This study reports the synthesis of two novel transition metal complexes using tetradentate 1,10-phenanthroline-2,9-dicarboxylic acid (H2PDA) as a main ligand and 1,10-phenanthroline (phen) as an auxiliary N-donor ligand.The structures of [FeCl(phen)(PDA)]∙H2O (1) and NH4[Fe(PDA)2]∙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 π–π 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 PDA2− ligand. Finally, the DFT calculations are devoted to analyze the π–π stacking interactions observed in compound 1 by comparing the interaction energies of phen···phen and PDA⋯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/N2 and CO2/CH4 mixtures. Higher adsorption capacity and greater selectivity for CO2 over N2 and CH4 was observed in 2, thus suggesting it is a promising candidate for carbon capturing.

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