A theoretical investigation of the fragment interaction and nonlinear optical and electronic properties of tris(β-diketonato)iron(III) complexes

This study presents the effects of substituent groups R-1 and R-2 of beta-diketonato ligands (R1COCHCOR2)(-) in [Fe(beta-diketonato)(3)] complexes, on the interaction energy between the ligand fragments and the central iron(III) atom, coupled with the nonlinear optical properties and electronic properties of the [Fe(beta-diketonato)(3)] molecules. The results show that a CF3 group on the ligand reduces the nucleophilicity of the Fe atom and favours stability of the mer isomer above the fac isomer, contrary to what is observed in other unsymmetrically substituted molecules without a CF3 substituent group. The reduction of the molecules results in the weakening of the Fe-O coordination bonds, but increasing strength of the C-O covalent bonds of the ligands. The molecules that have favourable experimental reduction potentials (more positive values) are characterised by lower ionisation potential of the reduced iron(II), lower inductive effect and a lower energy of the lowest occupied molecular orbital (LUMO) before reduction (iron(III)), but by higher chemical potential and higher LUMO energy level after reduction (iron(II)). The experimental reduction potentials are found to correlate significantly with the inter-fragment total energy of interaction (Delta E-tot) between the beta-diketonato ligands and the iron(III) atom (R-2 = 0.948), with a mean absolute deviation (MAD) of the predicted reduction potential of 0.073. Also, the contribution of electrical energy (EL) to the interaction energy (Delta E-tot) correlates significantly with the experimental reduction potential (R-2 = 0.951), resulting in an even better prediction of reduction potential (MAD = 0.067). It is observed that ring substituents (Ph, C4H3O, C4H3S), dipole moment and polarisability have a greater effect on the hyperpolarisability of the molecules, than merely the presence of a more withdrawing substituent, like CF3. Therefore, molecule [Fe(PhCOCHCOH)(3)] with a phenyl ring substituent is found to be the compound best suited for nonlinear optical (NLO) applications, extraordinarily higher than the other molecules of this study.