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 the N⋮N Bonds Assisted by Nb^III-calix[4]arene

Abstract

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 (μ-M) 2 ] (M = Li, 3; M = Na, 4; M = K, 5 ), which contain a very reactive Nb Nb unit [Nb Nb = 2.659(1) Å; 4 ], in THF. The reaction leads to the dinuclear complexes [{ p -Bu t -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 [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 (μ-N) 2 (μ-Na) 2 Na 2 ], which in solution is in equilibrium with the monomeric form 18, [{ p -Bu t -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 -Bu t -calix[4]-(O) 4 } 2 Nb 2 (μ-η 2:η 2 -N 2 )(μ-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) Å, Nb−Nb, 2.635(1) Å]. Complex 12 contains a μ-η 2:η 2 side-on N 2 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 N 2, carried out in toluene instead of THF or DME, gives a nitrido complex, [{ p -Bu t -calix[4]-(O) 4 } 3 Nb 3 (μ-N) 2 Na 3 (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 CHCl 3 or toluene, upon addition of TMEDA, into [{ p -Bu t -calix[4]-(O) 4 } 2 Nb 2 (μ-N) 2 {μ-Na 2 (TMEDA) 2 }Na 2 ], 16, and [{ p -Bu t -calix[4]-(O) 4 } 2 Nb 2 (μ-N)][Na(TMEDA) 2 ], 17 .