Dinitrogen Activation and Functionalization Affording Chromium Diazenido and Hydrazido Complexes

ConspectusThe activation and functionalization of N₂ to form nitrogen-element bonds have long posed challenges to industrial, biological, and synthetic chemists. The first transition-metal dinitrogen complex prepared by Allen and Senoff in 1965 provoked researchers to explore homogeneous N₂ fixation. Despite intensive research in the last six decades, efficient and quantitative conversion of N₂ to diazenido and hydrazido species remains problematic. Relative to a plethora of reactions to generate N₂ complexes, their functionalization reactions are rather rare, and the yields are often unsatisfactory, emphasizing the need for systematic investigations of the reaction mechanisms.In this Account, we summarize our recent work on the synthesis, spectroscopic features, electronic structures, and reactivities of several Cr-N₂ complexes. Initially, a series of dinuclear and trinuclear Cr(I)-N₂ complexes bearing cyclopentadienyl-phosphine ligands were accessed. However, they cannot achieve N₂ functionalization but undergo oxidative addition reactions with phenylsilane, azobenzene, and other unsaturated organic compounds at the low-valent Cr(I) centers rather than at the N₂ unit. Further reduction of these Cr(I) complexes leads to the formation of more activated mononuclear Cr(0) bis-dinitrogen complexes. Remarkably, silylation of the cyclopentadienyl-phosphine Cr(0)-N₂ complex with Me₃SiCl afforded the first Cr hydrazido complex. This process follows the distal pathway to functionalize the N_β atom twice, yielding an end-on η¹-hydrazido complex, Cr(III)═N-N(SiMe₃)₂. In contrast, upon substitution of the phosphine ligand in the Cr(0)-N₂ complex with a N-heterocyclic carbene (NHC) ligand, the corresponding reaction with Me₃SiCl proceeds via the alternating pathway; the silylation occurs at both N_α and N_β atoms and generates a side-on η²-hydrazido complex, Cr(III)(η²-Me₃SiN-NSiMe₃). Both silylation reactions are inevitably accompanied by the formation of Cr(III) hydrazido complexes and Cr(II) chlorides with a 2:1 ratio. These processes exhibit a peculiar '3-4-2-1' stoichiometry (i.e., treating 3 equiv of Cr(0)-N₂ complexes with 4 equiv of Me₃SiCl yields 2 equiv of Cr(III) disilyl-hydrazido complexes and 1 equiv of Cr(II) chloride). Upon replacing the monodentate phosphine and/or NHC ligand with a bisphosphine ligand, a monodinitrogen Cr(0) complex, instead of the bis-dinitrogen Cr(0) complexes, is obtained; consequently, the silylation reactions progress via the normal two-electron route, which passes through Cr(II)-N═N-R diazenido species as an intermediate and furnishes [Cr(IV)═N-NR₂]⁺ hydrazido as the final products. More importantly, this type of Cr(0)-N₂ complex can be not only silylated but also protonated and alkylated proficiently. All of the second-order reaction rates of the first and second transformations are determined along with the lifetimes of the intervening diazenido species. Based on these findings, we have successfully carried out nearly quantitative preparations of the Cr(IV) hydrazido species with unmixed or hybrid substituents.The studies of Cr-N₂ systems provide effective approaches for the activation and functionalization of N₂, deepening the understanding of N₂ electrophilic attack. We hope that this Account will inspire more discoveries related to the transformation of gaseous N₂ to high-value-added nitrogen-containing organic compounds.