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 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 NbNb unit [NbNb = 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.

Iron–dinitrogen coordination chemistry: Dinitrogen activation and reactivity

“…Complete cleavage of dinitrogen yielding a nitride ligand has been observed for some group 5 and 6 metals [93][94][95][96]. However, no such reaction has yet been observed for iron-dinitrogen complexes.…”

Terminal, Anionic Carbide, Nitride, and Phosphide Transition‐Metal Complexes as Synthetic Entries to Low‐Coordinate Phosphorus Derivatives

“…It should be noted that anionic niobium(v) nitride complexes have also been obtained by dinitrogen cleavage reactions. [44][45][46][47] Treatment of dimeric Na [48] This was converted in two steps and good overall yield into the desired iminophosphinimide precursor, [{(tBuHN)(Cl)P}NNb(NtBuAr) 3 ] which was to be subjected to dehydrohalogenation in the ultimate synthetic step. Dehydrohalogenation was effected by treatment with Li-[N(SiMe 3 ) 2 ], producing the desired tert-butyliminophosphinimide complex [(tBuN = PN)Nb(NtBuAr) 3 ] isolated in 66 % yield as bright yellow microcrystals.…”

Inner-sphere two-electron reduction leads to cleavage and functionalization of coordinated dinitrogen

“…Stoichiometric cleavage of the strong N-N bond has been observed for groups 5 and 6 metal complexes (20)(21)(22)(23)(24)(25), certain dinuclear zirconium dinitrogen complexes have been found to react with H 2 to generate N-H bonds (26)(27)(28), and terminal alkynes have been found to react with coordinated N 2 in a side-on bound dizirconium complex to generate a N-C bond (29). These processes have expanded the toolbox of fundamental reactions for coordinated dinitrogen.…”