The Journal of
(0
PhysicaI Chemistry
Copyright, 1992, by the American Chemical Society
VOLUME 96, NUMBER 15 JULY 23,1992
LETTERS Patterns for Additlon to Caot David A. Mxon,* Nobuyuki Matsuzawa,* Tadamichi Fukunaga, and Frederick N. Tebbe The Central Research and Development Department, Experimental Station, E. I. Du Pont de Nemours & Company, Inc., P. 0. Box 80328, Wilmington, Delaware 19880-0328 (Received: February 27, 1992; In Final Form: May 28, 1992)
The geometries of 1,7- and 1,9-C&, X = H, F, C1, Br, I, were optimized at the MNDO/AMl level. The energies show that the 1,9-structureis preferred for X = H, F and the 1,’l-structureis preferred for X = C1, Br. For X = I, no preference is predicted. In order to provide a better model for the energetics, the energy of each optimized structure was calculated at the local density functional level with a double numerical polarized basis set. The energies of the l,7-structure relative to the 1,9-structure are +6.8, +6.6, +2.5, -0.5, and -9.0 kcal/mol for X = H, F, C1, Br, and I, rcspectively. Addition patterns to Cso are analyzed in terms of these results.
The synthesis of Ca and other fullerenes has opened up a new area for the study of addition chemistry in carbon systems.*-7 Partly because of the large number of reaction sites, the chemistry of fullerenes can be quite complex. We believe that results from computational studies should prove to be useful for understanding factors that control the reghchemistry of addition to the exterior of the carbon framework. Semiempirical calculations on the addition of two hydrogen atoms to Ca showed that there were two addition sites of low energy.8 These correspond to direct addition to a C==C double bond (‘1,Zaddition” to a cyclohexatriene) to form the 19-isomer or to conjugate “1,4-addition” to a cyclohexatriene segment to form the 1,7-isomer, as shown in Figure 1. The 1,9-isomer is 4 kcal/mol more stable than the 1,7-isomer for C&, at the MNDO/AMl level? The 1$isomer, of course, does not change the bonding in the Ca framework other than to remove a r bond. However it does introduce eclipsing interactionsbetween groups at the two s$ carbons which are worth Contribution No.6166. *Presentaddma: SONY Corporation Rcsearcb Center, 174 Fujitsuka-cho, Hodogaya-ku, Yokohama 240, Japan.
0022-3654/92/2096-6107$03.00/0
about 3-5 kcal/mol, based on the barrier to rotation in C,H, and similar compounds? The 1,7-isomer introduces a double bond into a five-membered ring which does costs energy, 7-9 kcal/mol, a value similar to the average of 8.5 kcal/mol* that was calculated for the introduction of a double bond into a five-membered ring of Cso. For the case of the 1,7-isomer, no eclipsing interactions are present. Any other conjugate addition processes, such as ’1,6-addition” to a hexatriene segment, necessarily place more double bonds in five-membered rings, leading to further destabilization There will clearly be a balance between the electronic effect of introducing a double bond into a five-membered ring in the formation of the 1.7-isomer as opposed to the steric effect of eclipsed groups in the 19-isomer. This, of course, will be the case if one is adding any two doublet radicals or equivalently a compound with two moieties connected by a single bond.“ For the formation of metal complexes3 such as Ca[F’t(PR)3)2], (R = phenyl, ethyl; n = 1-6) and Cm[Os04(t-BuC6HSN),],the “1,4addition” cannot operate and only ‘1,Zaddition” is possible. We have performed a set of theoretical calculations on the 1,9and 1,7-addition to Cboof H2 and the halogens, X2, X = F, C1, 0 1992 American Chemical Society
