Cyclopalladated 2-phenyldihydrooxazole complexes with ethylenediamine, 2,2′-bipyridine, and bridging acetate ligands

Abstract

The 1H NMR, electronic absorption, and luminescence spectra, as well as voltammograms of the reduction and oxidation of the complexes [Pd(C∧N)(N∧N)]ClO4 and [Pd(C∧N)(μ-OOCCH3)]2 [where (C∧N)− is deprotonated 2-phenyl-4,5-dihydro-1,3-oxazole, and N∧N is ethylenediamine or 2,2′-bipyridine (bpy)] were compared. Magnetic nonequivalence of protons in the dihydrooxazole ring and upfield shift of the corresponding signals were observed as a result of anisotropic effect of the ring current in palladated phenyl substituents in the [Pd(C∧N)(μ-OOCCH3)]2 complex having a C 2 symmetry. One-electron reduction wave of [Pd(C∧N)bpy]+ was assigned to ligand-centered electron transfer to the π* orbital of 2,2′-bipyridine, and two oxidation waves of [Pd(C∧N)(μ-OOCCH3)]2 were attributed to successive one-electron oxidations of the palladium centers. Low-temperature (77 K) phosphorescence of [Pd(C∧N)En]+ and [Pd(C∧N)bpy]+ was ascribed to optical transition localized on the metal-complex fragment {Pd(C∧N)} and to interligand charge transfer between the chelating and cyclopalladated ligands. The formation of metal-metal bond in the complex [Pd(C∧N)(μ-OOCCH3)]2 gives rise to radiative decay of photoexcitation energy from two electronically excited states, one of which is localized on the {Pd(C∧N)} fragment, and the second corresponds to the charge transfer metal-metal-cyclopalladated ligand.