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(iPr)­C­(R)­N­(iPr)]}2(μ-N2) (Cp* = η5-C5Me5) (M = Nb, Ta), that proceeds in solution through an intramolecular “end-on-bridged” (μ-η11-N2) to “side-on-bridged” (μ-η22-N2) isomerization process to quantitatively provide the corresponding bimetallic bis­(μ-nitrido) complexes, {Cp*M­[N­(iPr)­C­(R)­N­(iPr)]­(μ-N)}2. It is further demonstrated that subtle changes in the steric and electronic features of the distal R-substituent, where R = Me, Ph and NMe2, 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(iPr)­C­(Me)­N­(iPr)]}2(μ-η11-N2) toward NN bond cleavage is rationalized in terms of a ground-state electronic structure that favors a significantly less-reduced μ-N2 fragment.