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Transition metal fullerene chemistry: The route from structural studies of exohedral adducts to the formation of redox active films
Pergamon
PIh S0022-3697(97)00043-7
J. Phys. Chem Solids Vol 58, No. 11, pp. 1633-1643, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/97 $17.00 + 0.00
TRANSITION METAL FULLERENE CHEMISTRY: THE ROUTE FROM STRUCTURAL STUDIES OF EXOHEDRAL ADDUCTS TO THE FORMATION OF REDOX ACTIVE FILMS ALAN L. BALCH*, V.J. CATALANO, D.A. COSTA, W.R. FAWCETT, M. FEDERCO, A.S. GINWALLA, J.W. LEE, M.M. OLMSTEAD, B.C. NOLL and K. W I N K L E R Contributionfrom the Departmentof ChemistryUniversityof California,Davis,CA 95616, USA Abstract--This article encompasses:(1) an update on recentdevelopmentson transitionmetal/fullereneadduct formation, (2) the use of metal complexesto form ordered crystallineadducts of fullerenesfor study by X-ray diffraction, (3) the characterizationof fullereneoxides, and (4) the formationof redox active filmsfrom fullerene oxides and from fullerenes.© 1997 ElsevierScience Ltd.
1. INTRODUCTION This paper will focus on recent research at the University of California, Davis on selected aspects of fullerene chemistry. This work began with a fascination with the chemical opportunities afforded by the novel exterior surface of C60 with its set of 30 olefinic C-C bonds at 6:6 ring functions [1, 2]. It has proceeded from the formation of organometallic adducts of C6o and higher fuilerenes onto the study of fullerene oxidation and fullerene oxidation products. In the study of the structures of higher fullerenes and fullerene oxides, the organometallic chemistry has been useful for the formation of crystalline adducts that can be studied by single-crystal X-ray diffraction. During our study of the fullerene epoxide, C6oO, we discovered that it formed a redox active film that coated and adhered to a variety of electrode surfaces. Thus the article will conclude with some information on these films. However, at the outset a brief update on the general area of transition metal/fullerene chemistry will be provided.
2. TRANSITIONMETAL/FULLERENECHEMISTRY: AN UPDATE A comprehensive overview of the chemistry of fullerenes with inorganic and organometallic complexes has recently been published and the reader is referred there for detailed information on this area [3]. Some of that work is covered in Fig. 1 which highlights routes to the formation of metal complexes bound to the outer surface
*Correspondingauthor.
of C6o. A significant number of metal complexes are now known to bond to C60 through an ~2-olefinic like bond as shown in Reaction (1) [3, 4]. The work of the Hawkins' laboratory has effectively studied the osmylation of C60 and higher fullerenes as set out in Reaction (3) [5]. Recently Rauchfuss and co-workers have shown that the S-S bond of $2Fe2(C0)6 adds across a 6:6 ring junction of C60 to form the new binuclear assembly shown in Reaction (4) [6]. This development may open up new organosulfur chemistry for the fullerenes. In this context it is interesting to note that the fullerenes have not yet been found to react with elemental sulfur although they do form co-crystals C6o.nSs [7]. In terms of multiple additions to C60, recent work has shown that binuclear metal complexes with small bridging ligands (CI-, H-) can bind to two adjacent olefinic sites on the fullerene surface [8]. (This complements earlier work on the addition of halo-bridged iridium complexes C60 [9].) Fig. 2 shows crystallographically determined structures for two such complexes, one with and the other without a direct metal-metal bond [8]. It should be possible to build up more complex organometallic structures on the fullerene surface which can be viewed as presenting an unusual convex surface upon which the synthetic chemist can work. Certainly the most remarkable recent development in transition metal fullerene chemistry has come from the laboratory of Y. Rubin who has been able to use an organometallic reaction to break fullerene C-C bonds in a C6o organic adduct [10]. The relevant chemistry is shown in Fig. 3. Part A shows the chemistry involved in the organic and then the organometallic functionalization of C60. Part B shows the crystallographically determined structure of the product, 0/5-CsH5)Co(C64H4). In
1633
1634
A.L. BALCH et al.
