Electronic effect of β-diketonato ligands on the redox potential of fac and mer tris(β-diketonato) iron(III) complexes: A density functional theory study and molecular electrostatic potential analysis

Experimental redox potentials of 16 derivatives of tris(beta-diketonato)iron(III) complexes (where beta-diketonato(sic)(R1COCHCOR2)degrees, with substituents R-1 and R-2 in different combinations of H, C4H3S, C4H3O, CH3, Ph, CF3, or C (CH3)(3)), and 11 additional isomers, were studied theoretically in terms of the electronic properties, substituent effects, electron affinity, and molecular electrostatic potential (MESP) analysis, using density functional theory methods. The computational methods reproduced the experimental reduction potential to a very high level of accuracy, especially when the M062X functional was used (with mean absolute deviation [MAD] = 0.054 and 0.093 and correlation R-2 = 0.978 and 0.981 obtained by application of two slightly different free energy cycles, respectively). The most negative computed reduction potential corresponds to the most negative reported experimental reductions, which is indicative of the least favorable reduction potential, also in most cases the most stable molecules energetically. The calculated reduction potentials of the fac isomers of the molecules were generally higher (less negative) than that of the mer isomers when one of the ligand substituents R-1 or R-2 was CF3 (M062X results), indicating better ease of reduction, although in many cases, the experimental reduction potential agreed better with the calculated reduction potential of the mer isomer instead. The calculated reduction potentials were also affected by the substituents in the order of CF3 > H > C4H3S > C4H3O > Ph > CH3 > C(CH3)(3) (the most negative value). The stronger the electron withdrawing tendency of the substituent, the more favorable (less negative value) the reduction potential becomes. In relation to the CH3-substituted molecule 1 as a reference, the molecules with electron withdrawing substituents resulted in an electron-deficient MESP iso-surface, in both the neutral state and reduced state. All the molecules in their reduced state were characterized with an electron-deficient MESP iso-surface compared with the reduced CH3-substituted molecule 1, with the deficiency increasing in mer compared with fac, for both the neutral and reduced molecules. The relative MESP values of Delta V-Fe in the reduced state of the molecules were able to predict the corresponding experimental reduction potential to a significant level of accuracy (with MAD = 0.091).