Spin-Equilibrium Behavior of Iron(III) Complexes with Various Monodentate Ligands and Bis(N,N′-3-Methoxysalicylideneaminopropyl) Methylamine

Abstract

The syntheses, structures, and magnetic properties of iron(III) complexes with the chemical formula of [Fe(X)L]Bph4 are reported, where X = monodentate ligand such as pyridine and imidazole derivatives, H2L = bis(N,N′-(3-methoxy-salicylideneaminopropyl)methylamine, and Bph4 − = tetraphenylborate, respectively. The magnetic susceptibilities were measured by a SQUID susceptometer in the temperature range 4-350 K under an external magnetic field of 1 T. On the basis of the magnetic susceptibility data, the complexes are classified into three groups (1)–(3). (1): high-spin complexes; The effective magnetic moments of [Fe(Im)L]Bph4 and [Fe(4-MeIm)L]Bph4 are constant at ca. 5.7 μB over the temperature range measured. (2): spin-equilibrium complexes between S = 5/2 and S =1/2; [Fe(4-NH2py)L]Bph4 showed an ideal spin-equilibriam behavior, as the μeff decreased from 5.5 μB at 300 K to 2.1 μ3 at 60 K. Hysteresis was not observed, indicating that the complex is classified as continuous type. The thermodynamic model of Slichter and Drickamer was applied and the intermolecular interaction parameter Γ, the enthalpy δH, and the entropy δS were estimated to be Γ = 2 kJ/mol, δH = 5.99 kJ/mol, and δS = 49.99 J/ mol K, respectively. (3): [Fe(py)L]Bph4 showed an unusual magnetic behavior. On lowering the temperature, the μeff decreased gradually from 5.69 μB at 300 K and reached to a plateau value of 3.89 μB at 60 K, whose value is too large for low-spin state and rather close to that of S = 3/2.