Thermodynamic Hydricities of Biomimetic Organic Hydride Donors

Thermodynamic hydricities (Δ G_H) 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 p K_a" 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 (Δ G_H) obtained in this work were compared with the enthalpy (Δ H_H) values obtained by others. The results indicate that, even though Δ H_H values exhibit the same trends as Δ G_H, the differences between room-temperature Δ G_H and Δ H_H 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.