Electrochemical and density functional theory study of bis(cyclopentadienyl) mono(β-diketonato) titanium(IV) cationic complexes

The electrochemical behaviour of fluorinated bis(cyclopentadienyl) mono(beta-diketonato) titanium(IV) complexes, of general formula [Cp2Ti(R'COCHCOR)]+ClO4- with Cp=cyclopentadienyl and R', R=CF3, C4H3S; CF3, C4H3O; CF3, Ph (C6H5): CF3, CH3; CH3, CH3; Ph, Ph and Ph, CH3 is described. Both metal and ligand based redox processes are observed. The chemically and electrochemically reversible Ti-IV/Ti-III couple is followed by an irreversible ligand reduction at a considerably more negative (cathodic) potential. A comparison of the ligand reduction in its free and chelated state indicates that the beta-diketonato ligand (R'COCHCOR)(-) in [Cp2Ti(R'COCHCOR)+ClO4- is electroactive at more negative potentials. A theoretical density functional theory (OFT) study shows that a highly localized metal centred frontier orbital dominates the Ti-IV/Ti-III redox chemistry resulting in a non-linear relationship between the formal redox potential (E-o') and the sum of the group electronegativities of the R and R' groups. chi(R) + chi(R'), of the ligand. Linear relationships, however, are obtained between the OFT calculated electron affinity (EA) of the complexes and chi(R) + chi(R'), the pK(a) of the free beta-diketones R'COCH2COR and the carbonyl stretching frequency, v(co), of the complexes. The DFT calculated electronic structure of the second reduced species [Cp2Ti(beta-diketonato)](-) shows that it is best described as Ti(III) coupled to a beta-diketonato radical. (C) 2010 Elsevier Ltd. All rights reserved.