A Detailed Kinetico-Mechanistic Investigation on the Palladium C-H Bond Activation in Azobenzenes and Their...

Palladium C-H bond activation in azobenzenes with R₁ and R₂ at para positions of the phenyl rings (R₁ = NMe₂, R₂ = H (L1); R₁ = NMe₂, R₂ = Cl (L2); R₁ = NMe₂, R₂ = I (L3); R₁ = NMe₂, R₂ = NO₂ (L4); R₁ = H, R₂ = H (L5)) and their monopalladated derivatives, using cis-[PdCl₂(DMF)₂], has been studied in detail by in situ ¹H NMR spectroscopy in N,N-dimethylformamide-d₇ (DMF-d₇) at room temperature; the same processes have been monitored in parallel via time-resolved UV-vis spectroscopy in DMF at different temperatures and pressures. The final goal was to achieve, from a kinetico-mechanistic perspective, a complete insight into previously reported reactivity results. The results suggest the operation of an electrophilic concerted metalation-deprotonation mechanism for both the mono- and dipalladation reactions, occurring from the coordination compound and the monopalladated intermediates, respectively. The process involves deprotonation of the C-H bond assisted by the presence of a coordinated DMF molecule, which acts as a base. For the first time, NMR monitoring provides a direct evidence of all the intermediate stages: that is, (i) coordination of the azo ligand to the Pd^II center, (ii) formation of the monopalladated species, and (iii) coordination of the monopalladated species to another Pd^II unit, which finally result in the (iv) formation of the dipalladated product. All of these species have been identified as intermediates in the dipalladation of azobenzenes, evidenced also by UV-vis spectroscopy time-resolved monitoring. The data also confirm that the cyclopalladation of asymmetrically substituted azobenzenes occurs by two concurrent reaction paths. In order to identify the species observed by NMR and by UV-vis spectroscopy, the final products, intermediates, and the Pd^II precursor have been prepared and characterized by X-ray diffraction and IR and NMR spectroscopy. DFT calculations have also been used in order to explain the isomerism observed for the isolated complexes, as well to assign their NMR and IR spectra.