Phosphine-Modified [FeFe]-Hydrogenase Mimics With Various –S(CH2)nS– Linker Lengths (n = 4, 5, 7, 8): Impact of PPh3 Ligand on Structure, Redox Properties, and Catalytic Activity

In this study, we synthesized a series of PPh₃-substituted diiron complexes with the general formula [Fe₂(CO)₅PPh₃{μ-S(CH₂)_nS}] (n = 4 (1), 5 (2), 7 (3), 8 (4)), bearing structural similarities to the active site of [FeFe]-hydrogenase. These complexes were derived from their parent analogues, [Fe₂(CO)₆{μ-S(CH₂)_nS}] (n = 4, 5, 7, 8), through ligand substitution. Their formation was verified through a combination of spectroscopic methods, elemental analysis, and X-ray crystallography, confirming their structural integrity and composition. Moreover, we examined how the PPh₃ ligand and the varying lengths of the dithiolato linker in complexes 1–4 affect their redox properties and catalytic performance. Using cyclic voltammetry, we evaluated their electrochemical behavior and proton reduction activity in the presence of acetic acid (AcOH). Consequently, the reduction potential shifts of complexes 1–4, in comparison to their precursors, followed the sequence: 3 > 2 > 1 > 4. Furthermore, at the highest AcOH concentration established, complex 3 demonstrates the most significant current enhancement (i_cat/i_p = 26.2). In comparison, complexes 1, 2, and 4 exhibit relatively similar i_cat/i_p values of 13.6, 13.0, and 11.9, respectively.