Direct transformation of dinitrogen: synthesis of N-containing organic compounds via N-C bond formation

Abstract

N-containing organic compounds are of vital importance to lives. Practical synthesis of valuable N-containing organic compounds directly from dinitrogen (N₂), not through ammonia (NH₃), is a holy-grail in chemistry and chemical industry. An essential step for this transformation is the functionalization of the activated N₂ units/ligands to generate N-C bonds. Pioneering works of transition metal-mediated direct conversion of N₂ into organic compounds via N-C bond formation at metal-dinitrogen [N₂-M] complexes have generated diversified coordination modes and laid the foundation of understanding for the N-C bond formation mechanism. This review summarizes those major achievements and is organized by the coordination modes of the [N₂-M] complexes (end-on, side-on, end-on-side-on, etc.) that are involved in the N-C bond formation steps, and each part is arranged in terms of reaction types (N-alkylation, N-acylation, cycloaddition, insertion, etc.) between [N₂-M] complexes and carbon-based substrates. Additionally, earlier works on one-pot synthesis of organic compounds from N₂ via ill-defined intermediates are also briefed. Although almost all of the syntheses of N-containing organic compounds via direct transformation of N₂ so far in the literature are realized in homogeneous stoichiometric thermochemical reaction systems and are discussed here in detail, the sporadically reported syntheses involving photochemical, electrochemical, heterogeneous thermo-catalytic reactions, if any, are also mentioned. This review aims to provide readers with an in-depth understanding of the state-of-the-art and perspectives of future research particularly in direct catalytic and efficient conversion of N₂ into N-containing organic compounds under mild conditions, and to stimulate more research efforts to tackle this long-standing and grand scientific challenge.

Keywords: N-containing organic compounds; N−C bond formation; dinitrogen transformation; metal-dinitrogen complex.

Figure 1.

The classification of this review. The N−C bond formation is reported (√) or not reported (×).

Scheme 1.

N-alkylation of end-on terminal N₂-Mo complexes by electrophiles. (a) N-methylation of N₂-Mo complex by MeOTs to afford methyldiazenido complex. (b) N-methylation of N₂-Mo complexes by MeOTs and MeOTf to afford methyldiazenido and N,N-dimethylhydrazido complexes.

Scheme 2.

N-alkylation of end-on terminal N₂-Fe, Co complexes by MeOTf or MeOTs. (a) N-methylation of N₂-Fe, Co complexes by MeOTs to afford methyldiazenido complexes. (b) N-methylation of N₂-Fe complexes by MeOTf to afford N,N-dimethylhydrazido complex.

Scheme 3.

N-alkylation of end-on terminal N₂-Mo, W complexes by in situ formed radicals. (a) N-alkylation of N₂-Mo, W complexes supported by diphosphine ligands. (b) N-alkylation of N₂-Mo complex supported by tetra-thioether ligand. (c) A plausible mechanism for the generation of 22.

Scheme 4.

Manganese-promoted direct conversion of N₂ into azomethane via the reaction between nucleophiles and N₂-Mn complex. (a) Simple Lewis formulas for end-on terminal N₂-M complexes. (b) A synthetic cycle for synthesis of azo-compound from N₂.

Scheme 5.

N-acylation of end-on terminal N₂-Mo, N₂-W complexes.

Scheme 6.

N−C bond formation from cycloaddition of end-on-bridged N₂-Ti complex with phenylallene, ^tBuNCO and CO₂.

Scheme 7.

N−C bond formation from the reactions of end-on-bridged N₂-Nb, Ta complexes with aldehyde or acetone. (a) The reaction of N₂-Nb complex with benzaldehyde. (b) The reaction of N₂-Ta complex with acetone.

Scheme 8.

N−C bond formation from the reactions of hydrazido Mo, W complexes with carbon-based reagents. (a) The reaction of N₂-Mo, W complexes with HX (X = Cl, Br and I) or HBF₄ to afford the hydrazido complexes. (b) Carbonylation of hydrazido complexes 35 and 36 to assemble N−C bond. (c) A synthetic cycle for synthesis of 1H-pyrrole from N₂.

Scheme 9.

An electrochemical cycle for synthesis of piperidine direct from N₂ via end-on terminal N₂-Mo, W complexes.

Scheme 10.

N-alkylation of side-on-bridged N₂-Hf complexes by EtBr or MeOTf. (a) N-ethylation of N₂-Hf complex by EtBr. (b) N-methylation of N₂-Hf complex by MeOTf.

