Oxalato-Nucleobase Extended Systems

The main structural feature common to all compounds is the presence of ID zigzag chains (Fig. 5) in which cw-[M(H20)(L)] units are sequentially bridged by bis-bidentate centrosymmetric oxalato ligands with dihedral angles 

The purine itself, without the exo-cyclic N6 amino group, possesses a great coordination versatility, but a searce number of complexes has been structurally characterized so far, probably because it is not usually present in biopolymers.In the isostructural compounds [M( i-ox)(H20)(pur-KA )] (M = Cu, Co, Mn and Zn), the neutral purine (pur) behaves as a N9-monodentate ligand (Fig. 6). 

Other cases of transition-metal ion binding to the imidazole ring N9 atom of the purine ligand have been reported in the monomer [TiCl(cp)2(pur-KA )] (cp is the cyclopentadienyl anion) with a purinate monoanion and a series of neutral dinuclear and mononuclear gold(I) complexes with phosphine and purin-9-ate synthesized under basic conditions. There are reported examples of Mn(II) binding to N donor sites of nueleobases in biopolymeric systems, but coordinative Mn-N linkages involving isolated nueleobases, as seen in 

FIGURE 6. (a) A fragment of the pol5mieric chain of compounds [M(p-ox)(H20)(pur-kA )] . (b) and (c) Perspective views showing the arrangement of the purine ligands within the chains. 

FIGURE 7. Projections of the cristal packing of the compounds with purine ligand showing the noncovalent interactions among the structural units (hydrogen bonds and n-n contacts). 

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