Dinuclear Organometallic Pd(II) Complexes with Iminopyridines: the Importance of Methyl Groups

Iminopyridines have been used as ancillary ligands for polymerization catalysts based on late-transition-metal complexes1 and as scaffolds for polydentate ligands to obtain dinuclear complexes.2 For several years we have been studying palladium complexes as catalysts for both CO/vinyl arene and ethylene/methyl acrylate copolymerization reactions.3 With the aim to develop better performing catalysts, we have now started to investigate dinuclear palladium complexes based on polydentate ligands (N-N'-N'-N) featured by two iminopyridyl compartments held together by a phenyl or a tetramethyl-substituted phenyl ring and being relatively positioned in either para or ortho with respect to each other. The iminopyridyl moieties derive by the condensation reaction of the proper diamine with either pyridine-2-aldehyde or 2-acetyl pyridine. Palladium complexes of general formula [Pd(Me)Cl]2[N-N'-N'-N] and {[Pd(Me)(MeCN)]2[N-N'-N'-N]}{PF6}2 have been synthesized. Their characterization in solution by NMR spectroscopy is not so trivial and thanks to the methyl groups on both the phenylene linker and the iminopyridyl compartment it was possible to unambiguously recognize the different isomers present. The solid state structure was solved for a neutral complex having the bidentate moieties in para position (fig. left), and for a dicationic derivative with the bidentate compartments in ortho to each other (Fig. right). The latter was shown to have a peculiar structure with a methylene group bridging the two palladium ions, and resulting from the neutral derivative through the C-H activation of one Pd-Me group and the dissociation of the second methyl from the palladium coordination sphere as a methane molecule.