The benzodiphosphaalkene ligand and its PdII and PtII complexes: Their synthesis, structure, and an ESR study of their reduction products

Article abstract of DOI:10.1021/ja00113a014

The new diphosphaalkene 1,3-bis[2-(2,4,6-tri-tert-butylphenyl)phosphanediylmethyl]benzene, L, has been synthesized. Due to the presence of two P=C bonds three isomers (EE, EZ, ZZ) were observed by ³¹P NMR, and the crystal structures of two of them could be determined (EE, ZZ). The electrochemical behavior of L has been studied by cyclic voltametry: a quasi-reversible reduction occurs at -1.89 V/SCE and corresponds to the formation of a radical anion which has been studied by ESR at variable temperature. The experimental ³¹P and ¹H hyperfine constants are consistent with free rotation about the P=C and C_{ph0sphaalkene}-C_benzene bonds at room temperature and agree with ab initio predictions. One of the isomers of L forms complexes with palladium(II) and platinum(II) ions. The crystal structures show that L is orthometalated and acts as a terdentate ligand by coordinating the metal with each phosphorus atom. These complexes are electrochemically reduced between -0.92 and -1.29 V, and the resulting paramagnetic species are studied by ESR in liquid and frozen solutions. This reduction process was shown to be a ligand-centered process, an appreciable part of the unpaired electron is localized on each of the phosphaalkene carbons (20%) and phosphorus atoms (5%).