The first water-based synthesis of Ce(iv)-MOFs with saturated chiral and achiral C4-dicarboxylate linkers

Abstract

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.