Solvent-dependent Ln(<scp>iii</scp>) clusters assembled by immobilization of CO₂ in air: zero-field single-molecule magnets and magnetic refrigerant materials

The automatic fixation of CO₂ in air played a key role in the construction of Dy(III) single-molecule magnets (SMMs) and Gd(III) magnetic refrigeration molecular materials when Ln(III)Cl₃ (Ln = Dy and Gd) was reacted with a new hydrazone Schiff base ligand {H₂L = (<i>E</i>)-<i>N</i>'-(4-fluoro-2-hydroxybenzylidene)pyrimidine-4-carbohydrazide}, which is condensed from pyrimidine-4-carbohydrazide and 4-fluorosalicylaldehyde. Surprisingly, the small difference in methanol and ethanol solvents leads to dramatic changes in the structures of these 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)₂Cl₂]·6EtOH (2) were obtained, respectively, and a tub-like Gd(III) octanuclear cluster [Gd₈L₈(CO₃)₄(MeOH)₃(H₂O)₅]·12MeOH·3CH₂Cl₂·3.25H₂O (3) and a trapezoidal pyramid Gd(III) pentanuclear cluster [Gd₅L₄(HL)(CO₃)O(OH)(EtOH)₅(H₂O)₂Cl]·EtOH·3H₂O (4) were yielded, respectively. Notably, 1 contains not only the carbonate anion, but also the monomethyl carbonate anion as the bridging ligand, which are formed by immobilizing CO₂ in air, while 2, 3 and 4 contain the carbonate bridging ligand only. Magnetic properties' investigations revealed that both 1 and 2 are zero-field SMMs, with <i>U</i>_eff/<i>k</i> values of 93.2 K and 133.5 K, respectively, while 3 and 4 show large magnetocaloric effects, with the largest -Δ<i>S</i>_m values of 27.49 J kg^-1 K^-1 at 2.0 K for Δ<i>H</i> = 7 T for 3 and 27.58 J kg^-1 K^-1 at 2.5 K for Δ<i>H</i> = 7 T for 4.