Synthesis, structures and luminescent properties of new Pb(ii)/M(i) (M = K, Rb and Cs) frameworks based on dicarboxylic acids: a novel icosahedral Pb6-M6 SBU

Abstract

Reactions of Pb(CH₃COO)₂·3H₂O, MNO₃ (M = K, Rb, and Cs) with m-H₂BDC (1,3-H₂BDC = 1,3-benzenedicarboxylic acid) in a mixed-solvent of DMF and methanol (v/v = 2/1) resulted in the formation of novel metal–organic frameworks [(Me)₄N]₂[Pb₆M₆(m-BDC)₉(OH)₂]·H₂O (1: M = K; 2: M = Rb; 3: M = Cs). With H₂SDBA (H₂SDBA = 4,4′-sulfonyldibenzoic acid), two new lead(II) coordination polymers, [K₂Pb(SDBA)₂(μ₃–H₂O)]·3H₂O (4) and [Cs₂Pb₂(SDBA)₃(DMF)] DMF (5), were isolated. These complexes have been fully characterized by elemental analysis, PXRD, FTIR, TGA and single crystal X-ray diffraction. Compounds 1–3 are 3-D coordination frameworks built by a novel 12-metal heterometallic Pb₆-M₆ cage that extends topologically into an eight-connected hex network viaisophthalic acid ligands. Compounds 4 and 5 are new 3-D lead(II) MOFs constructed by ladder-like Pb(II)/K(I)/SDBA²⁻ and rod-shaped Pb(II)/Cs(I)/SDBA²⁻ motifs, respectively. The Pb(II) ions in these complexes show different coordination geometries with various coordination numbers (5, 7 and 9). In addition, an ideal T-shaped μ₃-H₂O is observed in 4. The SDBA²⁻ ligands in 4 and 5 display three types of new coordination modes that have not been observed before. At room temperature, 1–3 exhibit strong emissions in the solid-state which can be attributed to ligand-to-metal charge transfer between the delocalized π bonds of carboxylate groups and p orbitals of lead(II) centers.