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One-Dimensional Oxalato-Bridged Cu(II), Co(II), and Zn(II) Complexes with Purine and Adenine as Terminal Ligands

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journal contribution
posted on 06.09.2004, 00:00 by Juan P. García-Terán, Oscar Castillo, Antonio Luque, Urko García-Couceiro, Pascual Román, Francesc Lloret
The reaction of nucleobases (adenine or purine) with a metallic salt in the presence of potassium oxalate in an aqueous solution yields one-dimensional complexes of formulas [M(μ-ox)(H2O)(pur)]n (pur = purine, ox = oxalato ligand (2-); M = Cu(II) [1], Co(II) [2], and Zn(II) [3]), [Co(μ-ox)(H2O)(pur)0.76(ade)0.24]n (4) and {[M(μ-ox)(H2O)(ade)]·2(ade)·(H2O)}n (ade = adenine; M = Co(II) [5] and Zn(II) [6]). Their X-ray single-crystal structures, variable-temperature magnetic measurements, thermal behavior, and FT-IR spectroscopy are reported. The complexes 14 crystallize in the monoclinic space group P21/a (No. 14) with similar crystallographic parameters. The compounds 5 and 6 are also isomorphous but crystallize in the triclinic space group P1̄ (No. 2). All compounds contain one-dimensional chains in which cis-[M(H2O)(L)]2+ units are bridged by bis-bidentate oxalato ligands with M...M intrachain distances in the range 5.23−5.57 Å. In all cases, the metal atoms are six-coordinated by four oxalato oxygen atoms, one water molecule, and one nitrogen atom from a terminal nucleobase, building distorted octahedral MO4OwN surroundings. The purine ligand is bound to the metal atom through the most basic imidazole N9 atom in 14, whereas in 5 and 6 the minor groove site N3 of the adenine nucleobase is the donor atom. The crystal packing of compounds 5 and 6 shows the presence of uncoordinated adenine and water crystallization molecules. The cohesiveness of the supramolecular 3D structure of the compounds is achieved by means of an extensive network of noncovalent interactions (hydrogen bonds and π−π stacking interactions). Variable-temperature magnetic susceptibility measurements of the Cu(II) and Co(II) complexes in the range 2−300 K show the occurrence of antiferromagnetic intrachain interactions.