Synthesis, electrochemical and DFT study of octahedral bis(β-diketonato)-titanium(IV) complexes

Abstract

The synthesis and electrochemistry of seven new and four known bis(β-diketonato)-titanium(IV) complexes are reported. Integrated electrochemical and quantum chemistry techniques have been used to show that the reduction potential of this series of eleven bis(β-diketonato)-titanium(IV) complexes, [Ti(R1COCHCOR2)2biphen], are significantly influence by the electronic properties of the 1,3-β-diketonato-substituents (R1, R2), due to the efficient β-diketonato-metal π-conjugated character of the lowest unoccupied molecular orbital (LUMO) of the complexes. Electron density in the redox center is controlled by both inductive and resonance effects as seen by the spin density plots. Electron-withdrawing substituents make the reduction potentials more positive by stabilizing the reduced species. Both metal and ligand based redox processes are observed; the reversible metal TiIV/TiIII reduction is followed by a considerably more negative, irreversible ligand reduction. The density functional theory calculated electronic structure of the first reduction is a TiIII species while the second reduced species, is TiIII coupled to a β-diketonato radical. A number of important relationships between the electronic properties of the substituents and the redox behaviour have been developed. The experimentally measured reduction potentials, Epc, gave excellent correlation in the linear relationship between experimental Epcvs. calculated LUMO energies, ELUMO (R2=0.98).