Electron-transfer catalysis of ligand substitution in triiron clusters. The role of the bridging ligand in anion radical intermediates

Abstract

The polynuclear cluster Fe/sub 3/(CO)/sub 9/(..mu../sub 3/-PPh)/sub 2/ (I) undergoes rapid ligand substitution by electron-transfer catalysis (ETC) under conditions in which the thermal process is nonexistent. X-ray crystallography and /sup 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 tuning the reduction potential specifically to generate catalytic amounts of the anion radical Fe/sub 3/(CO)/sub 9/(PPh)/sub 2//sup -/ (I/sup -/) in either acetonitrile or tetrahydrofuran. Transient ESR spectroscopy of I/sup -/ and three related paramagnetic intermediates establish the sequential transformation of anion radicals as they evolve in the ETC mechanism. The rate-limiting rearrangement of I/sup -/ by the slippage of a phosphinidene cap from ..mu../sub 3/ ..-->.. ..mu../sub 2/ coordination underscores the key role of the bridging ligand in the substitution process. The importance of this critical transformation related 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.