Two new coordination polymers of Tl(I) & Hg(II), based on nicotinic acid ligand: synthesis, characterization, crystal structure determination & DFT calculation

Two new complexes, [Tl(pyc)]_n (1) and {[Hg(pyc.H)(μ-Br)₂].C₂H₅OH}_n (2) have been synthesized by the reaction of one and two equivalent of pyridine 3-carboxylic acid (pyc.H) ligand with Tl₂(CO₃) (1) and HgBr₂ (2), respectively. The complexes were fully characterized by elemental analysis, UV, FT-IR, FT-NMR and emission spectroscopies and their structures were studied by the single-crystal X-ray diffraction method. According to X-ray analysis, complex 1 (τ₅ = 0.915) exhibit slightly distorted trigonal bipyramidal geometry around Tl(I) and complex 2 (τ_{5 =} 0.062) shows a slightly distorted square pyramidal geometry around Hg(II). There are intermolecular hydrogen bonding (for complexes 1 and 2) and π-π contacts (for complex 2) which play a significant role in the stabilization of the crystal structure. Luminescence studies revealed the emission properties of free ligand and both complexes in solution. Moreover, in this study, density functional theory (DFT) was performed for Tl(I) coordination polymer 1. The optimized geometry of this complex is shown in good agreement by single crystal X-ray data. Molecular properties including bond lengths, bond angles, and HOMO-LUMO energy levels, were analyzed. Moreover, the UV-Vis spectra were analyzed using time-dependent density functional theory (TD-DFT) method. The tendency of the donor-acceptor interactions in the complex 1 was examined using natural bond orbital (NBO) analysis. In addition, the partial density of states (PDOS) calculation indicates that the π-character of the aromatic pyridine ligand plays role in dominating the valence bands with negligible participation of the Tl orbitals.