Catalytic Cycle for N–CN Bond Cleavage by Molybdenum Silyl Catalyst: A DFT Study

Abstract

Using hybrid density functional theory calculations with the B3LYP functional, the reaction mechanisms for cleavage of R2N–CN (R = H, Me) bonds in the presence of unsaturated molybdenum(II) silyl catalyst, Cp(CO)2MoSiMe3 (Cp = η5-C5H5), were studied. The catalytic cycle takes place in two stages; the first involves cleavage of the R2N–CN bond. The favorable sequence of reactions for this stage is as follows: (i) coordination of a nitrile through the lone pair of electrons on the nitrile nitrogen atom (NCN) to give an end-on complex; (ii) isomerization of the end-on complex to a side-on complex; (iii) migration of the silyl group to NCN to form a stable Mo–C–NCN three-membered-ring intermediate with an Mo–NCN dative bond; (iv) dissociation of NCN from Mo and coordination of an amino N atom (NNR2) to Mo, leading to an Mo–C–NNR2 three-membered-ring intermediate; and (v) cleavage of the R2N–C bond to form a silylisocyanide complex. The second stage involves the regeneration of the active catalyst through two σ-metathesis steps. In the first, Cp(CO)2MoNR2 reacts with HSiMe3 to give Cp(CO)2MoH and R2NSiMe3, and in the second, σ-metathesis of Cp(CO)2MoH with HSiMe3 regenerates Cp(CO)2MoSiMe3. Step (iv) in the first stage possesses the largest activation energy and is the rate-determining step. The activation energies for this step for the reactions of H2NCN and Me2NCN were calculated to be 36.4 and 38.3 kcal/mol, respectively, based on potential energies with zero-point energy correction. After dissociation of the silylisocyanide ligand from the silylisocyanide complex, it will be isomerized to silylcyanide, as in previous studies. The catalytic cycle for the cleavage of R2N–CN bond is compared with that of MeO–CN bond. The effects of the metal atoms are also discussed.