Hydrogen-bonding and resonance stabilisation effects in cationic bis(iminium) phenoxide diacids.

The synthesis and characterisation of a bis(iminium)phenoxide diacid cation [4-Bu-CH-2,6-(HCN(H)Dipp)-1-O] ([HL]), is discussed. [HL][BF] (1) and [HL][HN{B(CF)}] (2) were synthesised in high yields protonation of the bis(imino)phenol conjugate base with ethereal HBF or Bochmann's acid ([H(OEt)][HN{B(CF)}]). Both species were fully characterised using NMR and IR spectroscopy as well as X-ray crystallography. The cationic fragment adopts an unusual tautomeric form in which both acidic protons are located on the nitrogen atoms: [HN〈O〉NH]. This bis(iminium) phenoxide tautomer is stabilised by delocalisation of electron density from oxygen, into the extended π-system of the planar cation, and was found to be 22.6 and 263.1 kJ mol lower in energy (Δ) than the alternative [N〈OH〉NH] and [N〈OH〉N] tautomers respectively. Topological analysis confirmed the presence of two electrostatic N⋯O hydrogen bonds which contribute -111.2 kJ mol towards the stabilisation of the diacid. The p values of the cations were estimated, from NMR experiments, to be 4.2 in THF (1) and 11.4 in acetonitrile (2).