C–CN vs C–H Activation: Actual Mechanism of the Reaction between [(dippe)PtH]₂ and Benzonitrile Evidenced by a DFT Computational Investigation

In this paper we have carried out a DFT computational investigation on the reaction of [(dippe)]­PtH]2 (1b) with benzonitrile (PhCN) leading to the products (dippe)­Pt­(H)­(2-C6H4CN) (2) and (dippe)­Pt­(Ph)­CN (5), which formally result from benzonitrile C–H and C–CN activation, respectively. Actually, DFT results indicate a process following a stepwise mechanism that satisfactorily explains the experimental evidence. 5 is a very stable species (19.1 kcal mol–1 below reactants and significantly more stable than compound 2). Computations clearly show that 5 does not represent an intermediate of the process eventually leading to the final products (dippe)­Pt­(H)­CN (3) and (dippe)­Pt­(CN)­(C6H4CN) (4). The favored path leading to product 3 originates directly from 1b, which is in equilibrium with the adduct 2. The highest energy transition state that must be overcome to give 3 is 29.1 kcal mol–1 above the reactants. Surmounting this transition structure can be considered a feasible task at the working temperature of 140 °C. Product 3 can be obtained only when a second PhCN molecule is involved in the process. PhCN behaves like a hydrogen carrier: it provides the hydrogen finally bonded to platinum in 3 and contributes to form a benzene molecule, which is released in the course of the reaction, as experimentally observed. This PhCN molecule can be considered as a catalyst of the process. Its involvement explains why, when 2 is heated in the absence of PhCN, no reaction is observed. Only in the presence of PhCN can 1b, which is in equilibrium with 2, complete the process to give 3.