Di-aqua-bis-[5-(2-pyrazin-2-yl)tetra-zolato]copper(II)-pyrazine-2-carbo-nitrile (1/2).

The title compound, [Cu(C5H3N6)2(H2O)2]·2C5H3N3, is a 1:2 co-crystal between the mononuclear complex di-aqua-bis-[5-(pyrazin-2-yl)tetra-zolato]copper(II) and the reagent pyrazine-2-carbo-nitrile which was used in the synthesis. The Cu(II) atom is located on an inversion centre and has a distorted octa-hedral [4 + 2]-coordination environment formed by four N atoms of two chelating bidentate 5-(pyrazin-2-yl)tetra-zolate ligands at shorter distances and two water O atoms at longer distances. The Cu(II) complex molecules are held together by O-H⋯N hydrogen bonds and π-π stacking inter-actions [centroid-centroid distance 3.6139 (8) Å], forming layers parallel to (100). These layers alternate with layers of pyrazine-2-car-bo-nitrile mol-ecules and both are held together via C-H⋯N hydrogen bonds and further π-π stacking inter-actions.

(E)-1-[(2,4,6-Tri-bromo-phen-yl)diazen-yl]naphthalen-2-ol.

The title azo mol-ecule, C16H9Br3N2O, adopts a trans conformation with respect to the azo N=N double bond. An intra-molecular O-H⋯N hydrogen bond forms an S(6) ring motif. The dihedral angle between the naphthalene ring system and the benzene ring is 33.80 (16)°. In the crystal, mol-ecules are stacked in columns along the a axis by π-π inter-actions [centroid-centroid distances = 3.815 (3) and 3.990 (3) Å].

1-[(E)-2-(2-Hy-droxy-5-methyl-phen-yl)diazen-2-ium-1-yl]naphthalen-2-olate.

The title zwitterion, C17H14N2O2, crystallizes with two independent mol-ecules in the asymmetric unit, both of which are approximately planar, the dihedral angles between the benzene ring and the naphthalene ring system being 4.39 (12)° in one mol-ecule and 5.83 (12)° in the other, and show an E conformation with respect to the azo double bond. An intra-molecular N-H⋯O hydrogen bond in each molecule helps to establish their near planar conformation. In the crystal, mol-ecules are linked through O-H⋯O hydrogen bonds into infinite chains running along the a-axis direction. In addition, the chains are stacked along the b axis via π-π inter-actions between the benzene and the naphthalene rings of adjacent mol-ecules, the centroid-centroid distances being 3.722 (3) and 3.823 (4) Å.

(E)-1-(2-Phenyl-diazen-2-ium-1-yl)naph-thalen-2-olate.

In the title zwitterionic compound, C16H12N2O, the dihedral angle between the phenyl ring and the naphthalene ring system is 17.85 (8)°; an intra-molecular N-H⋯O hydrogen bond occurs. In the crystal, π-π stacking is observed between naphthalene ring systems of adjacent mol-ecules, the centroid-centroid distance being 3.6486 (11) Å.