Elsevier

Tetrahedron

Volume 76, Issue 50, 11 December 2020, 131703
Tetrahedron

Outlook of nitrogen fixation by carbene

https://doi.org/10.1016/j.tet.2020.131703Get rights and content

Abstract

From the first N2-metal complex discovered in the 1960s to the borylene N2 complex reported very recently, chemists have been trying to activate N2 under milder conditions by investigating many approaches or strategies. Among them, carbene species has been considered a good choice to construct N–C bond to obtain N-containing organic compounds directly. Since the first glimpse of the N2 exchange of diazomethane, chemists have obtained a deeper understanding of the rebound reaction between carbene and N2. This review mainly summarizes the progress in the field of N2 fixation by using carbene species, both experimentally and theoretically. We believe this review will provide readers with an in-depth understanding of the state-of-the-art and perspectives of future research particularly in the use of carbene species for activation and transformation of N2 into N–C containing organic compounds.

Introduction

Nitrogen is an essential element in maintaining human living and civilization. Dinitrogen (N2) is the richest and “cheapest” source of nitrogen, which is inexhaustible. However, it is exceptionally stable and thus difficult to utilize. Accordingly, there is no doubt that realizing activation and direct transformation of N2 under mild conditions is a grand scientific problem that people need to solve.

Nature has already developed the means to realize N2 fixation under mild conditions through the nitrogenases [1,2], whereas industry has been relied on the Haber-Bosch process to transfer N2 to ammonia (NH3) for the production of fertilizer in the presence of a transition-metal catalyst [3,4]. Moreover, with the discovery of the first metal-N2 complex in 1965 [5], many catalysts for N2 fixation were synthesized based on metallic reducing agents [6,7]. Over 50 years of development, chemists have concluded that the peculiar ability of certain transition metal complexes to bind N2 gets benefit from their advantageous combination of occupied and unoccupied d orbitals, which are of reasonable energy and symmetry to synergically back-donate to and accept electron density from N2 (Fig. 1A). Up until recently, H. Branuschweig et al. demonstrated that modification of the electronic environment of the B atom, which is stabilized by CAAC ligand [CAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene], could enable N2 binding and reduction at a B center (Fig. 1B) [8,9]. Beyond metallic catalysts, the non-metallic process of N2 fixation affords another way to solve the grand scientific problem.

Carbenes as highly active species, play an important role in the field of transition metal catalysis and main-group chemistry [10,11]. The electronic structures of carbene are shown in Fig. 2. In 1932, Mulliken recognized that methylene should have two, low-lying electronic states [12]. One of these states is the triplet (A-σ [1]π1 in Fig. 2). In the A-σ [1]π1 state, two nonbonding electrons separately occupy the hybridized σ orbital and the carbon 2p-π atomic orbital (AO) with the spins of the two electrons are parallel. Another low-lying state is the singlet (B-σ [2]π0 in Fig. 2). In the B-σ [2]π0 state, both nonbonding electrons occupy the σ orbital; whereas, the π orbital consists of pure carbon 2p-π AO. Each one of them could be the ground state with selectively stabilized by substituents of carbene [13]. Except for these two stable states, there are two high energy unstable electronic states: the open-shell C-σ [1]π1 state, which has the same orbital occupancy as the A-σ [1]π1 state with antiparallel spins of the two nonbonding electrons; the highest energy D0π2 state configuration, both nonbonding electrons occupy the lower energy π orbital, whereas, the σ orbital consists of pure carbon σ MO. In the general system, such as all alkyl substituted carbene, both of them (C-σ [1]π1 state and D0π2 state) are computed to be 33–59 kcal/mol higher than that of the stable electronic configuration (A-σ [1]π1 or B-σ [2]π0) [14]. Compared with the compounds mentioned above, D0π2 carbene also has unoccupied sp [2] orbital and occupied p orbital [15]. It is possible to activate N2 with appropriate energy and symmetry (Fig. 3).

Chemists have investigated the process of N2 fixation catalyzed by carbenes for decades before borylenes. This review mainly summarizes the progress in the field of direct transformation of N2 to N-containing organic compounds by using carbenes from two aspects: experimentally and theoretically. We expect to inspire researchers to develop the synthesis of the unique σ0π2 carbene to realize activation and transformation of N2 to N-containing organic compounds under mild conditions.

