Temperature dependence of the acid–base equilibrium constants of substituted pyridine N-oxides in acetonitrile

Temperature and Solvent Effects on H2 Splitting and Hydricity: Ramifications on CO2 Hydrogenation by a Rhenium Pincer Catalyst

“…Metal-ligand cooperative examples include 1,2-addition of H2 across Fe-N or Fe-B bonds (ΔΗ˚1 ≈ -9 kcal•mol -1 in toluene 11 or benzene 9 ) and 1,3-addition of H2 in (PNP * )RuH(CO) (ΔΗ˚1 = -17.4 kcal•mol -1 in THF; PNP* = 2-( t Bu2PCH2)-6-( t Bu2PCH)-C5H3N). 10 The entropy of H2 splitting by 2 is far more unfavorable than any comparable systems reported in the literature. The ∆S˚1 of 2 varies from -63.3 cal•mol -1 •K -1 in MeCN to -76.0 cal•mol -1 •K -1 in toluene.…”

“…These solvents not only span a range of polarity and donor ability, but THF and toluene are commonly employed in catalysis and were the solvents used in thermodynamic studies of H2 splitting by metal-ligand cooperative systems. 10,11 While MeCN is not often employed in hydrogenation catalysis, it is a classic solvent for thermochemical studies. 12,34,35 Solutions of dicarbonyl 2 and various organic bases were prepared in each of the three solvents and placed under 1 atm H2 to probe for hydrogen cleavage reactivity.…”

“…5,7,8 Enthalpy and entropy of reaction parameters have been determined for a few catalysts that split H2 via the metal-ligand cooperative mechanism. [9][10][11] However, to our knowledge no analogous temperature-dependent thermochemical data is available for catalysts that follow an outer sphere H2 splitting mechanism, precluding comparisons between mechanisms. Scheme 1.…”

“…The first hydricity values in tetrahydrofuran (THF), a solvent commonly used in hydrogenation studies, have only very recently appeared. 10,13,14 Irrespective of solvent, so far ΔG°Hvalues have only been reported at the standard state temperature of 25 ˚C, where few catalysts operate.…”

Temperature and Solvent Effects on H2 Splitting and Hydricity: Ramifications on CO2 Hydrogenation by a Rhenium Pincer Catalyst

“…The pKa of DBU was assumed to be temperature-independent, based on studies of nitrogen heterocycles in MeCN. 43,44 Although the pKa change is likely negligible, ∆G2 decreases by 3 kcal•mol -1 as the temperature increases over the studied range (SI Table S3). The temperature-dependent free energy of H2 heterolysis, ∆GH2, was estimated based on the free energy of H + reduction to Hin water at a given temperature (based on temperaturedependent reduction potential data), 45 coupled with the ion transfer free energy of H + and Hfrom water to MeCN at the same given temperature (see SI Section VII for derivation).…”