Reaction Mechanism of the C⋮N Triple Bond Cleavage ofβ-Ketonitriles on a Molybdenum(0) Center1

The molybdenum dinitrogen complex trans -[Mo(N 2 ) 2 (dppe) 2 ] ( 2 ) reacted with 2−2.5 equiv of various β -ketonitriles at room temperature to afford the (nitrido)(nitrile-enolato) complexes trans -[Mo(N)(NCCR 1 COR 2 )(dppe) 2 ] ( 4; R 1 = H, R 2 = 4-MeOCOC 6 H 4, 4-ClC 6 H 4, 4-Tol, 4-MeOC 6 H 4, 2-C 4 H 3 O, 2-C 4 H 3 S, Pr i; R 1 = CN, R 2 = Me, Ph; dppe = Ph 2 PCH 2 CH 2 PPh 2 ) via the C⋮N triple bond cleavage of the nitriles on the molybdenum center. On the other hand, the reaction of complex 2 with 2 equiv of pivaloylacetonitrile at room temperature led to the isolation of the (alkylideneamido)(nitrile-enolato) complex trans -[Mo(NCHCH 2 COBu t )(NCCHCOBu t )(dppe) 2 ] ( 5k ), which further underwent the cleavage of the C N double bond of the alkylideneamido ligand to give the corresponding (nitrido)(nitrile-enolato) complex trans -[Mo(N)(NCCHCOBu t )(dppe) 2 ] together with 4,4-dimethyl-1-penten-3-one. Furthermore, treatment of 2 with large excess amounts of 4-chlorobenzoylacetonitrile followed by anion metathesis with [NHEt 3 ][OTf] (OTf = OSO 2 CF 3 ) resulted in the isolation of the cationic (imido)(nitrile-enolato) complex trans -[Mo{NCH 2 CH 2 CO(C 6 H 4 Cl-4)}{NCCHCO(C 6 H 4 Cl-4)}(dppe) 2 ][OTf] ( 6c + [OTf] - ). The solid-state structures of 4h ·1.5C 2 H 4 Cl 2 (R 1 = CN, R 2 = Me), 5k ·C 6 H 6, and 6c + [OTf] - were determined by single-crystal X-ray analyses. The detailed NMR analysis of the reaction of 2 with aroylacetonitriles revealed that the (alkylideneamido)(nitrile-enolato) complexes trans -[Mo(NCHCH 2 COR)(NCCHCOR)(dppe) 2 ] ( 5 ) act as the key intermediates for the C⋮N triple bond fission, and the rate constants for the conversion of the complexes 5 into the nitrido complexes 4 showed good correlation with the Hammett σ p or σ a constants for the aroyl substituents, where positive ρ values were obtained ( ρ p, 1.42; ρ a, 0.41). A reaction mechanism for the nitrido complex formation is proposed, which includes (1) the substitution of a dinitrogen ligand in 2 with a β -ketonitrile molecule, (2) the fast protonation of the nitrile ligand by a second β -ketonitrile molecule leading to the formation of complex 5, (3) the relatively slow proton shift from the α -position of the carbonyl group to the amido carbon in the alkylideneamido ligand to form an enolated imido ligand, and (4) the fast elimination of a vinyl ketone from the imido ligand giving the nitrido complex 4 as the final product.