Scheme 11.

Scandium-promoted direct conversion of N₂ into hydrazine derivatives via the reaction between MeOTf and N₂-Sc complex.

Scheme 12.

N−C bond formation of side-on N₂-Zr complex by reaction with alkynes.

Scheme 13.

N−C bond formation from the reactions of the side-on N₂-Zr, Hf complexes with isocyantes and CO₂. (a) The reaction of N₂-Hf complex with PhNCO. (b) The reaction of N₂-Hf complex with CO₂. (c) The reaction of N₂-Zr complex with CO₂.

Scheme 14.

CO-induced N₂ scission and functionalization at side-on N₂-Zr and Hf complexes. (a) The reaction of N_2-Zr, Hf complexes and CO. (b) A plausible mechanism for this CO-induced N₂ scission and functionalization reaction.

Scheme 15.

N−C bond formation of CO- and N₂-derived oxamidide complexes. (a) Thermolysis of oxamidide complex 71. (b) The reaction of oxamidide complex 64 with CO₂ and ^tBuNCO. (c) N-alkylation of the oxamidide complex 76 to afford N,N^′-dialkyloxamides.

Scheme 16.

N−C bond formation of CO- and N₂-derived Hf-nitride complexes.

Scheme 17.

CO-induced N₂ scission and functionalization at side-on N₂-U complexes. (a) The reaction between CO and N₂-U complex 90 with μ-nitride ligand. (b) The reaction between CO and N₂-U complex 92 with μ-oxo ligand.

Scheme 18.

N-alkylation of side-on-end-on N₂-Ta complex by BnBr.

Scheme 19.

N−C bond formation from the cycloaddition reactions of side-on-end-on N₂-Ta complexes with heteroatom 1,2-cumulenes. (a) The reaction of N₂-Ta complex 94 with carbodiimide, carbon disulfite and isothiocyanates. (b) A plausible mechanism for the generation of 97 and 98.

Scheme 20.

N-methylation of Nb-nitride by reaction with MeI.

Scheme 21.

N-alkylation of Mo-nitride by reaction with MeI.

Scheme 22.

Re-promoted conversion of N₂ into nitriles via N-alkylation of Re-nitride.

Scheme 23.

N-methylation of Fe-nitride by reaction with MeOTs.

Scheme 24.

Conversion of N₂ into nitriles via N-acylation of Mo-nitride.

Scheme 25.

Titanium-promoted direct conversion of N₂ into nitriles via the reaction between acyl chlorides and Ti-nitride.

Scheme 26.

Niobium-promoted direct conversion of N₂ into nitriles via the reaction between acyl chlorides and Nb-nitride.

Scheme 27.

Vanadium-promoted direct conversion of N₂ into potassium cyanate via the reaction between CO and N₂-V complex.

Scheme 28.

N−C bond formation via the reaction of silyl-imido complexes with CO₂.

Scheme 29.

Metal-ligand cooperative N-atom transfer of a Re-nitride.

Scheme 30.

Photolytic cleavage of end-on bridging N₂-Mo, W and Re complexes into nitrides. (a) Photolytic cleavage of end-on bridging N₂-Mo, W complexes 140. (b) Photolytic cleavage of end-on bridging N₂-Re complexes 143.

Scheme 31.

Electrochemical reduction involved synthetic cycle of direct conversion of N₂ into benzamide and benzonitrile.

Scheme 32.

Ti-promoted transformation of N₂ into amines via ill-defined intermediates. (a) The reaction of Cp₂TiCl₂ with aryllithium reagents under N₂ to afford aromatic amines. (b) The reaction of diaryltitanocenes Cp₂TiAr₂ with alkali or alkaline metal under N₂ to afford aromatic amines. (c) Synthesis of organic amines from the reaction between ketones and a supposed titanium nitride species.

Scheme 33.

Ti-promoted N−C bond formation via ill-defined N-silylation titanium species. (a) Preparation of N-silylation titanium complexes 148 from one-pot reaction of TiCl₄ or Ti(OⁱPr)₄ with Li and TMSCl under N₂ or dry air. (b) The reaction between 148 and keto-carbonyl compounds to afford nitric heterocycles. (c) Palladium-catalyzed synthesis of aryl- or allyl- amines and amide derivatives from 148 and aryl or allyl halides in the absence or the presence of CO.