Synthesis, Electrochemistry, Spectroelectrochemistry, and Solid-State Structures of Palladium Biferrocenylphosphines and Their Use in C,C Cross-Coupling Reactions

The series of biferrocenyl-functionalized phosphines Bfc­(PR₂)/Bfc­(SePR₂) (R = C₆H₅ (6/14), C₆H₄-2-CH₃ (7/15), ^cC₄H₃O (8/16), ^cC₆H₁₁ (9/17); Bfc = 1′-biferrocenyl, Fe­(η⁵-C₅H₅)­(η⁵-C₅H₄)­Fe­(η⁵-C₅H₄)₂) and biferrocenyl diphosphines bfc­(PR₂)₂/bfc­(SePR₂)₂ (R = C₆H₅ (10/18), C₆H₄-2-CH₃ (11/19), ^cC₄H₃O (12/20), ^cC₆H₁₁ (13/21); bfc = 1′,1‴-biferrocenyl, (Fe­(η⁵-C₅H₄)₂)₂) have been prepared by consecutive synthesis methodologies. The reaction of 6–9 with [Pd­(Et₂S)₂Cl₂] (22) gave the appropriate palladium dichloride complexes trans-[Pd­(Bfc­(PR₂))₂Cl₂] (R = C₆H₅ (23), C₆H₄-2-CH₃ (24), ^cC₄H₃O (25), ^cC₆H₁₁ (26)). The structures of 15, 16, 21, 23, and 25 in the solid state were determined by single-crystal X-ray diffraction studies, showing that the structural parameters of these molecules correspond to those of related seleno phosphines and phosphino palladium dichloride complexes. Additionally, all complexes were characterized by cyclic voltammetry using [ⁿBu₄N]­[PF₆] and [ⁿBu₄N]­[B­(C₆F₅)₄] as supporting electrolytes. Phosphines 6–9 and seleno phosphines 14–17 show mostly irreversible redox processes involving the phosphorus and the selenium atom, both being able to form radicals leading to dimerization and other follow-up reactions. In contrast, palladium complexes 23–26 show in both electrolyte solutions a reversible behavior, although the iron centers were oxidized at more positive potentials in comparison to free Bfc or bfc phosphines. UV/vis/near-IR spectroelectrochemical measurements were carried out with 25 and 26. At potentials between 300 and 700 mV IVCT bands typical for [Bfc]⁺ are observed, reflecting intermetallic communication between the two ferrocene moieties within the biferrocenyl phosphine units, two of which are present. No further bands were found, indicating that no electronic communication between the biferrocenyl moieties along the P–Pd–P unit exists in the mixed-valent species. The palladium complexes are suitable catalysts in the Suzuki reaction of 2-bromotoluene (27) or 4′-chloroacetophenone (28) with phenylboronic acid (29). They can also be applied in the Heck C,C cross-coupling of iodobenzene (32) with tert-butyl acrylate (33). Depending on the steric (estimated by the Tolman cone angle) and electronic properties (estimated by ¹J_{³¹P⁷⁷Se}) of the phosphine ligands, the activity of the corresponding palladium complexes can be predicted. It was found that bulky and electron-rich cyclohexyl- and o-tolyl-containing complexes are the most active catalysts in the appropriate Suzuki and Heck reactions.