Thermodynamic Hydricities of Biomimetic Organic Hydride Donors

Abstract

Thermodynamic hydricities (ΔGH–) in acetonitrile and dimethyl sulfoxide have been calculated and experimentally measured for several metal-free hydride donors: NADH analogs (BNAH, CN-BNAH, Me-MNAH, HEH), methylene tetrahydromethanopterin analogs (BIMH, CAFH), acridine derivatives (Ph-AcrH, Me2N-AcrH, T-AcrH, 4OH, 2OH, 3NH), and a triarylmethane derivative (6OH). The calculated hydricity values, obtained using density functional theory, showed a reasonably good match (within 3 kcal/mol) with the experimental values, obtained using “potential pKa” and “hydride-transfer” methods. The hydride donor abilities of model compounds were in the 48.7–85.8 kcal/mol (acetonitrile) and 46.9–84.1 kcal/mol (DMSO) range, making them comparable to previously studied first-row transition metal hydride complexes. To evaluate the relevance of entropic contribution to the overall hydricity, Gibbs free energy differences (ΔGH–) obtained in this work were compared with the enthalpy (ΔHH–) values obtained by others. The results indicate that, even though ΔHH– values exhibit the same trends as ΔGH–, the differences between room-temperature ΔGH– and ΔHH– values range from 3 to 9 kcal/mol. This study also reports a new metal-free hydride donor, namely, an acridine-based compound 3NH, whose hydricity exceeds that of NaBH4. Collectively, this work gives a perspective of use metal-free hydride catalysts in fuel-forming and other reduction processes.