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) and Ag(7)2 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) and Ag(7)2 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.