Novel 3D anionic heterometallic frameworks based on trinuclear CoII and trinuclear LnIII motifs displaying slow magnetic relaxation and selective adsorption of methylene blue

Abstract

Five novel 3D anionic Co^IILn^III heterometallic organic frameworks (HMOFs) [(CH₃)₂NH₂]₄[Co₃Ln₃(OH)(TZI)₆(H₂O)₆]·2DMA·16H₂O (CoLn, Ln = Eu (1), Gd (2), Tb (3), Dy (4), Ho (5), H₃TZI = 5-(4-(tetrazol-5-yl)phenyl)isophthalic acid) have been synthesized under solvothermal conditions and structurally characterized by elemental analysis, FTIR, PXRD, TGA and single-crystal X-ray diffraction. Structure analyses show that these CoLn HMOFs (1–5) are isomorphous and contain two different metal motifs: one is a triangular trinuclear Co₃(μ₃-OH)(μ₂-tetrazolate)₆ cluster, and the other is a linear trinuclear Ln₃(COO)₁₂ cluster. The adjacent Co^II and Ln^III clusters are connected by the TZI ligands to give a 3D heterometallic framework with a (3,3,6,10)-connected (4·6·8)(4²·5)(4³·5³·6⁴·8⁵)(4⁷·5¹⁰·6⁶·7⁸·8¹⁰·9⁴) topology. Magnetic investigations indicated that CoDy (4) and CoHo (5) display slow magnetic relaxation behaviors under a zero dc field. The most intriguing feature is that these CoLn HMOFs (1–5) contain microporous structures with [(CH₃)₂NH₂]⁺ cations being located in channels, which can successfully separate MB⁺ cationic dye molecules from mixed dye molecules including Rh B⁺, BV14⁺, NR⁰ and MO⁻ (adsorption – take CoTb (3) as an example). At equilibrium, the removal percentage of MB⁺ with CoTb (3) in a mixture of dyes is about 97.5% and the mechanism of adsorption of MB⁺ is also explored. Such 3d–4f anionic HMOFs are unusual.