Fundamental study of (ferrocenylethynyl)phosphines: Correlation of steric and electronic effects in C,C cross-coupling reactions

Abstract

Complexes [Pd(Cl)(μ-Cl)(P(CCFc)R2)]2 (Fc = (η5-C5H4)(η5-C5H5); 6e, R = tBu; 6f, R = cC6H11) and [PdCl2(P(CCFc)R2)2] (7a, R = C6H5; 7b, R = 2-MeC6H4; 7c, R = 2,4,6-Me3C6H2; 7d, R = cC4H3O; 7e, R = tBu; 7f, R = cC6H11) are accessible by the reaction of P(CCFc)R2 (3a–f) with either [PdCl2(cod)] (5) (cod = cyclo-1,5-octadiene) or [PdCl2(SEt2)2] (8). The spectroscopic, mass-spectrometric and cyclovoltammetric data of 6 and 7 were investigated and the structures of four complexes (6e, 6f, 7b, 7c) in the solid state determined. Complexes 7a–f are mononuclear with palladium in a square-planar environment and show a cis- (7b) or trans-configuration (7c) with linear FcCCP moieties in the solid state. In contrast, 6e and 6f are forming dimers with a planar Pd2P2Cl2(μ-Cl)2 core with the ferrocenylethynyl ligands positioned above and below this plane. Electrochemical studies of phosphines 3a–3f and the appropriate seleno phosphines 4a–4f show after oxidation follow-up reactions, while a reversible behavior is found for the corresponding palladium complexes 6 and 7. UV–Vis/NIR and IR spectro–electrochemical measurements of 7f indicate moderate electronic interactions between the ferrocenyl units. All complexes are catalytically active in Heck (reaction of iodo benzene with tert-butyl acrylate) and Suzuki (2-bromo toluene or 4′-chloro acetophenone with phenyl boronic acid) C,C cross-couplings. The influence of the electronic (1J(31P–77Se)) and the steric (Tolman cone angle) properties of the phosphine ligands on the activity of the respective palladium catalysts will be discussed.