Four series of lanthanide coordination polymers based on the tetrabromobenzene-1,4-dicarboxylate ligand: structural diversity and multifunctional properties

Abstract

Four series of lanthanide-based coordination polymers (LnCPs), namely [Ln(Br₄bdc)_1.5(MeOH)₃] (1_Ln; Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy), [Ln₂(Br₄bdc)₂(NO₃)₂(MeOH)₄] (2_Ln; Ln = Ce, Pr, Nd, Sm), [Ln(Br₄bdc)(NO₃)(MeOH)] (3_Ln; Ln = Gd, Tb, Dy), and [Ln₂(Br₄bdc)₃(H₂O)_2.3(MeOH)_2.7] (4_Ln; Ln = Gd, Tb, Dy) have been synthesized by reacting hydrated lanthanide(III) salts with tetrabromobenzene-1,4-dicarboxylic acid (H₂Br₄bdc) in different solvents under solvothermal conditions. The structural diversity found in the system mainly resulted from the effects of anions, solvents, and the variation in the ionic radii of the lanthanide(III) ions. Compounds in series 1_Ln feature a two-dimensional (2D) layered structure with sql topology based on {(Ln(COO)₂)₂(μ-COO)₂} secondary building units (SBUs). Compounds in series 2_Ln and 3_Ln comprise, respectively, infinite uniform and alternate chains of {Ln(COO)₂}_n SBUs that are assembled into a similar network topology to 1_Ln. Meanwhile, compounds in series 4_Ln feature 3D coordination networks of a pcu α-Po topological net consisting of binuclear {Ln₂(COO)₃} SBUs. The formation of polymeric networks in series 1_Ln–4_Ln is facilitated by the numerous coordination sites of the ligand Br₄bdc²⁻ and the fact that its bromine atoms can participate in the formation of various types of intermolecular interactions. The solid-state photoluminescence studies on Eu- (1_Eu) and Tb- (1_Tb, 3_Tb, 4_Tb) containing compounds indicate that the Br₄bdc²⁻ ligands can efficiently sensitize Eu³⁺ and Tb³⁺ emission. Notably, such compounds exhibit highly sensitive fluorescence sensing for acetone, water, and Fe³⁺ ions via the fluorescence quenching effect. As the representatives of the series, activated 1_Eu, 2_Pr, 3_Tb, and 4_Tb show the maximum CO₂ uptake capacities of 170.4, 273.7, 255.3, and 303.5 cm³ g⁻¹, respectively, at 50 bar and 298 K with good repeatability of the adsorption–desorption properties. Magnetic studies indicate that the Gd- and Dy-based compounds 1_Gd, 1_Dy, 3_Gd, 3_Dy, and 4_Gd show simple paramagnetic behaviours, whereas compound 4_Dy exhibits weak ferromagnetic interactions.