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((i)Pr)C(R)N((i)Pr)]}2(μ-N2) (Cp* = η(5)-C5Me5) (M = Nb, Ta), that proceeds in solution through an intramolecular "end-on-bridged" (μ-η(1):η(1)-N2) to "side-on-bridged" (μ-η(2):η(2)-N2) isomerization process to quantitatively provide the corresponding bimetallic bis(μ-nitrido) complexes, {Cp*M[N((i)Pr)C(R)N((i)Pr)](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((i)Pr)C(Me)N((i)Pr)]}2(μ-η(1):η(1)-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.