Structural and spectroscopic investigations of 1,1,3,3-tetrakis(alkylamino)allyl cations, methylene-bis(N,N′-dialkylformamidinium) dications and related formamidine derivatives

Abstract

Treatment of pentachlorocyclopropane under basic conditions with either of the primary amines, Pr ⁱ NH ₂ or Bu ^t NH ₂ , displaces all the chlorine atoms and yields methylene-bis(N,N′-dialkylformamidine), CH ₂ {C(NR)NHR} ₂ (R=Pr ⁱ or Bu ^t ). A facile protonation converts the molecule into the 1,1,3,3-tetrakis(alkylamino)allyl cation, [CH{C(NHR) ₂ } ₂ ] ⁺ to which a further proton can be added to generate the corresponding methylene-bis(N,N′-dialkylformamidinium) ion. Barriers to rotation about the partial CN double bonds of these dications, [CH ₂ {C(NHR) ₂ } ₂ ] ² (R=Pr ⁱ or Bu ^t ), of 80 and 74 kJ mol ^-1 , respectively, are deduced from variable temperature NMR data. ¹ H and ¹³ C NMR studies of the kindred methylene-bis(formamidine) molecules show evidence of tautomerism as well as conformational changes in solution. Hydrolysis of methylene-bis(N,N′-di-tert-butylfor mamidine) gives N,N′-di-tert-butylformamidine. The crystallographically determined structure of N,N′-di-tert-butylformamidinium chloride, [CH(NHBu ^t ) ₂ ]Cl, is compared with other members of this series.