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

Abstract

The series of biferrocenyl-functionalized phosphines Bfc(PR 2 )/Bfc(SePR 2 ) (R = C 6 H 5 (6/14), C 6 H 4 -2-CH 3 (7/15), c C 4 H 3 O (8/16), c C 6 H 11 (9/17); Bfc = 1′-biferrocenyl, Fe(η 5 -C 5 H 5 )(η 5 -C 5 H 4 )Fe(η 5 -C 5 H 4 ) 2 ) and biferrocenyl diphosphines bfc(PR 2 ) 2 /bfc(SePR 2 ) 2 (R = C 6 H 5 (10/18), C 6 H 4 -2-CH 3 (11/19), c C 4 H 3 O (12/20), c C 6 H 11 (13/21); bfc = 1′,1‴biferrocenyl, (Fe(η 5 -C 5 H 4 ) 2 ) 2 ) have been prepared by consecutive synthesis methodologies. The reaction of 6−9 with [Pd(Et 2 S) 2 Cl 2 ] (22) gave the appropriate palladium dichloride complexes trans-[Pd(Bfc(PR 2 )) 2 Cl 2 ] (R = C 6 H 5 (23), C 6 H 4 -2-CH 3 (24), c C 4 H 3 O (25), c C 6 H 11 ( 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 [ n Bu 4 N][PF 6 ] and [ n Bu 4 N][B(C 6 F 5 ) 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 1 J 31 P 77 Se ) of the phosphine ligands, the activity of the corresponding palladium complexes can be predicted. It was found that bulky and electron-rich cyclohexyland o-tolyl-containing complexes are the most active catalysts in the appropriate Suzuki and Heck reactions.

Functionalised Biferrocene Systems towards Molecular Electronics

“…These studies indicate that the biferrocene motif can be a better conductor than a single ferrocene unit, for example allowing electron transfer between terminating metals through 1,1′‐bis(ethynyl)biferrocene systems . The versatility of uses, the excellent electronic characteristics and the variation in synthetic strategies that can be achieved, make the biferrocene motif an attractive unit within the field of molecular electronics …”

Reactivity of Planar‐Chiral α‐Ferrocenyl Carbocations towards Electron‐Rich Aromatics

“…However, the different types of inter‐ and intramolecular weak interactions do not effect the P=S distance that remains within 1.952(3) and 1.9668(16) Å. For 1c the P=Se distance is increased to 2.116(2) Å, which is within the range for ferrocenyl seleno phosphines , , …”

Synthesis and Molecular Structures of Ferrocene and Zirconocene Featuring Bis(di(3,5-di-tert.-butyl)phenylphosphino) Groups

“…Synthetic protocols for the phosphine moieties incorporation into ferrocene molecules are established quite well, which allows for preparation of various phosphines in a high yield. Bulky ferrocenyldiphosphines are widely used in different catalytic processes . In order to evaluate the bulkiness of the particular ligand, buried percent volume (%V bur ), a technique complementary to Tolman cone angle, was developed ,…”