(1)
~
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+
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Fig. 1. Routes to the formation of transition metal complex/fullereneadducts. For (1) and (2) see Ref. [3]. For (3) see Ref. [5]. For (3) see Ref. [6]. the latter the C(1)-C(59) bond has opened up to a distance of 2.41 .~, whereas the comparable distance in C60 is 1.36 ,A,.This result is likely to spark interest in the organometallic reactivity of chemically modified
Rul•
fullerenes which offer even more sites for chemical reaction than the highly symmetric fullerenes themselves and the opportunity to markedly alter the geometry within the core fullerene.
H
Fig. 2. Adducts of C6o with two metal centers bound to adjacent olefinic sites on a six-memberedface (from Ref. [8]).
Transition metal fullerene chemistry
,E
1635
[4+4] ,.
PhCI
2
1
3a
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Fig. 3. Organometallicchemistry leading to rupture ofa C-C bond in the fullerene core. (Bottom)Structure of 075-CsHs)Co(C64H4). (Top) Chemical reactions leading to the formation of (~5-CsHs)Co(C64H4)(from Ref. [10]). 3. METAL COMPLEXES FOR THE FORMATION OF ORDERED CRYSTALLINE SOLIDS SUITABLE FOR SINGLE-CRYSTAL X-RAY DIFFRACTION STUDIES
The uniformity of the exterior surface of the high symmetrical fullerenes frequently introduces orientational disorder into solid fullerenes [1 1]. By appending groups to the surface of the fullerenes which reduce the intrinsic fullerene symmetry, it is possible to obtain ordered solids which are amenable to study by singlecrystal X-ray diffraction. We have employed the adduct formation with Vaska's complex, Ir(CO)CI(PR3)2, as shown in eqn (2) to prepare such adducts [12-17]. C6o + Ir(CO)CI(PR3)2 ~- (~/2_ C6o)Ir(CO)CI(PR3)2 (2)
The reaction is reversible, and dissolution of such adducts generally results in their dissociation into the free fullerene and the parent iridium complex. Fig. 4 compares the structures of three such adducts: (r/2-C6o)Ir(CO)Cl(PPh3)2 [12], (r/2-C70)Ir(CO)Cl(PPh3)2 [13] and (7/2-C84)Ir(CO)CI(PPh3)2 [14]. The relative size differences between these fullerenes are readily apparent. Crystallographic characterization of these adducts not only determines important metric parameters but also gives insight into the sites of chemical reactivity within the fullerenes. Thus, in C6o the situation is fairly simple, there are two bond types, those at 6:6-ring junctions and those at 6:5-ring junctions. The iridium binds to a 6:6-ring junction. In C7o there are eight types of C - C bonds, four of which involve 6:6-ring junctions. In this
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A.L. BALCH et al.
Fig. 4. A comparison of the structures of (r/2-C60)Ir(CO)Cl(PPh 3)2 (Ref [ 12]), (T/2-C701r(CO)CI(PPh 3)2 (Ref. [ 13]) and (~/2-C84)Ir(CO)CI(PPh3)2 (Ref. [14]), as determined by X-ray crystallography. case the adduct formation occurs at one of the poles of the fullerene where the carbon atoms are most pyramidalized [13]. In C84 there are even more choices for reactivity and it appears that bond localization is responsible for creating sites to which the iridium complex preferentially adds [14]. In fact, the situation with C84 is complex. There are 24 individual isomeric forms of C84, each of which obey the isolated pentagon rule [18]. Spectroscopic data supported by theoretical calculations of stability indicate that the Ca4 obtained from vaporization of graphite consists predominantly of two isomers of Dzd and D2 symmetry. The structures of these two are shown in Fig. 5. The crystallographic data from the black crystals of (~/2-Cg4)Ir(CO)CI(PPh3)2 reveal the structure shown at the bottom of Fig. 5 [14]. The D2d isomer of Cs4 is bound to the iridium through a 6:6-ring junction that lies at the end of the principle C2 axis of the fullerene. In this case, crystallization has lead to a partial separation of the C84 isomers, and it is the D2d, rather than the more prevalent D2 isomer, which is found in the solid adduct. This is particularly significant since the Ca4 isomers so far have resisted chromatographic separation. Due to the weak fullerene binding that Vaska's complex exhibits, only single addition products with 1/1 fullerene/Ir ratio have been found to form [12-14]. However, alteration in the substituents on the phosphine ligands can increase the reactivity of the iridium atoms in this sort of complex, and with Ir(CO)CI(PMe2Ph)2 it has been possible to obtain double addition products of C6o and C70 as shown in Fig. 6 [15, 16]. For C60, double addition leads to the formation of the statistically least probable "para" addition product. Symmetry considerations, crystal packing, and solubility play important roles in determining which of the eight possible double addition products crystallizes. In the four examples of
