Development and Outlook of Carbenes with Inverted Electronic Configuration (σ0π2)★

doi:10.6023/A25050158

Acta Chimica Sinica ›› 2025, Vol. 83 ›› Issue (9): 1025-1034.DOI: 10.6023/A25050158 Previous Articles Next Articles

Perspective

反转电子态(σ⁰π²)卡宾的发展及其研究展望^★

刘郅勍, 刘柳^*()

南方科技大学化学系深圳 518055

投稿日期:2025-05-11 发布日期:2025-06-23

作者简介:

刘郅勍, 南方科技大学2021级本科生, 目前在刘柳课题组开展主族元素化学相关研究.

刘柳, 南方科技大学化学系研究员, 长期致力于双亲性主族元素化学的前沿探索, 聚焦亲核/亲电双功能主族元素分子体系的设计合成与转化研究, 并用其模拟过渡金属的电子结构和化学行为, 系统拓展主族元素的基础化学认知边界. 曾在2011年与2016年先后获得厦门大学学士、博士学位, 期间曾在加州大学圣地亚哥分校进行博士研究生联合培养. 随后赴多伦多大学和加州大学伯克利分校开展博士后研究(2016-2020), 2020年9月加入南方科技大学组建独立研究团队. 2021年入选国家级海外青年人才计划; 获得中国化学会—英国皇家化学会青年化学奖、中国化学会黄耀曾金属有机化学青年奖、中国化学会青年化学奖、中国化学会青委会菁青化学新锐奖、日本化学会杰出讲座奖、广东省青年科技创新奖. 目前担任《Inorganic Chemistry Frontiers》、《化学学报》、《EurJIC》、《Chinese Chemical Letters》青年编委.

^★ “中国青年化学家”专辑.

基金资助:
国家自然科学基金(22350004)

Development and Outlook of Carbenes with Inverted Electronic Configuration (σ⁰π²)^★

Zhiqing Liu, Liu Leo Liu^*()

Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China

Received:2025-05-11 Published:2025-06-23
Contact: * E-mail: liuleoliu@sustech.edu.cn
About author:
^★ For the VSI “Rising Stars in Chemistry”.
Supported by:
National Natural Science Foundation of China(22350004)

Carbenes (R₂C:) represent a unique class of neutral divalent carbon species that formally possess six valence electrons at the central carbon atom, thereby deviating from the octet rule. These species have long occupied a central role in organic, organometallic, and materials chemistry owing to their rich electronic structures and versatile reactivity. The vast majority of isolated stable carbenes adopt a singlet ground-state electronic configuration described as σ²π⁰, wherein a lone pair occupies an in-plane σ orbital and the out-of-plane π orbital remains unoccupied. This configuration underpins the pronounced nucleophilicity observed in classical N-heterocyclic carbenes and their analogs. A limited number of carbenes with a σ¹π¹ open-shell ground state have been spectroscopically characterized, often exhibiting radical-like behavior and enhanced reactivity. In sharp contrast, carbenes with an inverted electronic configuration—σ⁰π²—remain extraordinarily rare and conceptually intriguing. These carbenes possess two electrons fully occupying the out-of-plane π orbital, leaving the in-plane σ orbital vacant. Such a configuration inverts the typical orbital occupancy pattern, potentially leading to electrophilic or ambiphilic reactivity profiles. To date, four principal design strategies have been proposed to stabilize σ⁰π² carbenes: (1) cyclic di(imino)carbenes; (2) di(boryl)carbene; (3) metal-coordinated cyclic di(phosphino)carbenes; and (4) dicationic carbones. However, despite extensive conceptual exploration, only one stable and isolable σ⁰π² carbene has been structurally authenticated to date. This species features a rigid RhP₂C four-membered metallacycle, wherein the carbene carbon exhibits an unprecedented ¹³C NMR chemical shift below －30 parts per million, representing the most upfield resonance recorded for a carbene center. This carbene demonstrates divergent reactivity patterns uncharacteristic of classical σ²π⁰ congeners. It reacts with both Lewis acids and Lewis bases to yield coordination adducts and π-complexes, respectively, and participates in bond-forming processes such as ketenimine generation from isocyanides. Its demonstrated ambiphilicity offers promise for the activation of inert small molecules, including dinitrogen (N₂)—a long-standing goal in main group chemistry. This perspective provides a systematic overview of the emerging chemistry of σ⁰π² carbenes, critically analyzing structural parameters, stabilization frameworks, and reactivity paradigms. We further highlight the prospective roles of such carbenes in bond activation and novel ligand design, envisioning their integration into next-generation main-group and transition-metal cooperative systems.

Key words: carbene, inverted electronic configuration, synthetic chemistry

Cite this article

Zhiqing Liu, Liu Leo Liu. Development and Outlook of Carbenes with Inverted Electronic Configuration (σ⁰π²)^★[J]. Acta Chimica Sinica, 2025, 83(9): 1025-1034.

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Fig. & Tab. 19

Figure 1. Pivotal milestones in the evolution of traditional carbenes and carbenes with inverted electronic configuration

Figure 2. Electronic configurations of carbenes

Figure 3. The strategy employed for stabilizing the singlet state of carbenes Green arrows represent the inductive effect while red arrows denote the mesomeric effect

Figure 4. Strategies to stabilize carbenes with a σ0π2 electronic configuration

Figure 5. (a) Photolysis or thermolysis of 1 leading to 2 and trapping 2 with benzene derivatives; (b) Photoreactions of 2 and its derivatives

Figure 6. The depiction of Least-Motion Path

Figure 7. Photooxidation of 2-diazo-2H-imidazole (1)

Figure 8. The synthesis and reactivity of compound 12

Figure 9. Diborylcarbene intermediate 15 and its reactions

Figure 10. Synthesis of compound 20

Figure 11. The synthesis attempt and trapping of free diborylcarbene 23

Figure 12. The synthesis and reactivity of compound 26

Figure 13. The formation and trapping reaction of transient carbene 30

Figure 14. The synthesis and resonance structure of complex 36

Figure 15. Reaction of compound 37 with silver or sodium salt with the aid of a Lewis base

Figure 16. (a) Comparison of the π-donating ability for noncyclic and cyclic diaminophosphino substituents; (b) Frontier molecular orbital diagrams depicting the electronic structure of planar phosphine substituents with the idealized D3h and C2v point groups

Figure 17. Examples of carbenes and their analogs stabilized by cyclic diamino-substituted phosphino substituents

Figure 18. (a) The synthesis of carbene 45; (b) The reactivity of carbene 45; (c) Schematic representing the essence of stabilization

Figure 19. (a) The resonance structures of stannyne 49; (b) Some of the frontier molecular orbitals of stannyne 49; (c) Schematic representing the essence of the stabilization to the σ0π2 configuration at the carbene center

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反转电子态(σ⁰π²)卡宾的发展及其研究展望^★

Development and Outlook of Carbenes with Inverted Electronic Configuration (σ⁰π²)^★

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反转电子态(σ0π2)卡宾的发展及其研究展望★