5817
J. Phys. Chem. 1992, 96, 5817-5818
From Buckyballs to Bunnyballs: A Theoretical Analysis of Adduct- Induced Electronic Effects Y.C.Farm,+D.Singh,? and S . A. Jamen* Department of Chemistry and Materials Science, Temple University, Philadelphia, Pennsylvania 191 22 (Received: December 17, 1991; In Final Form: February 24, 1992)
Buckminsterfullerene, Cso,has attracted great interest in multidisplinary fields. The electronic correlation between Csoand the bunnyballs is not well understood. This work provides a theoretical study of electronic properties of Csoand the bunnyball with hypothetical metal substitutions to understand the correlation and electronic effects of adducts on the bunnyball. In this analysis the osmylated Csoand two hypothetical Ru and Mn analogs were analyzed by extended HGckel methods. These are chosen because of the unique chemistries of their oxides, i.e., Ru04, Mn04-, and GO4. The molecular orbital interaction diagrams of the bunnyballs are presented. The results show substitution critically affects electronic population in the Ca unit and frontier states in the complexed Cm
In the last few years intensive studies based on Cm, buckminsterfullerene, have been performed. The results on the icosahedral structure’ and superconductivity when doped with metal ion^^.^ have attracted great interest across many disciplines. Although the sowerball framework was proposed by Kroto, Smalley, and co-workers in 1985, the crystal structure of Cm was difficult to determine due to its spherical symmetry and rapid rotational motion. Hawkins et al. reported a synthesis of a 1:l &-osmium adduct, Cm(Os04)(4-tert-butylpyridine)2,4 named the b~nnyball.~ A crystal structure of this osmylated Cm was reported which confirms the framework of Cma4However, no theoretical and little experimental work has been performed to assess the influence of the “bunny ears” (the adduct) on the electronic nature of the osmylated CW Previous computational approaches have focused on calculations of the HOMWLUMO gap and geometrical analysis in unsubstituted buckyballs.6 Several of these reports relied on simple Hiickel methods and thus rcceived criticism as the absolute energies obtained depend on parameterization. This work provides a detailed molecular orbital description of the effect of addition of the adduct and “substitution” of metal oxide on the buckyball. Though the extended Hiickel method’ is selected, an analysis of relative effects in molecular orbital composition and orbital energies is valid. The structures used in these calculations were taken from the crystal structure for the bunnyball, and thus small deviations from the exact “spherical“ or icosahedral symmetry are observed. This makes exact assignments of degeneracies impossible for those states shown in Figures 2 and 3. These orbital energies are within 0.05 eV. In addition, metal substitution should not affect the crystal structure since the average metal-oxygen distances are similar: 0s-O = 2.03 A, Ru-O = 2.02 A, and Mn-O = 2.10 A with the same coordination. Within the extended Hiickel method limitation, the metal substitution using the same crystal data is valid. In this study, the approach is to treat the “bunnyball” as a complex formed from the ‘buckyball” and the osmium adduct, the bunny ears. A typical interaction diagram is constructed showing how orbitals of the buckyball near the frontier are affected by substitution. In addition, this work is extended to hypothetical systems based on ruthenium and manganese, utilizing models suggested by organic and organometallicchemistries. In general, Ru04 is a very strong but not very selective oxidant. Mn04- is a strong oxidant which can convert an alkene to an alcohol or acid depending on conditions. GO4is a selective oxidant which usually reacts with the electron-rich double bond and terminates at diol. The structure of the bunnyball used in the study is shown in Figure 1.* Our calculation on the pristine buckyball shows a delocalized orbital structure with a HOMO-LUMO splitting of 1.56 eV and Daniel Swern Fellow.
TABLE I
A(charge) metalb chemical HOMO HOMO-LUMO species energy (eV) splitting (eV) adducta ( a b charge)’
cso cso-os CM-RU
-11.11 -11.12 -1 1.33 -11.20
1.56 0.98 0.05 0.69
0.91 4.98 1.90
1.70 (4.76) 2.05 (8.96) 1.80 (4.60)
Ca-Mn “The number of electrons transferred from the Csoto the adduct, the fragment attached to the CMunit. bThenumber of electrons gained by the metal when the bunnyball formed compared to that of the metal in the adduct fragment. ‘The calculated charge of metals in the bunnyball.
appears independent of spacing between neighboring Cbounits. This value is consistent with the experimentally determined value from photoemission studies of 1.7 eVa9 The general molecular orbital picture for the buckyball is shown in Figure 2c. After the linear combinations are formed, those orbitals generated near the frontier are combinations of C 2p orbitals. The total 2s contribution at the frontier is minimal. The focus of our discussion is to determine how substitution will alter the chemical properties of the buckyball. In Figure 2a,b, the orbitals of the “adduct” or bunny ears and the orbitals of the complex are shown. Also represented are the orbital interactions between components. Since the structure was generated from real crystal data, there are no symmetry correlations. What is apparent in this figure is that the HUMO-LUMO splitting and energy of the HOMO orbitals are effected by substitution as shown in the table. The orbital features that drive these effects can be described in terms of bonding interactions and dopant effects. In the case of the osmium and manganese adducts, the states near the frontier level are not significantly affected by interaction with the bunny ear ligand. The states produced by bonding interactions between the osmium and manganese adducts and the buckyball lie well below the frontier states. The apparent compression of the HOMO-LUMO splitting energy in the complex is due to contributions of nonbonding orbitals from the bunny-eared ligand as shown in Figure 2a,b, which come in at energies between the HOMO and LUMO of the pristine buckyball. Generally, the Cmunit tends to donate electron density to the adducts. There is experimental evidence that oxidation of the buckyball produces stable cations of Cm in solution; e.g., Dubois and his co-workers have found that Csocan undergo an irreversible four-electronoxidation.’O However, the net loss of electron density in the Cso unit is small for Os and Mn relative to the Ru analog. The fragment analysis shows that the bridging oxygens between the C60and the adduct tend to be “oxidized”. The origin of electron gain on the metal shown in Table I is mainly from the bridging oxygens and the carbons on pyridine rings.
