Theoretical Insight into Oxidation of Anilines to Azobenzenes Catalyzed by Hexamolybdate: Outer-Sphere Electron and Proton Transfer

Recently, the catalytic activity of Lindqvist-type hexamolybdate [Mo6O19]2– in the oxidation of an aniline derivative (LPhNH2, L = substituent) was demonstrated by Wei and co-workers (Angew. Chem. Int. Ed. 2021, 60, 6382–6385). Herein, taking phenylamine (PhNH2) oxidation to azobenzene (PhNNPh) as a model reaction, we report the density functional theory investigation of the catalytic mechanism of [Mo6O19]2– and illustrate the critical experimental phenomena. During the catalytic reaction, once the preassociation of [Mo6O19]2– and PhNH2 takes place, electron transfer and proton transfer immediately proceed to form an N radical intermediate. The higher the highest occupied molecular orbital energy of the substrate, the easier the formation of the N radical intermediate. The N–N bond formation proceeds via the second PhNH2 nucleophilic attack on the N radical intermediate. The substituent position and the N reaction site of the substrate have a significant effect on the second PhNH2 nucleophilic attack process. In the reaction process, the six MoVI of [Mo6O19]2– are still hexacoordinated, which is defined as the outer-sphere pathway. One of the factors determining the product selectivity is the electrostatic repulsion between LPhNH2 and the N radical intermediate. The experiment reveals that the product yield is increased by the addition of Na2S2O3, while the catalytic reaction is completely deactivated with Na2CO3 or K3PO4. Based on the proposed mechanism, the experimental observation was rationalized. The S2O32– part of Na2S2O3 has a similar function as the electron-withdrawing substituent due to its low lowest unoccupied molecular orbital (LUMO) energy, which reduces the LUMO energy of the N radical intermediate and thus facilitates PhNH2 nucleophilic attack, while the CO32– part of Na2CO3 or PO43– part of K3PO4 has an undesirable effect on the electrophilicity of the N radical intermediate, resulting in the interruption of the catalytic reaction. This work would provide a detailed understanding of the catalytic reaction.