Synthesis and Structural Characterization of Fe(Ii) Complexes with Hemilabile Phosphine-Based P,O Donor Ligands: A Combined Experimental and Computational Study

Iron(II) complexes of the type [FeCl 2 (P∩X) 2 ] [P∩X = o -(diphenylphosphino)benzaldehyde{PPh 2 ( o -C 6 H 4 CHO)}( 1 ) and o -(diphenylphosphino)benzoic acid {PPh 2 ( o -C 6 H 4 COOH)}( 2 )] have been synthesized by the reaction of FeCl 2 .4H 2 O with respective ligands in DMF solution under refluxing condition. The complexes were characterized by elemental analysis, ESI-mass, Fourier Transform Infrared spectra (FTIR), UV-Vis, 1 H and 31 P{ 1 H} NMR spectroscopy. From the FTIR measurements, though the structures are predicted to be trans , it is not confirmed as crystal structures of the complexes are not yet available. In order to confirm their stable structures, computational calculations were performed to provide detailed molecular-level comparison of the structures, frontier molecular orbitals, electronic, and optical properties using first-principles density functional theory (DFT) method. The gas phase calculations indeed confirm the trans -isomers to be the minimum energy geometries compared to cis -isomers, with the energy difference of 8-9.3 kcal/mol between the geometrical isomers. The natural bonding orbital calculation on trans -isomers showed that intramolecular ligand → metal charge transfer stabilizes the complexes. Further, time dependent DFT (TDDFT) calculations were employed to interpret the optical properties of the trans -isomers of the complexes. Our combined integrative approach provided consistent agreement at both experimental and theoretical levels and showed that the trans structures are the most preferred structures for the two complexes.