Geometric and Electronic Structures of Phenoxyl Radicals Hydrogen Bonded to Neutral and Cationic Partners

• Published in: Chemistry : a European journal 2012-04, Vol.18 (17), p.5416-5429
• Main Authors: Orio, Maylis, Jarjayes, Olivier, Baptiste, Benoit, Philouze, Christian, Duboc, Carole, Mathias, Jenny-Lee, Benisvy, Laurent, Thomas, Fabrice
• Format: Article
• Language: English
• Subjects: Benzimidazoles - chemistry; Cations - chemistry; Chemical Sciences; Chemistry; Crystallography, X-Ray; density functional calculations; Electrochemistry; electron transfer; Electronic structure; Hydrogen Bonding; Hydrogen bonds; Imines - chemistry; Magnetic Resonance Spectroscopy; Mathematical analysis; or physical chemistry; Oxidation; Phenols - chemistry; phenoxyl radicals; Quantum Theory; Radicals; Tensors; Theoretical and; X-rays
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201102854

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. Phenol–benzimidazole conjugates which mimic the tyrosine–imidazole pair of photosystem II have been synthesised and oxidised. The properties of the resulting phenoxyl radicals, that is, their oxidation potentials, electronic structures and EPR g tensors (see figure), are subtly modulated by the charge and position of the hydrogen‐bonding partner, the strength of the hydrogen‐bonding interaction and the deviation from planarity of the molecule.