Sterically Demanding 8-(Diphenylphosphino)quinoline Complexes of Group 10 Metal(II): Synthesis, Crystal Structures, and Properties in Solution.

Several series of platinum(II), palladium(II), and nickel(II) complexes bearing 8-(diphenylphosphino)quinoline (PQ) or its 2-methyl or 2-phenyl derivatives (PQ or PQ) were synthesized, and their crystal structures and behaviors in solution were investigated. Most of the complexes [M(PQ)]X (M = Pt, Pd, or Ni; R = H, Me or Ph; X = monoanionic ions) characterized in this study have an approximately square-planar coordination geometry with two bidentate ,-chelating or monodentate -donating quinolylphosphine ligands in the (,) configuration. A large steric requirement from the Me or Ph substituent introduced at the 2-position of the quinoline ring gives the resulting complexes severe distortion. The Pt and Pd complex cations maintained the square-planar coordination geometry, but the M center was displaced from the chelating ligand plane. This bending of the chelate coordination makes the M-N(quinoline) bond weaker, as demonstrated by the longer M-N bonds. In accord with the bond weakening, the partial dissociation of the PQ or PQ chelates by substitution with halide anions were observed using UV-vis spectroscopy and X-ray crystallography. In contrast, the PQ complexes were stable in solution toward the addition of halide anions; the intramolecular π-π stacking interaction between the coordinating quinolyl and the 2-substituted phenyl rings protects the M center from nucleophilic attack. In the corresponding Ni complexes, the steric congestion arising from the mutually -positioned PQ ligands resulted in a large tetrahedral distortion around the Ni center. However, the intramolecular π-π stacking interaction was still effective in the PQ complex, and this interaction can explain some unusual robustness and electrochemical properties of the Ni-PQ complex.