The [NHN] − homoconjugated anions: X-ray diffraction, IR and multinuclear MR studies

Abstract

X-ray diffraction, IR and NMR studies are reported on chemically symmetric [NHN] − anions formed by 1,8-bis(4-toluenesulphonamido)naphthalene, 1,8-bis(trifluoroacetamido)naphthalene, 1,8-diamino-2,4,5,7-tetranitronaphthalene and di-4-nitroimidazolyl-5-sulphide after abstraction of a proton by means of the proton sponge (DMAN), and in the 2:1 adducts of 4,5-dicyanoimidazole and 2-amino-4,5-dicyanoimidazole with DMAN as well as in the conjugated tetrazole-tetrazolate anion. The anionic NHN − hydrogen bonds studied so far are markedly weaker as compared with NHN + cationic ones of similar geometry. They are characterized by an asymmetric, in the scale of vibrations, H-atom localization in the bridge with the protonic (NHN) − band located usually in the region 2600–3000 cm −1. The δ 1H value in acetonitrile of the bridge proton in [NHN] − anions studied until now is between 14.1 and 16.4 ppm, while that in protonated DMAN is 18.7 ppm. The studies on 15N chemical shift and 15N 1H coupling constant yielded additional information on the symmetry of [NHN] − bridges and localization of the bridge H-atom.