Synthesis, Structure, Ligand Dynamics, and Catalytic Activity of Cationic [Pd(η3-allyl)(κ2(E,N)-EN-chelate)]+ (E = P, O, S, Se) Complexes

A series of cationic palladium allyl complexes of the type [Pd(η3-allyl)(κ2(E,N)-EN-chelate)]+ containing several heterodifunctional EN (E = P, O, S, Se) ligands based on N-(2-pyridinyl)aminophosphines and oxo, thio, and seleno derivatives thereof are prepared. These complexes are studied by one- and two-dimensional NMR techniques together with X-ray and DFT calculations. Variable-temperature and phase-sensitive 1H,1H NOESY NMR measurements reveal both allyl and EN ligand dynamics. In the case of palladium, PN complexes' η3 to η1 isomerization takes place by opening the η3-allyl group selectively at the trans position with respect to the phosphorus center, while for EN (E = O, S, Se) complexes an “apparent” allyl rotation is observed proceeding with Pd–E and Pd–N bond breaking. DFT calculations indicate that both isomerization processes are solvent assisted, in agreement with the NMR data. In addition, the use of the new palladium allyl complexes has been examined as catalysts for Suzuki–Miyaura coupling of various aryl bromides and arylboronic acids. [Pd(η3-CHPhCHCH2)(ON-Ph)]+, bearing an η3-cinnamyl ligand, is one of the most efficient catalysts, converting aryl bromides and arylboronic acids at 80 °C with a catalyst loading of 0.1 mol % quantitatively into the expected biaryl products.