Deferasirox HOF and its in situ metallization affording a new family of lanthanide coordination polymer-based MHOFs

Abstract

Pure organic HOFs (hydrogen-bonded organic frameworks) lack stronger coordination bonds and the presence of some functional sites in their parent structures. Therefore, it is important to graft functional metal ions into single HOF structures via coordination bonds in order to develop highly stable and multifunctional MHOFs (metallo hydrogen-bonded organic frameworks). Deferasirox (DFX), which is used as a trifurcated hydrogen-bonded linking agent, is suitable for the construction of dual-self-complementary HOFs. In the absence of deprotonated reagents, in situ metallization of DFX HOFs with lanthanide ions in aqueous solutions directly produced a new family of lanthanide coordination polymer-based MHOFs, namely, [Ln₂(μ-H₂O)(μ-H₂L)₆]_n (Ln = Ce (DFX-Ce), Eu (DFX-Eu), Gd (DFX-Gd)) and [Tb₄(μ-H₂O)₂(μ-H₂L)₁₀(H₂O)(H₂L)₂·2H₂O]_2n (DFX-Tb) (H₂L = 4-[3,5-bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]-benzoate anion). Structural studies revealed that these four new compounds exhibited cruciform-like 1D coordination polymer chains with the MHOF structures owing to the presence of pendant flexible robust phenol groups. Solid-state photoluminescence of DFX-Ce exhibited the dominant emission of the free DFX ligand accompanied by the 5d → 4f transitions of Ce³⁺, while DFX-Eu and DFX-Tb displayed the obvious characteristic emissions for the corresponding lanthanide ions based on 4f → 4f transitions. In particular, since the lowest triplet-state energy levels of the DFX ligand matched well with the resonance level of Tb³⁺, DFX-Tb exhibited highly efficient photoluminescence compared with DFX-Eu. The strongly emissive characteristic of DFX-Tb indicated its potential application in fluorescent materials. Furthermore, magnetic property studies of DFX-Gd revealed the weak antiferromagnetic interaction between neighboring Gd³⁺ ions and the significant magnetocaloric effect with a large magnetic entropy change. To the best of our knowledge, no lanthanide coordination polymers have been successfully synthesized from the DFX ligand or its derivatives thus far, and the presented in situ metallization of the DFX HOF with the corresponding lanthanide ions provides a new route for the synthesis of lanthanide coordination polymer-based MHOFs with interesting structures and multifunctional properties in the future.