Structural insights on the molecular recognition patterns between N6-substituted adenines and N-(aryl-methyl)iminodiacetate copper(II) chelates☆

For a better understanding of the metal binding pattern of N6-substituted adenines, six novel ternary Cu(II) complexes have been structurally characterized by single crystal X-ray diffraction: [Cu(NBzIDA)(HCy5ade)(H2O)] · H2O (1), [Cu(NBzIDA)(HCy6ade)(H2O)] · H2O (2), [Cu(FurIDA)(HCy6ade)(H2O)] · H2O (3), [Cu(MEBIDA)(HBAP)(H2O)] · H2O (4), [Cu(FurIDA)(HBAP)]n (5) and {[Cu(NBzIDA)(HdimAP)] · H2O}n (6). In these compounds NBzIDA, FurIDA and MEBIDA are N-substituted iminodiacetates with a non-coordinating aryl-methyl pendant arm (benzyl in NBzIDA, p-tolyl in MEBIDA and furfuryl in FurIDA) whereas HBAP, HCy5ade, HCy6ade and HdimAP are N6-substituted adenine derivatives with a N-benzyl, N-cyclopentyl, N-cyclohexyl or two N-methyl groups, respectively. Regardless of the molecular (1–4) or polymeric (5–6) nature of the studied compounds, the Cu(II) centre exhibits a type 4 + 1 coordination where the tridentate IDA-like chelators adopt a mer-conformation. In 1–5 the N6-R-adenines use their most stable tautomer H(N9)adenine-like, and molecular recognition consists of the cooperation of the Cusingle bondN3(purine) bond and the intra-molecular interligand N9single bondH · · · O(coordinated carboxy) interaction. In contrast, N6,N6-dimethyl-adenine shows the rare tautomer H(N3)dimAP in 6, so that the molecular recognition with the Cu(NBzIDA) chelate consist of the Cusingle bondN9 bond and the N3single bondH · · · O intra-molecular interligand interaction. Contrastingly to the cytokinin activity found in the free ligands HBAP (natural cytokinin), HCy5ade and HCy6ade, the corresponding Cu(II) ternary complexes did not show any activity.