Hydrogen Evolving Mono‐Nuclear Manganese Complexes with Carefully Positioned Pyridine Base Ligands

Abstract

AbstractEnormous efforts have been made by the scientific community in the development of bio‐mimetic/bio‐inspired hydrogen‐evolving complexes as catalysts for an alternative hydrogen energy carrier and a renewable energy sources. In this regard, new bio‐inspired mono‐nuclear Mn(I) carbonyl model complexes fac‐[(Mn(CO)3(κ2‐SN2C7H5)(κ1‐PPh2Py))] 1 and fac‐[(Mn(CO)3(κ2‐S2NC7H4)(κ1‐PPh2Py))] 2 with 2‐mercaptobenzimidazole (N,N) and 2‐mercaptobenzothiazole (S,N) ligands and a N‐based phosphine ligand (PPh2Py) have been synthesized, spectroscopically characterized and evaluated as electro‐catalysts for hydrogen generation. In contrast to related mono‐nuclear (N,N/S,N)−Mn−PPh3 complexes,, the newly introduced pyridine nitrogen atom of the PPh2Py ligand placed in an axial position in the second coordination sphere of the Mn(I) complexes, provides a first site for protonation and enables a fast intra‐molecular proton transfer to the central metal atom with a weaker acid. Theoretical DFT calculations enable a detailed picture of catalytic pathways, assign redox transitions and structural changes. Both the molecular complexes were tested as electro‐catalysts for hydrogen generation in CH3CN. They effectively catalyzed the electrochemical proton reduction with the weaker acetic acid as the proton source and displayed a turnover frequency (TOF/s−1; O.P., η/V) of 610; 1.02 and 615; 1.12, respectively. The computer‐aided design of dedicated proton transfer pathways from second coordination sphere ligands is thus able to allow a hydrogen evolution reactivity with a weaker acid (acetic acid).