From Azobenzene Coordination to Aryl−Halide Bond Activation by Platinum

This contribution describes the reactivity of Pt(PEt₃)₄ with (4-bromo-phenyl)-pyridin-4-yl-diazene. η²-Coordination of Pt(PEt₃)₂ to the NN moiety is kinetically preferable and followed by an aryl−halide bond activation process. This quantitative transformation proceeds under mild reaction conditions in solution and in the solid state. Mechanistic studies in solution indicate that the metal insertion into the aryl−halide bond is the rate-determining step. The reaction obeys first-order kinetics in the η²-coordination complex with ΔG^⧧_298K = 24.6 ± 1.6 kcal/mol, ΔH^⧧ = 26.5 ± 1.6 kcal/mol, and ΔS^⧧ = 6.6 ± 5.0 eu. No effect on the reaction progress and NMR line shape has been observed in the presence of excess PEt₃. However, competition experiments with the η²-coordination complex and PhBr reveal that the product ratio can be altered by the presence of PEt₃, indicating that the two aryl−halide bond activation processes proceed via different mechanistic pathways. Numerical analysis of a series of competition experiments fits a reaction scheme involving a unimolecular transformation from the η²-coordination complex to the product of aryl−halide oxidative addition. This “ring-walking” process is kinetically accessible as shown by density functional theory (DFT) calculations at the PCM:PBE0/SDB-cc-pVDZ/PBE0/SDD level of theory.