Isoreticular Chemistry of Group 13 Metal-Organic Framework Compounds Based on V-Shaped Linker Molecules: Exceptions to the Rule?

Following the concept of isoreticular chemistry, we carried out a systematic study on Ga-containing metal-organic frameworks (MOFs) using six V-shaped linker molecules of differing sizes, geometries, and additional functional groups. The linkers included three isophthalic acid derivatives (m-H2BDC-R, R = CH3, OCH3, NHCOCH3), thiophene-2,5-dicarboxylic acid (H2TDC), and two 4,4'-sulfonyldibenzoic acid derivatives (H2SDBA, DPSTA). The crystal structures of seven compounds were elucidated by a combination of model building, single-crystal X-ray diffraction (SCXRD), three-dimensional electron diffraction (3D ED), and Rietveld refinements against powder X-ray diffraction (PXRD) data. Four new Ga-MOFs that are isoreticular with their aluminum counterparts, i.e. Ga-CAU-10-R (Ga(OH)(m-BDC-R); R = OCH3, NHCOCH3), Ga-CAU-11 (Ga(OH)(SDBA)), and Ga-CAU-11-COOH (Ga(OH)(H2DPSTC)), were obtained. For the first time large single crystals of a MOF crystallizing in the CAU-10 structure type could be isolated, i.e. Ga-CAU-10-OCH3, which permitted a detailed structural characterization. In addition, the use of 5-methylisophthalic acid and thiophene-2,5-dicarboxylic acid resulted in two new Ga-MOFs denoted Ga-CAU-49 and Ga-CAU-51, respectively, which are not isostructural with any known Al-MOF. The crystal structure of Ga-CAU-49 ([Ga4(m-HBDC-CH3)2(m-BDC-CH3)3(OH)4(H2O)]) contains an unprecedented rod-shaped inorganic building unit (IBU) of the formula ∞1{Ga16(OH)18O60}, composed of corner-sharing GaO5 and GaO6 polyhedra. In Ga-CAU-51 ([Ga(OH)(C5H2O2S)]) chains of alternating cis and trans corner-sharing GaO6 polyhedra form the IBU. A detailed characterization of the title compounds was carried out, including nitrogen gas and water vapor sorption measurements. Ga-CAU-11 was the only compound exhibiting porosity toward nitrogen with a type I isotherm, a specific surface area of aS,BET = 210 m2/g, and a micropore volume of Vmic = 0.09 cm3/g. The new MOF Ga-CAU-51 exhibits exceptional water sorption properties with a reversible S-shaped isotherm and a high uptake around p/p0 = 0.38 of mads = 370 mg/g.

Metal-Dependent and Selective Crystallization of CAU-10 and MIL-53 Frameworks through Linker Nitration.

The reaction of the V-shaped linker molecule 5-hydroxyisophthalic acid (H2 L0 ), with Al or Ga nitrate under almost identical reaction conditions leads to the nitration of the linker and subsequent formation of metal-organic frameworks (MOFs) with CAU-10 or MIL-53 type structure of composition [Al(OH)(L)], denoted as Al-CAU-10-L0, 2, 4, 6 or [Ga(OH)(L)], denoted as Ga-MIL-53-L2 . The Al-MOF contains the original linker L0 as well as three different nitration products (L2 , L4 and L4/6 ), whereas the Ga-MOF mainly incorporates the linker L2 . The compositions were deduced by 1 H NMR spectroscopy and confirmed by Rietveld refinement. In situ and ex situ studies were carried out to follow the nitration and crystallization, as well as the composition of the MOFs. The crystal structures were refined against powder X-ray diffraction (PXRD) data. As anticipated, the use of the V-shaped linker results in the formation of the CAU-10 type structure in the Al-MOF. Unexpectedly, the Ga-MOF crystallizes in a MIL-53 type structure, which is usually observed with linear or slightly bent linker molecules. To study the structure directing effect of the in situ nitrated linker, pure 2-nitrobenzene-1,3-dicarboxylic acid (m-H2 BDC-NO2 ) was employed which exclusively led to the formation of [Ga(OH)(C8 H3 NO6 )] (Ga-MIL-53-m-BDC-NO2 ), which is isoreticular to Ga-MIL-53-L2 . Density Functional Theory (DFT) calculations confirmed the higher stability of Ga-MIL-53-L2 compared to Ga-CAU-10-L2 and grand canonical Monte Carlo simulations (GCMC) are in agreement with the observed water adsorption isotherms of Ga-MIL-53-L2 .

