Competitive role of aryldiazonium cation and aryldiazene radical in PdIV catalysed C–C coupling reactions: DFT insights

Aryldiazonium cation undergoes C–C coupling reactions through instant formation of Pd^IV aryldiazenido complex with the precursor [(Tp*)Pd^IIMe₂]^- complex (Tp*\(=\)tris(3,5-dimethyl-1-pyrazolyl)borate). This Pd^IV arylazenido complex is the first high-valent Pd^IV complex in C–C coupling reactions which can decompose further into aryldiazonium cation and Pd^II complex and follow a series of reactions via a two-electron or ionic path. Alternatively, it can decompose into aryl radical (AniR) and follow one-electron or radical path forming the C–C coupled product. The possibility of these two mechanisms were proposed by Fekl and co-workers [Dalton Trans. 2017, 46, 4004–4008] and in this work, DFT calculations have been performed to clarify the mechanism as well as to probe the competitive role of aryldiazonium cation and aryl radical (AniR) in this reaction. In the two-electron pathway the process follows sequentially oxidative addition, transmetallation, oxidative addition and dinitrogen extrusion, and reductive elimination to form the C–C coupled product 4,4′-dimethoxybiphenyl (P1) and 4-methoxy toluene (P3). In the one-electron or radical pathway, 4-methoxyphenyl radicals are formed directly and they recombine to give P1. There are other products including [(Tp*)Pd^IVMe₃] (P2) and ethane (P4) formed in the reaction. QTAIM calculations reveal that methyl group migrates as a cation in the transmetallation step of the two-electron path. N₂ extrusion passes through a six membered cyclic transition state involving orbital and CH--.π interactions, and reductive elimination passes through a three-membered cyclic transition state. NBO calculations explain the nature of metal-ligand bonding of the species involved in the reaction path. A close inspection of the activation barriers shows the one-electron pathway seems to be favoured over two-electron path because it is low lying and everything becomes irreversible once aryl radical is formed, which quickly undergoes completely irreversible coupling, whereas the first several steps of the two-electron pathway are all reversible. This is in agreement with the experiment and calculations further clarify that the proposed Pd^IV diaryldiazenido complex is not feasible. Computations thus reveal that aryldiazonium cation starts the reaction by forming [(Tp*)Pd^IVMe₂(pmbd)](RC) complex and this complex reacts favourably through aryl radical to form the products.

Graphical abstract

Aryldiazonium cation readily reacts with the precursor Pd^II complex to give Pd^IV aryldiazenido complex and this complex undergoes C–C coupling reaction via radical pathway through aryl radical and forms 4, 4′-dimethoxybiphenyl as a major product, and ethane and 4-methoxytoluene as minor products following Pd^II/Pd^IV catalytic cycle.