(Metallocenylphosphane)palladium Dichlorides – Synthesis, Electrochemistry and Their Application in C–C Coupling Reactions

(Metallocenylphosphane)palladium Dichlorides – Synthesis, Electrochemistry and Their Application in C–C Coupling Reactions:

AbstractThe synthesis and characterization of a series of metallocenylphosphanes of the type PR₂Mc/Se=PR₂Mc [Mc = Fc = Fe(η⁵-C₅H₄)(η⁵-C₅H₅), R = C₆H₅ (3a/4a), 2-MeC₆H₄ (3b/4b), c-C₄H₃O (3c/4c), tBu (3d/4d), c-C₆H₁₁ (3e/4e); Mc = Rc = Ru(η⁵-C₅H₄)(η⁵-C₅H₅), R = C₆H₅ (6a/7a), 2-MeC₆H₄ (6b/7b), c-C₄H₃O (6c/7c), c-C₆H₁₁ (6d/7d)] and their palladium complexes [PdCl₂(PR₂Mc)₂] [Mc = Fc, R = C₆H₅ (9a), 2-MeC₆H₄ (9b), c-C₄H₃O (9c), tBu (9d), c-C₆H₁₁ (9e); Mc = Rc, R = C₆H₅ (10a), 2-MeC₆H₄ (10b), c-C₄H₃O (10c), c-C₆H₁₁ (10d)] is reported. The solid-state structure of 4b confirms the tetrahedrally distorted geometry at phosphorus with the o-tolyl groups indicating steric congestion, which is confirmed by ¹H and ¹³C{¹H} NMR spectroscopy. Phosphanes 3, 4, and 9 were characterized by cyclic voltammetry with [N(nBu)₄][B(C₆F₅)₄] as the supporting electrolyte. In general, the first oxidation occurs at the phosphane metallocenyl unit(s), although the appropriate Pd complexes are oxidized at more positive potentials. Depending on the phosphane or selenophosphane, follow-up reactions occur, which are discussed. In contrast, the palladium complexes show reversible redox behavior. UV/Vis/NIR spectroelectrochemical studies carried out on 9b indicate an electrostatic interaction between the two terminal ferrocenyl groups. All of the palladium complexes were examined as catalysts in Heck and Suzuki C–C cross-coupling and showed high catalytic activities. These results can be correlated to the electronic (¹J) parameters of the selenophosphanes.