Purines ligands

Systems Based on Two Different Ring Types 13.2.6.4.1 Purine ligand systems... 

As a free ligand, purine (68) in the neutral form has been studied only to a limited extent. The acidic hydrogen on the nitrogen is in fact labile and may also occur on N-9 and N-l. Upon deprotonation the purine anion (78) results, and now four nitrogen atoms are available for metal-ion binding. Bridging between two metal ions has been observed quite frequently for the deprotonated purine ligands. 

Figure 24 Schematic structure of copper(II) dimeric species held together by four bridging purine ligands (after ref. 28).

The effect of Pt coordination on the acid-base properties of the purine ligands / -ethylguanine, N -methylhypoxanthine, inosine, Af -methyladenine, and trimethyladenine has been quantitatively evaluated The acidity constants of the complexes were calculated by curve-fitting procedures, using the pH (pD) dependence of the H NMR shift data. It was found that Pt coordinated to the N -site acidifies the N-7 unit to the same extent as N -coordinated Pt acidifies the N-1 site. 

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). 

It should be pointed out that the parallel orientation of the purine ligands with respect to the metal-oxalato framework locates the nonprotonated minor groove N3 atom over the carbon-carbon bond of one centrosymetric oxalato ligand with a mean N3 C distance of 3.0 A and a dihedral angle between the pyrimidinic ring and the oxalato plane of 90° (Fig. 8). This fact precludes the involvement of the potential hydrogen-bonding N3 atom in any other interaction. 

As seen for compound [Co(p-ox)(H20)(pur-KA ] , this crystal structure is flexible enough to allow the random partial subtitution of the purine ligands for adenine ligands giving an isomorphous compound with formula [Co(p-ox)(H20)(pur-KA o.76(7 f-ade-KA o.24] - Its crystallographic parameters are similar, but the orientation of the terminal purine ligands and the partial replacement of purine by adenine (6-aminopurine) with an additional exocyclic... 

---