0022-365419212096-5817$03.00/0 0 1992 American Chemical Society
Fann et al.
5818 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
-
4 i,
10
I
o
I 1
H
- 11 2Rj
Ru Adduct ....
“Bunnyball” Buckyball
Multi-state interaction -- Single-state intBraCtiOn
-
I-,
I31
Unresolved, nondegenerated states within 0 05 BV
3 degenerate states
Figure 3. Molecular orbital interaction diagram of C60(Ru04)(4-tertbutylpyridine),. Figure 1. Structure of the “bunnyball” (C,(M0,)(4-tert-b~tylpyridine)~; M = Os, Ru, Mn).
..:. ...
8 ..
.’.’
.......... 3 .:::: , -( 4 ) ............ ............. (51 I31 6 ... ,/.: ,.... ... , - ,
...........
-
14
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...............
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Os Adduct
“Bunnyball”
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I31
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.
Ha
-
r)
Ilb
-
( 451)
(3) (3)
Buckyball llc
-.
Single-state interaction
Unresolved, nondegenerated stales within 0 05 eV 3 degenerate states
Figure 2. Molecular orbital interaction diagram of the osmylated C60: H, highest occupied molecular orbital.
The ruthenium adduct shows a greater effect of “self-doping”. In Figure 3, the fragment orbitals of the ruthenium adduct are shown. From this figure, it is apparent that several unoccupied, nonbonding orbitals of the ruthenium adduct are centered at an energy lower than the HOMO of the pristine buckyball. As nonbonding-type orbitals, they are not strongly affected upon complexation with the Cmfragment, and thus these states showing no significant perturbation add in below the frontier in the complex. The effect realized is that states in the complex originating from the c 6 0 unit, e.g., near the frontier, are depopulated, and the ruthenium adduct/fragment is essentially reduced relative to
the “free” moiety. This is shown in Table I. Note the effect to be emphasized is the trend not the absolute energies or charges as the extended Huckel method does not produce reliable absolute energies and tends to exaggerate the absolute charges. An analysis of the overlap population shows that local bonding effects between the adduct and C6*moiety are similar, regardless of metal substitution. Since the carbon-based orbitals near the HOMO are nonbonding, changes in population do not affect the integrity of the C60unit. The detailed analyses of this work will be described elsewhere.’ In this work, we have shown that substitution may affect the properties of the resulting complex in multiple ways. The work presented on the adducts suggests the buckyball fragment is oxidized as opposed to alkali c60 adducts, i.e., oxidized vs reduced c60. The level of oxidation will critically effect the resultant properties of the material. Further synthetic efforts might produce crystalline materials in which one will find greater utility in various applications. Finally, though the extended Huckel method is limited, the conclusions drawn suggest that further substitution of c60 will produce materials of unique chemical properties. References and Notes (1) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smalley, R. 1985, 318, 162. (2) Tanigaki, K.; Ebbesen, T. W.; Saito, S.; Mizuki, J.; Tsai, J. S.; Kubo, Y.; Kuroshima, S. Nature 1991, 352, 222. (3) Stephens, P.; Mihaly, L.; Whetten, R. L.; Huang, S. M.; Kaner, R.; Deiderich, F.; Holczer, K. Nature 1991, 351, 632. (4)Hawkins, J. M.; Meyer, A.; Lewis, T. A.; Loren, S.; Hollander, F. J. Science 1991, 252, 312. ( 5 ) Curl, R. F.; Smalley, R. E. Sci. Am. 1991, Oct, 54. (6) Fowler, P. W.; Lazzeretti, P.; Zanasi, R. Chem. Phys. Lett. 1990,165, 79. Klein, D. J.; Schmalz, T. G.; Hite, G. E.; Seitz, W. A. J. Am. Chem. SOC. 1986,108, 1301. (7) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397. Hoffmann, R.; L i p scomb, W. M. Zbid. 1962, 36, 3179; 1962, 37, 2872. (8) .The graphic was produced by the KGNGRAF program, a part of Modern Techniques in Computational Chemistry (MOTECC-90) package. We thank Dr. Hawkins at the University of California, Berkeley, for his generosity in providing the X-ray data of the osmylated Cm. (9) Sohmen, E.; Fink, J.; Kratschmer. Submitted for publication in Europhys. Lett. (10) Dubois, D.; Kadish, K. M.; Flanagan, S.; Wilson, L. J. J. Am. Chem. SOC.1991, 113, 7773. (1 1) Singh, D.; Fann, Y. C.; Jansen, S. A. To be published.
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