6108 The Journal of Physical Chemistry, Vol. 96, No. 15, 1992
Letters TABLE Ik Relative Energies for 1 , 7 - C a 2 As Compared to 1.9-C,& in kilocalories Per mole relative relative energies energies X AM1 LDF X AMI LDF H +4.4 +6.8 Br -1.2 -0.5 F +4.2 +6.6 I +0.2 -9.0 CI -0.9 +2.5 ~~
TABLE III: Geometry Parametersa for 1 , 9 - C G 2 and l,2-C2H,X2
J
"1,2-addition"
M
\*
@ @ \
\
w
w
/
1,g-isomer
w
"1,4-addition"
1,7-isomer
Figure 1. Numbering system of the carbons of C,, and the 1,9- and 1,7-isomers of C 6 a 2 . Only a part of the Cm framework is shown. TABLE I: Rotation Barriers (Trans-Eclipsed) in 1,Z-Disubstituted Ethanes in kilocalories Per mole rotational rotational barrier barrier X AMI LDF X AM1 LDF H 1.2 2.8 Br 2.8 9.6 F 2.6 6.6 I I .7 9.7 CI 4.0 8.7
Br, I, in order to gain a better understanding of factors controlling addition patterns. The initial calculations were done at the semiempiricallevel with the AM1 parametrization of the MNDO Hamiltonian.Io We used this parametrization scheme instead of the PM-3 parametrization" because there is too much attraction between nonbonded halogens at the PM-3 level. This leads to rotation barriers in 1,2-disubstituted ethanes that are too 10w.l~ Geometries of the molecules were fully optimized at this level. We used the program system MOPAC version 5.0"J4for these calculations on a Cray-YMP computer. Barriers to rotation in the 1,Zdisubstituted ethanes are still low even with the AMI parametrization (Table We thus performed local density functional (LDF) calculationsl5 at the final AM1 geometries.'6 The LDF method is an ab initio method that includes electron correlation but only scales as N ,where N is the number of basis functions. The LDF calculations were done with the program DMoll' on a Cray-YMP computer with a double numerical basis set augmented by polarization functions. For C,, this basis set includes 840 basis functions. No symmetry was used in the calculations. The exchange-correlation potential of von Barth and Hedin was used.'* The relative energies of the 1,7- and 1,9-isomersof C G Zare shown in Table 11. At the AMI level, it is clear that addition of F2 follows the result of hydrogenation, favoring "1,2-addition" to form the 1,9-isomer. For C1, addition, the 1,7-isomer ("1,4addition") is favored over the 1,g-isomer by about 1 kcal/mol. A similar result is predicted for the addition of Brz with formation of the 1,'l-isomer being favored by an additional 0.3 kcal/mol as compared to the Clz result. For Iz addition, the two isomers are predicted to be nearly isoenergetic. The LDF results generally follow the same qualitative trends, but the quantitative results are somewhat different. The energy difference between the 1,9- and 1,7-isomers is increased for H2 addition by 2.4 kcal/mol from the AM1 result to 6.8 kcal/mol.
X H F CI Br
I
e(C2H4X2) (AMI) 111.1 112.2 116.4 117.2 116.0
e(C2H4X2) e(c6d(2) (LDF) (AMI) 111.8 107.4 110.6 106.6 115.7 112.7 118.1 113.2 119.7 112.2
d(C2H4X2) 4C6d(2) (AMI) (AMI) 2.3 1 2.59 3.06 3.26 3.31
2.23 2.41 2.94 3.11 3.14
Angles in degrees and distances in angstroms.
