Geometric and electronic structures of phenoxyl radicals hydrogen bonded to neutral and cationic partners

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.