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

Abstract

In this paper we have carried out a DFT computational investigation on the reaction of [(dippe)]­PtH]₂ (<b>1b</b>) with benzonitrile (PhCN) leading to the products (dippe)­Pt­(H)­(2-C₆H₄CN) (<b>2</b>) and (dippe)­Pt­(Ph)­CN (<b>5</b>), 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. <b>5</b> is a very stable species (19.1 kcal mol^–1 below reactants and significantly more stable than compound <b>2</b>). Computations clearly show that <b>5</b> does not represent an intermediate of the process eventually leading to the final products (dippe)­Pt­(H)­CN (<b>3</b>) and (dippe)­Pt­(CN)­(C₆H₄CN) (<b>4</b>). The favored path leading to product <b>3</b> originates directly from <b>1b</b>, which is in equilibrium with the adduct <b>2</b>. The highest energy transition state that must be overcome to give <b>3</b> 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 <b>3</b> 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 <b>3</b> 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 <b>2</b> is heated in the absence of PhCN, no reaction is observed. Only in the presence of PhCN can <b>1b</b>, which is in equilibrium with <b>2</b>, complete the process to give <b>3</b>