Synthesis of a Carbene-Stabilized (Diphospha)aminyl Radical and Its One Electron Oxidation and Reduction to Nonclassical Nitrenium and Amide Species

• Published in: Journal of the American Chemical Society 2023-04, Vol.145 (16), p.9223-9232
• Main Authors: LaPierre, Etienne A., Watanabe, Lara K., Patrick, Brian O., Rawson, Jeremy M., Tuononen, Heikki M., Manners, Ian
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.3c01408

Herein, we report the synthesis of an acyclic carbene-stabilized diphospha­(aminyl) PNP radical CAACMePNPCAACMe 4 (CAACMe = 1-[2,6-bis­(isopropyl)­phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene) by a facile one-pot, seven-electron reduction of hexachlorophosphazene chloride [Cl3PNPCl3]­[Cl]. The PNP radical 4 features a conjugated framework with spin density primarily localized on the central nitrogen atom as well as the flanking carbenes. Unlike other tripnictogen radicals, 4 undergoes facile one-electron oxidation and reduction to yield nonclassical nitrenium and amide species [5]+ and [6]−, respectively. The cation [5]+ exhibits conformational flexibility in the solution state between the expected W-shaped geometry [5 b ]+ and a previously unobserved linear heteroallene-type structure [5 a ]+, which was characterized in the solid state. The equilibrium was explored both computationally and experimentally, showing that [5 a ]+ is favored over [5 b ]+ both enthalpically (ΔH = −2.9 × 103 ± 80 J mol–1) and entropically (ΔS = 4.2 ± 0.25 J mol–1 K–1). The formal amide [6]− displays remarkable flexibility in its coordination chemistry due to the presence of multiple Lewis basic centers, as evidenced by the structure of its potassium complex K 2 6 2 , which exhibits μ, κ-P, κ-P, and η3-PNP coordination modes. Protonation of [6]− leads to the formation of an amine 7, which features a trigonal planar geometry around nitrogen.