A study of the second coordination sphere in 8-azaxanthinato salts of divalent metal aquacomplexes

The interaction of the 4,6-dimethyl and 4-monomethyl derivatives of 1,2,3-triazolo[4,5-d]pyrimidin-5,7-dione (which may be named also as purine derivatives, 1,3-dimethyl-8-azaxanthine, Hdmax and 3-methyl-8-azaxanthine, H3max) with the divalent cations of Mn, Co, Ni, Zn and Cd in aqueous media generate solids with formulation ML2 · 6H2O. The crystal structure of the Mn and Cd compounds with dmax and the Cd compound with 3max have been determined by single-crystal X-ray diffraction revealing that the compounds are salts with [M(H2O)6]2+ as cations and dmax− or 3dmax− as anions. The second-sphere interactions in these compounds have been analysed, consisting in a network of very well-defined hydrogen bonds, with all available potential donor and acceptor positions involved. The topology of the motifs generated by these hydrogen bonds has been characterised, adapting to the second coordination sphere concepts usually applied to the first (chelate, bridge, monodentate, …). Monodimensional (tapes) superstructures with the building blocks rather tightly bounded appear in the compounds with dmax− as anion, whereas the corresponding superstructure in the Cd compound with 3max− is bidimensional. These superstructures further interact among them in a less tight fashion to generate the three dimensional crystal structures. Powder X-ray diffraction strongly suggests that Mn, Co, Zn and Cd compounds of each ligand are isostructural, so the results of the samples determined by single-crystal X-ray diffraction may be extended to all these compounds. On the other hand, powder X-ray diffraction indicates that the nickel compounds have a different structure and the spectroscopic data for these compounds suggest that the ligand is directly attached to the metal for them.