double addition of this type so far examined, all have the "para" arrangement [16, 17]. The process of crystallization selects this one compact geometric arrangement from an array of addition products that exist in solution. For C70, as for C6o, steric factors preclude placing the two added iridium complexes in close proximity. Thus, in (CT0){Ir(CO)CI(PMe2Ph)2}2 the two iridium centers are bound to identical sites at opposite poles of the fullerene [15]. The reversible nature of the fullerene/Ir(CO)Cl(PR3)2 binding is nicely demonstrated by the 31p {IH} NMR spectra shown in Fig. 7 for solutions obtained from Cro{Ir(CO)CI(PEt3)2}2 [17]. At room temperature, the broad spectrum (A) shows two resonances in dynamic exchange with one another. Upon cooling, the resonances sharpen as shown in trace B, In trace B the resonance at 21 ppm is that of free Ir(CO)CI(PEt 3)2 while the complex set of resonances at - 1 5 to - 2 0 ppm result from the single addition product and several isomers of double addition products. Addition of C 60to this solution produces the spectrum shown in trace C. The equilibrium has shifted so that all of the iridium complex now is present as the single addition product, (~/2-C60)Ir(CO)CI(PEt3)2. The binding of Vaska-type complexes to fullerenes produces relatively small distortions of the fullerene geometry. Those distortions that do occur are largely confined to the region in the immediate vicinity of the iridium center. Fig. 8 shows a line drawing of the structure of C60{Ir(CO)CI(PPhMe2)2}2 onto which is superimposed an ideal C60 molecule. The observable deviations occur at the opposite ends. Binding of the Vaska-type complex results in a radical displacement of the iridium-bound carbon atoms away from the fullerene core by ca. 0.2 ,~. As a consequence, the distance between the mid-points of the "para" C - C
Transition metal fullerene chemistry
1637
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D2 (22)
cs 83 C6 C65
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s
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Fig. 5. Top ideal structures of the D2d (23) and D2 (22) isomers of C84 that are the most prevalent forms in the generated fullerene preparations. The numbers in parenthesis refer to the tabulation of C s4 isomers of Manolopoulos and Fowler [ 18, 19]. Bottom: the crystallographically determined structure of (r/2-Cs4)Ir(CO)CI(PPh3)2.The three two-fold axes pass through the centers of bonds between the circles that are hatched (from Ref. [14]). bonds that are attached to the iridium centers is 7.33 .A, which can be compared to the corresponding distance of 6.94 ,~ in C6o itself. Additionally, the C - C bonds that are involved in binding the iridium center are elongated by ca. 0.14 ,~. Recent work in our laboratory suggests that cocrystallization of fullerenes with Co(OEP) (OEP is the dianion of octaethylporphyrin) offers another way of obtaining fullerenes in ordered crystalline forms [20]. Fig. 9 shows a view of the structure of C60{Co(OEP)}2 [20]. Several features are noteworthy. Two of the metalloporphyrins cradle the fullerene, but there is some asymmetry in the positioning of the fullerene between these two porphyrins. The approaches of the fullerenes to the cobalt atoms are two long for covalent bond formation where C o - C distances in the range 19-2.2 ,~ would be expected. However, the fullerene/
porphyrin contacts are much shorter than normal van der Waals contacts, which should be in the 3.0-3.3-,~ range. Finally despite the lack of covalent bonding to anchor the fullerene, the C60 moiety is fully ordered in this structure. The formation of this fullerene/porphyrin aggregate is interesting not only for its structural information, but also because of its relation to the use of porphyrins in the chromatographic separation of fullerenes [21, 22]. Silica-based chromatographic supports which have been functionalized through covalent binding of porphyrins and metalloporphyrins have utility in fullerene separation and purification. Specific porphyrin/fullerene interactions have been postulated to be responsible for the chromatographic utility. Fig. 10 shows a conceptual drawing that illustrates the proposed fullerene/porphyrin interaction [22]. Examination of
1638
A.L. BALCH et al.
