A series of coordination polymers with tuned terphenyl tetracarboxylates and bis-pyridyl ligands with different flexibilities manifesting fluorescence properties and photocatalytic activities

Abstract

A family of new two-dimensional (2D) and three-dimensional (3D) coordination polymers, namely, [Co(H₂L)(bpe)₂]_n (1), [Co₂(L)(bpmp)(H₂O)]_n (2), [Co(H₂L)(bpfp)(H₂O)₂]_n (3), {[Ni(H₂L)(bpe)(H₂O)₂]·2H₂O}_n (4), [Ni(L)_0.5(bpmp)_0.5]_n (5), {[Cd(H₂L)(bpe)_0.5(H₂O)₂]·0.5(bpe)}_n (6) and {[Cd₂(L)(bpfp)_0.5]·H₂O}_n (7) was constructed using [1,1′ : 4′,1′′-terphenyl]-2′,4,4′′,5′-tetracarboxylic acid (H₄L), the transition metals and the bis-pyridyl ligands (bpe = 1,2-bis(pyridin-4-yl)ethene, bpmp = N,N′-bis(4-pyridyl)piperazine and bpfp = bis(4-pyridylformyl)piperazine) with different flexibilities. These complexes were characterized by elemental analyses, IR spectra, thermogravimetric (TG) analyses, powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. Complex 1 comprises a 2D coordinated layer that consists of Co^II metal ions and H₂L ligands, and monodentate bpe ligands extend on both sides of the network. Complex 2 is a (4,6)-connected 3D framework based on Co-L 2D sheets with four types of different quadrilaterals and bridging bpmp ligands. When bpmp is replaced by bpfp, complex 3 possesses a 2D (4,4) network containing “H”-type 1D Co-H₂L chains and 1D Co-bpfp zigzag chains. Complex 4 is similar to complex 3 except that it contains 1D Ni-bpe linear chains. Complex 5 displays a 3D framework with symmetrical polygonal windows, in which there are 2D grids composed of quadrilaterals of the same size. Complex 6 exhibits a 2D {6³} layer including hexagonal grids, and some non-coordinated bpe ligands are present in the network. When bpfp is used in complex 7, a complicated 3D {4¹⁵·6¹³}₂{4²·6³·8}{4⁷·6³}₄ framework is formed. In this structure, all the coordinated atoms from L and bpfp ligands coordinate with Cd^II metal ions. Furthermore, the 2D layers of complexes 1, 3, 4 and 6 are ultimately expanded into 3D supramolecular frameworks via hydrogen-bonding interactions. The flexibility of the bis-pyridyl ligands indicates a consequential influence on the final architectures of complexes 1–7. Moreover, their electrochemical behaviors and fluorescence properties have been investigated. Photocatalytic properties reveal that complex 3 exhibits good catalytic activities for the degradation of the organic dye methylene blue (MB).