Comparison of 8-methylquinoline and benzo[h]quinoline complexes of palladium(II) with those of related ligands. Crystal and molecular structure of aqua(benzo[h]quinoline)[2-(dimethylaminomethyl)phenyl-N]palladium(II) perchlorate

Abstract

Benzo[h]quinoline (bquin) does not cleave the chloride bridge of [{Pd(dmp)Cl}₂][dmp = 2-(dimethylaminomethyl)phenyl-N], 8-methylquinoline (8Me-quin) does so if in large excess, while other ligands L (pyridine, 2-methylpyridine, or 7-methylquinoline) give monomeric [Pd(dmp)Cl(L)] quantitatively. Measured equilibrium constants for the bridge-cleavage reaction indicate that the Pd⋯H–C interactions in bquin and 8Me-quin complexes are strongly destabilising. Rotations about the Pd–N bonds in [Pd(dmp)Cl(L)] are slow and no faster than equilibria involving dissociation to [{Pd(dmp)Cl}₂]. Stable complexes of bquin and of 8Me-quin can, however, be readily formed if the strongly bridging chloride ligands are replaced by nitrate or perchlorate. For example, stoicheiometric addition of bquin to an acetone solution derived by addition of Ag[ClO₄] to [{Pd(dmp)Cl}₂] gives the title complex, [Pd(dmp)(bquin)(OH₂)][ClO₄], the crystal and molecular structure of which has been determined by single-crystal X-ray diffraction. The crystals are triclinic, a= 11.050(4), b= 9.808(4), c= 11.412(5)Å, α= 77.04(4), β= 110.90(4), γ= 97.00(3)°, and Z= 2, space group P. The structure was solved by standard heavy-atom methods and refined by least squares, using 1976 observed data, to R 0.053. The co-ordination is essentially square planar with mutually trans nitrogen atoms. The most notable feature is the short H¹⁰(bquin)⋯Pd distance [2.09(6)Å], but consistent with this being a non-bonding interaction is the ca. 10° tilt of the bquin (Pd–N–C 111 and 131°) in a direction that lessens this clash. From a comparison of 1,10-phenanthroline and other authentic bidentate ligands with 8Me-quin and bquin, it is concluded that the latter ligands are unidentate.