Syntheses of new di-nuclear and tetra-nuclear lanthanide 9-anthracenecarboxylates: Magnetocaloric effect, slow magnetic relaxation and luminescent properties

Lanthanide complex-based materials with magnetic and luminescent properties have received a great deal of attention, owing to their importantly potential applications in diverse fields. In this work, the 9-anthracenecarboxylic acid (9-Haca) as bridging ligand was applied in combination with optically active π-conjugated 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz) to construct a series of new di-nuclear and tetra-nuclear lanthanide 9-anthracenecarboxylates [La2(9-aca)6(tptz)2(μ-H2O)]·0.25H2O (1), [Ln2(9-aca)6(tptz)2(H2O)2]·0.5AQ·CH3COOH·2H2O [Ln = Sm (2a), Eu (2b), Gd (2c), Tb (2d), Dy (2e), Tm (2f); AQ = 9,10-anthracenedione] and [Ln4(9-aca)10(tptz)2(CH3COO)2]·H2O [Ln = Eu (3a), Gd (3b), Dy (3c)]. 1 displays a di-nuclear centrosymmetric moiety [La2(9-aca)6(tptz)2(μ-H2O)] built up from two complex [La(9-aca)2(tptz)] groups bridged by one H2O molecule and two syn-syn-μ2-η1:η1-9-aca‒ bridging ligands, while 2a‒2f are isostructural and comprise another di-nuclear centrosymmetric moieties [Ln2(9-aca)6(tptz)2(H2O)2] based on two complex [Ln(9-aca)2(tptz)(H2O)] groups connected by two syn-syn-μ2-η1:η1-9-aca– bridging ligands. 3a‒3c are isostructural and show tetra-nuclear centrosymmetric moieties [Ln4(9-aca)10(tptz)2(CH3COO)2] containing three types of coordination modes of 9-aca– ligands. The present compounds offer the only examples of lanthanide 9-anthracenecarboxylates decorated by optically active tptz. Magnetic investigations of 2a, 2c‒2f, and 3b‒3c indicate that 3c is typical of slow relaxation of the magnetization, while 2c and 3b show significant magnetocaloric effect (MCE) with the value of –ΔSm of 5.26 J/(kg·K) at 4 K and 15.65 J/(kg·K) at 2 K (ΔH = 7 T), respectively. The luminescent properties were also studied and reveal that the characteristic luminescent properties of the π-conjugated aromatic ligands are introduced into magnetic lanthanide 9-anthracenecarboxylates, simultaneously exhibiting slow magnetic relaxation (or MCE) and luminescent properties.