Alkali Metal Induced Rupture of a Phosphorus−Phosphorus Double Bond. Electrochemical and EPR Investigations of New Sterically Protected Diphosphenes and Radical Anions [ArPPAr]•-

The synthesis and characterization of the new diphosphenes DmtP PDmt ( 2a ) (Dmt = 2,6-dimesityl- p -tolyl) and DxpP PDxp ( 3a ) (Dxp = 2,6-di( m -xylyl)phenyl) are described. The electrochemical behavior of 2a, 3a, and DmpP PDmp ( 1a ) (Dmp = 2,6-dimesitylphenyl) has been examined by cyclic voltammetry. Compounds 1a − 3a display reversible reductions in THF (room temperature, 0.1 M n Bu 4 NBF 4 ) at −2.08, −2.15, and −2.08 V vs SCE, respectively. Chemical and electrochemical reduction of solutions of 1a − 3a led to the stable radical anions [DmpPPDmp] • - ( 1b ), [DmtPPDmt] • - ( 2b ), and [DxpPPDxp] • - ( 3b ) which were studied by EPR spectroscopy. The EPR data indicate the unpaired spins reside in P P π* molecular orbitals. Chemical reduction of 1a − 3a with either Na metal or sodium naphthalenide yields sodium salts Na[ArPPAr], which show additional EPR signals that have been attributed to the presence of ion-paired species. No ion pairing was detected by EPR spectroscopy for the corresponding magnesium, potassium, or lithium salts of 1a . Reduction of DmpP PDmp with excess potassium metal in THF led to dark, EPR silent, solutions. Analysis of such solutions by 31 P NMR indicated the presence of DmpP(H)K. Quenching of this species with degassed H 2 O or CF 3 COOH affords DmpPH 2 .