On the Question of Stability, Conjugation, and “Aromaticity” in Imidazol-2-ylidenes and Their Silicon Analogs † Christoph Heinemann, ‡ Thomas Mu 1 ller, § Yitzhak Apeloig,* ,§ and Helmut Schwarz* ,‡ Contribution from the Institut fu ¨ r Organische Chemie der Technischen UniVersita ¨ t Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany, and Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel ReceiVed July 14, 1995 X Abstract: Thermodynamic, structural, and magnetic criteria, the properties of the charge distributions, and low- energy ionization processes are theoretically analyzed to learn about the role of π-electron delocalization in recently synthesized stable singlet carbenes (Arduengo et al. J. Am. Chem. Soc. 1991, 113, 361) and silylenes (Denk et al. J. Am. Chem. Soc. 1994, 116, 2691) of the imidazol-2-ylidene type and also in related model systems. The different approaches show consistently that cyclic electron delocalization does indeed occur in the CdC unsaturated imidazol- 2-ylidene systems, in particular with respect to the corresponding C-C saturated imidazolin-2-ylidenes. However, the conclusion regarding the degree of conjugation and aromaticity depends on the criteria used, being quite small according to the “atoms-in-molecules” charge analysis but more significant according to the energetic and the magnetic properties. According to all criteria, the aromatic character of imidazol-2-ylidenes is less pronounced compared to benzene or the imidazolium cation. π-Electron resonance is found to be less extensive in the silylenes compared to their carbene analogs. Introduction For a long time, species involving divalent carbon and silicon atoms were considered to be “elusive” in the sense that they could only be directly observed by spectroscopic techniques, either in the gas-phase 1 or in low-temperature matrices, 2 but could not be isolated in macroscopic amounts at room temper- ature. However, the recent syntheses of stable crystalline singlet carbenes (imidazol-2-ylidenes 1 with R ) adamantyl, aryl or tert-butyl; 3 imidazolin-2-ylidenes 2 with R ) aryl 4 ) and silylenes (3, 4 with R ) tert-butyl 5,6 ) have changed our views of these species. These discoveries have stimulated research on the electronic structure of these novel divalent species and on the reasons for their unusual stability. 7-15 The present study addresses the question whether the CdC- unsaturated Arduengo-type carbenes 1 (Scheme 1) and their silicon analogs 3 benefit from “aromaticity”, i.e. from cyclic 6π-electron delocalization. A critical role of the CdC double bond in stabilizing 1 (and 3) might have been suspected intuitively from the fact that in the carbon case the corresponding imidazolin-2-ylidenes 2, in which the olefinic backbone of the five-membered ring is transformed into a saturated hydrocarbon moiety, had not been isolated until mid 1995 4 despite numerous attempts. During the 1960s, Wanzlick and co-workers showed that compounds of type 2 could be generated in solution in a very similar manner to 1, 16 but in the absence of scavenger reagents they could only observe carbene dimerization products. The carbene itself was never isolated. However, very recently, actually in a paper which came to our attention when this work † Dedicated to Prof. Chava Lifshitz, Jerusalem, on the occasion of