S
Fig. 8. A superposition of the structure of C60{Ir(CO) CI(PPhMe2)2} 2(solid lines) and an ideal C60 molecule (dashed lines). Note that the distortion of the fullerene is confined to the region near the iridium atoms.
II (
~-~
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4. FULLERENE OXIDATION AND FULLERENE OXIDE STRUCTURES
C37
Fig. 6. The structures of C6o{Ir(CO)CI(PMe2Ph)2}2 (from Ref. [16]) and CT0{Ir(CO)CI(PMe2Ph)}2 (from Ref. [15]).
particle size effects on the chromatographic efficiency led to the suggestion, illustrated by this drawing, that the fullerene could interact with a pair of closely spaced porphyrin appendages [22]. The C60{Co(OEP) }2 structure in Fig. 9 is an interesting model for this interaction.
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Any technological application of the fullerenes is likely to be effected or limited by oxidative degradation by atmospheric oxygen. We and others have been exalaining oxidation of C6o under a variety of conditions [23-25]. As a result, the oxides C600 [23, 24], C6oO2 [25], C700 [26, 27] and Ci2oO [28-30] have all received serious structural study by spectroscopic and crystallographic investigation. Structures in the form of line drawings of each of these are shown in Fig. 11. Initial work in this laboratory focused on crystallographic characterization through the use of adducts with Vaska's complex. This was done since C6oO itself, like C6o, shows orientational disorder [31]. The crystallographic work agrees with earlier spectroscopic studies which show that C600 has an epoxide structure with the oxygen atom lying over a 6:6-ring junction [32, 33]. Fig. 12 shows the structure of (~/2-C6oO)Ir(CO)CI(PPh3)2 [32] and (7/2-C6o)Ir(CO)Cl(AsPh3)2 [33]. Both show some disorder in the oxygen atom position with two partially
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Fig. 7. The 31p { IH} NMR spectra of C6o{Ir(CO)CI(PEt3)2}2 in 6:4 o-dichlorobenzene/toluene at (A) 20, (B) -80 and (C) -80°C in the presence of an excess of added C60 (from Ref. [171).
Fig. 9. A view of the structure of C6o{Co(OEP)}2 (from Ref. [20]).
Transition metal fullerene chemistry
1639
O e
B
Fig. 10. Proposed porphyrin/fullerene interactions in chromatographic supports with porphyrin units appended to the surface (from Ref. [22]). occupied sites for the triphenylphosphine complex [32] and four such sites for the triphenylarsine complex [33]. Notice that the epoxide functionality provides an electronic effect which influences the site of binding of the iridium complex and which generally places that group on a 6:6-ring junction that is immediately adjacent to the epoxide. Oxidation of C60 with m-chloroperoxybenzoic acid yields at least six oxidation products but some control can be exercised to produce predominantly C6oO and a single isomeric form of C6002 [25]. 13C NMR spectroscopy reveals that this isomer has C~ symmetry. Crystallographic investigation through the formation of an adduct with Vaska's complex (Fig. 13) shows that this isomer of C6002 has a bis-epoxide structure in which the two oxygen atoms lie over two 6:6-ring junctions that are part of a common six-membered ring. Thus, the epoxide groups are as close together as possible. As Fig. 13 shows (part A), there are seven partially occupied oxygen atom sites in the crystal with all lying over 6:6-ring junctions. Careful analysis of the fractional occupancies of these sites shows that the pattern can only be explained if one isomer with the two epoxide groups immediately adjacent to each other is present. Part B of Fig. 13 shows the predominant adduct that is present in this solid. Oxidation of C7o with m-chloroperoxybenzoic acid or by photooxygenation with rubrene as photosensitizer yields C700 [26, 27]. JaC NMR and 3He NMR studies (the latter of the endohedral He@C700) indicate that two isomers of C700 are formed and that these have the epoxide groups attached to the C~-Cb and Co-Co bonds of the fullerene as shown in Fig. 11 [26]. Crystallographic examination of the adduct, (7/2-C 700)Ir(CO)CI(PPh3) 2, reveals a complex structure which involves disorder in the orientation of the C70 unit and in the oxygen atom site [27]. Fig. 14 shows the two orientations that are observed [27]. These occupy a common site and differ in location of the
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Fig. 11. Line drawings of C6oO, C6oO2 (the known isolated isomer), C7oO(four isomers), and C 1200. long axis (originally the C5 axis) of the fullerene. Additionally, there are two sites for the epoxide oxygen atom. The structural work indicates that the oxygen atom does not reside in the equatorial belt of C70, where theoretical work indicates that the molecule would have the greatest stability [34]. Rather, the crystallographic work is consistent with the presence of the two isomeric forms (1 and 2) of C700 shown in Fig. 11. Recently a chromatographic separation of these isomers has been achieved [27].
