3,5-Bis((4′-carboxylbenzyl)oxy)benzoilate-based coordination polymers: their synthesis, structural characterization, and sensing properties

Abstract

With the assistance of various organic bases, hydrothermal reactions of transition-metal salts with a kind of tripodal tricarboxylic acid, 3,5-bis((4′-carboxylbenzyl)oxy)benzoic acid (H₃(bcbob)), at different pH values were carried out, producing four new bcbob-based 3-D coordination polymers: [Ni₂(Hbcbob)₂(bimb)₂] (bimb = 1,4-bis(imidazol-1-yl-methyl)benzene) 1, [Zn₃(bcbob)₂(bpe)₂] (bpe = 1,2-bis(4-pyridyl)ethene) 2, [H₂(bpp)][Mn₂(bcbob)₂] (bpp = 1,2-bis(4-piperidyl)propane) 3, and [Cd₂(OH)(bcbob)(bpe)_0.5]·H₂O 4. X-ray single-crystal diffraction analysis reveals that (i) in 1, the bcbob molecules link the Ni²⁺ centers into a 2-D layer with a (6,3) net, and then the bimb molecule serves as the pillar linker; (ii) in 2, the bpe molecule is only used to stabilize the 3-D network. The bcbob molecules link two types of tetrahedral Zn²⁺ ions into a 3-D network with a pcu topology; and (iii) the 3-D networks of 3 and 4 are both built up from carboxylate-bridged rod-shaped secondary building units (SBUs) by the bcbob molecules (acting as a 3-connected node), and three types of 1-D channels are observed. However, since the rod-shaped SBUs contain different structures, the side walls of the channels exhibit different arrangements. In particular, the side wall in 4 adopts a helical arrangement. The bcbob molecules act as the side wall of the channel. In 3, the H₂(bpp)²⁺ molecules occupy the space of the largest channels, while in 4, the bpe molecules occlude one type of smaller channels by coordinating to the Cd²⁺ ions on the side wall. The photoluminescence analysis reveals that 2 and 4 can serve as chemosensors to detect the ppm-grade nitrobenzene (250 ppm for 2, 150 ppm for 4).