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

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 LnIII 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 LnIII ions, nine-coordinated LnIII ions in distorted spherical capped square antiprism geometry and eight-coordinated LnIII 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 −ΔSm = 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 TbIII characteristic emission peaks, while for complex 1, EuIII typical emission peaks were not observed.