Synthesis, equilibrium studies and structural characterisation of the Zn(II) complexes, with trimethylene-N6,N6′-bisadenine

The new compound trimethylene-N-6, N-6'-bisadenine (L), in which two adenine molecules are linked together by a trimethylene bridge that connects the N-6 atoms, has been prepared. Reaction of L with HgCl2 and ZnCl2 in concentrated HCl solution leads to crystalline solids. The X-ray characterisation of the Hg(II) complex (H2L)[HgCl4].3H(2)O reveals that it is an outer-sphere complex in which the ligand is protonated at N(1) and N(1'). In contrast, the structure of the complex [H2L(ZnCl3)(2)].2H(2)O shows the ligand co-ordinated to two different Zn(II) ions through the N(7) of both adenine fragments, the protons being located on the N(I) atoms. The latter compound constitutes the first crystallographic evidence of an inner sphere complex with bis-adenines and, for this reason, an equilibrium study was carried out on the Zn(II)-L-H+ system. Potentiometric studies indicate that L is protonated in aqueous solution to form HL+ and H2L2+ with log K-H values of 4.42 and 3.35 (25degreesC, 0.10 M KNO3). The data from potentiometric titrations in the presence of Zn2+ can be analysed considering the formation of the species LZn2+, HLZn3+, LZn24+, and HLZn25+, whose stability constants exceed the value expected for a monodentate interaction of the metal ion with adenine and Suggest the Possibility of a polydentate behaviour of L in the pH range 2.5-5.0. In contrast, spectrophotometric titrations carried out under conditions similar to those used in the synthetic work (1 M HCl) can be fitted with a model involving exclusively the H2LZn4+ and H2LZn26+ species with log K-M values reasonable for the interaction of Zn(II) with the N(7) of the protonated adenine fragments. Despite the H2LZn26+ species has a low stability, the spectrophotometric results are in agreement with its formation under the conditions in Which the solid complex was prepared. (C) 2002 Elsevier Science Inc. All rights reserved.