Synthesis and structural characterization of [H(OEt 2) 2] +[(C 3H 3N 2){B(C 6F 5) 3} 2] − — a Brønsted acid with an imidazole-derived ‘non-coordinating’ anion

Abstract

Lithium imidazolide, generated by NH deprotonation of imidazole with n-butyllithium, reacts with two molar equivalents of tris(pentafluorophenyl)borane with NB bond formation to yield the product Li +[(C 3H 3N 2){B(C 6F 5) 3} 2] − ( 6a, isolated in >70% yield). The analogous reaction sequence starting from 4,5-dimethylimidazole gives the corresponding salt 6b. Its THF coordination product [Li(THF) 4] +[(C 3HMe 2N 2){B(C 6F 5) 3} 2] − ( 6b·THF) was characterized by X-ray diffraction. Deprotonation of benzimidazole followed by the addition of two B(C 6F 5) 3 equivalents gave the corresponding benzimidazolide-based anion, isolated as the lithium compound 6c. The lithium salts 6 of the large ‘non-nucleophilic’ anion system [(C 3HR 2N 2){B(C 6F 5) 3} 2] − were employed in the generation of Group 4 metallocene cations by salt metathesis. Treatment of 6a with HCl in diethyl ether afforded the product [H(OEt 2) 2] +[(C 3H 3N 2){B(C 6F 5) 3} 2] − ( 9), that was also characterized by X-ray crystal structure analysis. The Brønsted acid 9 was used to generate the Group 4 metallocene cation system [Cp 2Zr(CH 3)(OEt 2)] + ( 11) (with [(C 3H 3N 2){B(C 6F 5) 3} 2] − anion) starting from dimethylzirconocene.