Dinitrogen rearranging over a metal-oxo surface and cleaving to nitride: From the end-on to the side-on bonding mode, to the stepwise cleavage of theN N bonds assisted by Nb-III-calix[4]arene

Four-electron reduction of dinitrogen has been achieved in the reaction of N-2 With [{p-Bu-t-calix-[4](O)(4)}(2)Nb-2(mu-M)(2)] (M = Li, 3; M = Na, 4; M = K, 5), which contain a very reactive Nb=Nb unit [Nb=Nb = 2.659(1) Angst rom; 4], in THF. The reaction leads to the dinuclear complexes [{p-Bu-t-cal ix[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) Angstrom, N-N = 1.390(17) Angstrom; 7], containing the hydrazi do [N-2](4-) anion. Two-electron reduction of the hydrazido tetraanion in 7 using sodium metal leads to the nitrido species 11, [{p-Bu-t-calix[4]-(O)( 4)}(2)Nb-2(mu-N)(2)(mu-Na)(2)Na-2], which in solution is in equilibrium wit h the monomeric form 18, [{p-Bu-t-calix[4]-(O)(4)}Nb equivalent to N-NaSn]. Two distinct pathways have been identified as a function of the solvent us ed (THF or DME), In the case of DME, the key intermediate 12 has been inter cepted as green crystals: The formation of 12, [{p-Bu-t-calix[4]-(O)(4)}(2) Nb-2(mu-eta(2):eta(2)-N-2)(mu-Na)(3)Na], 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) Angstrom, Nb-Nb, 2.635(1) Angstrom]. Complex 12 contains a mu -eta 2:eta(2) side-on N-2 group, bonded between two niobiums and preorganiz ed to be transformed into two nitrido anions. Heating in pyridine (90 degre es C) converts 12 to 11, with simultaneous cleavage of the Nb-Nb and N-N bo nds, the heating causing two-electron transfer from the Nb-Nb to the N-N bo nd. The reaction of 4 with N-2, carried out in toluene instead of THF or DM E, gives a nitrido complex, [{p-Bu-t-calix[4]-(O)(4)}(3)Nb-3(mu-N)(2)Na-3(T HF)(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 CHC l3 or toluene, upon addition of TMEDA, into [{p-Bu-t-calix[4]-(O)(4)}(2)Nb- 2(mu-N)(2){mu-Na-2-(TMEDA)(2)}Na-2], 16, and [{p-Bu-t-calix[4]-(O)(4)}(2)Nb -2(mu-N)][Na(TMEDA)(2)], 17.