Just as found at the AM1 level, the energy difference between the 1,9- and 1,7-isomers decreases slightly for Fz addition, but there is still a strong preference for the 19-isomer. At the LDF level, the 1,9-isomer is still favored for C12addition as compared to the opposite prediction at the AM1 level. For Br, addition, the 1,7-isomer is favored by 0.5 kcal/mol at the LDF level in semiquantitative agreement with the AM1 result. The LDF and AM1 results for I2 addition show the largest difference with the qualitative result being different. The LDF results predict that the 1,7-isomer ("1,4-additionn) is strongly favored over the 1,9isomer. This is in accord with the larger steric bulk of iodine. The energetic results show that as the steric bulk of the group increases, the preference for " 1,t-addition" to form the 1,9-isomer decreases significantly. This is due to steric repulsion between the two eclipsing groups (F-strain), which may be estimated from rotation barriers in 1,2-disubstitutedethanes (see Table I). This trend can be seen most clearly at the LDF level. Examination of geometry parameters, however, reveals an additional important effect. Table I11 compares the XCC bond angles of 1,2-disubstituted ethanes in the eclipsed conformation with the corresponding angles in the 1,9-isomerof C G z . The angles in C G 2 are all smaller than those in the eclipsed ethanes. The C6,, cage framework sterically blocks the bending of the XCC bond and makes such deformation (B-strain)I9energetically more costly than having shorter internuclear distances. This in turn results in greater steric repulsion (F-strain) between the two eclipsing groups. This is probably most pronounced for X = Br and I. Hydrogenation, fluorination, and chlorination of C, have been reported to yield mixtures containing many different species.IJ,Sa,b This is consistent with the magnitude of the calculated energy differences between the "1,2- and 1,4-additionsn,and it is possible that both types of additions occur with the result that mixtures are produced. Furthermore, the additions are exothermic (see below) and kinetically controlled products may persist. Under certain conditions,5caddition of Br2 to C6, leads to a single compound, CwBrz4,of high symmetry ( Th).The structure has only " 1,eaddition" sites" and is devoid of contiguously placed bromine atoms. Our computational results for addition of a single Br, show that the "1,4-additionn is only slightly favored, but the B-strain associated with the "1 &addition" makes the placement of bromines on more than two contiguous carbon atoms on the C,, surface highly improbable. Beyond this constraint, there are no other obvious energetic reasons as to why the subsequent bromination selects the sites to reach the final high symmetry structure.
The Journal of Physical Chemistry, Vol. 96, No. 15, 1992 6109
Letters
TABLE I V Reaction Energies in kilocalories per mole reaction 2
X H F C1
Br I
(LDF) 28.5 32.4 39.8 39.0 38.3
reaction energies reaction 3 reaction 1 (expt) (Hess’s law) -4.1 -32.6 -84.5 -1 16.9 -42.9 -3.1 -28.9 +10.1 -11.5 +26.8
n
9 9 9 7 7
In order to better understand possible equilibration processes, it would be useful to know the energetics of reaction 1. However, AE for reactions such as reaction 1 are difficult to calculate C60 x2 l,n-C& (1) accurately because the bonds that are being made and broken are significantly different; it is not isodesmic. One can calculate the energy of reaction 2, which is isodesmic, from the LDF resultsI2 Cm CH2XCH2X l , n - C d 2 C2H4 (2) C2H4 X2 CH2XCH2X (3) and make the approximation that AE AH. The energy of reaction 3 can be calculated from experimental data?’ and addition of (2) and (3) yields (1). The energetics of the various reactions are given in Table IV. The results in Table IV show that reaction 2 is endothermic for all X and the endothermicity increases from H to C1 and then is approximately constant. The largest source of error in the calculation of the energy of reaction 2 at the LDF level is the energy of c6& This is because the geometry employed in the calculation was obtained at the AM1 level. The geometries of C2H4 and Cs0 do not strongly depend on the level of calculation,2’ and the errors in the energies of CH2XCH2X range from 0.5 to 4.0 kcal/mol due to the use of the AM1 geometries. Because of the need for larger amounts of relaxation in C d 2 , the error in this energy due to the use of the AM1 geometries is expected to be larger. This suggests that the errors in the energy of reaction 2 could be at least 5 kcal/mol, especially for X = Br and I, and this error is in the direction of making the reaction less endothermic.22 We also note that we could expect an additional error on the order of 2-3 kcal/mol because of our use of the LDF method in predicting the isodesmic reaction energies,lsfalthough we expect a much smaller error in predicting the relative energies of two closely related isomers such as those calculated above. Reaction 3 is exothermic for all X with F2 showing the largest