Multicenter Bonding in Organic Chemistry. Geometry-Sensitive 3c-2e Bonding in (C‚‚‚H‚‚‚C) Fragments of Organic Cations Robert Ponec* and Gleb Yuzhakov Institute of Chemical Process Fundamentals, Czech Academy of Sciences, Prague 6, Suchdol, 165 02 Czech Republic
Dean J. Tantillo Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616
[email protected] Received October 13, 2003
This paper reports the application of a new general tool for the study of multicenter bonding, namely the so-called generalized population analysis, to the investigation of interesting geometry dependent variation of 3c-2e bonding in the (C‚‚‚H‚‚‚C) fragments of ingeniously designed organic cations I and II. This phenomenon was previously characterized by the correlation between experimental 1 H NMR chemical shifts of the central hydrogen in the (C‚‚‚H‚‚‚C) fragment and the changes in the corresponding C-H-C bond angle. The observed values of both chemical shifts and C-H-C angles are shown herein to correlate with the calculated 3-center bond indices but the dependence displays splitting into two separate lines according to the type of corresponding cation. Introduction There is probably no other concept that contributed to the development of modern chemistry more remarkably than the classical Lewis tenet of chemical bonds formed by shared electron pairs.1 Although this concept, which originally considered only the sharing between two atoms, was remarkably successful in the rationalization of the structures of a wealth of both organic and inorganic molecules, there are also a number of examples where this classical model fails. This is due to the fact that the spectrum of possible bonding situations is apparently much more rich, and in addition to the traditional model of two-center two-electron (2c-2e) bonding, one encounters also more complex nonclassical situations where electron pairs are shared between more than two atoms. This phenomenon is generally known as multicenter bonding, and its usefulness was clearly demonstrated in the seminal studies by Lipscomb,2-4 who used the idea of the so-called three-center two-electron (3c-2e) bond to rationalize the structures of electron-deficient boranes. Although the existence of 3c-2e bonding has since that time been discovered also in many othersespecially inorganics systems, like metallic clusters, lithiated hydrocarbons, or bridged metal halides,5-7 examples of this bonding in organic chemistry are much more scarce and the substances that contain these bonds are relatively un* Address correspondence to this author. (1) Lewis, G. N. J. Am. Chem. Soc. 1916, 38, 762. (2) Lipscomb, W. N. J. Chem. Phys. 1954, 22, 985. (3) Lipscomb, W. N. Science 1997, 196, 1047. (4) Lipscomb, W. N. Acc. Chem. Res. 1973, 8, 257. (5) Ponec, R.; Roithova, J.; Sannigrahi, A. B.; Lain, L.; Torre, A.; Bochicchio, R. Theochem 2000, 505, 283. (6) Sannigrahi, A. B.; Kar, T. K. Theochem 2000, 496, 1. (7) Ponec, R.; Cooper, D. L. Int. J. Quantum Chem. 2004, 97, 1002.
stable.8,9 Probably the best known examples of 3c-2e bonding in organic molecules can be found in the nonclassical carbonium ions participating during the ionization of 2-norbornyl derivatives and closely related species.10 In addition to these well-known systems with cyclic (C‚‚‚H‚‚‚C) arrays, ingeniously designed organic cations with acyclic (C‚‚‚H‚‚‚C) fragments were reported some time ago by Sorensen (I)11,12 and McMurry (II).13,14
These molecules are interesting not only because of their relatively high stability, but also because the character of the 3c-2e bonding in these systems appeared to be extremely sensitive to variations of the geometry of the structures supporting the (C‚‚‚H‚‚‚C) fragment.15 (8) Olah, G.; Surya Prakash, G. K.; Williams, R. E.; Field, L. D.; Wade, K. Hydrocarbon Chemistry; Wiley-Interscience: New York, 1987. (9) DeKock, R.; Bosma, W. B. J. Chem. Educ. 1983, 65, 194. (10) Brown, H. C. Acc. Chem Res. 1983, 16, 432. (11) Kirchen, R. P.; Sorensen, T. S. J. Chem. Soc., Chem. Commun. 1978, 769. (12) Sorensen, T. S.; Whitworth, S. M. J. Am. Chem. Soc. 1990, 112, 8135. (13) McMurry, J. E.; Hodge, C. N. J. Am. Chem. Soc. 1984, 106, 6450. (14) McMurry, J. E.; Lectka, T.; Hodge, C. N. J. Am. Chem. Soc. 1989, 111, 8867. (15) McMurry, J. E.; Lectka, T. Acc. Chem. Res. 1992, 25, 47. 10.1021/jo035506p CCC: $27.50 © 2004 American Chemical Society
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J. Org. Chem. 2004, 69, 2992-2996
Published on Web 03/20/2004
Multicenter Bonding in Organic Chemistry
Our aim in this study is to characterize the nature of the 3c-2e bonding in systems I and II. Our goal is to correlate the observed variations in 1H NMR chemical shifts and C-H-C angles with the changes in computed 3-center bond indices. For this purpose we use the recently proposed formalism of the so-called generalized population analysis,16 which was designed specifically as a tool to detect multicenter bonding in molecules and to determine the extent of its delocalization.
ity of 3c-2e bonding in the (C‚‚‚H‚‚‚C) fragment of organic cations I and II to variations of the geometric arrangement in this fragment.14 The systematic change of the geometry of the (C‚‚‚H‚‚‚C) fragment in both types of species was ensured by the variation of the number of bridging CH2 groups in the cations I and II. Consistent with the notation of the previous study15 we specify the structures of considered species as I (n ) x) for x varying between 5 and 8 and II [4.4.4], II [5.4.4], II [6.3.3], and II [6.4.2].
Theoretical The term generalized population is a generic name that has been coined for the whole family of approaches attributing certain physical or chemical meaning to mono-, di-, tri-, and generally k-atomic contributions resulting from the partitioning of the identity (1) for a given value of k.
1 2
Tr(PS)k ) N )
k-1
(k) (k) ∆(k) ∑ ∑ ∆AB...K A + ∑ ∆AB + ... A A
