| Summary: | Addition of the bulky redox-active diphosphine 1,8-bis(diphenylphosphino)naphthalene (dppn) to [Fe<sub>2</sub>(CO)<sub>6</sub>(µ-edt)] (<b>1</b>) (edt = 1,2-ethanedithiolate) affords [Fe<sub>2</sub>(CO)<sub>4</sub>(κ<sup>2</sup>-dppn)(µ-edt)] (<b>3</b>) as the major product, together with small amounts of a P⁻C bond cleavage product [Fe<sub>2</sub>(CO)<sub>5</sub>{κ<sup>1</sup>-PPh<sub>2</sub>(1-C<sub>10</sub>H<sub>7</sub>)}(µ-edt)] (<b>2</b>). The redox properties of <b>3</b> have been examined by cyclic voltammetry and it has been tested as a proton-reduction catalyst. It undergoes a reversible reduction at <i>E</i><sub>1/2</sub> = −2.18 V and exhibits two overlapping reversible oxidations at <i>E</i><sub>1/2</sub> = −0.08 V and <i>E</i><sub>1/2</sub> = 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 <i>E</i><sub>p</sub> = −2.00 V in the presence of CF<sub>3</sub>CO<sub>2</sub>H. 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.
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