Cadmium(II) Coordination Polymers of 4-Pyr-poly-2-ene and Carboxylates: Construction, Structure, and Photochemical Double [2 + 2] Cycloaddition and Luminescent Sensing of Nitroaromatics and Mercury(II) Ions

Solvothermal reactions of Cd(NO 3 ) 2 ·4H 2 O with 4-pyr-poly-2-ene (ppene) in the presence of benzene-1,2-dicarboxylic acid (1,2-H 2 BDC), benzene-1,3-dicarboxylic acid (1,3-H 2 BDC), benzene-1,4-dicarboxylic acid (1,4-H 2 BDC), benzene-1,3,5-tricarboxylic acid (1,3,5-H 3 BTC), or 4,4′-oxidibenzoic acid (4,4′-H 2 OBA) 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 [Cd 2 (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), [{Cd 2 (4-tp-3-lad)(1,4-BDC) 2 }·2MeCN] n ( 3a ), and [{Cd 2 (4-tp-3-lad)(1,3,5-HBTC) 2 }·(ppene)] n ( 4a ) in a single-crystal-to-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 two-dimensional (2D) (4,4) networks, in which one-dimensional (1D) [Cd 2 (1,2-BDC) 2 ] n or [Cd 2 (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 [Cd 2 (1,3-BDC) 2 ] n double chains by ppene ligands. Compounds 3a and 5 contain 3D frameworks assembled by linking 2D [Cd 2 (1,4-BDC) 2 ] n or [Cd 2 (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 Hg 2+ in aqueous solutions.