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

Abstract

Abstract:Two pentacoordinate mononuclear iron carbonyls of the form (bdt)Fe(CO)P 2 [bdt = benzene-1,2-dithiolate; P 2 = 1,1'-diphenylphosphinoferrocene (1) or methyl-2-{bis(diphenylphosphinomethyl)amino}acetate (2)] were prepared as functional, biomimetic models for the distal iron (Fe d ) of the active site of [FeFe]-hydrogenase. Xray 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 1 is in a distorted trigonal bipyramidal arrangement, and that of 2 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 1 and 2, respectively. Although coordinatively unsaturated, the complexes display only weak, reversible binding affinity towards CO(1 bar). However, ligand centered protonation by the strong acid, HBF 4 .OEt 2 , triggers quantitative CO uptake by 1 to form a dicarbonyl analogue [1(H)-CO] + that can be reversibly converted back to 1 by deprotonation using NEt 3 . Both crystallographically determined distances within the bdt ligand and DFT calculations suggest that the iron centers in both 1 and 2 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 1(H) + susceptible to external CO binding.3

Diiron Complexes [Fe2(CO)5(μ‐pdt/Mebdt)(L)] Containing a Chelating Diphosphine Ligand L=(Oxydi‐2,1‐phenylene)bis(diphenylphosphine): Bioinspired [FeFe] Hydrogenase Model Complexes

“…[23] The absorption spectra of complexes 1 , 3 and 4 were recorded in dichloromethane (Figure and Table ). The transitions in the complexes are mainly from the two iron centers to the dithiolate linkers or to the chelating bidentate phosphine ligand …”

“…This was probably due to the presence of P=O in 4 that can act as a proton relay and direct the transfer of protons to the iron centre. A similar complex [Fe 2 (CO) 5 (μ‐pdt)(PPh 3 )] with a monodentate phosphine ligand (PPh 3 ) has not been reported to be catalytically active . Monosubstitution with PPh 3 in a Mebdt complex is not known.…”

Electrocatalytic Hydrogen Production by a Nickel(II) Complex with a Phosphinopyridyl Ligand

“…Structural and functional studies of the active sites of [FeFe]hydrogenase have led to the understanding that the highly efficient catalytic activity is provided by proton trapping and the provision of sites near the active center, as well as a hydride‐ion interaction site at the iron center . More recently, hydrogen generation by hydrogenase model complexes using inexpensive metals such as iron, cobalt, and nickel have also been investigated ,…”

Biomimetic Complexes for Production of Dihydrogen and Reduction of CO2

“…Roy and coworkers used the 1,1 0 -bis(diphenylphosphino)ferrocene (dppf) ligand to create the analogous [(κ 2 -dppf)Fe(CO)(κ 2 -bdt)] complex (Fig. 12b) [57]. The dppf ligand is unique in that, due to the geometric constraints and rigidity of the ferrocene, it has a larger bite angle than most chelating phosphines.…”

“…12 Structures of key mononuclear, fivecoordinate iron complexes capable of proton reduction. (a) Fe(CO)(bdt) (PPh 2 NPh 2 ), (b) (bdt)Fe (CO)(dppf), and (c) (bdt)Fe (CO)(P 2 ) for P 2 is O,O 0 -(CH 3 CH 2 ) 2 -2-{bis-(diphenylphosphinomethyl) amino}-acetate [57,72,73] Biomimetic…”