The reactions of the precursor complex [Fe2(CO)6(μ‐bdt)] F with PPh3, PPh2Me, PPh2H have been investigated. Treatment of F with the phosphine ligands yielded both mono‐ and disubstituted complexes [Fe2(CO)5(μ‐bdt)(PPh3)] (1), [Fe2(CO)4(μ‐bdt)(PPh3)2] (2), [Fe2(CO)5(μ‐bdt)(PPh2Me)] (3), [Fe2(CO)4(μ‐bdt)(PPh2Me)2] (4), [Fe2(CO)5(μ‐bdt)(PPh2H)] (5) and [Fe2(CO)4(μ‐bdt)(PPh2H)2] (6). Crystal structures have been reported for complexes 1–3. Complexes 1, 3 and 5 participate in electrocatalytic proton reduction in the presence of two distinct acids of varying strengths: HClO4 and CF3CO2H.

The mono-substituted complex [Fe2(CO)5(μ-naphthalene-2-thiolate)2(P(PhOMe-p)3)] was prepared taking after the structural principles from both [NiFe] and [FeFe]-hydrogenase enzymes. Crystal structures are reported for this complex and the all carbonyl analogue. The bridging naphthalene thiolates resemble μ-bridging cysteine amino acids. One of the naphthyl moieties forms π-π stacking interactions with the terminal bulky phosphine ligand in the crystal structure and in calculations. This interaction stabilizes the reduced and protonated forms during electrocatalytic proton reduction in the presence of acetic acid and hinders the rotation of the phosphine ligand. The intramolecular π-π stabilization, the electrochemistry and the mechanism of the hydrogen evolution reaction were investigated using computational approaches.

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Reaction of the precursor complexes [Fe2(μ‐SCH2Ph)2(CO)6] 1 and [Fe2(μ‐SEt)2(CO)6] A with the phosphine ligand (L=P(PhOMe‐p)3) yielded the mono‐substituted complexes [Fe2(μ‐SCH2Ph)2(CO)5(P(PhOMe‐p)3)] 2 and [Fe2(μ‐SEt)2(CO)5(P(PhOMe‐p)3)] 3. All the complexes were characterized by various spectroscopic techniques. X‐ray crystal structure has been reported for complex 3. The reduction potentials of the phosphine substituted complexes 2 and 3 appeared at more negative potentials in comparison to the precursor complexes 1 and A. Complexes 1–3 were catalytically active towards proton reduction in the presence of acids.

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.