Dinitrogen Activation and Functionalization Affording Chromium Diazenido and Hydrazido Complexes

Gao-Xiang Wang, Zhu-Bao Yin, Junnian Wei,* Zhenfeng Xi*

Acc. Chem. Res., 2023. DOI: 10.1021/acs.accounts.3c00476.

        The 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 Nβ 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 Nα and Nβ 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.

       氮氣的高效活化與功能化長期以來被認為是學術界和工業界的“聖杯”。自1965年第一例過渡金屬氮氣配合物被合成以來，均相固氮化學家一直緻力于利用模型配合物來實現溫和條件下N-H、N-C、N-Si等氮雜原子鍵的構建。在過去的60多年裡，有超過600個過渡金屬末端氮氣配合物的單晶結構被報道，其中僅有不到8%的氮氣配合物能發生後續兩步親電衍生化反應生成二氮烯（diazenido）與亞肼類（hydrazido）衍生物，這些氮氣的衍生化反應往往得不到令人滿意的收率。這兩步計量反應是整個氮氣催化還原循環的起始步驟，研究氮氣到二氮烯再到亞肼等中間體的定量高效轉化過程至關重要。

       在過去的4年間，席振峰課題組使用金屬鉻以及環戊二烯基配體，膦配體和卡賓配體合成了一系列金屬鉻氮氣Cr-N₂模型配合物，這些鉻氮氣配合物的合成、光譜學特征、電子結構和反應化學總結如下：

       （1）雙核和三核的一價鉻氮氣配合物Cr(I)-N₂難以發生氮氣的衍生化反應，與苯矽烷、偶氮苯以及炔烴等不飽和底物反應時，氮氣會作為離去基團離去，反應表現為一價鉻到三價鉻的氧化加成（Chem. Commun. 2019, 55, 9641−9644）；

       （2）繼續還原一價鉻氮氣配合物可以得到氮氣活化程度更高的零價鉻氮氣配合物Cr(0)-N₂，環戊二烯基膦配體零價鉻氮氣配合物與三甲基氯矽烷Me₃SiCl反應得到了首例鉻亞肼基配合物。該反應遵循遠端路徑（distal pathway）機制，矽基化末端N_β原子兩次，生成了端基配位的三價鉻亞肼Cr(III)=N-N(SiMe₃)₂配合物（J. Am. Chem. Soc. 2019, 141, 4241−4247）；

       （3）使用π酸性更弱的氮雜卡賓（NHC）配體替換環戊二烯基膦配體中的單膦配體後，零價鉻氮氣配合物與三甲基氯矽烷的反應轉變為交替路徑（alternating pathway），N_α與N_β兩個氮原子均被矽基化，生成側基配位的三價鉻亞肼Cr(III)(ƞ²-Me₃SiN-NSiMe₃)配合物（J. Am. Chem. Soc. 2023, 145, 7065−7070）；

       （4）上述兩類雙矽基化反應遵循“3-4-2-1”的化學計量比，即三分子零價鉻氮氣配合物與四分子三甲基氯矽烷反應，生成兩分子三價鉻亞肼配合物與一分子二價鉻氯化物：3 Cr(0)-N₂ + 4 Me₃SiCl → 2 Cr(III)[N₂(SiMe₃)₂] + 1 Cr(II)-Cl。該反應過程存在理論66.7%的收率極限，使用雙膦配體替換單齒膦或卡賓配體，可以制備配位飽和的零價鉻氮氣配合物，飽和的配位構型一定程度上抑制了奇電子反應路徑的發生。雙膦配體零價氮氣配合物的親電衍生化反應遵循兩步雙電子過程，先生成二價鉻的二氮烯，再轉化為四價鉻的亞肼：Cr(0)-N₂ → Cr(II)-N=N-R → Cr(IV)=N-NR₂。不僅是矽基化反應，質子化反應和甲基化反應同樣能定量進行。所有的二級反應速率常數以及二氮烯轉瞬中間體的壽命均由低溫快速紫外可見光譜測量得到。基于這些實驗結果，最終以接近當量（82%-96%）的轉化收率實現了四價鉻亞肼配合物的高效制備（J. Am. Chem. Soc. 2023, 145, 9746−9754）。

       上述對鉻氮氣配合物的功能化反應研究不僅加深了對遠端和交替路徑反應機制的理解，而且為合理設計氮氣到高附加值含氮有機物的高效催化循環奠定了堅實的基礎。