Cadmium–1,4-cyclohexanedicarboxylato coordination polymers bearing different di-alkyl-2,2′-bipyridines: syntheses, crystal structures and photoluminescence studies

Abstract

Four coordination polymers have been synthesized using self-assembly solution reactions under ambient conditions, reacting Cd(II) ions with 1,4-cyclohexanedicarboxylic acid in the presence of different 2,2′-bipyridine co-ligands: {[Cd(H₂O)(e,a-cis-1,4-chdc)(2,2′-bpy)]·H₂O}_n (1); [Cd₂(H₂O)₂(e,a-cis-1,4-chdc)₂(4,4′-dmb)₂]_n (2); {[Cd(e,a-cis-1,4-chdc)(5,5′-dmb)]·H₂O·CH₃OH}_n (3) and {[Cd(e,e-trans-1,4-chdc)(4,4′-dtbb)]·CH₃OH}_n (4), where 1,4-chdc = 1,4-cyclohexanedicarboxylato, 2,2′-bpy = 2,2′-bipyridine, 4,4′-dmb = 4,4′-dimethyl-2,2′-bipyridine, 5,5′-dmb = 5,5′-dimethyl-2,2′-bipyridine and 4,4′-dtbb = 4,4′-di-tert-butyl-2,2′-bipyridine. Crystallographic studies show that compound 1 has a 1D structure propagating along the crystallographic b-axis; the Cd ion in 1 is six-coordinated with a distorted-octahedral coordination sphere. Compound 2 has two crystallographic different Cd ions and both are six-coordinated with a distorted-octahedral coordination sphere. Compound 3 exhibits a seven-coordinated Cd ion having a distinctive distorted-monocapped trigonal prismatic geometry. In compound 4, the Cd ion is also seven-coordinated in a distorted monocapped octahedral geometry. Compounds 2, 3 and 4 possess rhombic-shaped dinuclear units (Cd₂O₂) as nodes to generate larger cycles made up of four dinuclear units, a Cd₄ motif, bridged by four 1,4-chdc ligands, accomplishing, thus, 2D structures. Remarkably, in compound 4 the 1,4-chdc ligand conformation changes to the equatorial, equatorial trans, unlike the other compounds where the bridging ligand conformation is the more typical equatorial, axial cis. The solid state luminescence properties of 1–4 were investigated; polymers 3 and 4 exhibited a strong blue emission (λ_em = 410–414 nm) compared to 1 and 2; structure-related photoluminescence is attributed to the degree of hydration of the compounds. Furthermore, Cd-polymer 3 suspended in acetone allows the fluorescence selective sensing of acetonitrile over common organic solvents such as alcohols and DMF, based on turn-on fluorescence intensity with a limit of 53 μmol L⁻¹.