Free Radical Chemistry of Phosphasilenes

Abstract

Understanding the characteristics of radicals formed from silicon‐containing heavy analogues of alkenes is of great importance for their application in radical polymerization. Steric and electronic substituent effects in compounds such as phosphasilenes not only stabilize the Si=P double bond, but also influence the structure and species of the formed radicals. Herein we report our first investigations of radicals derived from phosphasilenes with Mes, Tip, Dur, and NMe2 substituents on the P atom, using muon spin spectroscopy and DFT calculations. Adding muonium (a light isotope of hydrogen) to phosphasilenes reveals that: a) the electron‐donor NMe2 and the bulkiest Tip‐substituted phosphasilenes form several muoniated radicals with different rotamer conformations; b) bulky Dur‐substituted phosphasilene forms two radicals (Si‐ and P‐centred); and c) Mes‐substituted phosphasilene mainly forms one species of radical, at the P centre. These significant differences result from intramolecular substituent effects.

Free Radicals Formed by H Atom Addition to Allenes as Determined by Muon Spin Spectroscopy

“…In principle, two-step ionic pathways exist in the end-of-track muon radiolysis spur: either μ + attachment, followed by charge neutralization, or initial electron attachment, followed by muon addition. , One possible indication of such an ionic pathway is delayed formation of the radical product. This is rare, but in recent years, we have reported several instances of related behavior, where the amplitudes of precession frequencies vary sinusoidally with the delay between the muon stop and the time window used to Fourier transform the data. ,− Careful examination of the allene data set revealed some effect for radicals 2b and 2d but not the others. The phenomenon of oscillating signal amplitudes is under examination in other systems where the effect is more marked.…”

“…This is rare, but in recent years we have reported several instances of related behaviour, where the amplitudes of precession frequencies vary sinusoidally with the delay between the muon stop and the time window used to Fourier transform the data. 14,[36][37][38] Careful examination of the allene data set revealed some effect for radicals 2b and 2d but not the others. The phenomenon of oscillating signal amplitudes is under examination in other systems where the effect is more marked.…”

Muon Spin Rotation/Resonance (μSR) for Studying Radical Reactivity of Unsaturated Organophosphorus Compounds

“…Recently, Cai, Scheschkewitz, and Percival and co‐workers reported synthesis and μSR study of silyl‐substituted bulky phosphasilenes 5 (Figure 11 a). [15] In this study, TF‐μSR and intensive DFT calculations including potential energy surface (PES) scans were carried out.…”

Probing Radical Addition to 1‐Phosphabutadienes by Employing Muonium as a “Light Isotope” of Hydrogen

“…[16,17] Probing the addition of Mu to unsaturated phosphorus-containing molecules by μSR spectroscopy has very recently emerged as a powerful tool to detect novel P-based radicals that cannot be studied by more conventional methods. [18][19][20][21][22] The technique involves irradiating samples with beams of positive muons (μ + ) to generate muoniated radicals which are then analysed spectroscopically. The positive muon has spin 1 = 2 and, thus, the muonium atom (i. e. μ + + e À = Mu) may be envisaged as a light isotope of hydrogen.…”

“…While early work focused solely on organic species, more recently μSR spectroscopy has proven to be invaluable in the study of unsaturated organosilicon and organogermanium radicals [16,17] . Probing the addition of Mu to unsaturated phosphorus‐containing molecules by μSR spectroscopy has very recently emerged as a powerful tool to detect novel P‐based radicals that cannot be studied by more conventional methods [18–22] …”