Amidines from cyclic amines and nitriles in the presence of zinc(ii): other nitriles in place of acetonitrile

Abstract

[Zn(quin)₂(H₂O)] (quin⁻ = an anionic form of quinoline-2-carboxylic acid, quinaldinate) was reacted with secondary cyclic amines (piperidine or pyrrolidine) and nitriles (propionitrile or benzonitrile) to form a series of products, including two types of amidine compound. Amidines result from the nucleophilic addition of amine to nitrile, promoted by zinc(II) species. Complexes of zinc(II) with intact amines were also obtained, with compositions of [Zn(quin)₂(amine)] and trans-[Zn(quin)₂(amine)₂]. The nature of the products depended heavily upon the reaction conditions. Propionitrile reaction systems afforded only one zinc(II) amidine complex, [Zn(quin)₂(pipepropioam)] (5) (pipepropioam = amidine, obtained from piperidine and propionitrile), and two amidinium salts, pipepropioamH[Zn(quin)₃] (6) and pyropropioamH[Zn(quin)₃] (7) (pipepropioamH⁺, pyropropioamH⁺ = protonated amidines). Benzonitrile reactions led to zinc(II) complexes with both benzoamidines, [Zn(quin)₂(pipebenzoam)] (8) and [Zn(quin)₂(pyrobenzoam)] (9) (pipebenzoam, pyrobenzoam = amidines from benzonitrile and amines, piperidine or pyrrolidine, respectively). Both benzoamidine complexes displayed polymorphism, with pairs of polymorphs being labeled 8a/8b or 9a/9b. [Zn(quin)₃]⁻ salts with protonated benzoamidines, pipebenzoamH[Zn(quin)₃] (10) and pyrobenzoamH[Zn(quin)₃] (11) were also isolated. X-Ray structure analysis has revealed that the [Zn(quin)₃]⁻ ions of 6, 7 and 11 featured zinc(II) in a five-coordinate environment. In the case of pyrrolidinobenzoamidine, a co-crystal, pyrobenzoamH[Zn(quin)₃]·[Zn(quin)₂(pyrobenzoam)] (12), which is composed of both the amidinium salt and the amidine complex, was obtained. Interestingly, the metal ion of [Zn(quin)₃]⁻ in 12 was in a six-coordinate environment.