Hydrogenase Biomimetics with Redox-Active Ligands: Synthesis, Structure, and Electrocatalytic Studies on [Fe2(CO)4(κ2-dppn)(µ-edt)] (edt = Ethanedithiolate; dppn = 1,8-bis(Diphenylphosphino)Naphthalene)

Abstract

Addition of the bulky redox-active diphosphine 1,8-bis(diphenylphosphino)naphthalene (dppn) to [Fe2(CO)6(µ-edt)] (1) (edt = 1,2-ethanedithiolate) affords [Fe2(CO)4(κ2-dppn)(µ-edt)] (3) as the major product, together with small amounts of a P–C bond cleavage product [Fe2(CO)5{κ1-PPh2(1-C10H7)}(µ-edt)] (2). The redox properties of 3 have been examined by cyclic voltammetry and it has been tested as a proton-reduction catalyst. It undergoes a reversible reduction at E1/2 = −2.18 V and exhibits two overlapping reversible oxidations at E1/2 = −0.08 V and E1/2 = 0.04 V. DFT calculations show that while the Highest Occupied Molecular Orbital (HOMO) is metal-centred (Fe–Fe σ-bonding), the Lowest Unoccupied Molecular Orbital (LUMO) is primarily ligand-based, but also contains an antibonding Fe–Fe contribution, highlighting the redox-active nature of the diphosphine. It is readily protonated upon addition of strong acids and catalyzes the electrochemical reduction of protons at Ep = −2.00 V in the presence of CF3CO2H. The catalytic current indicates that it is one of the most efficient diiron electrocatalysts for the reduction of protons, albeit operating at quite a negative potential.

Insights into the Two‐Electron Reductive Process of [FeFe]H2ase Biomimetics: Cyclic Voltammetry and DFT Investigation on Chelate Control of Redox Properties of [Fe2(CO)4(κ2‐Chelate)(μ‐Dithiolate)]

“…Shape and distribution of LUMO of 1 is compatible with the main geometry rearrangement observed upon reduction, namely the elongation of Fe−Fe distance by 0.100 Å. A similar FMO pattern is also typical of related systems with a redox‐active ligand [15, 24] …”

“…[15] Shape andd istributiono fL UMOo f1 is compatible with them aing eometry rearrangemento bservedu ponr eduction,n amelyt he elongation of FeÀFe distance by 0.100 .AsimilarF MO pattern is also typicalo fr elated systems with ar edox-activel igand. [15,24] As expected, LUMO of 1 andS OMO of 1 À À have as imilar shape ( Figure 3B). Non-innocence of the phenanthroline ligand is suggestedb yt he fact that these orbitals, both in 1 and 1 À À ,a re metal-and phenanthroline-centered frontiero rbi-tals.…”

Highly efficient electrocatalytic proton-reduction by coordinatively and electronically unsaturated Fe(CO)(κ2-dppn)(κ2-tdt)

“…The IR spectrum shows single carbonyl absorption at 1918 cm -1 , being similar to those observed for related complexes [6][7][8]. On the basis of the solid-state structure, the two phosphorus atoms of the dppn ligand would be expected to be inequivalent, but the 31…”

“…Nevertheless, 2 is found to be a highly efficient proton-reduction catalyst generating hydrogen from CF 3 CO 2 H at relatively mild potentials. Work is continuing to address the mechanism of proton-reduction, especially regarding the role of the redox-active dppn ligand [31], and hence to further develop this class of coordinatively and electronically unsaturated complexes as proton-reduction catalysts.…”

Insights into Triazolylidene Ligands Behaviour at a Di-Iron Site Related to [FeFe]-Hydrogenases

“…For these reasons, such systems have aroused the interest of chemists for several years, allowing novel insights into the organometallic chemistry of di-iron complexes involving bimetallic and metal-ligand cooperativities [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. In this context, various redox ligands have been combined with such bio-inspired di-iron systems but very few of them display non-innocent behaviour and act as the [Fe4S4] cubane in the H-cluster [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ]. A few years ago, the intriguing redox non-innocent behaviour of a N-heterocyclic carbene (NHC) was proposed for the two-electron reduction of the complex [Fe 2 (CO) 5 (IMes)(µ-pdt)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene) through a simultaneous dual electron uptake by the di-iron site and the NHC ligand [ 30 ].…”