Unusual Polymeric Zn^II/Cd^II Complexes with 2,6-Diaminopurine by Synergistic Coordination of Nucleobases and Polycarboxylate Anions: Binding Behavior, Self-Assembled Pattern of the Nucleobase, and Luminscent Properties

To investigate the coordination behavior of 2,6-diaminopurine (Hdap) and to construct unique high-dimensional nucleobase-based complexes, six Hdap-based Zn^II/Cd^II polymers were obtained by incorporating aliphatic/aromatic polycarboxylate as coligands under hydrothermal conditions and were fully structurally characterized. Significantly resulting from the synergistic coordination of nucleobase and polycarboxylate groups, they are a tetranuclear Zn^II-core-based two-dimensional (2D) covalent layer for {[Zn₄(μ₂-Hdap)₂(tp)₃(μ₃-OH)₂]·2H₂O}_n (1), an eight-connected three-dimensional (3D) self-penetrating metal−organic framework (MOF) for [Zn₂(μ₂-Hdap)(tp)₂]_n (2), a 3D pillared-layer structure for {[Zn₂(μ₂-Hdap)(tm)(μ₂-OH)]·H₂O}_n (3), a one-dimensional (1D) linear double-chain motif for {[Zn(H₂dap)(H₂O)(btc)]·3H₂O}_n (4), a trinuclear Cd^II-cluster-based 2D aggregate for {[Cd₃(H₂O)₂(μ₃-dap)₂(ap)₂]·H₂O}_n (5), and a 1D Z-shaped chain for {[Cd(H₂dap)(H₂O)₂(tp)]·0.5tp·H₂O}_n (6), respectively (H₂tp = terephthalic acid, H₃tm = trimesic acid, H₃btc = 1,2,3-benzenetricarboxylic acid, and H₂ap = adipic acid). In the polymers, the neutral Hdap nucleobase presents bidentate μ₂-N3,N9 and μ₂-N7,N9 bridging modes to contribute to the tetranuclear Zn^II cluster in 1 and the [Zn₂(Hdap)]⁴⁺ subunit in 2 and 3, respectively. In contrast, acting as a terminal ligand by monodentate N9 and N7 binding patterns, the cationic H₂dap⁺ ligand just completes the metal coordination sphere in 4 and 6. More interestingly, the anionic dap⁻ molecule in 5 exhibits a tridentate μ₃-N3,N7,N9 bridging manner to devote to both the aggregate of the three Cd^II cores within the trinuclear cluster and the extension of an infinite 2D aggregate. Thus, the binding patterns, the protonation degree, and its corresponding tautomeric forms of the Hdap nucleobase essentially dominate the polymeric nature of 1−6. Additionally, the repeatedly observed hydrogen-bonding interactions produced by the exocyclic amino/endocyclic imino groups of the nucleobase and the carboxylate groups of the coligand favorably stabilize the high-dimensional order supramolecular architectures. At room temperature, complexes 1−6 exhibit intense luminescent emissions originated from a Hdap-based intraligand and/or photoinduced charge transfer upon cation binding, which offers the possibility for their applications as relevant antivirasic prodrugs.