Metal complexes of 6-pyrazolylpurine derivatives as models for metal-mediated base pairs

6-(3,5-Dimethylpyrazol-1-yl)purine has recently been introduced as an artificial nucleobase for the specific recognition of canonical nucleobases via the formation of a metal-mediated base pair. We report here the synthesis and structural characterization by single crystal X-ray diffraction analysis of a series of metal complexes of the corresponding alkylated model nucleobases 9-methyl-6-(3,5-dimethylpyrazol-1-yl)purine 2 and 9-methyl-6-pyrazol-1-yl-purine 7. The sterically more demanding ligand 2 forms the Cu(2+) complexes [Cu(2)(NO3)2] and [Cu(2)Cl2] with a 1:1 stoichiometry of ligand and metal ion. In contrast, ligand 7 forms complexes [Cu(7)2(NO3)](NO3) and [Ag(7)2](ClO4) with a 2:1 stoichiometry. The molecular structures of [Cu(2)(NO3)2] and [Cu(2)Cl2] confirm the previously suggested coordination pattern, i.e. Cu(2+) is coordinated via the pyrazole nitrogen atom and the purine N7 position. The fact that different relative orientations of the two ligands in [Cu(7)2(NO3)](NO3) and [Ag(7)2](ClO4) are observed allows the prediction that the corresponding metal-mediated homo base pairs should be stable both in regular antiparallel-stranded and in the rare parallel-stranded double helices.