O,O'-chelated titanium(IV) complexes: a synthetic, kinetic, electrochemical and structural study
Synthetic routes to prepare fluorinated tetrahedral mono-β-diketonato titanium(IV) complexes, [Cp2Ti(β)+], octahedral bis-β-diketonato titanium(IV) complexes, Ti(β)2Cl2 and Ti(β)2biphen, as well as dimeric and tetrameric titanium(IV) complexes, were developed and optimised. All complexes were fully characterised, inter alia with UV/vis, IR, 1H and 19F NMR and X-ray crystallography. Further characterisation of the complexes was done by means of electrochemical and kinetic techniques. Solution phase studies using variable temperature 1H and 19F NMR show that all octahedral Ti(β)2Cl2 and Ti(β)2biphen complexes exist in solution as an equilibrium mixture of three cis isomers which rearrange via fast (on NMR timescale) intramolecular exchange processes. The CF3-containing Ti(β)2Cl2 complexes further participate in facile monomer dimer equilibrium, involving the interconversion of chemically distinct compounds. Substitution and exchange reactions involving the ligand(s) in the octrahedrally coordinated bis(β-diketonato)Ti(IV) complexes were investigated by means of UV/Vis and NMR kinetics. Experimental and computational data were mutually consistent indicating that the substitution processes proceeded via a seven-coordinate transition state according to an associative mechanism. The ligand exchange equilibria studies of Ti(β)2biphen complexes showed that the formation of mixed-ligand complexes is a random statistical process when the exchanging β-diketonato ligands contain the same number of CF3 groups, while the equilibrium favours the mixed ligand complex when they differ by one CF3 group, Electrochemical (cyclic voltammetric) studies on the Ti(IV) complexes and the β-diketones were performed in the weakly coordinating CH3CN or DCE/[NBu4][PF6] medium. The reduction of the Ti(IV) complexes Cp2Ti(biphen), Ti(β)2biphen and Ti(β)2Cl2 which form negatively charged reduced species, are to varying degrees chemically unstable. However, the [Cp2Ti(β)]+ complexes, forming a neutral reduced species, are chemically stable. The redox process is electrochemically reversible for Cp2Ti(biphen), Ti(β)2biphen and [Cp2Ti(β)]+ and irreversible for Ti(β)2Cl2 The reduction of the uncoordinated β-diketone, forming an unstable radical anion, is not coupled with a reverse oxidation process. However, when the β-diketone contains two aromatic side groups, the radical anion is stabilised long enough that the redox process, becomes electrochemically reversible (ΔEp < 90 mV) with an increasing peak aniodic current at higher scan rates. The electron density on the titanium(IV) metal centre was manipulated by changing the R groups on the coordinated β-diketonato ligand (RCOCHCOR¢) from electron donating (R = CH3, Ph) to strongly electron withdrawing (R = CF3). The formal reduction potential, E0', of the Ti(IV) complexes (or the reduction potential, Epc, of the ligands), correlated to parameters related to electron density on the Ti centre, i.e., χR + χR' (sum of the group electronegativities of R and R' groups on the β-diketonato ligand R'COCHCOR-) and pKa of the β-diketones. Calculated (DFT) ionisation potentials of [Cp2Ti(β)]+ complexes showed a linear correlation to χR + χR', allowing one to predict electronegativities of the R and R¢ groups if the calculated ionization potential is known.