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Synthesis, Design, and Applications of Lanthanide-based Metal-Organic Structures


This work focuses on lanthanide-based inorganic-organic structures. The properties of lanthanide-based metal organic frameworks (Ln-MOFs) and layered rare earth hydroxides (LREHs) are reviewed and compared. Three distinct projects are presented herein: 1) the solvothermal syntheses of a series of Ln-MOFs with the ligand biphenyl-4,4-dicarboxylate (BPDC), 2) the solvothermal syntheses of a series of Ln-MOFs with the ligand 2,6-naphthalenedicarboxylate (NDC), and 3) the hydrothermal syntheses of a series of neodymium-based LREHs with increasing α,ω-alkanedisulfonate (ADS) carbon chain lengths.

The first project, which we refer to as Ln-BPDC (structures SLUG-43–48), is an isomorphous series of six anionic frameworks with the general structure [Ln(BPDC)2–][NH2(CH3)2+] (Ln = La, Ce, Nd, Eu, Gd, Er). The Ln(III) metal centers exhibit eight-coordinate binding to six different BPDC2- ligands. The anionic framework is charge-balanced by a dimethylammonium cation. The materials all possess the same 3-D structure and crystallize in the orthorhombic space group Pbcn. All exhibit thermal stability up to 300 °C and decompose to Ln2O3 after 800 °C. The Eu-BPDC structure exhibits strong fluorescence in the 612-620 nm range and a quantum yield of 2.11%.

The second project, we refer to as Ln-NDC (structures SLUG-49–52). This project consists of four neutrally charged structures that each crystallize in distinct space groups. Their formulas are [La6(NDC)9(DMF)3•6 DMF], [Nd2(NDC)3(DMF)2], [Eu2(NDC)3(DMF)2•DMF], and [Gd4(NDC)6(DMF)4]. The Ln(III) centers exhibit different coordination numbers (ranging from seven to nine), the NDC ligand exhibits multiple binding modes, and DMF solvent molecules are found either coordinated or floating within the structures. Despite these differences, the NDC-based structures exhibit similar thermal decomposition profiles and infrared spectra. The Eu-NDC structure exhibits a sharp red-orange luminescence at 613 nm and a quantum yield of 3.56%.

Lastly, the Nd-ADS project (structures SLUG-28–30) consists of three LREHs made of [Nd2(OH)4(OH2)22+] layers with interlamellar α,ω-[–O3S(CH2)nSO3–] anions (n = 2 to 4). These LREHs show an increase in thermal stability with increasing alkanedisulfonate chain length. As an initial example of anion exchange, all three materials show exchange for adipate, –O2C(CH2)4CO2–.

Several insights on the structural differences between Ln-MOFs and LREHs are proposed. In comparing the mentioned projects, we suggest the differences between the isomorphous Ln-BPDC series and the diversity of structures in the Ln-NDC series are due to the rigidity of the NDC ligand and the synthesis temperatures. The dimensionality of lanthanide-based materials (3-D or 2-D) are affected by reaction pH. This work expands the chemistry of lanthanide MOFs and LREHs.

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