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

Experimental data support a mechanism for N N bond cleavage within a series of group S bimetallic dinitrogen complexes of general formula, {Cp*M[N(Pr-i)-C(R)N(Pr-i)]}(2)(mu-N-2) (Cp* = eta(5)-C5Me5) (M = Nb, Ta), that proceeds in solution through an intramolecular endon-bridged (mu-eta(1):eta(1)-N-2) to side-on-bridged (mu-eta(2):eta(2)-N-2) isomerization process to quantitatively provide the corresponding bimetallic bis(mu-nitrido) complexes, {Cp*M[N(Pr-i)C(R)N(Pr-i)](mu-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(Pr-i)C(Me)N-(Pr-i)]}(2)(mu-eta(1): eta(1)-N-2) toward N N bond cleavage is rationalized in terms of a ground-state electronic structure that favors a significantly less-reduced mu-N-2 fragment.