Indenyl–titanium–trichloride complexes: correlations between electrochemical and UV–vis-spectroscopic data

Substituent effects in several titanium(IV)–trichloride half-sandwich complexes with η5-coordinated cyclopentadienyl or indenyl ligands including (η5-C5H5)TiCl3 (1), (η5-C9H7)TiCl3 (2), (η5-1-SiMe3C9H6)TiCl3 (3), (η5-1-MeC9H6)]TiCl3 (4), (η5-4-MeC9H6)]TiCl3 (5), [η5-5,6-(CH2)3C9H5]TiCl3 (6), (η5-2-SiMe3C9H6)TiCl3 (7), (η5-1,3-Me2-C9H5)TiCl3 (8), (η5-4,7-Me2C9H6)TiCl3 (9), [η5-3,4-(CH2)3C9H5]TiCl3 (10), (η5-1-SiMe34,7Me2C9H4)TiCl3 (11) and (η5-1,3,4,7-Me4C9H3)TiCl3 (12) were studied with cyclic voltammetry (CV) and UV–vis spectroscopy. Cyclic voltammograms of 1–12 showed current waves caused by the reversible redox process of the Ti(IV)/(III) redox couple at electrode potentials typical for d0-titanium centers. In UV–vis spectra, two major absorption bands between 400–430 and 530–590 nm were observed. The latter is assigned to a ligand to metal charge transfer (LMCT). A comparison shows that the alkylation or Me3Si-substitution (complexes 3–12) induces a shift of the corresponding absorption to longer wavelengths indicating a smaller energy gap between the frontier orbitals. Based on the experimental data an additive increment system for Me3Si and alkyl-substituted η5-indenyl derivatives of titanium(IV)–trichloride species is proposed.