Two di-tert-butylphenols incorporating an N-methylbenzimidazole moiety in the ortho or para position have been synthesised (MeOH and pMeOH, respectively). Their X-ray structures evidence a hydrogen bond between the phenolic proton and the iminic nitrogen atom, whose nature is intra- and intermolecular, respectively. The present studies demonstrate that MeOH is readily oxidised by an intramolecular PET mechanism to form the hydrogen-bonded phenoxyl-N-methylbenzimidazolium system (MeOH) .+ , whereas oxidation of pMeOH occurs by intermolecular PET, affording the neutral phenoxyl benzimidazole (pMeO). system. The deprotonations of MeOH and pMeOH yield the corresponding phenolate species (MeO)- and (pMeO)-, respectively, whilst that of the previously reported HOH (analogous to MeOH but lacking the N-methyl group) produces an unprecedented hydrogen-bonded phenol benzimidazolate species, as evidenced by its X-ray structure. The latter is believed to be in equilibrium in solution with its tautomeric phenolate form, as suggested by NMR, electrochemistry and DFT studies. The one-electron oxidations of the anions occur by a simple ET process affording phenoxyl radical species, whose electronic structure has been studied by HF-EPR spectroscopy and DFT calculations. In particular, analysis of the g1 tensor shows the order 2.0079>2.0072>2.0069>2.0067 for (MeO)., (HO)., (MeOH).+ and (HOH).+, respectively. (MeO). exhibits the largest g1 tensor (2.0079), consistent with the absence of intramolecular hydrogen bond. The g1 tensor of (HO). is intermediate between those of (MeOH).+ and (MeO). (g1=2.0072), indicating that the phenoxyl oxygen is hydrogen-bonded with a neutral benzimidazole partner.
Geometric and electronic structures of phenoxyl radicals hydrogen bonded to neutral and cationic partners