Symmetric heteropolynuclear Ti(IV)/Cu(I) complexes exhibiting stepwise electrochemical reductions to Ti(III) species

The heterotrinuclear complexes, [(CpTiCl(2)Cp-PPh(2))(2)Cu](+) (2) (as PF(6)(-) salt) and [CpTiCl(2)Cp-PPh(2)](2)CuCl (3), containing two electron-poor Ti(IV) fragments and one electron-rich Cu(I) center, and a tetrametallic species, [(CpTiCl(2)Cp-PPh(2))Cu(mu-Cl)](2) (4), were synthesized and characterized. The trinuclear nature of 2 and 3 was demonstrated by X-ray crystallography for which the three metallic centers are held together by two CpPPh(2) ligands. Weak Cl...Cu interactions are noted in 3, whereas two stronger Cl-Cu bridges are depicted in the cationic complex 2. The tetranuclear complex 4 contains two Cu atoms bridged together by two chloride ligands and connected to one Ti atom by one CpPPh(2) ligand and one mu-Cl ligand. Despite the short distances in the Ti-Cl-Cu bridges, notably in 2 and 4, the NMR spectra reveals evidence of fluxion in solution attributed to the lability of the mu-Cl ligands. Electrochemical experiments performed on 2 (rotating disk electrode and cyclic voltammograms) demonstrated the presences of two successive 1-electron reductions generating a first structurally unsymmetrical paramagnetic species [(CpTiCl(2)Cp-PPh(2))(2)Cu](0) (2'), confidently characterized by electron paramagnetic resonance (EPR) as a Ti(III)-Cu(I)-Ti(IV) system, and a second one, which is tentatively assigned to a symmetric neutral complex formulated as [CpTiClCp-PPh(2)](2)CuCl (5) with an EPR signature indistinguishable from that of 2' (i.e., Ti(III)-Cu(I)-Ti(III) system with a similar electronic environment). Density functional theory (DFT) computations examining the nature of the frontier orbitals and the geometry confirmed the presence and lack of symmetry in 2 and 2', respectively. The detection of chemical/electrochemical mechanisms in the electrochemical studies provides a clear explanation for the stepwise reduction behavior in these systems through chloride ligand transfer from Ti to Cu prior to the first reduction step. This transfer process is fully reversible upon re-oxidation. The electrochemical properties of complexes 3 and 4 are also reported.