Four-electron reduction of dinitrogen has been achieved in the reaction of N2 with [{p-But-calix[4]-(O)4}2Nb2(μ-M)2] (M = Li, 3; M = Na, 4; M = K, 5), which contain a very reactive NbNb unit [NbNb = 2.659(1) Å; 4], in THF. The reaction leads to the dinuclear complexes [{p-But-calix[4]-(O)4}2(Nb⋮N−N⋮Nb)]2- M+
2 (M = Li, 6; M = Na, 7; M = K, 8) [Nb⋮N = 1.747(12) Å, N−N = 1.390(17) Å; 7],
containing the hydrazido [N2]4- anion. Two-electron reduction of the hydrazido tetraanion in 7 using sodium
metal leads to the nitrido species 11, [{p-But-calix[4]-(O)4}2Nb2(μ-N)2(μ-Na)2Na2], which in solution is in
equilibrium with the monomeric form 18, [{p-But-calix[4]-(O)4}Nb⋮N−NaS
n
]. Two distinct pathways have
been identified as a function of the solvent used (THF or DME). In the case of DME, the key intermediate 12
has been intercepted as green crystals. The formation of 12, [{p-But-calix[4]-(O)4}2Nb2(μ-η2:η2-N2)(μ-Na)3Na], results from the reduction of Nb(V) to Nb(IV), which temporarily stores the two electrons in an Nb−Nb
bond [N−N = 1.403(8) Å, Nb−Nb, 2.635(1) Å]. Complex 12 contains a μ-η2:η2 side-on N2 group, bonded
between two niobiums and preorganized to be transformed into two nitrido anions. Heating in pyridine (90
°C) converts 12 to 11, with simultaneous cleavage of the Nb−Nb and N−N bonds, the heating causing two-electron transfer from the Nb−Nb to the N−N bond. The reaction of 4 with N2, carried out in toluene instead
of THF or DME, gives a nitrido complex, [{p-But-calix[4]-(O)4}3Nb3(μ-N)2Na3(THF)2], 15, which can be
formed equally well from the reaction of 4a with 7a carried out in a 1:2 molar ratio in toluene. Complex 15
breaks up in CHCl3 or toluene, upon addition of TMEDA, into [{p-But-calix[4]-(O)4}2Nb2(μ-N)2{μ-Na2(TMEDA)2}Na2], 16, and [{p-But-calix[4]-(O)4}2Nb2(μ-N)][Na(TMEDA)2], 17.
