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

Abstract

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.

Biologically inspired 3Fe4S cluster as structural mimics of FeMoco M‐cluster

“…The Fe-S bond lengths are between 2.23 and 2.32 Å, which agrees with the average Fe-S bond of 2.3 Å in the bdt ligated iron clusters. [29][30][31][32][33] The molecule has a C s symmetry with each iron coordinated to five different arrangements of ligand. The diphenylphosphide ligand spans a Fe-Fe edge whereas the two benzenedithiolate ligands take up two different coordination modes: one (S (1) and S (2)) spanning only a single Fe-Fe bond, while the second (S (3) and S (4)) caps two different Fe-Fe bonds.…”

Mononuclear manganese complexes as hydrogen evolving catalysts

“…Approaches to control catalytic performance and catalyst stability by intra-molecular non-covalent (Natarajan et al, 2017) (Natarajan et al, 2022) and covalent (Pullen et al, 2019) interactions or the reaction mechanism by introducing additional sites of protonation (Wilson et al, 2006;Liu et al, 2013;Pandey et al, 2021) or modifications of the first coordination shell ligands will allow the design of more efficient catalyst candidates for sustainable hydrogen production.…”

Synthesis, Characterization and Electrochemical Studies of bis(Monothiolato) {FeFe} Complexes [Fe2(μ‐SC6H4‐OMe‐m)2(CO)5L] (L = CO, PCy3, PPh3)

“…Furthermore, most of the studies have considered mainly dithiolates as the linkers [1,19–50] . Only recently, the focus has shifted to monothiolate‐based {FeFe} systems.…”