Stable Open-Shell Phosphorane Based on a Redox-Active Amidodiphenoxide Scaffold.

Abstract

The synthesis and redox reactivity of pentacoordinate phosphorus compounds incorporating a redox-active ONO amidodiphenoxide scaffold [ONO = N,N-bis(3,5-di-tert-butyl-2-phenoxide)amide] are described. Dichloro- and diphenylphosphoranes, 2·Cl2 and 2·Ph2, respectively, are synthesized and crystallographically characterized. Cyclic voltammograms of 2·Cl2 show only a single irreversible oxidation (Epa = +0.83 V vs Cp2Fe0/+), while the diphenyl analogue 2·Ph2 is reversibly oxidized at lower applied potential (E1/2 = +0.47 V vs Cp2Fe0/+). Chemical oxidation of 2·Ph2 with AgBF4 produces the corresponding radical cation [2·Ph2]•+, where electron paramagnetic resonance spectroscopy and density functional theory calculations reveal that the unpaired spin density is largely ligand-based and is highly delocalized throughout the ONO framework of the paramagnetic species. The solid-state structures indicate only minor geometrical changes between the neutral 2·Ph2 and oxidized [2·Ph2]•+ species, consistent with fast self-exchange electron transfer, as observed by NMR line-broadening experiments.