µSR studies for radical reactions of unsaturated organophosphorus compounds

In this paper, the µSR studies of some unsaturated organophosphorus compounds containing heavier congeners of cyclobutane-1,3-diyl and anthracene are reviewed by discussing the usefulness of µSR for main group chemistry. The regioselective addition of muonium (Mu = [µ⁺e^–]) to one of the skeletal phosphorus atoms in an electron-donating air-stable crystalline 1,3-diphosphacyclobutane-2,4-diyl leading to the paramagnetic 4-membered P-heterocycle was characterized by the Δ₁ (ΔM =  ± 1) resonance signal observed by muon (avoided) level-crossing resonance (µLCR). Meanwhile, a crystalline 1,3-diphosphacyclobutane-2,4-diyl bearing an electron-deficient nitrogen heterocyclic unit was analyzed by transverse-field muon spin rotation (TF-µSR) to characterize predominant muoniation at the skeletal radical carbon centre. A 9-phosphaanthracene bearing the trifluoromethyl (CF₃) stabilizing groups at the peri positions was also investigated from the views of radical reactivity, and the regioselective addition of muonium to the skeletal sp²-type phosphorus atom was characterized by the muon hyperfine coupling (hfc) constant observed by TF-µSR and the Δ₀ (ΔM = 0) signal of µLCR. The light mass of muon (Mass = 0.1134 amu) causes the larger zero-point energy and promotes the high-energy molecular structure in which the fused aromatic rings are almost flat, although the CF₃ groups would prefer the non-planar saddle-like 9-phosphaanthracene skeleton.