Hydride Affinities of Arylcarbenium Ions and Iminium Ions in Dimethyl Sulfoxide and Acetonitrile

Abstract

Gibbs free energies for scission of C−H bonds leading to carbanion and proton (mode a), radical pair (mode b), and carbenium ion and hydride (mode c) have been determined for a series of acidic C−H bonds in ca. 45 weak acids. This involved the use of the equilibrium acidities (or homolytic bond dissociation enthalpies), redox potentials in DMSO or MeCN solution, and the appropriate thermodynamic cycles in the two solvents. The introduction of electron-donating groups generally results in small-to-negligible effects on ΔG R - values (mode a scission) but in a relatively large stabilizing influence on the ΔG R + values for heterolytic cleavage to hydride and carbenium ion (mode c). Electron-withdrawing groups exert large stabilizing effects on ΔG R − , but their effects on ΔG R + are dependent on the nature of the substituents. The heterolytic processes a are usually favored in solution due to the strong solvation of the proton. Homolytic scission (mode b) is usually more favorable than the corresponding heterolytic process c, but they can be comparable when strong electron-donating groups such as dialkylamino are present. Indeed, heterolytic cleavage to hydride and carbenium ion was ca. 10 kcal/mol more favorable than homolytic cleavage for a series of highly stabilized 2-benzoyl-N,N‘-dialkylperhydropyrimidines.

Thermodynamic and Kinetic Hydricity of Ruthenium(II) Hydride Complexes

“…For example, DuBois has already noted the correlation of hydricity with the d 8 /d 9 Ni(II)/(I) potential. 81 Figure 10 shows the correlation between our results and hydricity values reported by DuBois, 12,16,81−86 Bruno, 87 Saveánt, 88 and Moiroux 89 for various kinds of hydride donors [three sets of monohydride complexes containing Ni(II), Pd(II), and Re(I) for which the first reduction potentials are available and which consist of more than two complexes in each set; NAD + model compounds; triarylcarboniums; and H 2 ] vs the second term on the right-hand of eq 17 (see also Supporting Information, Table S4). Lines of unit slope are imposed on the points of each class plotted; their zero intercepts correspond to a BDFE for each class at zero electrochemical driving force.…”

Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

“…As described in Section 3.1 above, use of this approximation has led to revision of the H + /H • reduction potentials in different solvents. The H • /H − reduction potentials in water362 and in DMSO and MeCN363 have been reported, allowing the p K a of H 2 to be estimated by eq 22. H 2 is very weakly acidic, though significantly more acidic than methane.…”

Computational Studies of [NiFe] and [FeFe] Hydrogenases

“…25(1) kcal/ mol. 41 As shown in Table 1, this value places the H-H bond among the strongest single bonds. 42 Because most new H-X bonds will generally be weaker than the H-H bond, there is often little or no thermodynamic driving force for the cleavage of the H-H bond.…”