Synthesis, Structural Characterization, and Catalytic Evaluation of Palladium Complexes with Homologous Ferrocene-Based Pyridylphosphine Ligands

A ferrocene-based phosphinopyridine, Ph2PfcPy (1; fc = ferrocene-1,1′-diyl, Py = 2-pyridyl), was newly synthesized by Negishi coupling of Ph2PfcZnCl with PyBr. Its homologous compound Ph2PfcCH2Py (2) was obtained by reductive dehydroxylation of Ph2PfcCH(OH)Py (4), the latter resulting via reaction of in situ generated Ph2PfcLi with PyCHO. Depending on the stoichiometry, compounds 1 and 2 react with [PdCl2(cod)] (cod = η2:η2-cycloocta-1,5-diene) to give P,N-chelate and bis-phosphine complexes, [PdCl2(L-κ2P,N)] (5, L = 1; 6, L = 2) and [PdCl2(L-κP)2] (7, L = 1; 8, L = 2), respectively. Analogously, [(LNC)PdCl]2 or [(LNC)Pd(MeCN)2]ClO4 (LNC = [(2-dimethylamino-κN)methyl]phenyl-κC1) react with 1 and 2 to afford complexes featuring these compounds as P-monodentate ligands ([(LNC)PdCl(L-κP)]: 9, L = 1; 10, L = 2) or P,N-chelating donors ([(LNC)Pd(L-κ2P,N)]ClO4: 11, L = 1; 12, L = 2), respectively. With the exception of compound 9, which undergoes self-ionization in solution, all complexes are defined air-stable solids and were characterized by elemental analysis and conventional spectroscopic methods (multinuclear NMR, IR, and MS). The crystal structures of 4, 5, 7·CH2Cl2, 8, 11, and 12 were determined by X-ray crystallography, revealing structural differences resulting from a more flexible geometry of the methylene-spaced ligand 2. The catalytic potential of the Pd complexes 5 and 6 and their in situ generated counterparts (Pd(OAc)2/L, L = 1, 2) was studied in Suzuki−Miyaura cross-coupling of 4-bromotoluene (13) with phenylboronic acid and in cyanation of the same substrate with K4[Fe(CN)6]. The results were compared with those obtained under identical conditions with analogous catalysts based on the related donor-symmetric ligand 1,1′-bis(diphenylphosphino)ferrocene.