Hydrogenation of esters catalyzed by a Mn(I) complex incorporating a dual proton-responsive CNN ligand

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Abstract
A manganese(I) complex featuring a picoline-derived CNN pincer ligand with two acidic sites ‒the methylene linker and a secondary amino group‒ has been synthesized. In the presence of an alkoxide base, this complex catalyzes the hydrogenation of esters to the corresponding alcohols under relatively mild conditions (1.0 mol% Mn, 10 mol% KOR, 20 bar H2, 110 oC). Stoichiometric reactions of the Mn(I) precursor with an excess of strong base (3 equiv of KHMDS or KOtBu) yield a manganate(I) species (Mn-CNN**) via the deprotonation of both acidic sites of the pincer ligand. X-ray photoelectron spectroscopy (XPS) evinces that the Mn-CNN** complex features a Mn center coordinated by a dianionic CNN** ligand. Finally, detailed DFT calculations were carried out on the two key steps of the catalytic cycle ‒H2 activation and hydride transfer to the ester substrate‒ considering neutral and anionic manganese species. The results indicate that ester hydrogenation proceeds via a Mn(I)-mediated anionic–dianionic manifold, where ligand deprotonation facilitates both H2 cleavage and hydride transfer.
Funding Information
  • Agencia Estatal de Investigación (TED2021-129181B-I00)
  • Agencia Estatal de Investigación (PID2022-136570OB-I00)