Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Generation and characterization of a fairly stable triplet carbene

Nature volume 412, pages 626–628 (2001)Cite this article

Abstract

Most molecules are held together by covalent bonds—electron pairs jointly shared by the two atoms that are linked by the bond. Free radicals, in contrast, have at least one unpaired electron. In the case of carbon-based radicals, the carbon atom at the radical centre no longer makes four bonds with other atoms as it would do in its normal, tetravalent state. The presence of unpaired electrons renders such radicals highly reactive, so they normally occur only as transient intermediates during chemical reactions. But the discovery1,2 by Gomberg in 1900 of triphenylmethyl, the first relatively stable free radical containing a central trivalent carbon atom, illustrated that radicals with suitable geometrical and electronic structures can be stable. Compounds containing a divalent carbon atom that uses only two of its four valence electrons for bonding are usually less stable than Gomberg-type radicals with trivalent carbon3,4,5. Although the role of these so-called carbenes in chemical reactions has long been postulated, they were unambiguously identified only in the 1950s. More recently, stable carbenes have been prepared6,7, but the singlet state of these molecules6,7,8,9,10,11,12, with the two nonbonding valence electrons paired, means that they are not radicals. Carbenes in the second possible electronic state, the triplet state, are radicals: the two nonbonding electrons have parallel spins and occupy different orbitals13,14. Here we report the preparation and characterization of a triplet carbene, whose half-life of 19 minutes at room temperature shows it to be significantly more stable than previously observed triplet carbenes15,16,17.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Figure 1: Structure of triplet di(9-anthryl)carbene (32a), generated by photolysis of the precursor diazomethane (1a), and the corresponding trimer (3).
Figure 2: Structure of triplet bis(9-anthryl)carbene (32b), generated by photolysis of the precursor diazomethane (1b), and the corresponding carbene dimer (4).
Figure 3: Ultraviolet/visible spectrum after photolysis (λ = 308 nm) of the precursor diazomethane (1b) in degassed benzene at 20 °C.
Figure 4: Structure of 32b and Gomberg's radical.

References

  1. Gomberg, M. Triphenylmethyl, ein Fall von dreiwerthigem Kohlenstoff. Ber. Deutsch. Chem. Ges. 33, 3150–3163 (1900).

    Article  CAS  Google Scholar 

  2. Gomberg, M. An instance of trivalent carbon: Triphenylmethyl. J. Am. Chem. Soc 22, 757–771 (1900).

    Article  Google Scholar 

  3. Staudinger, H. & Kupfer, O. Über Reaktionen des Methylenes. Diazomethan. Ber. Deutsch. Chem. Ges. 45, 501–509 (1912).

    Article  CAS  Google Scholar 

  4. Hine, J. Carbon dichloride as an intermediate in basic hydrolysis of chloroform. A mechanism for substitution reactions at a saturated carbon atom. J. Am. Chem. Soc. 72, 2438–2445 (1950).

    Article  CAS  Google Scholar 

  5. Doering, W. v. E. & Hoffmann, A. K. Addition of dichlorocarbene to olefins. J. Am. Chem. Soc. 76, 6162–6165 (1954).

    Article  CAS  Google Scholar 

  6. Igau, A., Grützmacher, H., Baceiredo, A. & Bertrand, G. Analogous α,α′–bis-carbenoid triply bonded species: Synthesis of a stable λ3-phosphinocarbene-λ5-phosphaacetylene. J. Am. Chem. Soc. 110, 6463–6466 (1988).

    Article  CAS  Google Scholar 

  7. Arduengo, A. J. III, Harlow, R. L. & Kline, M. A stable crystalline carbene. J. Am. Chem. Soc. 113, 361–363 (1991).

    Article  CAS  Google Scholar 

  8. Bourissou, D., Guerret, O., Gabbäi, F. P. & Bertrand, G. Stable carbenes. Chem. Rev 100, 39–91 (2000).

    Article  CAS  Google Scholar 

  9. Arduengo, A. J. III. Looking for stable carbenes: The difficulty in starting anew. Acc. Chem. Res. 32, 913–921 (1999).

    Article  CAS  Google Scholar 

  10. Regitz, M. Stable carbenes—illusion or reality. Angew. Chem. Int. Edn Engl. 30, 674–676 (1991).

    Article  Google Scholar 

  11. Dagani, R. Preparation of stable divalent species raises issues of electronic structure. Chem. Eng. News 2, 20–22 (1994).

    Article  Google Scholar 

  12. Heinemann, C., Müller, T., Apeloig, A. & Schwartz, H. On the question of stability, conjugation, and “aromaticity” in imidazol-2-ylidenes and their silicon analogs. J. Am. Chem. Soc. 118, 2023–2038 (1996).

