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

Complexes [Pd(Cl)(mu-Cl)(P(C CFc)R-2)](2) (Fc - (eta(5)-C5H4)(eta(5)-C5H5); 6e, R - Bu-t; 6f, R - (C6H11)-C-c) and [PdCl2(P(C CFc)R-2)(2)] (7a, R = C6H5; 7b, R = 2-MeC6H4; 7c, R = 2,4,6-Me3C6H2; 7d, R = (C4H3O)-C-c; 7e, R = Bu-t; 7f, R = (C6H11)-C-c) are accessible by the reaction of P(C CFc)R-2 (3a-f) with either [PdCl2(cod)] (5) (cod = cyclo-1,5-octadiene) or [PdCl2(SEt2)(2)] (8). The spectroscopic, mass-spectrometric and cyclo-voltammetric 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 FcC CP moieties in the solid state. In contrast, 6e and 6f are forming dimers with a planar Pd2P2Cl2(mu-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 ((1)J(P-31-Se-77)) and the steric (Tolman cone angle) properties of the phosphine ligands on the activity of the respective palladium catalysts will be discussed. (C) 2012 Elsevier B. V. All rights reserved.