Switching Site Reactivity in Hydrogenase Model Systems by Introducing a Pendant Amine Ligand.

Hydrogenases are versatile enzymatic catalysts with an unmet hydrogen evolution reactivity (HER) from synthetic bio-inspired systems. The binuclear active site only has one-site reactivity of the distal Fe atom. Here, binuclear complexes [Fe(CO)(μ-Mebdt)(P(4-CHOCH))] and [Fe(CO)(μ-Mebdt)(PPhPy)] are presented, which show electrocatalytic activity in the presence of weak acids as a proton source for the HER. Despite almost identical structural and spectroscopic properties (bond distances and angles from single-crystal X-ray; IR, UV/vis, and NMR), introduction of a nitrogen base atom in the phosphine ligand in markedly changes site reactivity. The bridging benzenedithiolate ligand Mebdt interacts with the terminal ligand's phenyl aromatic rings and stabilizes the reduced states of the catalysts. Although with monodentate phosphine terminal ligands only shows a distal iron atom HER activity by a sequence of electrochemical and protonation steps, the lone pair of pyridine nitrogen in acts as the primary site of protonation. This swaps the iron atom catalytic activity toward the proximal iron for complex . Density-functional theory (DFT) calculations reveal the role of terminal phosphines ligands without/with pendant amines by directing the proton transfer steps. The reactivity of is a thiol-based protonation of a dangling bond in and distal iron hydride mechanism, which may follow either an ECEC or EECC sequence, depending on the choice of acid. The pendant amine in enables a terminal ligand protonation and an ECEC reactivity. The introduction of a terminal nitrogen atom enables the control of site reactivity in a binuclear system.