Seven and eight-coordinate Fe(III) complexes containing pre-organized ligand 1,10-phenanthroline-2,9-dicarboxylic acid: Solvent effects, supramolecular interactions and DFT calculations

This study reports the synthesis of two novel transition metal complexes using tetradentate 1,10-phenanthroline-2,9-dicarboxylic acid (H(2)PDA) as a main ligand and 1,10-phenanthroline (phen) as an auxiliary N-donor ligand. The structures of [FeCl(phen)(PDA)]center dot H2O (1) and NH4[Fe(PDA)(2)]center dot 2MeCN (2) have been prepared under similar solvothermal conditions but using different solvents. They are characterized by elemental analysis, FT-IR spectroscopy, thermogravimetric analysis and single crystal X-ray diffraction. On the basis of crystallographic data, compounds 1 and 2 are very rare seven- and eight-coordination iron complexes and show distorted pentagonal bipyramidal (PBP) and distorted dodecahedron (DD) geometry around the metal centres, respectively. The X-ray diffraction analysis shows that complexes 1 and 2 feature a 3D supramolecular architecture which consist of different kinds of the non-covalent H-bonding and pi-pi stacking interactions. The different structural topology of both compounds suggests that the choice of solvent plays a significant role in the process of assembly. The influence of solvent on the structure of the complexes and gas adsorption research were also discussed. Furthermore, a CSD survey was carried out on the coordination numbers of Fe cation and on the complexes with PDA(2-) ligand. Finally, the DFT calculations are devoted to analyze the pi-pi stacking interactions observed in compound 1 by comparing the interaction energies of phen center dot center dot center dot phen and PDA center dot center dot center dot PDA assemblies. In addition, molecular electrostatic potential (MEP) and non-covalent interaction plot (NCIplot) computational tools are used to further characterize the interactions. Finally, the application of 1 and 2 in the separation of carbon dioxide was investigated using molecular simulations in CO2/N-2 and CO2/CH4 mixtures. Higher adsorption capacity and greater selectivity for CO2 over N-2 and CH4 was observed in 2, thus suggesting it is a promising candidate for carbon capturing.