exothermicity due to the weak F-F and strong C-F bonds. The energies for reaction 1 at this computational level show that addition of H2 and C12 is mildly exothermic whereas addition of F2 is strongly exothermic. Addition of Br2 is predicted to be endothermic, and addition of I2 is predicted to be strongly endothermic. The above discussion of errors for reaction 2 suggests that reaction 1 for X = H, F, and C1 will be more exothermic than calculated and for X = Br and I will be less endothermic than calculated. Reaction 1 also provides an estimate of the stability of the double bonds in Cm Our value for reaction 1 for H2,-3.7 kcal/mol, shows that hydrogenation of Caois much less exothermic than that of olefins. For comparison, heats of hydr~genation~~ of propene, (E)-2-butene, and 2,3-dimethyl-2-buteneare -29.8, -27.3, and -26.3 kcal/mol, respectively. The results indicate that although the C = C bonds in Cm are strained double bonds, the eclipsing hydrogens and angle strains in the hydrogenated c 6 0 result in the significant destabilization of 1 , 9 - C a 2compared to alkanes with relaxed geometries. Nevertheless, Caohydrogenation is more exothermic than hydrogenation of benzene, which is endothermic by 4.9 k c a l / m ~ l . ~ ~ The actual energy for reaction 1 for X = Br is probably 0-5 kcal/mol endothermic based on our calculated value and considering the errors in the calculations. Thus the CmBr2isomers can exist only if there is a sufficient kinetic barrier to loss of bromine. Reaction 1 represents only the first step in the addition of Br2 to C60to form CsOBr24.One cannot simply multiply the energy for reaction 1 by 12 to get the reaction energy for addition of 12 bromine molecules to Cm to form CmBr24.Semiempirical
+
-
+
--
+
+
-
calculations at the AM1 level for multiple additions of Br2to Cbo show that continued bromination leads to reactions that are more ex other mi^.^^ This suggests that the reaction energy will be thermoneutral or weakly exothermic for certain additions of Br2 to polybrominated Cao’s. This is probably due to the weaker double bonds in polysubstituted Cm’s as compared to those in Cm with its extended polyene structure. There is also the possibility of more geometry relaxation in the product in the more highly substituted systems leading to a more exothermic process. Thus, multiple bromination of Cm may be encouraged to proceed under thermodynamic eq~ilibration~~ until no more space is available on the c 6 0 surface for further bromine addition. The Th symmetry structure and the thermal instability of C60Br245C are consistent with this expectation. Any other isomeric C60Br24Kekult structures should be much less stable since they must contain bromines on at least three contiguous carbons. The further addition of bromine could occur with C-C bond rupture, but such a process has not yet been observed in Cm chemistry. q2-Metal complexes may be regarded as products exclusively of “1,2-additionn processes without the intervention of “1,4addition^".^ It is interesting to note that hexakismetal complexes of the same high Th symmetry as CmBr24can form, and the formation is also accompanied by thermal equilibration of intermediate complexes. Semiempiricalcalculations have shown that addition of H2 to Cm is preferred ‘1,2 and 1,4” with respect to a cyclohexatriene.* Our results show that the difference in energy for “1,2- and 1,4-additionnof hydrogen and halogens to Cm critically depends on the nature of the reagents. Fluorination is strongly exothermic, and products from multiple additions are expected to be mixtures of kinetic products from “1,2- and 1,Caddition” processes. Hydrogenation and chlorination are mildly exothermic and mixtures of 1,7- and 19-isomers could be present depending on reactions conditions. Chlorination differs somewhat from hydrogenation in that both F- and B-strain are expected to be larger. Thus, as the number of chlorines added becomes larger, steric effects could play a larger role, and under some specific conditions, “1,4addition” could become dominant, although we emphasize that under most conditions many structures will be observed. In the presence of excess reagent, bromination is expected to proceed under thermal equilibration. Even though the first addition of Br, is predicted to be endothermic, subsequent additions in certain steps are expected to be thermoneutral or exothermic leading to CsOBr24.The formation of the highly symmetric CboBr24isomer consisting only of the “1,4-additionnmode is consistent with the results of this study. Iodination is unlikely to occur based on thermodynamics, and only 1,Caddition” should be observed. Recently, intercalation of solid c 6 0 with I2 has been reported.26 No reaction between iodine and c60 was reported in this study consistent with our LDF calculations. It is interesting to note that both the “1,2-additionn (metal complex formation) and the “ 1,Caddition” (bromination) processes give rise under thermodynamic equilibrating conditions to highly symmetric structures.
