Luminescence and magnetocaloric effect of Ln4 clusters (Ln = Eu, Gd, Tb, Er) bridged by CO32− deriving from the spontaneous fixation of carbon dioxide in the atmosphere

Abstract

The synthesis and characterization of a new family of neutral tetranuclear Ln(III) complexes, [Eu₄L₄(CO₃)(acac)₂(H₂O)₂(CH₃OH)₂]·5H₂O·2CH₃OH (1), [Gd₄L₄(CO₃)(acac)₂ (CH₃OH)₄]·2CH₃OH·CH₂Cl₂ (2), [Tb₄L₄(CO₃)(acac)₂(H₂O)₂(CH₃OH)₂]·3H₂O·CH₃OH (3), and [Er₄L₄(CO₃)(acac)₂(H₂O)₂(CH₃OH)(DMF)]·H₂O·CH₃OH (4) (where acac⁻ is acetylacetonate, H₂L is 2-(hydroxyimino)-2-[(5-methyl-2-hydroxyphenyl)methylene]hydrazide and DMF = N,N-dimethyl formamide) are reported. For complexes 1–4, by absorbing and fixing CO₂ spontaneously under atmospheric conditions, with CO₃²⁻ as a bridging tetradentate ligand, the four Ln^III ions are bridged by two μ₂-O oxygen atoms from CO₃²⁻ to form a Ln₄ cluster. X-ray diffraction data reveal that the non-centrosymmetric (1, 3 and 4) or centrosymmetric (2) unit has two distinct Ln^III ions, nine-coordinated Ln^III ions in distorted spherical capped square antiprism geometry and eight-coordinated Ln^III ions in distorted triangular dodecahedron geometry. Magnetic study indicates that complex 2 can act as a cryogenic magnetic refrigerant and the magnetocaloric effect (MCE) was detected with the magnetic entropy change of −ΔS_m = 21.43 J kg⁻¹ K⁻¹ at ΔH = 7 T and 5 K. Furthermore, the fluorescence properties of complexes 1 and 3 were also investigated. The results prove that complex 3 can show Tb^III characteristic emission peaks, while for complex 1, Eu^III typical emission peaks were not observed.