Cadmium(II) Coordination polymers of 4-Pyr-poly-2-ene & Carboxylates: Construction, structure, & photochemical double [2 + 2] cycloaddition & luminescent sensing of nitroaromatics & mercury(II) Ions

Solvothermal reactions of Cd(NO3)₂4H₂O with 4-pyrpoly-2-ene (ppene) in the presence of benzene-1,2-dicarboxylic acid (1,2-H2BDC), benzene-1,3-dicarboxylic acid (1,3-H2BDC), benzene-1,4-dicarboxylic acid (1,4-H₂BDC), benzene-1,3,5-Tricarboxylic acid (1,3,5-H3BTC), or 4,4′-oxidibenzoic acid (4,4′-H₂OBA) afforded five Cd(II) coordination polymers [Cd(ppene)(1,2-BDC)]n (1), [Cd(ppene)(1,3-BDC)]n (2), [{Cd(ppene)(1,4-BDC)}MeCN]n (3) (Liu et al. Inorg. Chem. Commun. 2015, 58, 1-4), [{Cd(ppene)(1,3,5-HBTC)}0.5-(ppene)]n (4), and [Cd(ppene)(4,4′-OBA)]n (5), respectively. Upon UV light irradiation, 1, 3, and 4 can undergo a double [2 + 2] cycloaddition reaction to yield their corresponding photoproducts including [Cd2(4-Tp-3-lad)(1,2-BDC)2]n (1a, 4-Tp-3-lad = 2,3,5,6-Tetra(pyridin-4-yl)-bicyclo[2.2.0]hexane), [{Cd₂(4-Tp-3-lad)(1,4-BDC)₂}2MeCN]n (3a), and [{Cd₂(4-Tp-3-lad)(1,3,5-HBTC)₂}(ppene)]n (4a) in a single-crystalto-single-crystal manner. Compounds 1, 2, 4, 5, 1a, 3a, and 4a have been structurally characterized by elemental analysis, infrared spectroscopy, powder X-ray diffraction, and single-crystal X-ray diffraction. Compounds 1, 1a, 4 and 4a consist of twodimensional (2D) (4,4) networks, in which one-dimensional (1D) [Cd₂(1,2-BDC)₂]_n or [Cd₂(1,3,5-HBTC)₂]n chains are linked by ppene or 4-Tp-3-lad bridges. 2 has a three-dimensional (3D) 3-fold interpenetrated framework formed from bridging 1D [Cd₂(1,3-BDC)₂]_n double chains by ppene ligands. Compounds 3a and 5 contain 3D frameworks assembled by linking 2D [Cd₂(1,4-BDC)₂]_n or [Cd₂(4,4′-OBA)₂]_n networks by 4-Tp-3-lad or ppene bridges. In the three photoreactions, each ppene ligand had its two conjugated C?C bonds parallel to those of the neighboring ppene. The resulting ppene pair was transformed into the only product, 4-Tp-3-lad, through a unique double [2 + 2] cycloaddition. Photoluminescence properties of these compounds in water were also investigated. Compound 3a as a representative example was proven to be an efficient probe for dual detections of nitroaromatics and Hg²⁺ in aqueous solutions.