Tertiary phosphine‐supported diiron model complexes relevant to [FeFe]‐hydrogenases

In an effort to better probe the structure–function relationship on the biological [Fe 4 S 4 ] cluster of [FeFe]‐hydrogenases, a new library of tertiary phosphine‐supported diiron dithiolate complexes [Fe 2 ( μ ‐adt NR )(CO) 5 {P(C 6 H 4 X) 3 }] ( 1–3 ) with various substituents (X = F vs. H vs. Me), wherein adt NR is denoted as abbreviation for the (SCH 2 ) 2 N(C 6 H 4 C 2 H 4 OC(O)CH 2 C 6 H 4 Me) unit, is prepared as the active site mimics of [FeFe]‐hydrogenases. Notably, the substituent influences of tertiary phosphines on the structures and electrochemical behaviors of 1–3 are studied by spectroscopic and crystallographic techniques as well as cyclic voltammetry. For example, the electronic effect of tertiary phosphines [P(C 6 H 4 X) 3 ] is responsible for the electron density around diiron center and the key Fe–Fe as well as Fe–P lengths in 1–3 as revealed by IR spectroscopy and crystallographic analysis. Further, complexes 1–3 show the electrochemical activity for proton reduction into molecular hydrogen (H 2 ) using acetic acid (AcOH) as a proton source. Noticeably, F‐substituent‐containing diiron model complex 1 exhibits greater turnover frequency (i.e., turnover frequency value) and similar overpotential for electrocatalytic H 2 evolution relative to H‐ or Me‐substituent‐bearing diiron homologs 2 or 3 .