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

Abstract

Herein, we report the synthesis of an acyclic carbene-stabilized diphospha(aminyl) PNP radical CAAC^MePNPCAAC^Me <b>4</b> (CAAC^Me = 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 <b>4</b> features a conjugated framework with spin density primarily localized on the central nitrogen atom as well as the flanking carbenes. Unlike other tripnictogen radicals, <b>4</b> undergoes facile one-electron oxidation and reduction to yield nonclassical nitrenium and amide species [<b>5</b>]⁺ and [<b>6</b>]^-, respectively. The cation [<b>5</b>]⁺ exhibits conformational flexibility in the solution state between the expected W-shaped geometry [<b>5</b>_<b>b</b>]⁺ and a previously unobserved linear heteroallene-type structure [<b>5</b>_<b>a</b>]⁺, which was characterized in the solid state. The equilibrium was explored both computationally and experimentally, showing that [<b>5</b>_<b>a</b>]⁺ is favored over [<b>5</b>_<b>b</b>]⁺ both enthalpically (Δ<i>H</i> = -2.9 × 10³ ± 80 J mol^-1) and entropically (Δ<i>S</i> = 4.2 ± 0.25 J mol^-1 K^-1). The formal amide [<b>6</b>]^- 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 <b>K</b>_<b>2</b><b>6</b>_<b>2</b>, which exhibits μ, κ-P, κ-P, and η³-PNP coordination modes. Protonation of [<b>6</b>]^- leads to the formation of an amine <b>7</b>, which features a trigonal planar geometry around nitrogen.