Does reaction of three-coordinate molybdenum(III) with N2O proceed via thesame mechanism as with N-2? A theoretical study

Hybrid density functional (B3LYP) and integrated molecular orbital and mole cular mechanics (IMOMM) calculations were performed to elucidate the mechan ism of the reaction of nitrous oxide with the MoL3 complex, where L = NH2 a nd N[(Bu-t)(3,5-C6H3Me2)], respectively. The first step of the reaction is coordination of N2O to the Mo center of MoL3, leading to the formation of(N 2O)MoL3. From this complex, the reaction may proceeds via two distinct mech anisms, mononuclear and dinuclear. The mononuclear mechanism takes place vi a an N-N bond cleavage transition state and leads to formation of NMo(NH2)( 3) and free nitric oxide NO, which spontaneously reacts with another Mo(NH2 )(3) fragment, producing (NO)Mo(NH2)(3). The dinuclear mechanism starts by coordination of another MoL3 to (N2O)MoL3, leading to formation of L3Mo(N2O )MoL3, and splits further into two different paths, leading to N-O and N-N cleavage processes, among which the N-O activation is kinetically and therm odynamically more favorable. The experimental observation of the exclusive N-N cleavage is consistent only with the mononuclear mechanism. It was show n that inclusion of steric effects of the bulky amido ligands strongly favo rs the mononuclear mechanism by destabilizing the dinuclear reaction specie s much more than mononuclear species. The rate-determining step of the reac tion of N2O with bulky MoL3 in the mononuclear mechanism is the N-N bond ac tivation, which takes place with an 11 kcal/mol free energy of activation. Comparing these results with those for the reaction of N-2 with MoL3 on one side and with available experiments on another side, we conclude that reac tions of N2O and N-2 with MoL3 proceed via different mechanisms: nitrous ox ide reacts with MoL3 via a mononuclear mechanism, while dinitrogen reacts v ia a dinuclear mechanism.