Titanium thiosalicylate complexes: functional metalloligands for the construction of redox-active heterometallic...

The titanium complex [TiCp*(thiosal)(thiosalH)] (1) has been synthesised by reaction of [TiCp*Me₃], Cp* = η⁵-C₅Me₅, with thiosalicylic acid (H₂thiosal). Complex 1 reacts with [M(μ-OH)(COD)]₂ (M = Rh, Ir) to yield the corresponding early-late heterobimetallic complexes [TiCp*(thiosal)₂M(COD)] (M = Rh (2); Ir (3)). Carbon monoxide replaces the COD ligand in 2 and 3 leading to the respective dicarbonyl complexes [TiCp*(thiosal)₂M(CO)₂] (M = Rh (4); Ir (5)). Compound 4 reacts with PPh₃ to yield the monocarbonyl derivative [TiCp*(thiosal)₂Rh(CO)(PPh₃)] (6). The reaction of compound 1 with LiⁿBu yields the tetrametallic complex [{TiCp*(thiosal)₂Li}₂(THF)₃(H₂O)] (7). Compound 7 reacts with [RuCp*Cl(COD)] yielding the heterometallic complex [TiCp*(thiosal)₂RuCp*] (8). The molecular structures of compounds 4, 5 and 7 have been studied by X-ray diffraction. From cyclic voltammetric (CV) and square wave voltammetric (SWV) experiments, we observed that attachment of the titanium moiety of precursor 1 to a late transition metal moiety through the sulfur atoms has a significant influence on the reduction behaviour of the Ti(iv) metal centre. Thus, monometallic 1 exhibits an irreversible reduction process at -1.15 V vs. SCE, whereas the CVs of heterobimetallic compounds 2-6 are characterized by the reversible or quasi-reversible one-electron reduction of the Ti(iv)/Ti(iii) system, suggesting a significant stabilization of the Ti(iii) reduced species. Likewise, substitution of the M(COD) diolefin fragment in 2 and 3 by the M(CO)₂ carbonyl-containing moiety (in compounds 4 and 5) leads to a significant anodic shift in the titanium E_1/2 reduction redox potentials.