References and Notes (1) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byme, N. E.; Flanagan, S.; Haley, M. M.; O’Brien, S. C.; Pan, C.; Xiao, 2.;Billup, W. E.; Ciufolini, M. A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl, R. F.; Smalley, R. E. J . Phys. Chem. 1990, 94, 8634. (2) (a) Selig, H.; Lifshitz, C.; Peres, T.; Fischer, J. E.; McGhie, A. R.; Romanow, W. J.; McCauley, J. P., Jr.; Smith, A. B., 111. J . Am. Chem. Soc. 1991,123, 5475. (b) Holloway, J. H.; Hope, E. G.; Taylor, R.; Langley, J.; Avent, A. G.; Dennis, T. J.; Hare, J. P.; Kroto, H. W.; Walton, D. R. M. J . Chem. SOC.,Chem. Commun. 1991, 966. (3) (a) Fagan, P. J.; Calabrese,J. C.; Malone, B. Science 1991, 252, 1160. (b) Fagan, P. J.; Calabrese, J. C.; Malone, B. J . Am. Chem. Soc. 1991, 113, 9408. (c) Fagan, P. J.; Calabrese, J. C.; Malone, B. Acc. Chem. Res. 1992, 25, 134. (d) Hawkins, J. M.; Meyer, A.; Lewis, T. A.; Loren, S.;Hollander, E. J. Science 1991, 252, 312. (e) Hawkins, J. M.; Loren, S.; Meyer, A,; Nunlist, R. J . Am. Chem. SOC.1991, 113, 7770. (f) Balch, A. L.; Catalano, V. J.; Lee, J. W.; Olmstead, M. M.; Parkin, S.R. J . Am. Chem. Soc. 1991, 1 1 3 , 8 9 5 3 . (8) Koefod, R. S.;Hudgens, M. F.; Shapley, J. R. J . Am. Chem. SOC.1991, 113, 8957. (4) (a) Krusic, P. J.; Wasserman, E.; Parkinson, B. A.; Malone, B.; Holler, E. R., Jr.; Keizer, P. N.; Morton, L. R.; Preston, K. F. J . Am. Chem. SOC. 1991, 113, 6274. (b) Krusic, P. J.; Wasserman, E.; Keizer, P. N.; Morton,
6110
J . Phys. Chem. 1992, 96,6110-61 11
J. R.;Preston, K. F. Science 1991, 254, 1183. ( 5 ) (a) Olah, G. A,; Bucsi, I.; Lambert, C.; Aniszfeld, R.; Trivedi, N. J.; Scnsharma, D. K.; F'rakash, G. K.S.J. Am. Chem. Soc. 1991,113,9385. (b) Tebbe, F. N.; Becker, J. Y.; Chase, D. B.; Firment, L. E.; Holler, E. R.; Malone, B. S.;Krusic, P. J.; Wasserman, E. J . Am. Chem. Soc. 1991, 113, 9900. (c) Tebbe, F. N.; Harlow, R. L.; Chase, D. B.; Thorn, D. L.; Campbell, G. C., Jr.; Calabrese, J. C.; Herron, N.; Young, R. J., Jr.; Wasserman, E. Science 1992, 256, 822. (6) (a) Suzuki, T.; Li, Q.; Khemani, K. C.; Wudl, F.; Almaesson, 0. Science 1991, 254, 1186. (b) Bausch, J. W.; Prakash, G. K. S.;Olah, G. A. J. Am. Chcm.Soc. 1991, 113, 3205. (7) Hirsh, A.; Li, Q.;Wudl, F. Angew. Chem., In?. Ed. Engl. 1991, 30, 1309. (8) Matsuzawa, N.; Dixon, D. A.; Fukunaga, T. J. Phys. Chem., in press. (9) (a) Eliel, E. L. Stereochemistry of Carbon Compounds; Mdjraw-Hill: New York, 1%2. (b) Payne, P. W.; Allen, L. C. In Applications ofEl&ronic Structure Theory;Schaefer, H. F., 111, Ed.; Plenum Press: New York, 1977; Chapter 2, p 29. (10) Dewar, M. J. S.;Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J . Am. Chem. Soc. 1985, 107, 3902. (11) Stewart, J. J. P. J . Comput. Chem. 1989, 10, 209, 221. (12) Matsuzawa, N.; Dixon, D. A. J . Phys. Chem., submitted for publi-
cation. (13) Stewart, J. J. P. QCPE Program 455,1983. version 5.00, 1990. (14) Stewart, J. J. P. J . Comput.-Aided Mol. Design 1990, 4, 1. (15) (a) Parr, R. 0.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (b) Salahub, D. R.
