Structural consequences of the N7 and C8 translocation on the metal binding behavior of adenine.

7-Deaza-8-aza-adenine, namely 4-aminopyrazolo[3,4-d]pyrimidine (H4app), is a bioisoster of adenine (Hade) resulting from the translocation of N7 and C8 atoms on the purine moiety. With the aim of studying the influence of this translocation on the metal binding abilities of H4app, we have prepared and structurally characterized two ternary copper(II) complexes having H4app and one N-benzyl-iminodiacetate chelator (MEBIDA or FBIDA, with a methyl or fluoro group in para- of the benzyl aromatic ring): [Cu(2)(MEBIDA)(2)(μ(2)-N1,N8-H4app)(H(2)O)(2)]·4H(2)O (1) and [Cu(4)(FBIDA)(4)(μ(2)-N8,N9-H4app)(2)(H(2)O)]·3.5H(2)O (2). Furthermore, thermal, spectral, and magnetic properties have been also investigated. In 1, H(N9)4app is disordered over two equally pondered positions and the μ(2)-N1,N8 coordination mode is assisted by N6-H···O and N9-H···O intramolecular interactions, respectively. The acyclic nonlinear molecular topology of 2 is strongly influenced by two intramolecular H-bonding interactions (O-H···O-carboxylate) involving the apical aqua ligand of a terminal Cu(II) atom. Thus, both compounds have in common the Cu-N8 bond. In order to better understand our limited structural information, DFT calculations for the individual tautomers of H4app as well as mononuclear Cu(II) model systems have been carried out. According to previous results, we conclude that H(N9)4app is the most stable tautomer followed by H(N8)4app. When N9 and N8 are metalated, then the tautomer H(N1)4app can come into play as observed in compound 2. Likewise, the findings concerning compound 1 suggest that the formation of a Cu-N1 bond in H4app results was favored compared to neutral adenine, for which only one case has been reported with such coordination despite the large variety of related Cu(II)-Hade described in the literature.