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

Abstract

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)b2]Ap}·nH2O family (hereby A = SO42−), and the [Ce2(tar)2] dimeric motifs rig up the 2D positively charged 2∞[Ce2(H2O)2(tar)2]2+ 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)b2]Ap}·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.