Rationalizing the different products in the reaction of N2 with three-coordinate MoL3 complexes

Abstract

The reaction of N₂ with three-coordinate MoL₃ complexes is known to give rise to different products, N–MoL₃, L₃Mo–N–MoL₃ or Mo₂L₆, depending on the nature of the ligand L. The energetics of the different reaction pathways are compared for L = NH₂, NMe₂, N(ⁱPr)Ar and N(^tBu)Ar (Ar = 3,5-C₆H₃Me₂) using density functional methods in order to rationalize the experimental results. Overall, the exothermicity of each reaction pathway decreases as the ligand size increases, largely due to the increased steric crowding in the products compared to reactants. In the absence of steric strain, the formation of the metal–metal bonded dimer, Mo₂L₆, is the most exothermic pathway but this reaction shows the greatest sensitivity to ligand size varying from significantly exothermic, −403 kJ mol⁻¹ for L = NMe₂, to endothermic, +78 kJ mol⁻¹ for L = N(^tBu)Ar. For all four ligands, formation of N–MoL₃via cleavage of the N₂ bridged dimer intermediate, L₃Mo–N–N–MoL₃, is strongly exothermic. However, in the presence of excess reactant MoL₃, formation of the single atom-bridged complex L₃Mo–N–MoL₃ from N–MoL₃ + MoL₃ is both thermodynamically and kinetically favoured for L = NMe₂ and N(ⁱPr)Ar, in agreement with experiment. In the case of L = N(^tBu)Ar, the greater steric bulk of the ^tBu group results in a much less exothermic reaction and a calculated barrier of 66 kJ mol⁻¹ to formation of the L₃Mo–N–MoL₃ dimer. Consequently, for this ligand, the energetically and kinetically favoured product, consistent with the experimental data, is the nitride complex L₃Mo–N.