An experimental and theoretical study of the magnetic relaxation in heterometallic coordination polymers based on 6-methyl-2-oxonicotinate and lanthanide(iii) ions with square antriprismatic environment

Abstract

Two new isostructural compounds based on 6-methyl-2-oxonicotinate (6m2onic) ligand and sodium and lanthanide(III) ions are reported. The structural and chemical characterization reveals the following chemical formula: {[Ln(6m2onic)₂(μ-6m2onic)₂Na(H₂O)₃]·8H₂O}_n [where Ln(III) = Dy (1_Dy) and Er (2_Er)]. These compounds crystallize in the form of one-dimensional arrays held together into a hydrogen-bonded structure, in which 6m2onic ligands establish four O,O′ chelating rings with the lanthanide to render a distorted square antiprism (SAPR) geometry. Magnetic dc and ac susceptibility measurements confirm that 1_Dy and 2_Er behave as SIMs. Magnetic dilutions using Y(III) matrices have been made to achieve a Dy(III) counterpart (1_Y/Dy) that presents slow magnetic relaxation under zero dc field. Under an optimized H_dc field (of 1000 Oe and 1500 Oe for 1_Y/Dy and 2_Y/Er, respectively), 1_Y/Dy reveals the occurrence of two well-separated maxima, attributed to SR (U_eff = 65.2 K (45.3 cm⁻¹) and τ₀ = 2.76 × 10⁻⁹ s) and FR processes (U_eff = 23.2 K (16.1 cm⁻¹) and τ₀ = 1.40 × 10⁻⁸ s), whereas 2_Y/Er shows a multiple relaxation pathway that considers quantum tunnelling of the magnetization (QTM), Orbach, Raman and direct mechanisms. Ab initio calculations have been carried out to support the experimental evidence and to explain the lanthanide ion-dependent behaviour deepen the understanding of the magneto-structural relationship of the SAPR environment.