Linear trinuclear and tetranuclear Ln(<scp>iii</scp>) compounds aggregated by multidentate Schiff base ligands: insights into the magnetocaloric effect, slow magnetic relaxation behavior and catalytic CO₂ conversion

Ln-derived complexes have been intensively studied owing to their broad application scope in many fields (like magnetism, light, electricity, catalytic and biological activity, <i>etc</i>.). In this work, two new linear Ln₃ compounds 1 and 2 ([Ln₃(L)₂(dbm)₅(CH₃OH)]·CH₂Cl₂ (Ln(III) = Gd (1) and Dy (2); Hdbm = 1,3-diphenylpropane-1,3-dione; L₁ = 6-(hydroxymethyl)-<i>N</i>'-[(1<i>E</i>)-(6-methoxypyridin-2-yl)methylidene]pyridine-2-carbohydrazide)) and two new linear Ln₄ compounds 3 and 4 ([Ln₄(L)₂(dbm)₆(CH₃OH)₄] (Ln(III) = Gd (3) and Dy (4); HL₂ = 2-hydroxyl-3-methoxy-<i>N</i>'-[(1<i>E</i>)-(6-methoxypyridin-2-yl)methylidene]benzoic hydrazide)) were successfully synthesized. Crystallographic data show that the linear compounds 1 and 2 are trinuclear, and 3 and 4 are tetranuclear. The analysis of the magnetic characteristics discloses that the interactions among the neighboring Gd³⁺ ions in linear compounds 1 and 3 exhibit weak antiferromagnetic coupling. Moreover, linear compounds 1 and 3 have a considerable magnetic entropy variation with -Δ<i>S</i>_m of 21.04 J kg^-1 K^-1 for 1 and -Δ<i>S</i>_m = 24.25 J kg^-1 K^-1 for 3 (Δ<i>H</i> = 7 T at 2 K). Linear compounds 2 and 4 show slow magnetic relaxation behavior. What is particularly interesting is that linear compounds 1-4 are capable of catalyzing the cycloaddition of CO₂ to produce cyclic carbonates under moderate reaction conditions, and the yields were high. Notably, the heterogeneous catalysts 1-4 exhibit excellent cycling stability, maintaining their catalytic activity with an intact structure and no obvious performance decline in at least three reaction cycles. Based on what we know, linear compounds like 1-4, which possess both a magnetocaloric effect and slow magnetic relaxation behavior, as well as the ability to catalyze the conversion of CO₂, are rarely reported. Furthermore, they have the potential to become a bifunctional material in practical applications.