Stepwise Reduction of Dinitrogen Occurring on a Divanadium Model Compound: A Synthetic, Structural, Magnetic, Electrochemical, and Theoretical Investigation on the [VNNV]n+[n= 4−6] Based Complexes
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Abstract
This report details an extensive investigation on vanadium−dinitrogen complexes containing the [V(μ-N2)V]n+ skeleton with a variable oxidation state of the metal and reduction degree of dinitrogen, n varying from 6 → 5 → 4. This skeleton can be associated to alkali cations in ion-separated or ion-pair tight forms. The reaction of [(Mes)3V(thf)], 1 [Mes = 2,4,6-Me3C6H2], with Lewis acids [AlPh3, B(C6F5)3] removed the THF molecule leading under nitrogen to the formation of the diamagnetic complex [(Mes)3V(μ-N2)V(Mes)3], 4. The reaction of 1 with N2 also occurs under reducing conditions using Na or K metals. As supported by the electrochemical study, the preliminary stage is in both cases the formation of the vanadium(II) desolvated form [V(Mes)3]-, 5. The reaction of 5 with N2 in the case of potassium and in the presence of 1 leads to the compound {[(Mes)3V(μ-N2)V(Mes)3]-[K(digly)3]+}, 11, which, depending on the reaction conditions, undergoes a further reduction to [{K(digly)3(μ-Mes)2(Mes)V}2(μ-N2)], 12 [digly = diethylene glycol dimethyl ether]. Similarly, the reduction of 1 with sodium gave, depending on the workup mode, {[(Mes)3V(μ-N2)V(Mes)3]2-[Na(digly)2]+2}, 13, or {[(Mes)3V(μ-N2)(μ-Na)V(Mes)3]-[Na(digly)2]+}, 14, both containing the dianion [(Mes)3V(μ-N2)V(Mes)3]2-, 8. Complex 4 releases upon protonation exclusively N2, while complexes 11−14 release N2, N2H4, and NH3 in amounts depending on the degree of reduction of the dinitrogen and the structure of the complex. The electrochemical reduction of 1 shed light on the intermediate formation of 5 and its conversion to 8, which was oxidized to the monoanion [(Mes)3V(μ-N2)V(Mes)3]-, 7 (which is the precursor of 8 in the chemical reduction). The magnetic studies coupled with the theoretical interpretation are in agreement with the presence of a d2−d2 couple in 4, a d1−d1 in 12−14, and a d1−d0 in 11. This, along with the structural studies on 11−14, suggest a cumulenic structure [VNNV]+n [n = 5, 4] for 11−14.
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