A novel type of twisted antiparallel double-chain structure with stacking between the two strands. Structure, synthesis and magnetic properties of [{[Cu3L2(dien)(H2O)2]·3H2O}∞][L = 1H-1,2,4-triazole-3,5-dicarboxylate(3–), dien = 3-Azapentane-1,5-diamine]

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Abstract
The synthesis and structure of a completely new type of copper(II) polymer [{[Cu3L2(dien)(H2O)2]·3H2O}∞], where L = 1H-1,2,4-triazole-3,5-dicarboxylate(3–) and dien = diethylenetriamine (1,4,7-triazaheptane), is reported. The compound crystallizes in the tetragonal space group P21c, with a=b= 15.845(3), c= 22.743(1)Å and Z= 8. Least-squares refinement of 818 reflections with I > 2σ(I) and 174 parameters gave R= 0.0694, R′= 0.0969. The polymer is essentially a double chain, the building blocks of which are trinuclear copper(II) entities arranged zigzag clockwise and anticlockwise parallel to each other. The copper–copper distances within the trinuclear unit are Cu(1)ċ Cu(2) 5.937(4), Cu(1)ċ Cu(3) 4.014(6) and Cu(2)⋯ Cu(3) 5.937(4)Å. The two strands of the chain are linked to each other by apical Cu–O bonds with Cu(2)⋯ Cu(2′) 3.498(6)Å. The magnetic susceptibility data (5.8–284 K) are interpreted on the basis of an exchange Hamiltonian function giving J1[Cu(1)–Cu(3)]=–30.0 cm–1, J2[Cu(2)–Cu(2′)]=–1.5 cm–1 and g= 2.0. The nature and the strength of the antiferromagnetic exchange are discussed on the basis of the structural features, and compared with previously reported magneto-structural data for related copper(II) compounds with N1,N2- and N1,N4-linked 1,2,4-triazole ligands.