Field-assisted slow relaxation of magnetization in Cu(ii) complexes with pentaheterocyclic triphenodioxazine ligands: the quasi-one-dimensional versus the binuclear case

Abstract

Two new copper(II) complexes have been obtained by the reaction of sterically crowded triphenodioxazine ligands with Cu(II) hexafluoroacetylacetonate. Compound 1 is a co-crystal of Cu(hfac)₂ moieties and triphenodioxazine molecules within a quasi-one-dimensional crystal structure with only weak Cu⋯N short intermolecular interactions (Cu⋯N distances are equal to 2.69(1) and 2.75(1) Å). In contrast, complex 2 has a binuclear structure with two Cu(hfac)₂ units bridged through triphenodioxazine ligand with the covalent Cu–N bond length of 2.335(2) Å. The magnetic AC susceptibility data show that in spite of the absence of zero-field splitting in the Cu(II) ion having the ground state with spin S = 1/2, both compounds demonstrate slow magnetic relaxation at the applied magnetic field. For a quasi-one-dimensional complex 1, the optimal field for observing slow magnetic relaxation is 400 Oe, while for the binuclear complex it is 2500 Oe. The observed temperature dependence of the relaxation time in 1 is well described by the combination of Raman and direct mechanisms of relaxation, while for complex 2 the inclusion of Raman mechanism and quantum tunneling of magnetization is required to describe the relaxation behavior.