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 g 1 tensor shows the order 2.0079>2.0072>2.0069>2.0067 for ( MeO) ., ( HO) ., ( MeOH) .+ and ( HOH) .+, respectively. ( MeO) . exhibits the largest g 1 tensor (2.0079), consistent with the absence of intramolecular hydrogen bond. The g 1 tensor of ( HO) . is intermediate between those of ( MeOH) .+ and ( MeO) . (g 1=2.0072), indicating that the phenoxyl oxygen is hydrogen-bonded with a neutral benzimidazole partner.
- density functional calculations
- electron transfer
- electronic structure
- hydrogen bonds
- phenoxyl radicals