Electrochimica Acta 56 (2010) 257–264 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Electrochemical and density functional theory study of bis(cyclopentadienyl) mono(-diketonato) titanium(IV) cationic complexes Annemarie Kuhn a , Jeanet Conradie a,b,∗ a Department of Chemistry, University of the Free State, Bloemfontein 9300, South Africa b Center for Theoretical and Computational Chemistry and Department of Chemistry, University of Tromsø, N-9037 Tromsø, Norway article info Article history: Received 26 May 2010 Received in revised form 24 August 2010 Accepted 24 August 2010 Available online 17 September 2010 Keywords: Titanocene -Diketone Cyclic voltammetry Electron affinity DFT abstract The electrochemical behaviour of fluorinated bis(cyclopentadienyl) mono(-diketonato) titanium(IV) complexes, of general formula [Cp 2 Ti(R ′ COCHCOR)] + ClO 4 - with Cp = cyclopentadienyl and R ′ , R=CF 3 , C 4 H 3 S; CF 3 ,C 4 H 3 O; CF 3 , Ph (C 6 H 5 ); CF 3 , CH 3 ; CH 3 , CH 3 ; Ph, Ph and Ph, CH 3 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) poten- tial. A comparison of the ligand reduction in its free and chelated state indicates that the -diketonato ligand (R ′ COCHCOR) - in [Cp 2 Ti(R ′ COCHCOR)] + ClO 4 - is electroactive at more negative potentials. A theo- retical density functional theory (DFT) 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 ◦′ ) and the sum of the group electronegativities of the R and R ′ groups,  R +  R ′ , of the lig- and. Linear relationships, however, are obtained between the DFT calculated electron affinity (EA) of the complexes and  R +  R ′ , the pK a of the free -diketones R ′ COCH 2 COR and the carbonyl stretching frequency, v CO , of the complexes. The DFT calculated electronic structure of the second reduced species [Cp 2 Ti(-diketonato)] - shows that it is best described as Ti(III) coupled to a -diketonato radical. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Metallocene(IV) complexes are ‘bent-sandwich’ organometal- lic complexes where the bis(cyclopentadienyl) moieties are in a bent conformation with respect to the central metal atom. Titanocene(IV) complexes and in particular titanocene dichloride [Cp 2 Ti IV Cl 2 ] exhibit various catalytic applications for example as Ziegler–Natta catalyst [1,2]. Knowledge of the activity of these complexes towards oxidation (chemical and electrochemical) or substitution is necessary to study and understand their mech- anism of action during catalysis. Cyclic voltammetry (CV) is a very useful modern electroanalytical technique for the study of electroactive species [3,4]. We are interested in metallocene-- diketones [5,6] and metallocene--diketonate [7] complexes and how the electronegativity of the side groups on the -diketonate ligand influences the formal reduction potential of the metallocene couple We previously showed that the formal reduction potential of the Ti III/IV couple of a series of [Cp 2 Ti(CH 3 COCHCOR)] + com- ∗ Corresponding author at: Department of Chemistry, University of the Orange Free State, Bloemfontein 9300, South Africa. Tel.: +27 51 4012194; fax: +27 51 4446384. E-mail address: conradj@ufs.ac.za (J. Conradie). plexes (R = CF 3 , OCH 3 ,C 6 H 5 , CH 3 or Fc (ferrocenyl)) increased as the group electronegativity of the R group of the -diketonato lig- and increased [8], although no quantitative relationship could be establish. Here we have prepared and characterised a series of eight [Cp 2 Ti(R ′ COCHCOR)] + complexes with different R ′ and R groups and examined the redox behaviour using cyclic voltammetry. In order to predict redox potentials, enabling the design of organometal- lic molecules with particular redox potentials, a density functional theory (DFT) study was conducted. The “electron transfer” process was calculated in terms of the amount of energy released when an electron is added to the molecule. We have established a quan- titative relationship between the electrochemical potential (E ◦′ ) of the metal centred redox process and gas-phase electron affin- ity potentials in titanocene--diketonate complexes. Trends were also established for the interrelationships between electrochemi- cal, spectroscopic and calculated electron affinities. 2. Experimental 2.1. General NMR measurements were recorded on a Bruker Advance DPX 300 NMR spectrometer at 298 K. Chemical shifts are reported as 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.08.086