Aminophosphine-substituted diiron dithiolate complexes: Synthesis, crystal structure, and electrocatalytic investigation

Bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics [Fe2(μ-R2odt)(CO)4(κ2-diphosphine)] (R = Ph and H) with chelating diphosphines

In order to develop an attractive generation of bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics with chelating diphosphines, two new series of asymmetrically diphosphine-substituted diiron model complexes [Fe₂(μ-R₂odt)(CO)₄(κ²-diphosphine)] (3-5) with bulky Ph₂odt bridge and their reference counterparts (6-8) with common odt bridge were obtained from the Me₃NO-assisted substitutions of diiron hexacarbonyl precursors [Fe₂(μ-R₂odt)(CO)₆] (R₂odt = (SCHR)₂O, R = Ph (1) and H (2)) with different diphosphines such as (Ph₂P)₂NBn (labelled PN^BnP, Bn = benzyl), (Ph₂PCH₂)₂NBn (PCN^BnCP), and (Ph₂PCH₂)₂CH₂ (DPPP)), respectively. All the as-prepared complexes have been characterized by elemental analysis, IR plus NMR spectroscopies, and particularly by X-ray crystallography for 3-8. It is interesting to note that complexes 3 and 6 chelating by small bite-angle PN^BnP diphosphine have the favorable dibasal isomer whereas analogues 4, 5 and 7, 8 chelating by flexible backbone PCN^BnCP or DPPP ligands possess the main apical-basal isomer in solution or in the solid state. Further, the electrochemical properties of two pairs of representative complexes 3, 6 and 5, 8 are explored and compared by cyclic voltammetry (CV) in the absence and presence of trifluoroacetic acid (CF₃CO₂H) as proton source, indicating that the complete protonations of 3, 6 and 5, 8 with higher concentration of CF₃CO₂H lead to two new catalytic waves for the electrocatalytic proton reduction to hydrogen (H₂).

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.

[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.