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

This report details an extensive investigation on vanadium−dinitrogen complexes containing the [V(μ-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-Me 3 C 6 H 2 ], with Lewis acids [AlPh 3, B(C 6 F 5 ) 3 ] removed the THF molecule leading under nitrogen to the formation of the diamagnetic complex [(Mes) 3 V(μ-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(μ-N 2 )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-N 2 )], 12 [digly = diethylene glycol dimethyl ether]. Similarly, the reduction of 1 with sodium gave, depending on the workup mode, {[(Mes) 3 V(μ-N 2 )V(Mes) 3 ] 2 - [Na(digly) 2 ] + 2 }, 13, or {[(Mes) 3 V(μ-N 2 )(μ-Na)V(Mes) 3 ] - [Na(digly) 2 ] + }, 14, both containing the dianion [(Mes) 3 V(μ-N 2 )V(Mes) 3 ] 2 -, 8 . Complex 4 releases upon protonation exclusively N 2, while complexes 11 − 14 release N 2, N 2 H 4, and NH 3 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(μ-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 .