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

Abstract

Two di-tert-butylphenols incorporating an N-methylbenzimidazole moiety in the ortho or para position have been synthesised ((Me)OH and (pMe)OH, 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 (Me)OH is readily oxidised by an intramolecular PET mechanism to form the hydrogen-bonded phenoxyl-N-methylbenzimidazolium system ((Me)OH)(.+) , whereas oxidation of (pMe)OH occurs by intermolecular PET, affording the neutral phenoxyl benzimidazole ((pMe)O)(.) system. The deprotonations of (Me)OH and (pMe)OH yield the corresponding phenolate species ((Me)O)(-) and ((pMe)O)(-), respectively, whilst that of the previously reported (H)OH (analogous to (Me)OH 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 ((Me)O)(.), ((H)O)(.), ((Me)OH)(.+) and ((H)OH)(.+), respectively. ((Me)O)(.) exhibits the largest g(1) tensor (2.0079), consistent with the absence of intramolecular hydrogen bond. The g(1) tensor of ((H)O)(.) is intermediate between those of ((Me)OH)(.+) and ((Me)O)(.) (g(1)=2.0072), indicating that the phenoxyl oxygen is hydrogen-bonded with a neutral benzimidazole partner.

Bio‐inspired Photocatalytic Ruthenium Complexes: Synthesis, Optical Properties, and Solvatochromic Effect

“…The absorption spectra of Ru-1 and Ru-2 are shown in Figure 2a.T hey display electronic transitions at 275-295, 335-350 and 445-480 nm;t he first two peaks are attributed to p!p* transitions in the phenanthrolinea nd phenol groups, respectively, [11] and the third peak is the MLCT (metalt ol igand charge transfer) transition. [9] We note the identical magnitude of the MLCT transition in both complexes, as expected, as it is attributed to an electron transfer from the metallici on to one of the ligands, and is therefore independent of the ligand number.Y et, as the number of ligandsincreases, the amplitude of the p!p*t ransition grows, giving rise to ad istinction between the two similar complexes.T he MLCT band in the new complexes is red-shifted by 150 meV comparedw ith the commercial complex and its derivatives, [18,19] and therefore exhibits as uperior overlap with the solar spectrum( AM 1.5), as shown in Figure 2b.W ea ttribute the red shiftt ot he electron-donating phenolg roup, which destabilizes the t2g d-orbitals and therefore shifts them to higher energy,t hereby reducing the transition energy.…”

“…The absorption spectra of Ru‐1 and Ru‐2 are shown in Figure a. They display electronic transitions at 275–295, 335–350 and 445–480 nm; the first two peaks are attributed to π→π* transitions in the phenanthroline and phenol groups, respectively, and the third peak is the MLCT (metal to ligand charge transfer) transition . We note the identical magnitude of the MLCT transition in both complexes, as expected, as it is attributed to an electron transfer from the metallic ion to one of the ligands, and is therefore independent of the ligand number.…”

“…[5] Inspired by theseo bservations, ap ro-ligand was synthesized. [9] This pro-ligand contains ap henanthroline chelating unit, in addition to ar edox-activep henol-imidazole moiety,L H, and is referred here as Phen LH.T he redox-active moiety LH was shown to sustain as table phenoxylr adical in its free form, [10,11] and when coordinated to at ransition metal centert hrough its N,O unit. [12][13][14][15][16] Ah eteroleptic ruthenium(II) complex containing Phen LH was also synthesized, [Ru(bpy) 2 ( Phen LH)] 2 + (see Figure1).…”

Polymers Based on Stable Phenoxyl Radicals for the Use in Organic Radical Batteries

“…Phenoxyl radicals have been studied as products of the metabolism of β 2 ‐agonists salbutamol and fenoterol, for their hydrogen‐bonding capabilities, and in metal complexes . Additionally, these radicals were employed in poly(3‐radical‐substituted thiophene)s and poly(phenylene vinylene)s to study their magnetic and electrical properties .…”

Application of phenolic radicals for antioxidants, as active materials in batteries, magnetic materials and ligands for metal-complexes

“…Benzimidazole-phenoxyl compounds have been synthesized and investigated regarding the proton-coupled electron transfer reaction, which is believed to be the oxidation mechanism of tyrosyl if it is not coupled to a metal ion (Scheme 8). 101 Inter-and intramolecular hydrogen-bonds not involving the reactive positions can modulate the rate of H-abstraction by stabilizing or destabilizing the radical. 102 Intermolecular hydrogen-bonding between phenols and solvent molecule acceptors have been known to reduce the ability to donate hydrogen-radicals to other radicals.…”