Slow Relaxation of the Magnetization on Frustrated Triangular FeIII Units with S = 1/2 Ground State: The Effect of the Highly Ordered Crystal Lattice and the Counteranions

In order to understand how the different arrangements of highly ordered triangular Fe-III S = 1/2 systems with various types of diamagnetic and paramagnetic anions affect their static and dynamic magnetic properties, we have obtained by solvothermal synthesis four new mu(3)-oxido trinuclear Fe-III compounds: [Fe3O(Ac)(6)(AcNH2)(3)][BF4]center dot(CH3CONH2)(0.5)(H2O)(0.5) (1-BF4), [Fe3O(Ac)(6)(AcNH2)(3)][GaCl4] (1-GaCl4), [Fe3O(Ac)(6)(AcNH2)(3)][FeCl4] (1-FeCl4), and [Fe3O(Ac)(6)(AcNH2)(3)][FeBr4] (1-FeBr4), where Ac- = CH3COO- and AcNH2 = CH3CONH2. The organization of the triangular units is very varied, from segregated stacks to eclipsed equilateral triangular [Fe3O](+) units along the c-axis with intercalated [MX4](-) units. The ordering of the triangular species, together with disposition of the counteranions (intercalated or not), affects the static and dynamic magnetic properties of the [Fe3O](+) systems. Magnetic dc data also were satisfactorily fitted with a HDvV spin Hamiltonian, considering the existence of anisotropic phenomena (antisymmetric exchange and intermolecular interactions), in order to model the low-temperature region. From the antisymmetric exchange, we were able to obtain A(U-eff), which was used to model and rationalize the dynamic magnetic properties of these systems, reflecting that Orbach and Raman processes define the relaxation mechanism of these systems.