Electron transfer reactions of osmium(II) complexes with phenols and phenolic acids

Role of Hydrogen Bonding in Photoinduced Electron–Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand

“…The same opinion has recently been expressed by Hammarström and co-workers . Unfortunately, the erroneous procedure due to Meyer and co-workers has been uncritically adopted in several subsequent investigations of reactive quenching (including EPT from phenolic donors) that invoked precursor complex formation through H-bonding or ion pairing. − As a result, none of the reported equilibrium constants for H-bonding between excited metallocomplexes and EPT donors as well as rate constants for the EPT step within these H-bound structures can be considered reliable; only the products of these constants can be determined from the published data (see SI section S2 for details). In all of these studies, with two possible exceptions , (see SI section S2), no deviations of the Stern–Volmer plots (for I obs , τ obs , or both) from linearity were reported; therefore, it would appear that K 1 –P ≪ 1 M –1 and the H-bonding between the donor and acceptor in EPT reactions is very weak or absent altogether; in the latter case, K 1 –P would simply describe the formation of an encounter complex between noninteracting 1­(T) and phenol.…”

“…The inherent complexity of EPT and the short-range nature of proton transfer require the formation of a precursor complex with the reactants in close proximity and proper alignment for the EPT step, and hydrogen bonding is the principal interaction that can accomplish such a task. The participation of H-bonding in EPT reactions has been postulated in several studies of (i) intramolecular EPT in covalently linked molecular assemblies with the hydrogen bond (HB) donor and acceptor moieties within the H-bonding range, , (ii) “multiple-site” or “bidirectional” EPT where a donor transfers its proton to an H-bonded acceptor (basic solute or solvent) and an electron is transferred from the same donor to an oxidizing solute or electrode, − and (iii) photoinduced EPT from a donor to an electronically excited acceptor. − Here, we suggest and apply an improved mechanistic model for describing the latter systems; it explicitly accounts for the H-bonding interactions of both reactants with solvent and can be readily extended to include systems listed in (i) and (ii).…”

“…Equation suggests that the rates of reactive and physical quenching cannot be separated; only their combination k q app = k q + 1/τ o 1 –P – 1/τ o is experimentally determinable, and k q app can be equated to k q only under the assumption that H-bonding does not alter the natural lifetime of 1 ( T ); that is, τ o 1 –P = τ o . Although never mentioned or discussed, this assumption is implicit in all previous studies that postulated H-bonding between an excited metallocomplex and a quencher. − To investigate the validity of this assumption, we have compared the quenching of 1 ( T ) by p -methoxyphenol (MeOPhOH) and 2,2,2-trifluoroethanol (TFE). With their α 2 H values of 0.573 and 0.567, respectively, these compounds are HB donors of essentially the same strength, and their H-bonding to 1 ( T ) is expected to be quantitatively similar; this conjecture is supported by the computational data (Table S4).…”