In Ab Initio Methods in Quantum Chemistry-II; Lawley, K . P., Ed.; John Wiley & Sons: New York, 1987; p 447. (c) Wimmer, E.; Freeman, A. J.; Fu, C.-L.; Cao, P.-L.; Chou, S.-H.; Delley. B. In Supercomputer Research in Chemistry and Chemical Engineering, Jensen, K . F., Truhlar, D. G., Eds.; ACS Symposium Series 353; American Chemical Society: Washington, D.C., 1987; p 49. (d) Jones, R. 0.;Gunnarsson, 0. Rev. Mod.Phys. 1989,61,689. (e) Dixon, D. A,; Andzelm, J.; Fitzgerald, G.; Wimmer, E.; Delley, B. In Science and Engineering on Supercomputers; Pitcher, E. J., Ed.; Computa-
tional Mechanics Publications: Southampton, England, 1990; p 285. (f) Dixon, D. A.; Andzelm, J.; Fitzgerald, G.; Wimmer, E.; Jasien, P. In Density Functional Methods in Chemistry; Labnowski, J. K., Andzelm, J. W., Eds.; Springer-Verlag: New York, 1991; Cha ter 3, p 33. (16) SCF convergence criteria = 10- (PRECISE option in MOPAC). No symmetry constaints were used. (17) (a) Delley, B. J . Chem. Phys. 1990, 92, 508. DMol is available commercially from BIOSYM Technologies, San Diego, CA. The grid used in the calculations was obtained by using the MEDiuM parameter in DMol. (b) Delley, B. In Density Functional Methods in Chemistry; Labanowski, J. K.,Andzelm, J. W., Eds.; Springer-Verlag: New York, 1991; Chapter 11, p 101. (18) von Barth, U.; Hedin, L. J . Phys. C 1972, 5, 1629. (19) Brown, H. C. Record Chem. Progr. 1953, 14, 83. (20) The 1,3,5-trisubtituted trimethylmccyclohcxanestructure in C a r u is a consequence of a series of sequential '1,4-additions" under thermal equilibration (see ref 22). The pcntadienyl and cyclopcntadienyl radical structures (see ref 4) also can be regarded as results of the 1,haddition process. (21) Matsuzawa, N.; Dixon, D. A. J. Phys. Chem., submitted for publication. (22) The AMI energies for reaction 2 are -8.2, +22.3, +15.0, +10.6, and +7.2 kcal/mol for X = H, F, C1, Br, and I, respectively. Thcsc reactions are all more exothermic than predicted at the LDF level, and this will lead to exothermic values for reaction 1 for all X. (23) (a) Pedley, J. B.; Naylor, R. D.; Kirby, S.P. Thermochemical Data of Organic Compounds; Chapman and Hall: London, England, 1986. (b) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; MacDonald, R. A.; Syverud, A. N. J . Phys. Chem. Ref. Data 1985, 14, Suppl. 1. (24) Matsuzawa, N.; Dixon, D. A.; Fukunaga, T. J . Phys. Chem., sub mitted for publication. Such equilibration is suggested, for example, by the observation that freshly prepared partially brominated yellowish product, C60Br
