Dinitrogen activation in sterically-hindered three-coordinate transition metal complexes

Dinuclear metal systems based on sterically-hindered, three-coordinate transition metal complexes of the type ML₃ where the ancillary ligands L comprise bulky organic substituents, hold great promise synthetically for the activation and scission of small, multiply-bonded molecules such as N₂, NO and N₂O. In this study we have employed density functional methods to identify the metal/ligand combinations which achieve optimum activation and/or cleavage of N₂. Strong π donor ligands such as NH₂ and OH are found to produce the greatest level of activation based on N–N bond lengths in the intermediate dimer complex, L₃Mo(μ–N₂)MoL₃, whereas systems containing the weak or non-π donor ligands NH₃, PH₃, OH₂ and SH₂ are found to be thermodynamically unfavourable for N₂ activation. In the case of the Mo–NH₂ and W–NH₂ systems, a fragment bonding analysis reveals that the orientation of the amide ligands around the metal is important in determining both the spin state and the extent of dinitrogen activation in the intermediate dimer. For both systems, an intermediate dimer structure where one of the NH₂ ligands on each metal is rotated 90° relative to the other ligands, is more activated than the structure in which the NH₂ ligands are trigonally disposed around the metals. The level of activation is found to be very sensitive to the electronic configuration of the metal with d³ metal ions delivering the best activation along any one transition series. In particular, strong activation or cleavage of N₂ was calculated for the third row d³ metals systems involving Ta(II), W(III) and Re(IV), with the level of activation decreasing as the nuclear charge on the metal increases. This trend in activation reflects the size of the valence 5d orbitals and consequently, the capacity of the metal to back donate into the dinitrogen π* orbitals.