Synthesis and structural characterization of [H(OEt2)2]+[(C3H3N2){B(C6F5)3}2]− — a Brønsted acid with an imidazole-derived ‘non-coordinating’ anion

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+[(C3H3N2){B(C6F5)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]+[(C3HMe2N2){B(C6F5)3}2]− (6b·THF) was characterized by X-ray diffraction. Deprotonation of benzimidazole followed by the addition of two B(C6F5)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 [(C3HR2N2){B(C6F5)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(OEt2)2]+[(C3H3N2){B(C6F5)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 [Cp2Zr(CH3)(OEt2)]+ (11) (with [(C3H3N2){B(C6F5)3}2]− anion) starting from dimethylzirconocene.