A comparison of N₂ cleavage in Schrock's Mo[N₃N] and Laplaza-Cummins' Mo[N(R)Ar]₃ systems

The four-coordinate Mo-[N₃N] complex, [N₃N] = [{RNCH₂CH₂}₃N], R = 3,5-(2,4,6-iPr ₃C₆H₂)₂C₆H₃ (HIPT), which is capable of converting N₂ to ammonia catalytically, reacts with N₂ in a similar manner to Mo[N(R)Ar]₃ (R = tBu, Ar = 3,5-C₆H₃Me₂) to form a dinitrogen-bridged dimer intermediate, but unlike its three-coordinate counterpart, N₂ cleavage is not observed. To rationalise these differences, the reaction of N₂ with the model Mo[NH ₂]₃[NH₃] and full ligand Mo[N₃N] systems was explored using density functional theory and compared with the results of an earlier study involving the model three-coordinate Mo[NH ₂]₃ system. Although the overall reaction is exothermic, the final N-N cleavage step is calculated to be endothermic by 75 kJ mol ^-1 for the model system when the Mo-amine cap bond length is fixed to mimic the constraints of the ligand straps, but exothermic by 14 kJ mol ^-1 for the full ligand system. In the latter case, the slightly exothermic cleavage step can be attributed to the destabilization of the N ₂ bridged dimer relative to the nitride product owing to the steric effects of the bulky R groups. The activation barrier for N-N cleavage is estimated at 151 kJ mol^-1 for the model system, more than twice the calculated value for Mo[NH₂]₃, and even greater, 213 kJ mol^-1, for the full ligand [N₃N]Mo system. A bonding analysis shows that although the binding of the amine cap helps to stabilize the intermediate dimer, at the same time it destabilizes the metal d-orbitals involved in backbonding to the π* orbitais on N₂. As a result, backdonation is less efficient and N-N activation reduced compared to the three-coordinate system. Thus, the increased stability of the intermediate dimer on binding of the amine cap combined with the reduced level of N-N activation and higher kinetic barrier, explain why N-N cleavage has not been observed experimentally for the four-coordinate Mo[N₃N] system.