Synthesis and Electrochemical Investigation of Phosphine Substituted Diiron Phosphadithiolate Complexes

This work reports on ligand exchange reactions between a [FeFe] hydrogenase model containing the higher homologue (PhosDT) and phosphines selected to cover a variety of electronic properties and possible coordination modes. Additionally, the amount of the phosphines and the reaction temperature were varied to study the formation of complexes with multiple phosphines or altered binding modes. Due to steric effects caused by the position of the bridgehead, the phosphines bind preferentially at the more accessible iron centre on the phosphinate averted side. While all ligand exchanges resulted in a ligand-specific main product at room temperature, reflux conditions induced decomposition in case of PhosDT-(κ2-dppe) and PhosDT-(κ2-dppv) and a change in the binding mode for the dppm containing complex. Moreover, we highlight two novel iron complexes obtained as side products of the reactions with dppe and dppv, while in case of dppm an additional model with two bridging phosphine ligands was generated. Finally, the six novel phosphine substituted PhosDT models were electrochemically investigated, revealing a cathodic shift compared to the starting material due to the increased electron density at the iron atoms. Moreover, the models with monodentate ligands exhibit a different CV pattern for the FeIFeI/FeIFe0 process than complexes with bidentate phosphines.