Solvent-dependent Ln(III) clusters assembled by immobilization of CO2 in the air: zero-field single-molecule magnets and magnetic refrigerant materials

Abstract

The automatic fixation of CO₂ in the air plays a key role in the construction of Dy(III) single-molecule magnets (SMMs) and Gd(III) magnetic refrigeration molecular materials using Ln(III)Cl₃ (Ln = Dy and Gd) and a new hydrazone Schiff base ligand {H₂L = (E)-N'-(4-fluoro-2-hydroxybenzylidene)pyrimidine-4-carbohydrazide}, which is condensed from pyrimidine-4-carbohydrazide and 4-fluorosalicylaldehyde. Surprisingly, the small differences in methanol and ethanol solvents lead to dramatic changes in the structures of Ln(III) cluster complexes. When methanol and ethanol participated in the reaction, a trapezoidal pyramid Dy(III) pentanuclear cluster [Dy₅L₅(OH)₂(CO₃)(O₂COMe)(MeOH)₃(H₂O)]⸱3MeOH·3.5H₂O (1) and a triangular prism Dy(III) hexanuclear cluster [Dy₆L₆(CO₃)₂(EtOH)₂(H₂O)₂(Cl)₂]⸱6EtOH were obtained, respectively; and a tub-like Gd(III) octanuclear cluster [Gd₈L₈(CO₃)₄(MeOH)₃(H₂O)₅]⸱12MeOH·3CH₂Cl₂·3.25H₂O (3) for methanol and a trapezoidal pyramid Gd(III) pentanuclear cluster [Gd₅L₄(HL)(CO₃)(O)(OH)(EtOH)₅(H₂O)₂(Cl)]⸱EtOH⸱3H₂O (4) for ethanol were yielded. Notably, complex 1 contains not only the carbonate anion but also the monomethyl carbonate anion as bridging ligands, which are formed by immobilizing CO₂ in the air, while complexes 2, 3 and 4 contain the carbonate bridging ligand only. Magnetic properties’ investigations revealed that both 1 and 2 are zero-field SMMs, with U_eff/k values of 93.2 K and 133.5 K, respectively; while 3 and 4 show large magnetocaloric effects, with the largest −ΔS_m values of 27.52 Jkg^–1K^–1 at 2.0 K for ΔH = 7 T for 3 and 27.58 Jkg^–1K^–1 at 2.5 K for ΔH = 7 T for 4.