Effect of basic site substituents on concerted proton-electron transfer in hydrogen-bonded pyridyl-phenols
2012 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 116, no 50, 12249-12259 p.Article in journal (Refereed) Published
Separated concerted proton-electron transfer (sCPET) reactions of two series of phenols with pendent substituted pyridyl moieties are described. The pyridine is either attached directly to the phenol (HOAr-pyX) or connected through a methylene linker (HOArCH2pyX) (X = 4-NO2, 5-CF3, 4-CH3, and 4-NMe2). Electron-donating and -withdrawing substituents have a substantial effect on the chemical environment of the transferring proton, as indicated by IR and 1H NMR spectra, X-ray structures, and computational studies. One-electron oxidation of the phenols occurs concomitantly with proton transfer from the phenolic oxygen to the pyridyl nitrogen. The oxidation potentials vary linearly with the pK a of the free pyridine (pyX), with slopes slightly below the Nerstian value of 59 mV/pKa. For the HOArCH2pyX series, the rate constants ksCPET for oxidation by NAr3 •+ or [Fe(diimine)3]3+ vary primarily with the thermodynamic driving force (ΔG° sCPET), whether ΔG° is changed by varying the potential of the oxidant or the substituent on the pyridine, indicating a constant intrinsic barrier λ. In contrast, the substituents in the HOAr-pyX series affect λ as well as ΔG°sCPET, and compounds with electron-withdrawing substituents have significantly lower reactivity. The relationship between the structural and spectroscopic properties of the phenols and their CPET reactivity is discussed.
Place, publisher, year, edition, pages
2012. Vol. 116, no 50, 12249-12259 p.
Basic sites, Chemical environment, Computational studies, Diimines, Electron-donating, Electron-withdrawing substituents, H NMR spectra, Intrinsic barriers, One-electron oxidation, Oxidation potentials, Phenolic oxygen, Pyridyl, Spectroscopic property, Thermodynamic driving forces, X-ray structure, Complexation, Electron transitions, Hydrogen bonds, Nuclear magnetic resonance spectroscopy, Oxidation, Pyridine, Rate constants, Phenols
IdentifiersURN: urn:nbn:se:uu:diva-195173DOI: 10.1021/jp311388nOAI: oai:DiVA.org:uu-195173DiVA: diva2:607326