    Article  CAS  Google Scholar 

  13. Kirmse, W. (ed.) Carbene Chemistry 159–504 2nd edn (Academic, New York, 1971).

    Google Scholar 

  14. Jones, M & Moss, R. M. (eds) Carbenes Vol. II, 159–362 (Wiley, New York, 1975).

    Google Scholar 

  15. Tomioka, H. Persistent triplet carbenes. Acc. Chem. Res. 30, 315–321 (1997).

    Article  CAS  Google Scholar 

  16. Tomioka, H. Persistent triplet carbenes. Adv. Carbene Chem. 2, 175–214 (1998).

    Article  CAS  Google Scholar 

  17. Hirai, K. & Tomioka, H. A triplet carbene that can almost be bottled. J. Am. Chem. Soc. 121, 10213–10214 (1999).

    Article  CAS  Google Scholar 

  18. Murray, R. W., Trozzolo, A. M., Wasserman, E. & Yager, W. A. E.p.r. of diphenylmethylene, a ground-state triplet. J. Am. Chem. Soc. 84, 3213–3214 (1962).

    Article  CAS  Google Scholar 

  19. Itoh, K. Electron spin resonance of an aromatic hydrocarbon in its quintet ground state. Chem. Phys. Lett. 1, 235–238 (1969).

    Article  ADS  Google Scholar 

  20. Bandon, R. W., Closs, G. & Hutchison, C. A. Jr. Paramagnetic resonance in oriented ground-state triplet molecules. J. Chem. Phys. 37, 1878–1879 (1962).

    Article  ADS  Google Scholar 

  21. Sander, W., Bucher, G. & Wierlacher, S. Carbenes in matrices—spectroscopy, structure, and reactivity. Chem. Rev. 93, 1583–1621 (1993).

    Article  CAS  Google Scholar 

  22. Wasserman, E. et al. Electron paramagnetic resonance of 9,9′-dianthrylmethylene. A linear aromatic ground-state triplet methylene. J. Am. Chem. Soc. 93, 6335–6337 (1971).

    Article  CAS  Google Scholar 

  23. Astles, D. S. et al. The unusual reactivity of 9,9′-dianthrylcarbene. J. Org. Chem. 53, 6053–6057 (1988).

    Article  CAS  Google Scholar 

  24. Takahashi, Y. et al. Triplet di(9-anthryl)carbene undergoes trimerization. Angew. Chem. Int. Edn Engl. 39, 3478–3480 (2000).

    Article  CAS  Google Scholar 

  25. Tomioka, H. Strain and structure of sterically congested triplet carbenes. Adv. Strained Interesting Organic Mol. 8, 83–112 (2000).

    Article  CAS  Google Scholar 

  26. Gibbons, W. A. & Trozzolo, A. M. Spectroscopy and photolysis of a ground-state triplet molecule, diphenylmethylene. J. Am. Chem. Soc. 88, 172–173 (1966).

    Article  CAS  Google Scholar 

  27. Moritani, I., Murahashi, S.-I., Nishino, M., Kimura, K. & Tsubomura, H. Electronic spectra of the products formed by the photolysis of diazo compound at 77 K, possibly identified to carbenes. Tetrahedr. Lett. 4, 373–378 (1966).

    Article  Google Scholar 

  28. Closs, G., Hutchison, C. A. & Kohler, B. E. Optical absorption spectra of substituted methylenes oriented in single crystals. J. Chem. Phys. 44, 413–414 (1966).

    Article  ADS  CAS  Google Scholar 

  29. Iwamura, H. High-spin organic molecules and spin alignment in organic molecular assemblies. Adv. Phys. Org. Chem. 26, 179–253 (1990).

    CAS  Google Scholar 

Download references

Acknowledgements

We thank A. Nicolaides for critical reading of the manuscript. This work was supported by a Grant-in-Aid for Specially Promoted Research from the Ministry of Education, Science, Culture and Sports of Japan, the Nagase Science and Technology Foundation and the Mitsubishi Foundation.

Author information

Authors and Affiliations

  1. Chemistry Department for Materials, Faculty of Engineering, Mie University, Tsu, 514-8507, Mie, Japan

    Hideo Tomioka, Eri Iwamoto, Hidetaka Itakura & Katsuyuki Hirai

Authors
  1. Hideo Tomioka
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Eri Iwamoto
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Hidetaka Itakura
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Katsuyuki Hirai
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Hideo Tomioka.

