Fluorite and mixed-metal kagome-related topologies in metal-organic framework compounds: synthesis, structure, and properties

Abstract

Cluster around: Fluorite and mixed-metal Kagome-related three-dimensional metal-organic framework (MOF) compounds have been prepared by the reaction between Mn²⁺ ions and trimellitate anions. The octameric water clusters in the former can be reversibly adsorbed and both the compounds show interesting magnetic behavior. Two new three-dimensional metal-organic frameworks (MOFs) [Mn₂(μ₃−OH)(H₂O)₂(BTC)]·2H₂O, I, and [NaMn(BTC)], II (BTC=1,2,4-benzenetricarboxylate=trimellitate) were synthesized and their structures determined by single-crystal X-ray diffraction (XRD). In I, the Mn₄ cluster, [Mn₄(μ₃−OH)₂(H₂O)₄O₁₂], is connected with eight trimellitate anions and each trimellitate anion connects to four different Mn₄ clusters, resulting in a fluorite-like structure. In II, the Mn₂O₈ dimer is connected with two Na⁺ ions through carboxylate oxygen to form mixed-metal distorted Kagome-related two-dimensional —M—O—M— layers, which are pillared by the trimellitate anions forming the three-dimensional structure. The extra-framework water molecules in I are reversibly adsorbed and are also corroborated by powder XRD studies. The formation of octameric water clusters involving free and coordinated water molecules appears to be new. Interesting magnetic behavior has been observed for both compounds. Electron spin resonance (ESR) studies indicate a broadening of the signal below the ordering temperature and appear to support the findings of the magnetic studies.