A Comparison of N2 Cleavage in Schrock's Mo[N3N] and Laplaza–Cummins' Mo[N(R)Ar]3 Systems

Abstract

The four-coordinate Mo[N₃N] complex, [N₃N]=[{RNCH₂CH₂}₃N], R=3,5-(2,4,6-iPr₃C₆H₂)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[NH₂]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[NH₂]₃ system. Although the overall reaction is exothermic, the final NN cleavage step is calculated to be endothermic by 75 kJ mol⁻¹ for the model system when the Moamine 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 N₂ bridged dimer relative to the nitride product owing to the steric effects of the bulky R groups. The activation barrier for NN 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* orbitals on N₂. As a result, backdonation is less efficient and NN 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 NN activation and higher kinetic barrier, explain why NN cleavage has not been observed experimentally for the four-coordinate Mo[N₃N] system.