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 (PQH) or its 2-methyl or 2-phenyl derivatives (PQMe or PQPh) were synthesized, and their crystal structures and behaviors in solution were investigated. Most of the complexes [M­(PQR)2]­X2 (MII = PtII, PdII, or NiII; R = H, Me or Ph; X = monoanionic ions) characterized in this study have an approximately square-planar coordination geometry with two bidentate P,N-chelating or monodentate P-donating quinolylphosphine ligands in the cis(P,P) 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 PtII and PdII complex cations maintained the square-planar coordination geometry, but the MII 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 PQH or PQMe chelates by substitution with halide anions were observed using UV–vis spectroscopy and X-ray crystallography. In contrast, the PQPh 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 MII center from nucleophilic attack. In the corresponding NiII complexes, the steric congestion arising from the mutually cis-positioned PQR ligands resulted in a large tetrahedral distortion around the NiII center. However, the intramolecular π–π stacking interaction was still effective in the PQPh complex, and this interaction can explain some unusual robustness and electrochemical properties of the NiII–PQPh complex.