Fluorinated triazapentadienyl ligand supported ethyl zinc(ii) complexes: reaction with dioxygen and catalytic applications in the Tishchenko reaction

Abstract

Ethyl zinc complexes [N{(C₃F₇)C(Dipp)N}₂]ZnEt, [N{(C₃F₇)C(Cy)N}₂]ZnEt, [N{(CF₃)C(2,4,6-Br₃C₆H₂)N}₂]ZnEt and [N{(C₃F₇)C(2,6-Cl₂C₆H₃)N}₂]ZnEt have been synthesized from the corresponding 1,3,5-triazapentadiene and diethyl zinc. X-ray data show that [N{(C₃F₇)C(Dipp)N}₂]ZnEt has a distorted trigonal planar geometry at the zinc center. The triazapentadienyl ligand binds to zinc in a κ²-mode. The zinc-ethyl bonds of [N{(C₃F₇)C(Dipp)N}₂]ZnEt, [N{(C₃F₇)C(Cy)N}₂]ZnEt, [N{(CF₃)C(2,4,6-Br₃C₆H₂)N}₂]ZnEt and [N{(C₃F₇)C(2,6-Cl₂C₆H₃)N}₂]ZnEt readily undergo oxygen insertion upon exposure to dry air to produce the corresponding zinc-ethoxy or zinc-ethylperoxy compounds. The ethoxy zinc adducts {[N{(CF₃)C(2,4,6-Br₃C₆H₂)N}₂]ZnOEt}₂ and {[N{(C₃F₇)C(2,6-Cl₂C₆H₃)N}₂]ZnOEt}₂ as well as the ethylperoxy zinc adduct {[N{(C₃F₇)C(Cy)N}₂]ZnOOEt}₂ have been isolated and fully characterized by several methods including X-ray crystallography. They feature dinuclear structures with four-coordinate zinc sites and bridging-ethoxy or -ethylperoxy groups. The ethyl zinc complexes catalyze the Tishchenko reaction of benzaldehyde under solventless conditions affording benzyl benzoate. The reaction of ethyl zinc complexes with dioxygen and their catalytic behaviour in the Tishchenko reaction are affected by the electronic and steric factors of the triazapentadienyl ligand. {[N{(C₃F₇)C(Cy)N}₂]ZnOOEt}₂ is an excellent reagent for the epoxidation of trans-chalcone.