Self-assembly of octanuclear Ln(iii)-based clusters: their large magnetocaloric effects and highly efficient conversion of CO2

Abstract

The design and construction of high-nuclear lanthanide clusters with fascinating topology and functional properties have been an active area of research, however, the development of an effective approach for obtaining high-nuclear lanthanide clusters with multifunctional properties is still extremely difficult. Up to now, a systematic approach for guiding the further expansion of Ln(III)-based clusters showing good functional properties is lacking. Herein, we design and synthesize a polydentate Schiff base ligand (HL), which reacts with β-diketonate salts Ln(acac)₃·2H₂O, and a series of Ln₈ clusters [Ln₈(acac)₆(L)₂(μ₃-O)₆(μ₂-C₂H₅O)₄(μ₂-Hacac)₂]·2CH₃CN (Ln(III) = Gd (1), Dy (2), and Ho (3); HL = pyridine-2-carboxylic acid (5-hydroxymethyl-furan-2-ylmethylene)-hydrazide, Hacac = acetylacetone) have been successfully synthesized. Single-crystal X-ray diffraction studies reveal that clusters 1–3 are isostructural and can be viewed as a Ln₈ core bridged by eighteen μ₂-O atoms, six μ₃-O atoms and two μ₄-O atoms. Magnetic studies show that cluster 1-Gd₈ displays a large magnetocaloric effect with −ΔS_m = 46.14 J kg⁻¹ K⁻¹ (T = 2.0 K and ΔH = 7.0 T); cluster 2-Dy₈ exhibits single-molecule magnet behavior under zero-field conditions. It is worth mentioning that the −ΔS_m of cluster 1-Gd₈ is larger than that of most reported polynuclear Gd(III)-based clusters; the 2-Dy₈ cluster is one of the rare polynuclear Ln_n SMMs (n ≥ 8) under zero dc field. Importantly, these Ln(III)-based clusters (1–3) can catalyze the cycloaddition of CO₂ with epoxides with high efficiency under mild conditions; and cluster 1-Gd₈ as a catalyst could be reused at least three times without obvious loss of catalytic performance.