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

Abstract

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.