Ferromagnetic exchange interaction of the Cu–Cu dimer [Cu2(picditn)(CH3CO2)(H2O)2Cl][ZnCl4]·0.5H2O [picditn = 1,11-bis(2-pyridyl)-2,6,10-triazaundecane]‡

Abstract

Elemental analysis, IR and UV/VIS spectroscopy, magnetic susceptibility at room temperature and ESR at X- and Q-band at 300 and 77 K were carried out on the complex of formula C₂₀H₃₆Cl₅Cu₂N₅O_4.5Zn. Based on the elemental analysis, IR and UV/VIS results a bimetallic molecular structure is proposed where the acetate and the 1,11-bis(2-pyridyl)-2,6,10-triazaundecane groups bridge the copper(II) ions that locally have a square-based distorted pyramid geometry obeying approximately C_4v molecular symmetry or lower. The d–d transitions are tentatively assigned. From the measured magnetic moment, µ = 2.55 µ_B, the calculated exchange interaction J = +76 cm^–1, indicating an interaction of ferromagnetic character which is interpreted as being due to the accidental orthogonality of the bridged magnetic orbitals. The X- and Q-band spectra at 300 and 77 K show a single rather narrow resonance at g = 2.058, quite atypical of copper(II) monomeric species. The spectral features and their temperature behavior are fully rationalized in terms of a spin Hamiltonian with exchange, Zeeman and crystal-field interactions. The magnetic moment value and all the ESR features at 300 and 77 K are fully consistent with an exchange interaction of ferromagnetic character, J > 0, present in the Cu^II–Cu^II compound, and hence with the proposed mixed bridged dimeric molecular structure.