On the mechanistic behavior of highly efficient palladium-tetraphosphine catalytic systems for cross-coupling reactions: First spectroscopic and electrochemical studies of oxidative addition on Pd(0)/multidentate ferrocenylpolyphosphine complexes

Electrochemical studies carried out in conjunction with P-31 NMR spectroscopy on the palladium(II)/palladium(0) halogeno complexes of the tetraphosphine 1,1',2,2'-tetrakis(diphenylphosphino)-4,4'-di-tert-butylferrocene, termed Fc(P-4)Bu-t,are reported. Fc(P-4)Bu-t was chosen with regard to its good performances in pallado-catalyzed cross-coupling reactions, which resulted in catalytic turnover numbers (TONs) up to 1 000 000 [Organometallics 2003, 22, 4490-4499]. The complexes [PdX2{Fc(P)(4)Bu-t}] (X = Cl, Br, I) are described with their X-ray molecular characterization in the solid state and multinuclear NMR in solution. The electrochemical behavior of Fc(P-4)Bu-t and [PdX2(Fc{P)(4)Bu-t}] complexes was investigated by cyclic voltammetry and electrolysis: a formulation of the corresponding electro-generated Pd-0 species formed is proposed on the basis-of their NMR characterization. The oxidative addition reaction of phenyl iodide on these zerovalent palladium/ferrocenyltetraphosphine complexes was also studied by combining electrochemistry and P-31 NMR. The Pd(II) species that result from phenyl iodide oxidative addition are described, and their rate of formation under standard conditions is given; the halogeno phenyl products obtained are identified and characterized. These results are discussed in light of the knowledge existing for more classical systems combining Pd(II) halides or zerovalent palladium with n monodentate tertiary phosphines (n = 2-4). In this respect, the system using Fc(P-4)Bu-t is relevant since its four triarylphosphine groups (Ph2PCp) are formally similar to a 4-fold excess of the triarylphosphine triphenylphosphine (PPh3) as ligand. However, the phosphorus atoms of the tetraphosphine ligand present a mutual spatial proximity that is absent in monophosphines, due to their implantation on the ferrocene platform. Therefore, the possible effects of multidentarity are discussed. Finally, the importance of palladium anionic species is confirmed in this study, and the rate value of Ph-I oxidative addition (130 +/- 10 ms) on Pd(0) confirmed both the good reactivity and the enhanced stability provided to palladium species by the ferrocenyltetraphosphine ligand.