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-C₆H₂-2,6-(HCN(H)Dipp)-1-O]⁺ ([H₂L]⁺), is discussed. [H₂L][BF₄] (1) and [H₂L][H₂N{B(C₆F₅)₃}₂] (2) were synthesised in high yields via protonation of the bis(imino)phenol conjugate base with ethereal HBF₄ or Bochmann's acid ([H(OEt₂)₂][H₂N{B(C₆F₅)₃}₂]). 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^-1 lower in energy (ΔG) than the alternative [N〈OH〉NH]⁺ and [N〈OH₂〉N]⁺ tautomers respectively. Topological analysis confirmed the presence of two electrostatic N⁺H⋯O^- hydrogen bonds which contribute -111.2 kJ mol^-1 towards the stabilisation of the diacid. The pK_a values of the cations were estimated, from NMR experiments, to be 4.2 in THF (1) and 11.4 in acetonitrile (2).