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 = !5-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 "-metathesis steps. In the first, Cp(CO)2MoNR2 reacts with
HSiMe3 to give Cp(CO)2MoH and R2NSiMe3, and in the second, "-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.