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

Author:
Keane, Andrew J., Yonke, Brendan L., Hirotsu, Masakazu, Zavalij, Peter Y., Sita, Lawrence R.
Source:
Journal of the American Chemical Society 2014 v.136 no.28 pp. 9906-9909
ISSN:
1520-5126
Subject:
Gibbs free energy, energy, isomerization, nitrogen
Abstract:
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-η¹:η¹-N₂) to “side-on-bridged” (μ-η²:η²-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-η¹:η¹-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.
Agid:
5391907