Formation of 1D coordination polymers by reaction of a tetrazine ligand and PbX₂ (X: Br, I) salts: Spectral, structural and theoretical studies

Two lead(II) 1D-coordination polymers, [Pb-3(mu-BPTZ)(2)(mu(4)-Br)(mu(3)-Br)(mu-Br)(3)Br](n) (1) (BPTZ = 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine) and [Pb(HPMPH2)(mu-I)(2)](n) (2) (HPMPHz = zwitterionic form of N-(pyridin-2-ylmethylene) picolinohydrazonic acid, HPMPH) were prepared and identified by elemental analysis, FT-IR spectroscopy and single-crystal X-ray diffraction. The crystal structure of 1, is the first example for coordination of a tetrazine ligand to the lead(II) ion. The 1D chain in 1 is expanding by BPTZ and bromide bridges. Three different coordination environments were found for lead atoms: PbN4(mu(4)-Br)(mu(3)-Br)(p-Br)(2) with cube geometry; PbN2(mu(4)-Br) (mu(3)-Br)(mu(2)-Br)Br and PbN2(mu(4)-Br)(mu(3)-Br)(2)(mu(2)-Br)(2) with capped octahedral geometries. Ligands around these lead (II) ions are arranged in hemidirected mode. X-ray analysis of 2 revealed a ring opening hydrolysis reaction that converts BPTZ ligand to the HPMPHz. This structure is the first example for a PbN2OI4 coordination environment which is extended by iodide bridges and contains hemidirected pentagonal bipyramidal coordinated lead(II) ions. Also 2 is the first example containing zwitterionic form of HPMPH. In the crystal network of the complexes, in addition to hydrogen bonds there are pi-pi stacking interactions between pyridine rings. Optical properties of the compound 1 were investigated by UV-Vis diffuse reflectance and emission spectroscopy methods in solid state. According to the DFT calculated electronic band structures, the compounds are indirect band gap semiconductors. Linear optical properties of the compounds (including the dielectric function, absorption coefficient, reflectivity, refractive index, optical conductivity and energy loss function) are also investigated by DFT method.