Optical, electrochemical, and 1H NMR characteristics of Mono-, Bi-, and tetranuclear cyclopalladated complexes with 4,4′-bipyridyl

The [Pd(C∧N)(4,4′-bpy)Cl], [Pd(C∧N)NO3]2(μ-4,4′-bpy) and [Pd(C∧N)(μ-4,4′-bpy)]4(NO3)4 complexes (C∧N− are deprotonated forms of 2-phenylpyridine (ppy), 2-(2′’-thienyl)pyridine (tpy), and 2-phenylbenzothiazole (bt); bpy is 4,4′ bipyridyl) are synthesized and characterized by 1H NMR spectroscopy, electronic absorption and emission spectroscopy, and cyclic voltammetry. The upfield shifts of the chemical shift of a proton in the ortho-position to the donor carbon atom of the cyclopalladated ligand of complexes (Δδ = −(1.1–1.5) ppm) is assigned to the anisotropic effect of the circular current of the pyridine rings of 4,4′-bipyridyl orthogonal to the coordination plane. The characteristic long-wavelength absorption and phosphorescence bands of the complexes are assigned to the chromophore metal-complex fragment {M(C∧N)}. The quasi-reversible reduction waves of complexes are assigned to the ligand-centered processes of successive electron transfer to the π*-orbitals localized mainly on the coordinated pyridine components of 4,4′-bipyridyl.