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Effects of Substituents on the Stability of Phosphoranyl Radicals

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journal contribution
posted on 2005-11-10, 00:00 authored by Jennifer L. Hodgson, Michelle L. Coote
The effect of substituents on the geometries, apicophilicities, radical stabilization energies, and bond dissociation energies of P(CH3)3X (X = CH3, SCH3, OCH3, OH, CN, CF3, Ph) were studied via high-level ab initio molecular orbital calculations. Two alternative definitions for the radical stabilization energy (RSE) were considered:  the standard RSE, in which radical stability is measured relative to H−P(CH3)3X, and a new definition, the α-RSE, which measures stability relative to P(CH3)2X. We show that these alternative definitions yield almost diametrically opposed trends; we argue that α-RSE provides a reasonable qualitative measure of relative radical stability, while the standard RSE qualitatively reflects the relative strength of the P−H bonds in the corresponding H−P(CH3)3X phosphines. The P(CH3)3X radicals assume a trigonal-bipyramidal structure, with the X-group occupying an axial position, and the unpaired electron distributed between a 3pσ-type orbital (that occupies the position of the “fifth ligand”), and the σ* orbitals of the axial bonds. Consistent with this picture, the radical is stabilized by resonance (along the axial bonds) with configurations such as X- P•+(CH3)3 and X P(CH3)3. As a result, substituents that are strong σ-acceptors (such as F, OH, or OCH3) or have weak P−X bonds (such as SCH3) stabilize these configurations, resulting in the largest apicophilicities and α-RSEs. Unsaturated π-acceptor substituents (such as phenyl or CN) are weakly stabilizing and interact with the 3pσ-type orbital via a through-space effect. As part of this work, we challenge the notion that phosphorus-centered radicals are more stable than carbon-centered radicals.