Head-to-head and head-to-tail binding of a Pd(II)–methylthyminato complex: crystal structures and solution behavior of a novel cisplatin analogue

A novel cisplatin analogue was prepared as well as the equivalent palladium complex using the chelating ligand 3-chloro-6-(3-methylpyrazol-1-yl)pyridazine (cmpp). The reaction of [Pt(cmpp)(H2O)2]2+ and [Pd(cmpp)(H2O)2]2+ with the model nucleobase 1-methylthymine (1-MeT) was studied. The palladium compound forms a dinuclear complex with the model nucleobase, acting as a secondary bridging ligand via its N3 and O4 donor atoms. The resulting Pd(II)–cmpp–methylthyminato complex was characterized by X-ray structure analysis. Noteworthy is the fact that—depending on the use of different counter ions—it crystallizes in -two-different crystal systems. Bis(μ-1-methylthyminato-N3-O4)bis[(3-chloro-6-(3-methylpyrazol-1-yl)pyridazine)palladium(II)] hexafluorophosphate nitrate hydrate (4) (triclinic, space group P1̄) contains two crystallographically independent complex molecules within the asymmetric unit. One complex contains the model nucleobases in a head-to-head orientation, and the other binds the bridging methylthyminato ligands in a head-to-tail arrangement. Using perchlorate as a counter ion, the reaction of cmpp with 1-MeT leads to bis(μ-1-methylthyminato-N3-O4)bis[(3-chloro-6-(3-methylpyrazol-1-yl)pyridazine)palladium(II)] perchlorate hydrate (5) which crystallizes in the monoclinic space group P21/n. The complex cations in 5 all show a head-to-tail orientation concerning the bridging model nucleobases. The dynamic behavior of the analogous platinum compound and the coordination of the model nucleobase in solution was studied by 195Pt NMR.