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A neutral σ0π2 carbene enabling hydrogen activation via a σ-face pathway

Nature Chemistry (2026) Cite this article

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Abstract

Ground-state σ0π2 carbenes are among the least explored and least understood classes of low-valent carbon. Known examples of isolable σ0π2 carbenes remain largely limited to cationic compounds. Here we report the synthesis of a neutral σ0π2 carbene, namely a rhodadiphosphinocarbene featuring a planar RhP2C ring. In sharp contrast to small-molecule activation by conventional σ2π0 carbenes, which typically proceeds via nucleophilicity-driven, π-face, non-least-motion trajectories mandated by orbital-symmetry constraints, this neutral σ0π2 carbene cleaves H2 under ambient conditions via a σ-face pathway. Computations reveal an electrophilicity-driven early transition state characterized by minimal reorganization of the carbene framework and a concerted yet asynchronous H–H scission to form two C–H bonds. These findings show that a stable carbene can enable small-molecule activation through a σ-face pathway and deepen our understanding of electronic structure–reactivity correlations in low-valent carbon chemistry.

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Fig. 1: Conceptual overview, notable examples and present work.
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Fig. 2: Single-crystal X-ray diffraction solid-state structures of compounds 2 and 3a at 100 K with anisotropic displacement ellipsoids at the 40% probability level and selective labelling.
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Fig. 3: Frontier molecular orbitals and experimental two-dimensional deformation density maps.
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Fig. 4: Theoretical analysis.
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Data availability

Data supporting the findings of this study are provided in the Article or its Supplementary Information. Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2483382 (2) and 2483383 (3a). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures. Underlying data for Fig. 3, Extended Data Figs. 1 and 2 are available from Figshare at https://doi.org/10.6084/m9.figshare.31908739 (ref. 86).

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Acknowledgements

We thank the financial support from the National Natural Science Foundation of China (grants 22350004, 22271132 and 22501124), the Shenzhen Science and Technology Program (grant KQTD20240729102027009) and Guangdong Innovation and Entrepreneurial Research Team Program (grant 2021ZT09C278). We also acknowledge support from the SUSTech Core Research Facilities. F.F. thanks financial support from the China Postdoctoral Fellowship Program of CPSF (GZC20250636). The theoretical work was supported by the Center for Computational Science and Engineering at SUSTech. We thank X. Chang and Q. Liang for helpful discussions and assistance with crystallographic analysis.

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  1. These authors contributed equally: Fei Fan, Haoxiang Nong.

Authors and Affiliations

  1. Department of Chemistry, Southern University of Science and Technology, Shenzhen, China

    Fei Fan  (凡飞), Haoxiang Nong  (农皓翔), Miaomiao Zhou  (周苗苗) & Liu Leo Liu  (刘柳)

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  1. Fei Fan  (凡飞)
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Contributions

F.F. and H.N. contributed equally to this work. L.L.L. conceptualized and supervised the project. F.F. performed the experimental work. L.L.L., H.N., M.Z. and F.F. performed the computational work. F.F. and M.Z. performed the X-ray crystallographic analyses. L.L.L. wrote the paper with input from all authors. All authors discussed the results in detail and commented on the paper.

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Correspondence to Liu Leo Liu  (刘柳).

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Extended data

Extended Data Fig. 1 Electron localization function (ELF) analysis of the CP2Rh plane in compound 2.

a, Top view, showing minimal electron localization at the carbene carbon along the σ-orbital direction, consistent with a σ0 electronic configuration. b, Side view, revealing two symmetric regions of high ELF above and below the carbene carbon, indicative of a π2 electronic configuration.

Extended Data Fig. 2 Kinetic studies.

a, Full time-course kinetics of D2. b, Kinetic isotope effect (KIE) determined from the initial-rate regime (KIE = 1.52).

Extended Data Fig. 3 Natural localized molecular orbital (NLMO) analysis of transition state TS1.

Two dominant donor–acceptor interactions are identified: σ(H–H) → σ0 (carbene) donation (left) and π2 (carbene) → σ*(H–H) back-donation (right), highlighting the cooperative σ-donation/π-back-donation manifold operative in H–H activation. Mesityl substituents were simplified to methyl groups for clarity.

Supplementary information

Supplementary Information (download PDF )

Supplementary Figs. 1–55, SupplementaryTables 1–12. Supplementary Materials and methods, Experimental details and characterization data, and Computational details.

Supplementary Data 1

Cartesian coordinates for all calculated structures, including the SCF energies and free energies.

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Fan, F., Nong, H., Zhou, M. et al. A neutral σ0π2 carbene enabling hydrogen activation via a σ-face pathway. Nat. Chem. (2026). https://doi.org/10.1038/s41557-026-02147-0

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