Zn^II and Hg^II Complexes with 2,3-Substituted-5,6-di(1<i>H</i>-tetrazol-5-yl)pyrazine Ligands: Roles of Substituting Groups and Synthetic Conditions on the Formation of Complexes

In our continuing efforts to explore the effects of substituting groups of bridging ligands on the structure and properties of their complexes, two structurally related bis(tetrazolate) ligands, 2,3-diethyl-5,6-di(1H-tetrazol-5-yl)pyrazine (H2L1) and 2,3-diphenyl-5,6-di(1H-tetrazol-5-yl)pyrazine (H2L2), have been designed, and seven ZnII and HgII complexes with the two ligands, [Zn2(L1)2(H2O)4](H2O)4 (1), {[Zn2(L1)2](H2O)0.5}∞ (2), [Zn6(L2)6(H2O)12] (3), [Zn(L2)]∞ (4), [Hg(L1)Br]∞ (5), {[Hg4(L2)4(H2O)6](H2O)}∞ (6), and [Hg3(L2)2(H2O)0.5Br2]∞ (7), have been synthesized and characterized. 1 has a dinuclear structure in which ZnII is four-coordinated to furnish a distorted tetrahedral geometry, and in the crystal structure the dinuclear units are further assembled to form a three-dimensional (3D) framework by hydrogen bonds. 2 possesses a 3D framework with (4,122) topology in which the ZnII center is four-coordinated by N atoms of the ligand. 3 has a hexanuclear structure comprised of two trinuclear motifs as basic building units each of which has three types of coordination environments around the ZnII ions, and the hexanuclear units are interlinked by O−H···N hydrogen-bonding to generate a two-dimensional (2D) framework. 4 takes a 3D (4,8,10) topology, in which ZnII ions act as three-connected nodes and the L2 ligand takes a chelating−bridging mode being similar to that in 1. 5 possesses a 2D (4,8,8) layer structure with one kind of HgII center four-coordinated by one Br anion, two N atoms from two L1 ligands, and one weakly coordinated N atom from another L1 ligand. 6 has two kinds of one-dimensional chain structures with two kinds of HgII ions, and one kind of these chains is further assembled into 2D sheets via hydrogen-bonding interaction. 7 has a 2D layer structure with one kind of HgII center coordinated by a Br anion, the other one being seven-coordinated by six N atoms from four L2 ligands and one weakly coordinated O atom from a water molecule. In this work, 1, 3, 5, and 6 were synthesized by the conventional method at room temperature, while 2, 4, 7 were prepared under hydrothermal conditions. These results show that the substituting groups of ligands and the reaction conditions have great influences on the formation of these complexes. In addition, the luminescent properties of complexes 1−7 have been briefly studied.