4-Nitro-phenol-piperazine (2/1).

In the title adduct, C6H5NO3·0.5C4H10N2, the piperazine ring possesses inversion symmetry and has a chair conformation. Its mean plane makes a dihedral angle of 65.45 (7)° with the 4-nitro-phenol ring. In the crystal, the piperazine ring is linked to two 4-nitro-phenol mol-ecules via O-H⋯N hydrogen bonds. The mol-ecules are also linked via bifurcated N-H⋯(O,O) hydrogen bonds involving the NO2 O atoms, forming a two-dimensional network lying parallel to (102). The networks are linked via C-H⋯O hydrogen bonds, forming a three-dimensional structure.

l-Histidinium p-toluene-sulfonate.

In the title salt, C6H10N3O2 (+)·C7H7O3S(-), the imidazole ring makes a dihedral angle of 70.93 (12)° with the plane of the toluene ring. In the crystal, the ions are linked via N-H⋯O and weak C-H⋯O hydrogen bonds forming two-dimensional networks lying parallel to (001). These networks are linked via C-H⋯π inter-actions, forming a three-dimensional structure.

Bis(2-hy-droxy-eth-yl)ammonium picrate.

The asymmetric unit of the title salt, C4H12NO2 (+)·C6H2N3O7 (-), contain two bis-(2-hy-droxy-eth-yl)ammonium cations and two picrate anions. An intra-molecular N-H⋯O hydrogen bond occurs in each cation. In the crystal, mol-ecules are linked via O-H⋯O and N-H⋯O hydrogen bonds, which generate two R 2 (1)(6), an R 2 (2)(10) and an R 2 (2)(13) graph-set ring motifs. There are also a number of C-H⋯O hydrogen bonds present. The sum of these inter-actions leads to the formation a three-dimensional structure.

3-Methyl-4-nitro-phenol-4-dimethyl-amino-pyridine (1/1).

In the title adduct, C(7)H(7)NO(3) (.)C(7)H(10)N(2), the dihedral angle betwen the benzene ring and pyridine rings is 9.60 (8)° while the nitro group attached to the benzene ring makes a dihedral angle of 21.76 (13)°. The hydroxyl O atom deviates by 0.0247 (15) Šfrom the plane of the benzene ring. The crystal packing features O-H⋯N hydrogen bonds.