Cadmium(II) three-dimensional coordination polymers constructed from 1,3,5-tris(4-carboxyphenoxy)benzene: synthesis,...

Two three-dimensional (3D) Cd^II coordination polymers, namely poly[[di-μ-aqua-diaquabis{μ₅-4,4',4''-[benzene-1,3,5-triyltris(oxy)]tribenzoato}tricadmium(II)] dihydrate], {[Cd₃(C₂₇H₁₅O₉)₂(H₂O)₄]·2H₂O}_n, (I), and poly[[aqua{μ₆-4,4',4''-[benzene-1,3,5-triyltris(oxy)]tribenzoato}(μ-formato)[μ-1,1'-(1,4-phenylene)bis(1H-imidazole)]dicadmium(II)] dihydrate], {[Cd₂(C₂₇H₁₅O₉)(C₁₂H₁₀N₄)(HCOO)(H₂O)]·2H₂O}_n, (II), have been hydrothermally synthesized from the reaction system containing Cd(NO₃)₂·4H₂O and the flexible tripodal ligand 1,3,5-tris(4-carboxyphenoxy)benzene (H₃tcpb) via tuning of the auxiliary ligand. Both complexes have been characterized by single-crystal X-ray diffraction analysis, elemental analysis, IR spectra, powder X-ray diffraction and thermogravimetric analysis. Complex (I) is a 3D framework constructed from trinuclear structural units and tcpb^3- ligands in a μ₅-coordination mode. The central Cd^II atom of the trinuclear unit is located on a crystallographic inversion centre and adopts an octahedral geometry. The metal atoms are bridged by four syn-syn carboxylate groups and two μ₂-water molecules to form trinuclear [Cd₃(COO)₄(μ₂-H₂O)₂] secondary building units (SBUs). These SBUs are incorporated into clusters by bridging carboxylate groups to produce pillars along the c axis. The one-dimensional inorganic pillars are connected by tcpb^3- linkers in a μ₅-coordination mode, thus forming a 3D network; its topology corresponds to the point symbol (4².6².8²)(4⁴.6²)₂(4⁵.6⁶.8⁴)₂. In contrast to (I), complex (II) is characterized by a 3D framework based on dinuclear cadmium SBUs, i.e. [Cd₂(COO)₃]. The two symmetry-independent Cd^II ions display different coordinated geometries, namely octahedral [CdN₂O₄] and monocapped octahedral [CdO₇]. The dinuclear SBUs are incorporated into clusters by bridging formate groups to produce pillars along the c axis. These pillars are further bridged either by tcpb^3- ligands into sheets or by 1,4-bis(imidazol-1-yl)benzene ligands into undulating layers, and finally these two-dimensional surfaces interweave, forming a 3D structure with the point symbol (4.6²)(4⁷.6¹⁴). Compound (II) exhibits reversible I₂ uptake of 56.8 mg g^-1 with apparent changes in the visible colour and the UV-Vis and fluorescence spectra, and therefore may be regarded as a potential reagent for the capture and release of I₂.