her 60th birthday. ‡ Berlin. § Haifa. X Abstract published in AdVance ACS Abstracts, February 1, 1996. (1) For example: (a) Leopold, D. G.; Murray, K. K.; Lineberger, W. C. J. Chem. Phys. 1984, 81, 1048. (b) Srinivas, R.; Bohme, D. K.; Schwarz, H. J. Phys. Chem. 1993, 97, 13643. (c) McGibbon, G. A.; Kingsmill, C. A.; Terlouw, J. K. Chem. Phys. Lett. 1994, 222, 129. (d) Burgers, P. C.; McGibbon, G. A.; Terlouw, J. K. Chem. Phys. Lett. 1994, 224, 539. (e) McGibbon, G. A.; Burgers, P. C.; Terlouw, J. K. Int. J. Mass Spectrom. Ion Processes 1994, 136, 191. (2) For example: (a) Drahnak, T.; Michl, J.; West, R. J. Am. Chem. Soc. 1979, 101, 5527. (b) Maier, G.; Glatthaar, J.; Reisenauer, H. P. Chem. Ber. 1989, 122, 2403. (c) Gillette, G. R.; Noren, G.; West, R. Organo- metallics 1990, 9, 2925. (d) Veith, M.; Werle, E.; Lisowski, R.; Lo ¨ppe, R.; Schno ¨ckel, H. Chem. Ber. 1992, 125, 1375. (3) (a) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (b) Arduengo, A. J., III; Rasika Dias, H. V.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1992, 114, 5530. (c) For a brief review, see: Regitz, M. Angew. Chem. 1991, 103, 691; Angew. Chem., Int. Ed. Engl. 1991, 30, 674. (4) Arduengo, A. J., III; Goerlich, J. R.; Marshall, W. J. J. Am. Chem. Soc. 1995, 117, 11027. (5) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Belyakov, A. V.; Verne, H. P.; Haaland, A.; Wagner M.: Metzler, N. J. Am. Chem. Soc. 1994, 116, 2691. (6) Denk, M.; Green, J. C.; Metzler, N.; Wagner, M. J. Chem. Soc., Dalton Trans. 1994, 2405. (7) Arduengo, A. J., III; Rasika Dias, H. V.; Dixon, D. A.; Harlow, R. L.; Klooster, W. T.; Koetzle, T. F. J. Am. Chem. Soc. 1994, 116, 6812. (8) Arduengo, A. J., III; Dixon, D. A.; Harlow, Kumashiro, K. K.; Lee, C.; Power, W. P.; Zilm, K. W. J. Am. Chem. Soc. 1994, 116, 6361. (9) Arduengo, A. J., III; Bock, H.; Chen, H.; Denk, M.; Dixon, D. A.; Green, J. C.; Herrmann, W. A.; Jones, N. L.; Wagner, M.; West, R. J. Am. Chem. Soc. 1994, 116, 6641. (10) Dixon, D. A.; Arduengo, A. J., III J. Phys. Chem. 1991, 95, 4180. (11) Cioslowski, J. Int. J. Quantum Chem: Quantum Chem. Symp. 1993, 27, 309. (12) Heinemann, C.; Thiel, W. Chem. Phys. Lett. 1994, 217, 11. (13) Apeloig, Y.; Karni, M.; Mu ¨ller, T. In Organosilicon Chemistry II; VCH: Weinheim, in press. (14) Nyula ´szi, L.; Ka ´rpa ´ti, T.; Veszpre ´mi, T. J. Am. Chem. Soc. 1994, 116, 7239. (15) Heinemann, C.; Herrmann, W. A.; Thiel, W. J. Organomet. Chem. 1994, 475, 73. (16) (a) Wanzlick, H.-W.; Schikora, E. Angew. Chem. 1960, 72, 494. (b) Schikora, E. Ph.D. Thesis, Technical University Berlin, D83, 1961. (c) Wanzlick, H.-W. Angew. Chem. 1962, 74, 129. (d) Lemal, D. M.; Kawano, K. I. J. Am. Chem. Soc. 1962, 84, 1761. (e) Lemal, D. M.; Lovald, R. A.; Kawano, K. I. J. Am. Chem. Soc. 1964, 86, 2518. (f) Scho ¨nherr, H.-J. Ph.D. Thesis, Technical University Berlin, D83, 1970. Besides studies on the C-C-saturated imidazolin-2-ylidenes 2, this work also reports efforts to synthesize the C-C-unsaturated imidazol-2-ylidenes 1. However, upon deprotonation of 1,3-disubstituted imidazolium salts, which is basically the procedure used in refs 3a and 4, no carbenes could be isolated: (g) Scho ¨nherr, H.-J.; Wanzlick, H.-W. Chem. Ber. 1970, 103, 1037. 2023 J. Am. Chem. Soc. 1996, 118, 2023-2038 0002-7863/96/1518-2023$12.00/0 © 1996 American Chemical Society