Ancillary Tethering Influences σ3‑P vs σ5‑P Speciation and Enables Intermolecular S–H Oxidative Addition to Nontrigonal Phosphorus Compounds

The design and synthesis of a nontrigonal phosphorus­(III) triamide (2) bearing a peripheral ethylene bridge is described. By comparison to a compound lacking the ethylene bridge (1, (P­{N­[o-NMe-C6H4]2})), 2 is shown to exclusively form σ5-P oxidative addition products upon E–H addition (E = OR, SR) in preference to a σ3-P adduct from cooperative addition across one P–N bond. The resulting pentacoordinate phosphoranes have been characterized by multinuclear NMR spectroscopy and X-ray crystallography. DFT calculations on relative energies of σ3-P and σ5-P species indicate that the ethylene linker in 2 energetically destabilizes the σ3-P tautomer of the phosphorane product (2·[H]­[E]) by constraining rotation along the C–N bond, favoring formation of σ5-P phosphoranes by ring–chain tautomerism.