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 Fed atom. Here, binuclear complexes [Fe2(CO)5(μ-Mebdt)­(P­(4-C6H4OCH3)3)] 1 and [Fe2(CO)5(μ-Mebdt)­(PPh2Py)] 2 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 2 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 1 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 2 acts as the primary site of protonation. This swaps the iron atom
catalytic activity toward the proximal iron for complex 2. 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 1 is a thiol-based
protonation of a dangling bond in 1– and distal iron hydride mechanism, which may follow either an ECEC
or EECC sequence, depending on the choice of acid. The pendant amine
in 2 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.