Synthesis, crystal structure, EXAFS and magnetic properties of catena-poly[μ-tris(4-(2'-hydroxyethyl)-1,2,4-triazole-N¹,N²) copper(II)] diperchlorate trihydrate: relevance with the structure of the iron(II) 1,2,4-triazole spin transition molecular materials

[Cu(hyetrz)3](ClO4)2·3H2O (hyetrz = 4-(2-hydroxyethyl)-1,2,4-triazole) represents the first structurally characterized metal(II) chain compound containing triple N1,N2-1,2,4-triazole bridges. The structure has been solved at 298 K by single-crystal X-ray analysis. catena-Poly[μ-tris(4-(2-hydroxyethyl)-1,2,4-triazole-N1,N2)copper(II)] diperchlorate trihydrate (C12H27N9Cl2O14Cu) crystallizes in the monoclinic space group P21/n, a = 13.877(3) Å, b = 23.023(5) Å, c = 15.351(2) Å, β = 91.10(2)°, Z = 8 (Cu(II) units). The Cu(II) ions are linked by triple N1,N2-1,2,4-triazole bridges yielding a slightly alternating chain with Cu1−Cu2 = 3.853(2) Å and Cu2−Cu3 = 3.829(2) Å. The EXAFS results are consistent with the crystal structure. At 30 K, the EXAFS signature of the multiple scattering path Cu1−Cu2−Cu3 is clearly observed, confirming that such exceptional long distance EXAFS signals can be used to detect metal alignments in inorganic long chains when crystal structures are not available. The thermal behavior of this multiple scattering signal was compared to those of similar Fe(II) low-spin compounds, and the observed differences have been discussed by comparing their electronic (dynamic Jahn−Teller effect) and vibrational properties. The Cu(II) ions are weakly antiferromagnetically coupled with J = −1.18(2) cm-1 (based on the Hamiltonian, H = −J[∑i Si·Si+1]). The nature and the magnitude of the antiferromagnetic exchange have been discussed on the basis of the structural features.