In situ solvent and counteranion-induced synthesis, structural characterization and photoluminescence properties of Pb-based MOFs

Abstract

Based on an attempt to investigate the influence of solvents and counteranions on the structures and photoluminescence properties of Pb-based metal–organic frameworks, the hydrothermal reactions of the same amounts of Pb(NO₃)₂ and Pb(OAc)₂ with the ligand 5-tert-butylisophthalic acid (H₂tip) in the presence of water and a water–ethanol mixture have yielded three compounds [Pb(H₂O)(tip)]_n (1), [Pb₃(μ₄-O)(tip)₂]_n (2) and [Pb₄(μ₄-O)(tip)₃]_n (3) under similar reaction conditions (tip = 5-tert-butylisophthalate anion). These compounds represented the first examples of Pb(II) metal–organic frameworks with H₂tip. All of them have been characterized by means of FT-IR spectra, elemental analysis, single-crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis, and photoluminescence spectra. The reaction processes revealed that 1 and 2 had selectivity for specific solvents. And the selectivity for specific counteranions was also found in 2 and 3. Single-crystal X-ray diffraction showed that 1 and 3 crystallized in the orthorhombic crystal system with space groups of Iba2 and Pna2₁, respectively, while 2 crystallized in the monoclinic P2₁/c space group. The structure of 1 featured a 2D bilayer structure containing a uninodal sql-type topological motif with a Schlafli symbol of (4⁴). The 2D layer framework of 2 was constructed from a unique 8-connected hexanuclear cluster secondary building unit Pb₆O₂(COO)₈, resulting in a uninodal hxl-type topological motif with a Schlafli symbol of (3⁶·4⁶·5³). The structure of 3 could be described as a 3D microporous framework with a 6-connected tetranuclear cluster Pb₄O(COO)₆. Topological analysis revealed that 3 represented a uninodal dia-type topological motif with a Schlafli symbol of (6⁶). All of the solid state compounds exhibited the photoluminescence properties at room temperature. Furthermore, taking the emissions of the free ligand into consideration, the emissions 1 and 2 could be assigned to metal-centered s → p transition transitions, while the emissions of 3 were due to ligand-to-metal charge transfers between the delocalized p bonds of carboxylate groups and p orbitals of Pb(II) centers. Especially, the in situ solvent and counteranion-induced synthesis strategy reported here could afford us new opportunities in the rapid design of new materials with interesting structures and properties.