The title compound, [Cu(C5H3N6)2(H2O)2]·2C5H3N3, is a 1:2 co-crystal between the mononuclear complex di­aqua­bis­[5-(pyrazin-2-yl)tetra­zolato]copper(II) and the reagent pyrazine-2-carbo­nitrile which was used in the synthesis. The CuII atom is located on an inversion centre and has a distorted octa­hedral [4 + 2]-coordination environment formed by four N atoms of two chelating bidentate 5-(pyrazin-2-yl)tetra­zolate ligands at shorter distances and two water O atoms at longer distances. The CuII complex molecules are held together by O—H⋯N hydrogen bonds and π–π stacking inter­actions [centroid–centroid distance 3.6139 (8) Å], forming layers parallel to (100). These layers alternate with layers of pyrazine-2-car­bo­nitrile mol­ecules and both are held together via C—H⋯N hydrogen bonds and further π–π stacking inter­actions.

Two alkaline earth-tetrazole compounds, namely catena-poly[[[triaquamagnesium(II)]-μ-5,5'-(azanediyl)ditetrazolato-κ(3)N(1),N(1'):N(5)] hemi{bis[μ-5,5'-(azanediyl)ditetrazolato-κ(3)N(1),N(1'):N(2)]bis[triaquamagnesium(II)]} monohydrate], {[Mg(C(2)HN(9))(H(2)O)(3)][Mg(2)(C(2)HN(9))(2)(H(2)O)(6)]0.5·H(2)O}n, (I), and bis[5-(pyrazin-2-yl)tetrazolate] hexaaquamagnesium(II), (C(5)H(3)N(6))[Mg(H(2)O)(6)], (II), have been prepared under hydrothermal conditions. Compound (I) is a mixed dimer-polymer based on magnesium ion centres and can be regarded as the first example of a magnesium-tetrazolate polymer in the crystalline form. The structure shows a complex three-dimensional hydrogen-bonded network that involves magnesium-tetrazolate dimers, solvent water molecules and one-dimensional magnesium-tetrazolate polymeric chains. The intrinsic cohesion in the polymer chains is ensured by N-H...N hydrogen bonds, which form R(2)(2)(7) rings, thus reinforcing the propagation of the polymer chain along the a axis. The crystal structure of magnesium tetrazole salt (II) reveals a mixed ribbon of hydrogen-bonded rings, of types R(2)(2)(7), R(2)(2)(9) and R(2)(4)(10), running along the c axis, which are linked by R(2)(4)(16) rings, generating a 4,8-c flu net.

Two alkaline-earth coordination compounds, [Ba(C8H4N4O2)(H2O)4] n , (I), and [Sr(C8H4N4O2)(H2O)3] n , (II), from the one-pot hydrolysis transformation of benzoyl chloride and the in situ self-assembled [2 + 3] cycloaddition of nitrile are presented. These coordination compounds are prepared by reacting 4-cyanobenzoyl chloride with divalent alkaline-earth salts (BaCl2 and SrCl2) in aqueous solution under hydrothermal conditions. The mononuclear coordination compounds (I) and (II) show the same mode of coordination of the organic ligands. The cohesion of the crystalline structures is provided by hydrogen bonds and π-stacking interactions, thus forming three-dimensional supramolecular networks. The two compounds have a three-dimensional (3,6)-connected topology, and the structural differences between them is in the number of water molecules around the alkaline earth metals. Having the same emission frequencies, the compounds exhibit photoluminescence properties with a downward absorption value from (I) to (II).