Fine-Tuning the Energy Barrier for Metal-Mediated Dinitrogen NN Bond Cleavage

Experimental data support a mechanism for NN bond cleavage within a series of group 5 bimetallic dinitrogen complexes of general formula, {Cp*M­[N­(<sup><i>i</i></sup>Pr)­C­(R)­N­(<sup><i>i</i></sup>Pr)]}<sub>2</sub>(μ-N<sub>2</sub>) (Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) (M = Nb, Ta), that proceeds in solution through an intramolecular “end-on-bridged” (μ-η<sup>1</sup>:η<sup>1</sup>-N<sub>2</sub>) to “side-on-bridged” (μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>) isomerization process to quantitatively provide the corresponding bimetallic bis­(μ-nitrido) complexes, {Cp*M­[N­(<sup><i>i</i></sup>Pr)­C­(R)­N­(<sup><i>i</i></sup>Pr)]­(μ-N)}<sub>2</sub>. It is further demonstrated that subtle changes in the steric and electronic features of the distal R-substituent, where R = Me, Ph and NMe<sub>2</sub>, can serve to modulate the magnitude of the free energy barrier height for NN bond cleavage as assessed by kinetic studies and experimentally derived activation parameters. The origin of the contrasting kinetic stability of the first-row congener, {Cp*V­[N­(<sup><i>i</i></sup>Pr)­C­(Me)­N­(<sup><i>i</i></sup>Pr)]}<sub>2</sub>(μ-η<sup>1</sup>:η<sup>1</sup>-N<sub>2</sub>) toward NN bond cleavage is rationalized in terms of a ground-state electronic structure that favors a significantly less-reduced μ-N<sub>2</sub> fragment.