Suitable alkali cations improved Mn(I)-catalyzed hydrogenation of esters: An insight into the effect of bases and ligands’ ring sizes on catalytic performance

Pincer Mn(I)-complexes (Mn-1–Mn-9) bearing a family of ferrocenyl-based PNN-ligands containing the cycloalkyl anywhere ring sizes of from five to eight, were successfully employed as efficient catalysts for ester hydrogenation. Among them, Mn-1, in combination with 10 mol% base-containing Li⁺ or Na⁺ (especially NaBH₄ and LiH), proved to be the highest activity catalyst, further confirming its lowest Gibbs free energy values for H₂ activation. Control experiments and DFT calculations highlight the role and rule of cations and/or anions of bases during heterolytic H₂ cleavage and hydride transfer process for Mn-catalyzed hydrogenation. In particular, either NaBH₄ or LiH as base, their comparative performance was studied and calculated with Mn-1 as catalyst, two possible cycles of alkali cations participating in the hydrogenation pathway were proposed. Employing suitable alkali cations enhancing the Mn-H catalyst, various aromatic and aliphatic esters (37 examples) were hydrogenated to the corresponding bulk and fine alcohols, offering a turnover number (TON) reaching up to 6000.