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Download file# Effects of Substituents on the Stability of Phosphoranyl Radicals

journal contribution

posted on 10.11.2005, 00:00 authored by Jennifer L. Hodgson, Michelle L. CooteThe effect of substituents on the geometries, apicophilicities, radical stabilization energies, and bond dissociation
energies of

^{•}P(CH_{3})_{3}X (X = CH_{3}, SCH_{3}, OCH_{3}, OH, CN, CF_{3}, 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(CH_{3})_{3}X, and a new definition, the α-RSE, which measures stability relative to P(CH_{3})_{2}X. 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(CH_{3})_{3}X phosphines. The^{•}P(CH_{3})_{3}X radicals assume a trigonal-bipyramidal structure, with the X-group occupying an axial position, and the unpaired electron distributed between a 3p_{σ}-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^{•+}(CH_{3})_{3}and X^{•}P(CH_{3})_{3}. As a result, substituents that are strong σ-acceptors (such as F, OH, or OCH_{3}) or have weak P−X bonds (such as SCH_{3}) 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 3p_{σ}-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.