New Ce(iii) sulfate–tartrate-based MOFs: an insight into the controllable self-assembly of acentric metal–organic complexes

Hydrothermal reactions yielded five new Ce (III) sulfate–tartrate-based MOFs, namely, α-Ce2 (H2O) 2 (L-tar) 2 (SO4) ·4H2O 1, α-Ce2 (H2O) 2 (D-tar) 2 (SO4) ·4H2O 2, β-Ce2 (H2O) 2 (L-tar) 2 (SO4) ·4H2O 3, β-Ce2 (H2O) 2 (D-tar) 2 (SO4) ·4H2O 4 and Ce2 (H2O) 3 (L/D-tar) (SO4) 25. The two α-enantiomorphs (1, 2) crystallize in the polar monoclinic C2 space group, and the two β-enantiomorphs (3, 4) crystallize in the non-polar orthorhombic space group P21212, while 5 crystallizes in the monoclinic space group P21/c. The acentric MOFs 1–4 belong to the 3∞2∞M2 (H2O) x (tar) b2Ap·nH2O family (hereby A = SO42−), and the Ce2 (tar) 2 dimeric motifs rig up the 2D positively charged 2∞Ce2 (H2O) 2 (tar) 22+ networks of a four-connected uninodal (42·64) topology, which are further pillared by sulfate anions into the 3∞2∞Ce2 (H2O) 2 (tar) 2 (SO4) framework of the (4, 5) -connected dinodal (42·64) (42·67·8) topology with the lattice water molecules in the tunnels. Within 5, the Ce (tar) motifs rig up the 2D 2∞Ce2 (H2O) 3 (tar) 4+ patterns, which were stabilized by sulfate anions to form a 2D 2∞Ce2 (H2O) 3 (tar) (SO4) 2+ layer of a (3, 4) -connected bi-nodal (63) (66) topology, and the 2D layers are pillared by additional sulfate anions into the 3D 3∞2∞Ce2 (H2O) 3 (L/D-tar) (SO4) SO4 framework of an unprecedented (3, 4, 5, 6) -connected penta-nodal (4·62) (4·65) (4·67·82) (44·62) (45·68·82) topology. The results suggest that the acentricity of the 3∞2∞M2 (H2O) x (tar) b2Ap·nH2O MOF is crucially determined by the tartrate forms incorporated in the 2D 2∞M2 (H2O) x (tar) b2 sheet, and the distances between the 2D sheets are tunable by the judicious selection of the pillaring auxiliary ligands. The polar α-MOFs (1, 2) display a promising ferroelectricity with a remnant polarization (Pr) of ca. 0. 158 μC cm−2 for 1 (ca. 0. 323 μC cm−2 for 2), a coercive field (Ec) of ca. 27. 05 kV cm−1 for 1 (ca. 42. 00 kV cm−1 for 2), and saturation of the spontaneous polarization (Ps) occurs at ca. 0. 335 μC cm−2 for 1 (ca. 0. 400 μC cm−2 for 2). The magnetic behaviors of the title compounds obey a modified Curie–Weiss law χ = χTIP + C/ (T − θ) with χTIP = 200–230 × 10−6 cm3 mol−1, C = 0. 228–0. 329 cm3 K mol−1, θ = −0. 03– −0. 06 K, and the decrease of χMT with the lowering temperature and the negative value of θ may be due to the populations of the Stark levels and/or the possible antiferromagnetic interactions between the Ce3+ ions. Furthermore, the title complexes were subjected to microelemental analyses, IR spectroscopic measurements and thermal analyses, and the results are also discussed.