摘要:
A new series of square planar palladium(II) complexes with pincer ligands,pip(2)NCN(-)(pip(2)NCNH = 1,3-bis(piperidylmethyl)benzene) and pip(2)NNN (2,6-bis(piperidylmethyl)pyridine), has been prepared: Pd(pip(2)NCN)X (X = Cl, Br, I), [Pd(PiP(2)NCN)(L)](BF4) (L = pyridine, 4-phenylpyridine), and [Pd(pip(2)NNN)Cl]Cl. The X-ray crystal structures of Pd(pip(2)NCN)Br, [Pd(pi(2)NCN)(L)]BF4, and [Pd(PiP(2)NNN)Cl]Cl confirm the tridentate coordination geometries of the pincer ligands. For the PiP(2)NCN(-) 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(PiP(2)NNN)CI+ adopts a previously unobserved coordination geometry, effectively placing the metal center at an axial position on the N(piperidyl) atom. H-1 NMR and UV-Vis absorption measurements provide additional insight into the electronic structures of these complexes. The H-1 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 d pi 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. (c) 2005 Elsevier B.V. All rights reserved.