Electrochemical and density functional theory modeled reduction of enolized 1,3-diketones

Cathodic peak potentials (Epc) of ten enolized 1,3-substituted 1,3-diketones, R1COCHC(OH)R2 derivatives containing electron withdrawing and/or electron donating groups, were measured by cyclic voltammetry in acetonitrile. Quantum computational based methods are exploited to model experimentally measured reduction potentials (Epc) by comparing experimentally measured reduction potentials Epc to the calculated descriptors; LUMO energy (ELUMO), electron affinity (EA), electrophilicity index (ω) and relative group electronegativity (χ), obtained from calculated electronic energies of the neutral, anionic and cationic molecules. Observed reduction potentials gave excellent correlation in the linear relationship between experimental Epc and calculated ELUMO (R2 > 0.99). Electrochemical behaviour, in agreement with DFT results, show that aromatic β-diketones (containing aromatic side groups) are characterized by reversible CV's due to the stabilization of the radical anion while β-diketones containing aromatic and/or aliphatic groups feature irreversible CV's. The stability of the radical anion is supported by the π-conjugated nature of the LUMO orbitals. The power of the substituent group's inductive effect was determined by using the sum of experimental group electronegativities (Gordy scale) of the R1 and R2 groups (χR1 + χR2) and calculated Mulliken electronegativities (in eV). A non-linear relationship between the observed substituent's inductive power and reduction potential (Epc) was observed since the electron density in the redox centre is controlled by both inductive and resonance effects.

► Design of β-diketonato ligands with particular redox potentials from unique correlation between theoretical and exp potentials. ► Excellent correlation between experimental redox-potentials of 1,3-substituted β-diketones and calculated LUMO energies, electron affinities (EA) and global electrophilicity index (ω).