Supplementary information

Preparation of Di[9-(10-phenyl)anthryl]diazomethane (1b)

To a stirred and cooled solution of 9-phenyl-10-bromoanthracene (5.7 g, 17.1 mmol) in absolute Et2O (100 ml) was slowly added n-BuLi (1.7 M in n-hexane, 8.3 ml, 21.0 mmol) at °C under an atmosphere of Ar, and the mixture was stirred for 2 hours. To the stirred solution was added a solution of methyl formate (8.56 mmol) in absolute Et2O (5 ml) at 0°C and the mixture was gently refluxed overnight. A saturated NH4Cl solution was added carefully and the mixture was extracted with Et2O. The organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. Solid was recrystallized from n-hexane to give di[9-(10-phenyl)anthryl]methanol as a pale yellow crystal in 80 % yield; mp 155.7-158.7°C; 1H NMR (300 MHz, CDCl3) d 2.89 (d, J=2.94 Hz, 1 H), 7.26-7.34 (m, 9 H), 7.44-7.63 (m, 10 H), 7.67-7.72 (m, 4 H), 8.62-8.67 (m, 4 H); 13C NMR (75 Hz, CDCl3) ? 73.0, 124.6, 125.1, 125.9, 127.5, 128.0, 128.4, 130.0, 130.5, 131.2, 134.4, 139.2; HRMS (EI) calcd for C45H30O, 536.2140, found 536.2138 (M+).

To a stirred and cooled solution of the methanol (2.67 g, 4.98 mmol) in anhydrous benzene (30 ml) was introduced a stream of dry HCl at 0°C until TLC check indicated that all the methanol was consumed. Solvent was evaporated to dryness to leave di[9-(10-phenyl)anthryl]methyl chloride as a gummy solid, which was used to the next step without further purification, as the chloride was found to be rather unstable.

To a stirred and heated mixture of AgBF4 (1.08 g, 5.6 mmol) and urethane (7.6 g, 86 mmol) in anhydrous dioxane (5 ml) at 60°C was added a solution of the crude chloride (2.4 g, 4.3 mmol) in anhydrous dioxane (50 ml) under an atmosphere of Ar and the mixture was refluxed overnight. To the cooled mixture were added H2O (50 ml) and CH2Cl2 (20 ml) and filtered. The filtrate was extracted with CH2Cl2 (30 mlx3) and the combined organic layer was washed with H2O, dried over Na2SO4, and evaporated to dryness to leave yellow gummy solid. The mixture was column-chromatographed on silica gel eluted with n-hexane-CHCl3 (1:1) to obtain ethyl N-di[9-(10-phenyl)-anthryl]methylcarbamate as a yellow solid in 12 % yield; mp 166.8-168.2 °C; 1H NMR (300 MHz, CDCl3)?d 1.20 (t, J=7.17 Hz, 3 H), 4.13 (q, J=7.17 Hz, 2 H), 5.83 (d, J=7.17 Hz, 1 H), 7.26-7.32 (m, 9 H), 7.41-7.61 (m, 10 H), 7.68 (d, J=7.72 Hz, 4 H), 8.61 (d, J=8.08 Hz, 4 H) ;13C NMR (75 Hz, CDCl3) ? 15.3, 54.7, 61.3, 124.3, 124.5, 126.2, 127.5, 128.4, 130.0, 130.5, 131.3, 133.2, 139.1, 139.2; HRMS (EI) calcd for C44H33NO2, 607.2511, found 607.2561.

To a solution of N2O4 (9.4 g, 102 mmol) in anhydrous CCl4 (10 ml) at 0°C was added dried NaOAc (16.7 g, 204 mmol). A solution of the carbamate (308.4 mg, 0.51 mmol) in anhydrous CCl4 (10 ml) was slowly added to the stirred and cooled mixture and the mixture was stirred for 2 h at 0°C. The mixture was poured onto a large amount of H2O and extracted with CCl4 (30 mlx3). The organic layer was washed with saturated to dryness to give ethyl N-nitroso-N-di[9-(10-phenyl)anthryl]-methylcarbamate, which was used for the next step without further purification.

The nitroso compound was dissolved in anhydrous THF (30 ml) in a round-bottom flask and the whole system was deaerated by repeated cycles of evacuation and purge with Ar. To the stirred and cooled mixture was added t-BuOK (116.7 mg, 1.0 mmol) at 0°C and the mixture was stirred overnight at room temperature. The mixture was poured into H2O, washed with Et2O (30 mlx3) and the ethereal layer was washed with H2O, dried over Na2SO4 and evaporated to dryness. A brownish solid was purified by repeated chromatography on a gel permeation column (a Shodex GPC H-2001, 20 mmx50 cm) with CHCl3. Di[9-(10-phenyl)anthryl]diazomethane (1b) was obtained as a red solid in 7 % yield; mp 226.3-227.7°C (dec); 1H NMR (300 MHz, CDCl3) d 7.31-8.42 (m, 8 H), 7.47-7.64 (m, 10 H), 7.74 (dd, J=1.38, 7.56 Hz, 4 H), 8.34 (dd, J=1.38, 7.81 Hz, 4 H). 13CNMR (75 MHz, CDCl3) ? 56.7, 125.2, 125.3, 125.4, 126.5, 127.6, 128.2, 128.4, 130.1, 130.8, 131.3, 138.8, 138.9, IR(NaCl) n 2032 cm-1.

