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

The synthesis and characterisation of a bis(iminium)phenoxide diacid cation [4- t Bu-C 6 H 2 -2,6-(HCN(H)Dipp)-1-O] + ([H 2 t Bu,Dipp L] + ), is discussed. [H 2 t Bu,Dipp L][BF 4 ] (1) and [H 2 t Bu,Dipp L][H 2 N{B(C 6 F 5 ) 3 } 2 ] (2) were synthesised in high yields via protonation of the bis(imino)phenol conjugate base with ethereal HBF 4 or Bochmann's acid ([H(OEt 2 ) 2 ][H 2 N{B(C 6 F 5 ) 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 (Δ G ) than the alternative [N〈OH〉NH] + and [N〈OH 2 〉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 p K a values of the cations were estimated, from NMR experiments, to be 4.2 in THF (1) and 11.4 in acetonitrile (2).