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 H-1 NMR spectra of the Pd-2(1) complexes Pd2X2(dMPM)(2) (X = Cl, Br, or I; the P-P ligands in the Pd-2 complexes are always bridged, but for convenience, the mu-symbol is omitted) show the complexes to be fluxional in solution, the barriers to a ring-flipping process being DeltaGdouble dagger = 37.9, 39.0, and 43.2 +/- 0.9 kJ mol(-1) for the chloro, bromo, and iodo complexes, respectively. Treatment of Pd2X2(P-P)(2) (X = Cl or Br) with X-2 generates the stable, face-to-face Pd-2(II) derivatives trans-Pd2X4(P-P)(2), while oxidation of Pd2I2(P-P)(2) complexes with I-2 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).2CHCl(3), 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.