End-to-end azides as bridging ligands in lanthanide coordination chemistry: Magnetic and magnetocaloric properties of tetranuclear Ln4 (Ln = Gd, Dy) complexes exhibiting a rare rhombus topology

The syntheses and magnetic properties of two new isostructural tetranuclear lanthanide complexes, [Ln(4)(N-3)(4)(TPz)(8)](Ln = G(III) (1), Dy-III (2) and, Tpz = tris(pyrazolyl)borate) are reported. In these complexes the lanthanide ions are eight coordinate with distorted geometries and are bridged by end-to-end azido ligands in a rare rhombus topology. Direct current magnetic susceptibility studies revealed the presence of weak ferromagnetic exchange interactions between the metal ions which were quantified in the case of the isotropic Gd-4 analog to give J = 0.0080(2) cm(-1) and g = 2.01(1). Complex 2 exhibits frequency and temperature dependent out-of-phase (chi '') ac magnetic susceptibility signals under a 0.1 T applied field, suggestive of SMM behavior. Low temperature magnetization studies revealed that compound 1 exhibits an appreciable magnetic entropy change, -Delta S-m, which reaches a value of similar to 20.9 J kg(-1) K-1 at T=3 K for Delta H = 7 T. Theoretical calculations further support the experimental results. (C) 2018 Published by Elsevier Ltd.