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^MePNPNCAAC^Me <b>4</b>_<b>CAAC</b> (CAAC^Me = 1-[2,6-bis(isopropyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene) and IPrPNPNIPr <b>4</b>_<b>NHC</b> (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 <b>4</b>_<b>NHC</b> exhibiting increased conjugation relative to <b>4</b>_<b>CAAC</b>. The nature of the stabilizing carbene also influences the redox properties of the compound, with <b>4</b>_<b>CAAC</b> undergoing potassium-mediated reduction to the closed-shell P-P bonded dimer <b>K</b>_<b>2</b><b>5</b>_<b>2</b>, which upon treatment with Kryptofix-2,2,2 converts to the transient radical anion [K_crypt][<b>5</b>], the formal one-electron reduction product of <b>4</b>_<b>CAAC</b>. In contrast, <b>4</b>_<b>NHC</b> undergoes reversible one-electron oxidation to the stable radical cation [<b>6</b>_<b>NHC</b>][SbF₆]. 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.