Carbene dimmer (4) was isolated from a mixture obtained by the irradiation (l=308 nm) of 1a in a degassed benzene at room temperature; 300MHz 1H-NMR(CDCl3)d 8.85(t, J=10.84,11.00Hz, 8H),7.53-7.46(m, 12H), 7.35-7.28(m, 8H), 7.24-7.21(m, 4H), 7.11-7.05(m, 4H), 6.97-6.87(m, 12H), 6.39-6.34(m, 4H); 75.5MHz 13C-NMR(CDCl3): d 146.1, 139.5, 139.4, 137.5, 132.5, 131.0, 130.9, 130.5, 129.9, 129.4, 128.6,128.2, 128.0, 127.6, 127.1, 126.8, 126.3, 125.2, 124.0, 123.6, 123.5; UV/vis (CH2Cl2) lmax = 427nm (e = 15,600); HRMS(MALDI) calcd for C82H52, 1036.4069, found 1036.4083

EPR Measurements. The diazo compound was dissolved in 2-methyltetrahydrofuran (2-MTHF) (5X10-4 M), and the solution was degassed in a quartz cell by four freeze-degas-thaw cycles. The sample was cooled in a optical transmission EPR cavity at 77 K and irradiated with a Wacom 500 W Xe lamp using a Pyrex filter. EPR spectra were measured on a JEOL JES TE200 spectrometer (X-band microwave unit, 100 kHz field modulation). The signal positions were read by the use of a gaussmeter.

Low-Temperature UV/Vis Spectra. Low-temperature spectra at 77 K were obtained by using an Oxford variable-temperature liquid-nitrogen cryostat (DN 2704) equipped with a quartz outer window and a sapphire inner window. The sample was dissolved in dry 2-MTHF, placed in a long-necked quartz cuvette of 1-mm path length, and degassed by four freeze-degas-thaw cycles at pressure near 10-5 Torr. The cuvette was flame-sealed, under reduced pressure, placed in the cryostat, and cooled to 77 K. The sample was irradiated for several minutes in the spectrometer with a Halos 500-W high-pressure mercury lamp using a Pyrex filter, and the spectral changes were recorded at appropriate time intervals. The spectral changes upon thawing were also monitored by carefully controlling the matrix temperature with an Oxford Instrument Intelligent Temperature Controller (ITC 4).

TSP-601 flash spectrometer. The excitation source for the laser flash photolysis was a XeCl excimer laser. A Hamamatsu 150 W xenon short are lamp (L2195) was used as the probe source, and the monitoring beam guided using an optical fiber scope was arranged in an orientation perpendicular to the excitation source. The probe beam was monitored with a Hamamatsu R2949 photomultiplier tube through a Hamamatsu S3701-512Q linear image sensor (512 photodiodes used). Timing of the laser excitation pulse, the probe beam, and the detection system was achieved through an Iwatsu Model DS-8631 digital synchroscope, which was interfaced to a NEC 9801 RX2 computer. This allowed for rapid processing and storage of the data and provided printed graphics capabilities. Each trace was also displayed on a NEC CRT N5913U monitor.

A sample was placed in a long-necked Pyrex tube which had a side arm connected to a quartz fluorescence cuvette and degassed using a minimum of four freeze-degas-thaw cycles at pressure near 10-5 Torr immediately prior to being flashed. The sample system was sealed, and the solution was transferred to the quartz cuvette, which was placed in the sample chamber of the flash spectrometer. The concentration of the sample was adjusted so that it absorbed a significant portion of the laser light.

Figure S1

(GIF 12.5 KB)

ESR spectrum from photolysis of 1b in 2-methyltetrahydrofuran (2-MTHF) matrix. (a) ESR spectrum obtained by irradiation of 1b at 77 K before annealing. (b) same sample after annealing to 110 K. Same sample after annealing to (c) 240 K, (d) 270 K and (e) 300 K, respectively, and refreezing to 110 K.

Figure S2

(GIF 21.6 KB)

UV/vis spectrum from photolysis of 1b in 2-MTHF. (A) spectrum before and after irradiation of 1b at 77 K and after thawing to 110 K. (B) Same sample measured upon annealing . The temperatures are as indicated in the legends.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Tomioka, H., Iwamoto, E., Itakura, H. et al. Generation and characterization of a fairly stable triplet carbene. Nature 412, 626–628 (2001). https://doi.org/10.1038/35088038

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI: https://doi.org/10.1038/35088038

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing