Molecular and electronic structure of square planar complexes [PdII(tbpy)(L IPN,O)]0, [PdII(tbpy)(L ISQN,O)](PF6), and [PdII(tbpy)(L IBQN,O)](PF6)(BF4)·2CH2Cl2: an o-iminophenolato based ligand centered, three-membered redox series

Abstract

Three Pd^II complexes which are members of the same electron-transfer series have been synthesized. Refluxing of the reaction mixture containing equimolar amounts of PdCl₂, 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol (H₂L_N,O), 4,4′-di-tert-butyl-2,2′-dipyridyl (^tbpy), and 3 equiv of triethylamine in MeOH under an argon atmosphere followed by exposure to air and addition of KPF₆ after cooling to room temperature yields reddish brown crystals of paramagnetic (S = 1/2) [Pd(L^ISQ_N,O)(^tbpy)](PF₆) (2). Reaction of 2 with one equiv of [CoCp₂] in dry and degassed CH₂Cl₂ using anaerobic conditions gives diamagnetic [Pd(L^IP_N,O)(^tbpy)] (1), which is the one-electron reduced form of 2. One-electron oxidation of 2 in CH₂Cl₂ under argon with one equiv of NOBF₄ affords diamagnetic [Pd(L^IBQ_N,O)(^tbpy)](PF₆)(BF₄)·2CH₂Cl₂ (3). Complexes 1, 2, and 3 constitute three members of the same electron-transfer series. They are ideally suited to distinctly distinguish the geometrical and spectroscopic features of the N,O-coordinated, closed-shell, diamagnetic o-iminophenolate (L^IP_N,O)²⁻, the corresponding open-shell π-radical o-iminobenzosemiquinonate (L^ISQ_N,O)¹⁻˙ (S_rad = 1/2), and the closed-shell o-iminobenzoquinone (L^IBQ_N,O)⁰ forms. All complexes were characterized by X-ray crystallography (100 K), cyclic voltammetry, EPR, and UV-vis spectroscopy. Complex 2 exhibits three reversible electron transfer waves in the cyclic voltammogram. Structural characterization of complex 3 reveals an interesting strong ion pairing between the BF₄ anion and the complex dication with a short C–F distance of 2.7 Å.