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.

       氮气的高效活化与功能化长期以来被认为是学术界和工业界的“圣杯”。自1965年第一例过渡金属氮气配合物被合成以来，均相固氮化学家一直致力于利用模型配合物来实现温和条件下N-H、N-C、N-Si等氮杂原子键的构建。在过去的60多年里，有超过600个过渡金属末端氮气配合物的单晶结构被报道，其中仅有不到8%的氮气配合物能发生后续两步亲电衍生化反应生成二氮烯（diazenido）与亚肼类（hydrazido）衍生物，这些氮气的衍生化反应往往得不到令人满意的收率。这两步计量反应是整个氮气催化还原循环的起始步骤，研究氮气到二氮烯再到亚肼等中间体的定量高效转化过程至关重要。

       在过去的4年间，席振峰课题组使用金属铬以及环戊二烯基配体，膦配体和卡宾配体合成了一系列金属铬氮气Cr-N₂模型配合物，这些铬氮气配合物的合成、光谱学特征、电子结构和反应化学总结如下：

       （1）双核和三核的一价铬氮气配合物Cr(I)-N₂难以发生氮气的衍生化反应，与苯硅烷、偶氮苯以及炔烃等不饱和底物反应时，氮气会作为离去基团离去，反应表现为一价铬到三价铬的氧化加成（Chem. Commun. 2019, 55, 9641−9644）；

       （2）继续还原一价铬氮气配合物可以得到氮气活化程度更高的零价铬氮气配合物Cr(0)-N₂，环戊二烯基膦配体零价铬氮气配合物与三甲基氯硅烷Me₃SiCl反应得到了首例铬亚肼基配合物。该反应遵循远端路径（distal pathway）机制，硅基化末端N_β原子两次，生成了端基配位的三价铬亚肼Cr(III)=N-N(SiMe₃)₂配合物（J. Am. Chem. Soc. 2019, 141, 4241−4247）；

       （3）使用π酸性更弱的氮杂卡宾（NHC）配体替换环戊二烯基膦配体中的单膦配体后，零价铬氮气配合物与三甲基氯硅烷的反应转变为交替路径（alternating pathway），N_α与N_β两个氮原子均被硅基化，生成侧基配位的三价铬亚肼Cr(III)(ƞ²-Me₃SiN-NSiMe₃)配合物（J. Am. Chem. Soc. 2023, 145, 7065−7070）；

       （4）上述两类双硅基化反应遵循“3-4-2-1”的化学计量比，即三分子零价铬氮气配合物与四分子三甲基氯硅烷反应，生成两分子三价铬亚肼配合物与一分子二价铬氯化物：3 Cr(0)-N₂ + 4 Me₃SiCl → 2 Cr(III)[N₂(SiMe₃)₂] + 1 Cr(II)-Cl。该反应过程存在理论66.7%的收率极限，使用双膦配体替换单齿膦或卡宾配体，可以制备配位饱和的零价铬氮气配合物，饱和的配位构型一定程度上抑制了奇电子反应路径的发生。双膦配体零价氮气配合物的亲电衍生化反应遵循两步双电子过程，先生成二价铬的二氮烯，再转化为四价铬的亚肼：Cr(0)-N₂ → Cr(II)-N=N-R → Cr(IV)=N-NR₂。不仅是硅基化反应，质子化反应和甲基化反应同样能定量进行。所有的二级反应速率常数以及二氮烯转瞬中间体的寿命均由低温快速紫外可见光谱测量得到。基于这些实验结果，最终以接近当量（82%-96%）的转化收率实现了四价铬亚肼配合物的高效制备（J. Am. Chem. Soc. 2023, 145, 9746−9754）。

       上述对铬氮气配合物的功能化反应研究不仅加深了对远端和交替路径反应机制的理解，而且为合理设计氮气到高附加值含氮有机物的高效催化循环奠定了坚实的基础。