Stepwise Reduction of Dinitrogen Occurring on a Divanadium Model Compound: A Synthetic, Structural, Magnetic, Electrochemical, and Theoretical Investigation on the [V=N=N=V]n+ [n = 4-6] Based Complexes

This report details an extensive investigation on vanadium-dinitrogen complexes containing the [V(mu-N-2)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)(3)V(thf)], 1 [Mes = 2,4,6-Me3C6H2], with Lewis acids [AlPh3, B(C6F5)(3)] removed the THF molecule lending under nitrogen to the formation of the diamagnetic complex [(Mes)(3)V(mu-N-2)V(Mes)(3)], 4. The reaction of 1 with N-2 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 N-2 in the case of potassium and in the presence of 1 leads to the compound {[Mes)(3)V(mu-N-2)V(Mes)(3)](-)[K(digly)(3)](+)}, 11, which, depending on the reaction conditions, undergoes a further reduction to [{K(digly)(3)(mu-Mes)(2)(Mes)V}(2)(mu-N-2)], 12 [digly = diethylene glycol dimethyl ether]. Similarly, the reduction of 1 with sodium gave, depending on the workup mode, {[(Mes)(3)V(mu-N-2)V(Mes)(3)](2-)[Na(digly)(2)](2)(+)}, 13, or {[(Mes)(3)V(mu-N-2)(mu-Na)V(Mes)(3)](-)[Na(digly)(2)](+)}, 14. both containing the dianion [(Mes)(3)V(mu-N-2)V(Mes)(3)](2-), 8. Complex 1 releases upon protonation exclusively Nz. while complexes 11-14 release N-2, 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)(3)V(mu-N-2)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 d(2)-d(2) couple in 4, a d(1)-d(1) in 12-14, and a d(1)-d(0) in 11. This, along with the structural studies on 11-14, suggest a cumulenic structure [V=N=N=V](+n) [n = 5, 4] for 11-14.