Section snippets

Experimental studies of nitrogen activation by carbene species

The general method of producing a carbene is shown in Scheme 1 [16]. By using the diazoalkanes as the precursors, carbenes are easily obtained through a thermally or photochemically induced loss of N2; whereas, the reverse reaction is N2 fixation. The relevant research about this reverse reaction can be traced back over 50 years to Moore’s 1964 paper [17]. Moore demonstrated that matrix reaction of methylene with N2 to form diazomethane. Isotopic labeling experiments showed that the produced CH2

Conclusions

From the first N2-metal complex to the late borylene N2 complex, chemists have been trying to activate N2 under milder conditions. Till now, the most investigated progress is focused on ammonia and NxHy synthesis. Moreover, there are few reports on the direct synthesis of products containing N-X (X = Si, P) bond from N2, especially for N-containing organic compounds. The catalytic cycle for N–C bond formation has not been implemented yet.

Beyond metallic catalysts, non-metallic process of N2

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Nature Science Foundation of China (21988101).

Chun-Hai Wang received his B.Sc. degree in applied chemistry from Hebei University of Technology in 2012. Then he was employed in Pharmaron (Beijing) as a research associate from 2012 to 2014. Next, he studied at Lanzhou University since 2014, and received his Ph.D. degree in organic chemistry supervised by Prof. Shang-Dong Yang in 2019. Then he joined Prof. Zhenfeng Xi’s group as a Boya postdoctoral fellow at PKU since 2019. His current research focuses on dinitrogen activation and

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    Chun-Hai Wang received his B.Sc. degree in applied chemistry from Hebei University of Technology in 2012. Then he was employed in Pharmaron (Beijing) as a research associate from 2012 to 2014. Next, he studied at Lanzhou University since 2014, and received his Ph.D. degree in organic chemistry supervised by Prof. Shang-Dong Yang in 2019. Then he joined Prof. Zhenfeng Xi’s group as a Boya postdoctoral fellow at PKU since 2019. His current research focuses on dinitrogen activation and transformation under mild conditions.

    Zhu-Bao Yin received his B.Sc. degree in organic chemistry supervised by Prof. Da-Gang Yu at Sichuan University in 2017. He started his Ph.D. study at Peking University (PKU) under the supervision of Professor Zhenfeng Xi and Professor Wen-Xiong Zhang since 2017. His current research focuses on dinitrogen activation and transformation.

    Junnian Wei received his B.Sc. and Ph.D. in organometallic chemistry supervised by Prof. Zhenfeng Xi and Prof. Wen-Xiong Zhang at Peking University, China in 2010 and 2015. Then he joined Paula L. Diaconescu’s group as a postdoctoral fellow at UCLA. In 2017 he moved to UCSF and joined the Michael J. Evans’ group for his second-term postdoc research. In September 2020, he joined Prof. Zhenfeng Xi’s lab at PKU as an Associate Research Professor, focusing his interest on the direct transformation of N2 into N–C containing organic compounds.

    Wen-Xiong Zhang received his B.Sc. from Hunan Normal University in 1996, his M.Sc. from Guangxi Normal University in 1999, and his Ph.D. from Nankai University with professor Li-Cheng Song in 2003. He carried out postdoctoral research at Peking University with Prof. Zhenfeng Xi and at Riken in Japan with Prof. Zhaomin Hou. In 2007, he joined College of Chemistry at Peking University as an Associate Professor. In 2016, he became a full professor and was a recipient of the National Science Fund for Distinguished Young Scholars of China in 2017. From 2018, he is a Boya Distinguished Professor of Peking University. His research interests include design, synthesis and small molecule activation of rare-earth metallacycle; element organic chemistry focusing on B, Si, P and S chemistry; and synthetic organic chemistry to construct N- or P-containing heterocycles.

    Zhenfeng Xi received his B.Sc. degree from Xiamen University in 1983, M.Sc. degree under the supervision of Professor Douman Jin from the Henan Institute of Chemistry, Nanjing University, and Zhengzhou University in 1989, and Ph.D. degree under the supervision of Professor Tamotsu Takahashi from the Institute for Molecular Sciences (IMS), Okazaki, Japan in 1996. He took an Assistant Professor position at Hokkaido University, Japan in 1997. In 1998, he joined the College of Chemistry at Peking University, where he is now a Professor. In 2015, he was elected to the Chinese Academy of Sciences as an academician. He worked on the discovery and development of organo-di-metallic (dilithio in particular) reagents during the past two decades. Currently, he is focusing his interest on the direct transformation of N2 into N–C containing organic compounds.

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