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 degrees 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.