrogen bond, forming an infinite two-dimensional network parallel to (011).

The asymmetric unit of the title compound, C6H9N3, contains three crystallographically independent mol-ecules of similar geometry. All of the mol-ecules are almost planar, with r.m.s. deviations of 0.003, 0.016 and 0.005 Å. In the crystal, the mol-ecules are linked by N-H⋯N hydrogen bonds into zigzag ribbons parallel to the c axis, generating rings of R2(2)(8) graph-set motif.

In the title compound, C(5)H(5)ClN(2), a by-product in the synthesis of ethyl 2-(3-chloro-pyridin-2-yl)-5-oxopyrazolidine-3-carboxyl-ate, the amine groups form inter-molecular hydrogen-bonding associations with pyridine N-atom acceptors, giving centrosymmetric cyclic dimers. Short inter-molecular Cl⋯Cl inter-actions [3.278 (3) Å] also occur.