The first water-based synthesis of Ce(iv)-MOFs with saturated chiral and achiral C4-dicarboxylate linkers†
Six different chiral and achiral alkane dicarboxylic C4-acids, i.e. succinic acid (H2SUC), DL-2-methylsuccinic acid (H2MS), 2,3-dimethylsuccinic acid (H2DMS) and aspartic acid (D-, L- and DL-H2ASP), were used to obtain Ce(IV)-MOFs via microwave assisted reactions. In water-based syntheses, MOFs with three different topologies, denoted as UiO-66 (fcu), CAU-41 (bcu) and CAU-44 (bct), were obtained within 30 min under mild reaction conditions. The MOFs were fully characterized and their structures were refined from PXRD data. The chirality of the incorporated linker molecules was confirmed by circular dichroism spectroscopy. The optical activities were also investigated by second-harmonic generation (SHG) measurements. The use of H2MS, H2DMS and H2ASP leads to the formation of a UiO-66-type structure, and a hydrated form of the common hexanuclear cluster, i.e. [Ce6(μ3-O)4(μ3-OH)4(H2O)6]12+ was observed. Rietveld refinement of the PXRD data showed an ordered arrangement of the D- and L-enantiomers in Ce-UiO-66-DL-ASP. By employing H2SUC as the linker, the other two title compounds are obtained. CAU-41, [Ce6(μ3-O)4(μ3-OH)4(SUC)4(OH)4(H2O)4], exhibits the well-known eight-fold connectivity of the hexanuclear cluster, whereas in CAU-44, [Ce6(μ3-O)4(μ3-OH)4(SUC)4(NO3)2(OH)2(H2O)2], the connection of the clusters is also achieved by nitrate ions resulting in a framework with bct topology.