Stabilization of anti-aromatic and strained five-membered rings with a transition metal

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Abstract

Anti-aromatic compounds, as well as small cyclic alkynes or carbynes, are particularly challenging synthetic goals. The combination of their destabilizing features hinders attempts to prepare molecules such as pentalyne, an 8π-electron anti-aromatic bicycle with extremely high ring strain. Here we describe the facile synthesis of osmapentalyne derivatives that are thermally viable, despite containing the smallest angles observed so far at a carbyne carbon. The compounds are characterized using X-ray crystallography, and their computed energies and magnetic properties reveal aromatic character. Hence, the incorporation of the osmium centre not only reduces the ring strain of the parent pentalyne, but also converts its Hückel anti-aromaticity into Craig-type Möbius aromaticity in the metallapentalynes. The concept of aromaticity is thus extended to five-membered rings containing a metal–carbon triple bond. Moreover, these metal–aromatic compounds exhibit unusual optical effects such as near-infrared photoluminescence with particularly large Stokes shifts, long lifetimes and aggregation enhancement.

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Figure 1: Synthesis, structure and reactivity of osmapentalynes.
Figure 2: Aromaticity in osmapentalynes: downfield 1H chemical shifts and resonance structures.
Figure 3: NICS(0)zzcontributions of the four key occupied perimeter molecular orbitals of model complex 2′.
Figure 4: ISE evaluations of the anti-aromaticity of pentalene and pentalyne and the aromaticity of osmapentalyne models.
Figure 5: Photoluminescence of osmapentalyne 2a.

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Acknowledgements

This research was supported by the National Science Foundation of China (grant nos. 20925208, 21172184, 21175113 and 21273177), the National Basic Research Program of China (nos. 2012CB821600 and 2011CB808504), the Program for Changjiang Scholars and Innovative Research Team in University of China, and US-NSF Grant CHE 105-7466. The authors thank E. Meggers at Philipps-Universität Marburg, Zhenyang Lin at the Hong Kong University of Science and Technology, Yirong Mo at Western Michigan University, and Xinzheng Yang at the University of California, Berkeley, for their suggestions, and, in particular J. I-Chia Wu, Georgia, for instructive discussions and her assistance with aromaticity analyses.

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H.X. conceived the project. C.Z., M.L. and X.Z. performed the experiments. S.L. and Y.N. conducted the luminescence study of osmapentalynes. C.Z. and T.W. recorded all NMR data and solved all X-ray structures. H.X., C.Z. and T.W. analysed the experimental data. J.Z. conceived the theoretical work and, with M.-L.L., conducted theoretical computations. J.Z., X.L., Z.C., M.-L.L. and P.v.R.S. analysed and interpreted the computational data. J.Z., H.X., S.L. and C.Z. drafted the paper, with support from Z.C., X.L., T.W. and Z.X., as well as language editing by P.v.R.S. All authors discussed the results and contributed to the preparation of the final manuscript.

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Correspondence to Jun Zhu or Haiping Xia.

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The authors declare no competing financial interests.

Supplementary information

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Supplementary information (PDF 2404 kb)

Supplementary information

Crystallographic data for compound 2a. (CIF 26 kb)

Supplementary information

Crystallographic data for compound3a. (CIF 27 kb)

Supplementary information

Crystallographic data for compound 4. (CIF 27 kb)

Supplementary information

Crystallographic data for compound 5. (CIF 27 kb)

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Zhu, C., Li, S., Luo, M. et al. Stabilization of anti-aromatic and strained five-membered rings with a transition metal. Nature Chem 5, 698–703 (2013). https://doi.org/10.1038/nchem.1690

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