Low-spin, mononuclear, Fe(III) complexes with P,N donor hemilabile ligands: A combined experimental and theoretical study

Two new mononuclear Fe(III) complexes, [FeCl3{PPh2(p-C6H4NMe2)-P}3](1) (PPh2(p-C6H4NMe2): 4-(dimethylamino)phenyldiphenylphosphine) and [FeCl3(PPh2py-P)(PPh2py-P,N)] (2) (PPh2py: diphenyl(2-pyridyl)phosphine) were synthesized by reacting anhydrous FeCl3 with respective ligand in acetonitrile solution under refluxing condition. Both the complexes were characterized by elemental analysis, FAB-Mass, FTIR, UV–Vis, ESR, Cyclic Voltammetry and magnetic measurement. The FAB mass spectra of complexes 1 and 2 show molecular ion peak at m/z 1078 [M]+ and m/z 687 [M−1]+, respectively, indicating mononuclear nature of the complexes. UV–Vis spectra of the complexes were consistent with low-spin, octahedral geometry. The variable temperature magnetic susceptibility measurement (73–323 K) of these complexes is also consistent with the paramagnetic nature of the complexes with a ground state spin S = ½. The Fe(III) centers of these two complexes remain low-spin, both at room temperature and liquid nitrogen temperature, was also indicated by the ESR analysis. Cyclic Voltammetry of both the complexes show an irreversible oxidation wave attributed to Fe3+ → Fe4+ + e− along with the peak for ligand oxidation. Theoretical calculations (B3LYP) of the complexes show that for complex 1, a trans geometry of the two phosphorous atoms and for complex 2, a mer,cis structures are the most favored geometrical isomer. TDDFT calculations were performed to interpret the observed bands in the UV–Visible spectra.