Solution Behavior and Structural Diversity of Bis(dialkylphosphino)methane Complexes of Palladium

The preparation of dipalladium complexes containing sterically nondemanding diphosphine (P-P) ligands of the type R2PCH2PR2 where R = Me (dmpm) or Et (depm) is reported. Variable-temperature 1H NMR spectra of the PdI2 complexes Pd2X2(dmpm)2 (X = Cl, Br, or I; the P-P ligands in the Pd2 complexes are always bridged, but for convenience, the -symbol is omitted) show the complexes to be fluxional in solution, the barriers to a ring-flipping process being (delta)G(double dagger) = 37.9, 39.0, and 43.2 (plus-minus) 0.9 kJ mol^-1 for the chloro, bromo, and iodo complexes, respectively. Treatment of Pd2X2(P-P)2 (X = Cl or Br) with X2 generates the stable, face-to-face PdII2 derivatives trans-Pd2X4(P-P)2, while oxidation of Pd2I2(P-P)2 complexes with I2 generates a new type of symmetrically di-iodo-bridged, five-coordinate complexes Pd2I2((mu)-I)2(dmpm)2 and Pd2I2 ((mu)-I)2(depm)2. The molecular crystal structures of four dipalladium(II) complexes are described: trans-Pd2Cl4(dmpm)2·2CHCl3, trans-Pd2Br4 (dmpm)2, trans-Pd2Cl4(depm)2, and Pd2I2((mu)-I)2(dmpm)2. Solution NMR and UV-vis absorption spectra are consistent with the solid-state structures determined by X-ray diffraction. The stability of the dimeric Pd(II) complexes is attributed primarily to ligand steric factors.