Synthesis, structures and spectroscopic properties of palladium(II) complexes with tridentate piperidyl-containing pincer ligands

A new series of square planar palladium(II) complexes with pincer ligands, pip2NCN− (pip2NCNH = 1,3-bis(piperidylmethyl)benzene) and pip2NNN (2,6-bis(piperidylmethyl)pyridine), has been prepared: Pd(pip2NCN)X (X = Cl, Br, I), [Pd(pip2NCN)(L)](BF4) (L = pyridine, 4-phenylpyridine), and [Pd(pip2NNN)Cl]Cl. The X-ray crystal structures of Pd(pip2NCN)Br, [Pd(pip2NCN)(L)]BF4, and [Pd(pip2NNN)Cl]Cl confirm the tridentate coordination geometries of the pincer ligands. For the pip2NCN− complexes, each piperidyl ring adopts a chair conformation with the metal center at an equatorial position on the N(piperidyl) atom. However, one of the piperidyl groups of Pd(pip2NNN)Cl+ adopts a previously unobserved coordination geometry, effectively placing the metal center at an axial position on the N(piperidyl) atom. 1H NMR and UV–Vis absorption measurements provide additional insight into the electronic structures of these complexes. The 1H NMR spectra of Pd(pip2NCN)X (X = Cl, Br, I) are consistent with deshielding of the pip2NCN− ligand resonances along the Cl < Br < I series, in opposition to the relative halogen electronegativities. It is suggested that this trend is consistent with decreasing filled/filled repulsions between the dπ orbitals of the metal center and the lone pair orbitals of the halide ligands along this series. Electronic absorption spectra support the notion that ligand-to-metal charge-transfer states are stabilized in these palladium(II) complexes relative to their platinum(II) analogues.