The synthesis, structure and CO-releasing properties of a number of new tricarbonyl rhenium(i) complexes with 5-substituted-6-amino-1,3-dimethyluracils are reported and their structural features discussed on the basis of both spectral and X-ray crystallographic analyses. The 5-substituent library includes -N[double bond, length as m-dash]CH-2py (DAAUPic) and -CH[double bond, length as m-dash]N-N[double bond, length as m-dash]CH-2py (FDUHzPic) as additional metal binding components and chloride, acetonitrile or pyridine acting as ancillary ligands. The compounds have been identified by elemental analysis, NMR, MS and IR spectroscopy. In addition, [ReCl(CO)(DAAUPic)], [Re(CO)(FDUHzPic)py]ClO, [Re(CO)(FDUHzPic)py]PF, [ReCl(CO)(FDUHzPic)] and [ReCl(CO)(FDUHzPicH)(HO)] structures have been solved by X-ray diffraction methods. These studies have clearly shown that the preferred coordination mode to rhenium takes place through the (N1F,N52)-pyridin-2-yl-methyleneamine moiety, the uracil coordinative availability (O4-N51 or N6-N51) being used only to bind the second metal center. The CO-releasing ability of these rhenium compounds has been investigated by the reaction with myoglobin; the corresponding studies have revealed that two of the mononuclear complexes and their related binuclear analogues are able to release CO to a moderate extent. This ability has also been theoretically assessed through a QTAIM analysis. The results, although non-conclusive, may explain somehow possible different preferences in CO releasing power after a comparison between the nature of Re-CO links in mononuclear and binuclear compounds.

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The new complex formed by Cd(II) and the 1:2 Schiff-base-type ligand 2,6-bis[1-(4-amino-1,2,3,6-tetrahydro-1,3-dimethyl-2,6-dioxopyrimidin-5-yl)imino]ethylpyridine (DAPDAAU) has been chemically and structurally characterized by X-ray diffraction: the ion Cd(II) is surrounded by six nitrogen atoms from two DAPDAAU ligands which coordinates each one in a tridentate fashion through the pyridine ring (N1) and both azomethine nitrogen atoms (N5). The interaction of the Cd(II) complex (compound I) with calf-thymus DNA as observed by circular dichroism spectroscopy suggests the initial unwinding of the DNA double helix strongly depends on increasing incubation times and metal-to-nucleic acid molar ratios. Electrophoretic experiments indicate that the cadmium complex induces cleavage of the plasmid pBR322 DNA to give ulterior nicking and shortening of this molecule, as a result of the complex binding to DNA, resulting in the conclusion that compound I behaves as a chemical nuclease. Cytotoxic activity of the Cd(II) complex against selected different human cancer cell lines is specific and increases with increasing concentration of the metal compound; this fact indicates the potential antitumor character of the complex. When the culture medium is supplemented with compound I, a remarkable inhibition of the growing cell is observed, important cell degeneration appears before 48 h and abundant precipitates are formed that correspond to cell residues and denatured proteins.

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