Synthesis of Carbene-Stabilized PNPN Fragments and Their Carbene-Dependent Redox Properties

Abstract

Herein, we report the synthesis of carbene-stabilized 1,3-diaza-2,4-diphosphabutenes CAAC Me PNPNCAAC Me 4 CAAC (CAAC Me = 1-[2,6-bis(isopropyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene) and IPrPNPNIPr 4 NHC (IPr = 1,3-Bis(2,6-diisopropylphenyl)-imidazol-2-ylidene). The bonding in both systems is defined by a delocalized polar covalent π-system, with 4 NHC exhibiting increased conjugation relative to 4 CAAC . The nature of the stabilizing carbene also influences the redox properties of the compound, with 4 CAAC undergoing potassium-mediated reduction to the closed-shell P−P bonded dimer K 2 5 2 , which upon treatment with Kryptofix-2,2,2 converts to the transient radical anion [K crypt ][5], the formal one-electron reduction product of 4 CAAC . In contrast, 4 NHC undergoes reversible one-electron oxidation to the stable radical cation [6 NHC ][SbF 6 ]. Computational and spectroscopic analyses of both radical species are suggestive of unevenly delocalized spin, with the bulk of the spin density residing on phosphorus in both cases.

Synthesis of sterically congested unsymmetrical 1,2-dicarbonyl radicals through a stepwise approach

“…1a. 24–39 This approach can result in the synthesis of highly stable organic radicals that are resistant to air and heat. In our previous research, we synthesized 1,2-dicarbonyl radical cations derived from two identical NHCs or CAACs (Fig.…”