Electron-Transfer Catalysis of Ligand Substitution in Triiron Clusters. The Role of the Bridging Ligand in Anion Radical Intermediates

Article abstract of DOI:10.1021/ja00271a019

The polynuclear cluster Fe3(CO)9(μ₃-PPh)2 (I) undergoes rapid ligand substitution by electron-transfer catalysis (ETC) under conditions in which the thermal process is nonexistent.X-ray crystallography and (31)P NMR spectroscopy establish the stepwise substitution of the CO ligands by trimethyl phosphite to take place selectively at three separate iron centers.The high selectivity to the mono-substitution product II is achieved by turning the reduction potential specifically to generate catalytic amounts of the radical Fe3(CO)9(PPh)2^-. (I^-.) in either acetonitrile or tetrahydrofuran.Transient ESR spectroscopy of I^-. and three related paramagnetic intermediates establish the sequential tansformation of anion radicals as they evolve in the ETC mechanism.The rate-limiting rearrangement of I^-. by the slippage of a phosphinidene cap from μ₃ -> μ₂ coordination underscores the key role of the bridging ligand in the substitution process.The importance of this critical transformation relates to the formation of a 17-electron, coordinatively unsaturated iron center in the otherwise intact cluster.As such, it emphasizes the key role that the bridging ligand can play in cluster activation.