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Potentially Aromatic Metallocycles Kim K Iowa State University From the SelectedWorks of Mark S. Gordon June, 1988 Potentially Aromatic Metallocycles Kim K. Baldridge Mark S. Gordon Available at: https://works.bepress.com/mark_gordon/69/ 4204 J. Am. Chern. Soc. 1988, 110, 4204-4208 Potentially Aromatic Metallocycles Kim K. Baldridge and MarkS. Gordon* Contribution from the Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105. Received November 12, 1987 Abstract: Ab initio molecular orbital theory is used to characterize a series of metal-substituted benzene and cyclopentadiene structures, with the heteroatom taken from the block in the periodic table bounded by groups IV-VI and periods 2-5. Structures are predicted with the 3-21G* basis set and SCF wave functions. The calculated bond lengths and bond angles are in general within 0.04 A and 2°, respectively, of the available experimental values. As a measure of the de localization stabilization, !::.£and t::..H0 values for the appropriate bond separation and superhomodesmic reactions are calculated with 3-21 G* Hartree-Fock energies for these compounds and some smaller acyclic structures. I. Introduction Scheme I. Bond Separation Reactions a Group IV Considerable efforts have gone into the study of five- and X six-membered ring compounds. 1- 38 The theoretical and exper- 0 + SCH4 + XH4 (I) Baldridge, K. K.; Boatz, J. A.; Sakai, S.; Gordon, M.S. Annu. Rec. 2H 3C-CH3 + H3X-CH3 + 2H2C=CH2 + H2X=CH2 Phys. Chern .. in press. b Group V (2) Baldridge, K. K.; Gordon, M.S. Organometal/ics 1988, 7, 144-154. (3) Dewar, J. S.; Lo, D. H.; Ramsden, C. A. J. Am. Chern. Soc. 1975, 97, 1311. (4) Schlegel, H. B.; Coleman, B.; Jones, M. J. Am. Chern. Soc. 1978, 100, 6499. (5) Blustin, P. H. J. Organomet. Chern. 1979, 21. (6) Gordon, M.S.; Boudjouk, P.; Anwari, F. J. Am. Chern. Soc. 1983, 105, 4972. .. (7) Chandrasekhar, J.; Schleyer, P. v. R.; Baumgartner, R. 0. W.; Reetz. X M. T. J. Org. Chern. 1983, 38, 3453. (8) Baldridge, K. K.; Gordon, M.S. J. Organomet. Chern. 1984. 271, 369. g + XH2 + 4CH4 -- 2C2H4 + C2Hs + 2CH3XH (9) Chandrasekhar, J.; Schleyer, P. v. R. J. Organomet. Chern. 1985, 289, 51. (10) George, P.; Bock, C. W.; Trachtman, M. Tetrahedron Lell. 1985, 26, Scheme II. Homodesmic Reaction 5667. (II) Nagase, S.; Kudo, T.; Aoki, M. J. Chern. Soc., Chern. Commun. 1985, x-::::x'x I II + 3XH2=XH2 - 3XH2=XH-XH=XH2 1121. X.;:::x,....x (12) Sax, A.; Janoschek, R. Angew. Chern., Int. Ed. Engl. 1986, 98,634. (13) Damewood, J. R., Jr. J. Org. Chern. 1986, 51, 5028. Scheme III. Superhomodesmic Reactions (14) Sakurai, K.; Kitaura, K.; Nishimoto, K. Theor. Chim. Acta 1986,69, 23. a Group IV (15) George, P.; Bock, C. W.; Trachtman, M. Tetrahedron Lett. 1985, 26, X 5667. + 2XH2=CH-CH=CH2 + CH =CH-CH=CH (16) Garratt, P. J. Aromaticity; McGraw-Hill: London, 1971. 0 2 2 (17) Binsch, G. Naturwissenschaften 1973, 60, 369. (18) Maier, L.; Seyferth, D.; Stone, F. G. A.; Rochow, E. G. J. Am. Chern. 2XH2=CH-CH=CH-CH=CH2 + CH2=CH-XH=CH-CH=CH2 Soc. 1957, 79, 5884. b Group V (19) Sorensen, G. 0.; Mahler, L.; Rastrup-Andersen, J. J. Mol. Struct. 1974, 20, 119. I (20) Lohr, L. L.; Schlegel, H. B.; Morokuma, K. J. Phys. Chern. 1984, 88, 2 2 } - 2 1981. '\\'(:/{' + { 2XH -~H=CH f2XH 2 -CH=~H-CH=CH l (21) Clark, D. T.; Scanlan, I. W. J. Chern. Soc., Faraday Trans. 2 1974, CH2=CH-CH=CH2 CH2=CH-XH-CH=CH2j 70, 1222. R l (22) Palmer, M. H.; Findlay, R. H.; Moyes, W.; Gaskel, A. J. J. Chern. c Group VI Soc., Perkins Trans. 2 1975, 841. (23) Von Niessen, W.; Diercksen, G. H. F.; Cederbaum, L. S. Chern. Phys. 2XH-CH=CH2 } f2HX-CH=CH-CH=CH 1975, 10, 345. ~'(:/( + { 2l (24) Hodges, R. V.; Beauchamp, J. L.; Ashe, A. J., III; Chan, W. T., in CH2=CH-:-CH=CH2 press. M CH2=CH~CH=CH2 -l j (25) Ashe, A. J., III Ace. Chern. Res. 1978, II, 153. (26) Campos-Vallette, M. M.; Calvijo, C. Spectrosc. Lett. 1985, 78, 759. Scheme IV. Superhomodesmic Reaction (27) Aroca, R.; Robinson, E. H.; Ford, T. A. J. Mol. Struct. t976, 31, 117. (28) (a) Bak, B.; Christensen, D.; Dixon, W. B.; Hansen-Nygaat, C.; Rastrup-Andersen, J.; Schatt-Lander, M. J. Mol. Spectrosc. 1962, 9, 124. (b) Bak, B.; Christensen, D.; Hansen-Nygaard, L.; Rastrup-Andersen, J. J. Mol. Spectrosc. 1961, 7, 58. (29) Ashe, A. J., lll; Sharp, R. R.; Tolan, J. W. J. Am. Chern. Soc. 1976, 98, 5451. (30) Nekrasov, Y. S.; Zagorevskii, D. V.; Sizoi, V. F.; Denisov, F. S. J. Organomet. Chern. 1975, 97, 253. imental work involving benzene, substituted silabenzene, and the (31) Jutzi, P.; Kohl, F.; Kruger, C. Angew, Chern., Int. Ed. Engl. 1979. various isomers of disilabenzene have recently been reviewed.1.2 91, 81. (32) Badger, G. M. Aromatic Character and Aromaticity; Cambridge (33) Ashe, A. 1., III J. Am. Chern. Soc. 1971, 93, 3293. University: New York, 1969. (34) Ashe, A. J., III J. Am. Chern. Soc. 1979, 93, 6690. 