Strong Lewis acid air-stable cationic titanocene perfluoroalkyl(aryl)sulfonate complexes as highly efficient and recyclable catalysts for C–C bond forming reactions

Abstract

A series of strong Lewis acid air-stable titanocene perfluoroalkyl(aryl)sulfonate complexes Cp₂Ti(OH₂)₂(OSO₂X)₂·THF (X = C₈F₁₇, 1·THF; X = C₄F₉, 2·H₂O·THF; X = C₆F₅, 3) were successfully synthesized by the treatment of Cp₂TiCl₂ with C₈F₁₇SO₃Ag, C₄F₉SO₃Ag and C₆F₅SO₃Ag, respectively. In contrast to well-known titanocene bis(triflate), these complexes showed no change in open air over three months. TG-DSC analysis showed that 1·THF, 2·H₂O·THF and 3 were thermally stable at 230 °C, 220 °C and 280 °C, respectively. Conductivity measurements showed that these complexes underwent ionic dissociation in CH₃CN solution. X-ray analysis results confirmed that 2·H₂O·THF and 3 were cationic. ESR spectra showed that the Lewis acidity of 1·THF (1.06 eV) was higher than that of Sc³⁺ (1.00 eV) and Y³⁺ (0.85 eV). UV/Vis spectra showed a significant red shift due to the strong complex formation between 10-methylacridone and 2·H₂O·THF. Fluorescence spectra showed that the Lewis acidity of 2 (λ_em = 477 nm) was higher than that of Sc³⁺ (λ_em = 474 nm). These complexes showed high catalytic ability in various carbon–carbon bond forming reactions. Moreover, they show good reusability. Compared with 1·THF, 2·H₂O·THF and 3 exhibit higher solubility and better catalytic activity, and will find broad applications in organic synthesis.