Abstract:
During the last two decades, various examples of stable species, which cannot be represented by standard Lewis/ or Kekule structures, were prepared and studied experimentally. Different kinds of stable biradicals 1,2,3 are known. The first stable carbon-based Schlenk's diradical was prepared in 19154. The most stable singlet 1,3-diradical 2,2-dimetoxy substituted cyclopentane derivative known to date has a lifetime of microseconds at room temperature 2.
Molecular design of the stable triplet carbens 5 successfully completed by the "bottleable" diantracyl substituted triplet carbene with a lifetime more then week.
Formal biradical 2,5-diamino-1,4-benzoquinonediimine is a very stable crystalline compound.6 Azaacene (5,7-diphenyl-5H,12H-quinoxalino[2,3-b]phenazine) also possesses a singlet ground state. This is a result of the delicate tuning between covalent (biradical) and polar (zwitterions) contributions to the resulting electronic structure could.
Some of biradical Non-Kekule molecules show a pronounced zwitterionic structure, which was explained by model indicated that electron donor and acceptor substituents placed at opposite sides of a benzene ring (1,2,4,5-substitution) can result in electron transfer favouring a zwitterionic ground state.7
Singlet carbenes11were stabilized by electron donation from 1,3-substituents into the vacant p-AO of the central C atom. The first stable crystalline diaminocarbene was reported by Arduengo et al. in 1991.9 Numerous stable singlet carbenes were prepared during the last two decades.1,10
A substantial stabilization of singlet ground state and a corresponding increase of the singlet-triplet gap for non-Kekule molecules can be attained for molecules isoelectronic with the dianions of antiaromatic hydrocarbons. 2,5-diheterosubstituted-pentalenes and 1,5-di-heterosubstituted-cyclooctatetraenes are predicted to be stable persistent non-Kekule molecules11,12 due to strong stabilization by intra-molecular charge transfer.
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3 D. Scheschkewitz, H. Amii, H. Gornitzka, W. W. Schoeller, D. Bourissou, G. Bertrand, Science, 295, 1880 (2002)
4 W. Schlenk, M. Brauns, Ber. Dtsch. Chem. Ges. 48, 66, (1915)
5 E. Iwamoto, K. Hirai and H. Tomioka, J. Am. Chem. Soc., 125, 14664 (2003)
6 P.Braunstein, O.Siri, J.-P.Taquet, M.-M.Rohmer, M.Benard, R.J.Welter, R. J. Am. Chem. Soc., 125, 12246, (2003)
7 Y. Haas and S. Zilberg, J. Am. Chem. Soc., 126, 8991, (2004)
8 A. J. Arduengo, Acc. Chem. Res. 32, 913 (1999)
9 A. J. Arduengo, R. L. Harlow, M. Kline, J.Am.Chem.Soc., 113, 361, (1991)
10 W. Kirmse, Angew. Chem. Int. Ed., 43, 1767 (2004)
11 S. Zilberg and Y. Haas, J. Phys. Chem., A110 , 8397 (2006)
12 S. Zilberg, E. Tsivion, Y. Haas, J. Phys. Chem., A112, 12799 (2008)
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