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

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 pi-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)][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.