A Scandium MOF with an Unprecedented Inorganic Building Unit, Delimiting the Micropore Windows.

A new scandium metal-organic framework (Sc-MOF) with the composition of [Sc(OH)(OBA)], denoted as Sc-CAU-21, was prepared under solvothermal reaction conditions using 4,4'-oxidibenzoic acid (H2OBA) as the ligand. Single-crystal structure determination revealed the presence of the new inorganic building unit (IBU) {Sc8(µ-OH)8(O2C)16}. It is composed of cis-connected ScO6 polyhedra forming an eight-membered ring through bridging µ-OH groups. The connection of the IBUs leads to a 3D framework, containing 1D pores with a diameter between 4.2 and 5.6 Å. Pore access is limited by the size of the IBU, and in contrast to the isoreticular aluminum compound Al-CAU-21 [Al(OH)(OBA)], which is nonporous toward nitrogen at 77 K, Sc-CAU-21 exhibits a specific surface area of 610 m2 g-1. The title compound is thermally stable in air up to 350 °C and can be employed as a host for photoluminescent ions. Sc-CAU-21 exhibits a ligand-based blue emission, and (co)substituting Sc3+ ions with Ln3+ ions (Eu3+, Tb3+, and Dy3+) allows the tuning of the emitting color of the phosphor from red to green. Single-phase white-light emission with CIE color coordinates close to the ideal for white-light emission was also achieved. The luminescence property was utilized in combination with powder X-ray diffraction to study in situ the crystallization process of Sc-CAU-21:Tb and Sc-CAU-21:Eu. Both studies indicate a two-step crystallization process, with a crystalline intermediate, prior to the formation of Sc-CAU-21:Ln.

Influence of the substitution pattern of four naphthalenedicarboxylic acids on the structures and properties of group 13 metal-organic frameworks and coordination polymers.

Metal-organic frameworks containing Ga3+ ions and four differently substituted naphthalenedicarboxylates (ndc2-) have been synthesized and characterized. The Ga3+ ions are six-fold coordinated by oxygen atoms in all title compounds, but two different inorganic building units, i.e. trans corner-sharing and cis,trans edge-sharing octahedra are observed. Crystal structures were validated by Rietveld refinements against powder X-ray diffraction data. [Ga(OH)(1,4-ndc)]·2H2O crystallizes in a non-breathing MIL-53 type structure with two different pore sizes (5.5 × 5.5 Å and 9 × 9 Å). It is non-porous with respect to nitrogen but has a water adsorption capacity of about 155 mg g-1 and a thermal stability of up to 240 °C. The dense compound [Ga(OH)(1,8-ndc)] crystallizes in a new layered structure motif, which is related to the crystal structure of MIL-122 ([Al(OH)((O2C)4C6H2)]). The third and fourth compounds [Ga2(OH)4(2,3-ndc)]·H2O and [Ga(OH)(2,6-ndc)]·H2O are isoreticular to CAU-15 ([Al2(OH)4(2,3-bdc)]·H2O) and MIL-69 ([Al(OH)(2,6-ndc)]·H2O), respectively. The last two compounds are non-porous toward nitrogen but reversible dehydration was demonstrated. For comparison, the two new compounds [Al(OH)(1,8-ndc)] and [Al2(OH)4(2,3-ndc)]·H2O, which are isostructural to the newly described gallium compounds, were also synthesized and fully characterized. The Al-containing coordination polymers exhibit higher temperature stabilities compared to their isostructural Ga compounds.