Catalytic Hydrogen Evolution by Fe(II) Carbonyls Featuring a Dithiolate and a Chelating Phosphine

Two pentacoordinate mononuclear iron carbonyls of the form (bdt)­Fe­(CO)­P₂ [bdt = benzene-1,2-dithiolate; P₂ = 1,1′-diphenylphosphinoferrocene (<b>1</b>) or methyl-2-{bis­(diphenylphosphinomethyl)­amino}­acetate (<b>2</b>)] were prepared as functional, biomimetic models for the distal iron (Fe_d) of the active site of [FeFe]-hydrogenase. X-ray crystal structures of the complexes reveal that, despite similar ν­(CO) stretching band frequencies, the two complexes have different coordination geometries. In X-ray crystal structures, the iron center of <b>1</b> is in a distorted trigonal bipyramidal arrangement, and that of <b>2</b> is in a distorted square pyramidal geometry. Electrochemical investigation shows that both complexes catalyze electrochemical proton reduction from acetic acid at mild overpotential, 0.17 and 0.38 V for <b>1</b> and <b>2</b>, respectively. Although coordinatively unsaturated, the complexes display only weak, reversible binding affinity toward CO (1 bar). However, ligand centered protonation by the strong acid, HBF₄·OEt₂, triggers quantitative CO uptake by <b>1</b> to form a dicarbonyl analogue <b>[1­(H)-CO]⁺</b> that can be reversibly converted back to <b>1</b> by deprotonation using NEt₃. Both crystallographically determined distances within the bdt ligand and density functional theory calculations suggest that the iron centers in both <b>1</b> and <b>2</b> are partially reduced at the expense of partial oxidation of the bdt ligand. Ligand protonation interrupts this extensive electronic delocalization between the Fe and bdt making <b>1­(H)⁺</b> susceptible to external CO binding