0002-7863/88/1510-4204$01.50/0 © 1988 American Chemical Society Potentially Aromatic Metallocycles J. Am. Chern. Soc., Vol. 110, No. 13, 1988 4205 H 1.072 11.0831 Table I c N 0 Si p s 1.385 11.3961 Ge As Se Benzene 120,0"1120~ Sn Sb Te ....... c 11.072 In addition, several theoretical and experimental papers have H appeared in the literature relating to other metal-substituted H 1.073 benzenes. Included in these studies are those that focus on aro­ 1.390 matic character and the various methods by which aromaticity may be defined. 1H 7 In particular, significant attention has been given to the comparison of the properties of pyridine with ben­ Sllabenzene zene.15-19 Initial concepts of aromaticity have been derived from these studies. George et al. 15 calculated empirical resonance energies for benzene and pyridine from experimental !!:..Hr 0 data and from 6-31G(d) 38 self-consistent field (SCF) calculations. These results predict the aromatic stabilization of pyridine and H 1.073 benzene to be essentially the same. Among the studies performed 1.391 on phosphabenzene,2o-24 Lohr et al. 20 theoretically determined gas-phase proton affinities in a series of phosphorus compounds, including phosphabenzene and three protonated forms of phos­ Germlbenzene phabenzene, using a variety of basis sets. It was inferred from this study, in agreement with experiment, that P-site protonation is favored. 24 More extensive studies have been carried out by several 25 28 25 groups. - Ashe considered the series of compounds MC5H 5, M = CH, N, P, As, Sb. In this experimental study of a family of aromatic compounds in which carbon is bonded to an entire column of elements, the concept of aromaticity was addressed by a comparative study of their NMR spectra, structural data, and chemical behavior. This work will be considered in more detail Stannobenzene below. An investigation by Campos-Vallette and Clivijo26 involved the calculation of the force field and mean amplitudes of vibration and experimental determination of relative reactivities of the furan-like series MC H , M = 0, S, Te, Se. The ground-state 4 4 Figure 1. SCF/3-21 G* geometries for group IV six-membered rings. aromaticity order thiophene > tellurophene > selenophene > furan Angles are in degrees, and bond lengths are in angstroms. Experimental was determined by considering force fields and mean amplitudes values are given in parentheses. of vibration of the compounds. In the same work, the reactivity toward electrophilic substitution was determined to be in the order et al. 11 are the homodesmic42 (Scheme II) and the superhomo­ furan > selenophene > tellurophene > thiophene. desmic43 (Scheme IV) reactions. Note that these latter two types Various methods have been employed to determined aroma­ of reactions conserve successively larger fragments of the parent 46 ticity. One method used · to provide a quantitative measure of ring compounds. In particular, superhomodesmic reactions directly the delocalization stabilization in potentially aromatic compounds measure the effect of "cyclizing~ an already delocalized structure. is the calculation of the energy difference ~E for the appropriate Comparisons of the t::.E for these two types of reactions were 39 bond separation reaction. This type of reaction measures the performed for benzene, silabenzene, and hexasilabenzene at both stability of a parent molecule relative to isolated prototypical bonds. the SCF/3-21G and SCF/6-31 G(d) levels for both systems. 11 The Examples of these bond separation reactions are given in Scheme results for the homodesmic reactions differed from those of the 2 I. The aromaticities of benzene, silabenzene, and the three superhomodesmic reactions by less than 2 kcaljmol. The nu­ isomers of disilabenzene8 using such isodesmic reactions have merical results indicated that hexasilabenzene has approximately previously been considered in this laboratory with 3-21 G40 energies half the aromatic stabilization energy of benzene. 11 More general 41 at ST0-2G geometries (hereafter denoted 3-21G/ /ST0-2G). examples of superhomodesmic reactions are given in Scheme III. These reactions measure the stability of the parent molecule The present study considers the potential aromaticity in various relative to its simplest isolated prototypes (Scheme 1), such that ring structures containing the block of elements from the periodic each bond type is formally conserved. Such a reaction minimizes chart in Table I. Only the planar isomers of the six-membered the importance of correlation error in the calculation. ring structures have been considered. Group IV elements (Si, Ge, Two types of isodesmic reactions that have been used by Nagase Sn) all form analogues that are isovalent with benzene.
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