Synthesis, Structures, and Photoluminescence of Zinc(II), Cadmium(II), and Mercury(II) Coordination Polymers Constructed from Two Novel Tetrapyridyl Ligands

Self-assembly of N,N,N′,N′-tetra(4-pyridyl)-1,4-phenylenediamine (L1) and N,N-di(2-pyridyl)-N′,N′-di(4-pyridyl)-1,4-phenylenediamine (L2) with MX2 (M = Zn, Cd, and Hg; X = Cl, Br, and I) generated novel supramolecular structures with a rich structural diversity. In the crystal of L1, all pyridyl rings are involved in intermolecular π···π stacking and T-shaped C−H···π interactions to form a two-dimensional (2D) network. [CdL1Cl2]n·nH2O (4), [CdL1I2]n (6), [HgL1Cl2]n (7), [HgL1Br2]n (8), [Cd4(L2)4Cl8]n·2nDMF (13), [CdL2Br2]n (14), [CdL2I2]n (15), and [HgL2Cl2]n·0.5nDMF (16) are coordination polymers. In complex 4, L1 utilizes all its pyridyl nitrogens to coordinate with Cd(II) centers to afford an unprecedented three-dimensional (3D) binodal (3,4)-connected network with the Schläfli symbol of (8.102)2(84.102). Complexes 6, 7, 8, 14, and 15 have zigzag or centipede-like 1D coordination structures. For complex 13, each Cd has an octahedral coordination environment, and the Cd centers are linked by dichloro-bridges to form interesting infinite (CdCl2)∞ chains, which are further bridged by L2 ligands to form a 2D coordination network. Crystal of 16 has a one-dimensional (1D) chain structure. The chains are arranged in an alternate up−down−up mode, with all the pyridyl rings involved in π···π stacking interactions to afford a 3D structure which consists of hexagonal channels along the c-axis. [Hg2(L2)2Br4]·H2O (17) and [Hg2(L2)2I4]·H2O (18) have binuclear coordination moieties, which are linked with water molecules by hydrogen bonds to form 1D structures. From these results, it is demonstrated that the structures of the complexes and the coordination modes of L1 and L2 are strongly dependent on the metal cations and the anions. The Hg(II) atoms in these complexes have tetrahedral coordination environments, whereas the Cd(II) centers have octahedral coordination geometries when Cl− is used as the anion, affording 3D and 2D coordination networks. The photoluminescence of L1, L2, and the complexes measured in the solid state at room temperature indicated that the emission colors vary from violet to yellow, and the emission intensity varies to a large extent, which can be rationalized by the contribution of a conflicting coordination effect and the heavy atom effect.