5. ELECTROCHEMICAL STUDIES OF FULLERENE OXIDES: THE FORMATION OF REDOX ACTIVE
FILMS Electrochemical reduction with repeated cycling of C600, C6002 and C700 at the second reduction step results in the coating the electrode with orange brown
1640
A.L. BALCH et al.
C50~ C 5 3 ~
C6
C54C ~ 4 '
Fig. 12. The structures of (~2-C~O)Ir(CO)Cl(PPh3)2(from Ref. [321) and (~2-C~O)Ir(CO)CI(AsPh3)2(from Ref. [331).
films that strongly adhere to the electrode surface [35, 36]. Representative electrochemical data for the reduction of C600 are shown in Fig. 15 [36]. Repeated cycling as shown in Trace D leads to an increase in current during the reductive processes and the development of new features at ca. - 0 . 2 V range upon re-oxidation. In order to observe film formation it is necessary to reach the reduction potential for the second reduction process for C6oO. When the switching potential is set so that only the first one-electron reduction of C6oO occurs, no film formation occurs as seen in Trace A of Fig. 15. However, as the switching potential reaches more negative values (as seen in Traces B, C, and D) increases in current that accompany film growth during repeated cycling are observed. The second reduction, to form C 6002-, appears to involve a reaction that involves the epoxide unit, possibly opens that three-membered ring, and induces polymerization. The film that coats the electrode surface is insoluble in common organic solvents and in water. The infrared spectrum of the film shows features due to tetra(n-butyl ammonium) ions, and the fullerene units. Images of the film itself obtained by scanning and transmission electron microscopy show that the film involves the formation of spherical units of surprisingly uniform size. Fig. 16 shows a scanning electron micrograph of the surface of a gold electrode that has been
coated with film grown from C6oO. Piles of spherical units extend out from both edges of the electrode and meet at the center. These spherical units have a diameter that corresponds to a linear row of ca. 1000 buckyballs. However, at this time we do not know whether these spherical units are solid or hollow. The film is sufficiently stable that repeated electron microscopic views of the same structural features can be obtained. C12oO, which has recently been prepared by the thermal reaction between C6oO and C60 [28-30], is an interesting structural model for a linking group that may be present within these fuilerene films. A line drawing of the structure of C 12oO,which involves the formation of a tetrahydrofuran-like linkage between the two fullerenes, is shown in Fig. 11. Under electrochemical reduction, C 1200 does not act as a film-forming precursor [30]. This is not entirely surprising. The linkage between the two fullerene units is relatively unstrained and is hidden from the environment by the surrounding fullerene cages. Fig. 17 compares the electrochemical reduction of C~o with that of C 1200 [30]. In the potential range utilized here, C6o shows three reversible, one-electron reductions as can be seen in Trace a of Fig. 17. Trace b shows the reductive behavior for C 120O. Waves corresponding to those of C60 show broadening and resolution into pairs of reduction processes. Further examination has shown that there are three
Transition metal fullerene chemistry
f
1641
°
~20B
Fig. 13. The structures of (~t2-C6oO2)Ir(CO)CI(PPh3)2as determined by X-ray crystallography. (A) The location of the seven oxygen atom sites. (B) The principle form of the molecules in the solid (from Ref. [25]). pairs of one-electron reductions in the electrochemical cycles shown in Trace b of Fig. 17. These pairs correspond to the single-electron reductions of C60, but in the case of C l20 we see successive additions of single electrons to each ball within the fullerene "dimer". The resolution observed indicates that some degree of communication between the two fullerene units exists in C 1200. Work is proceeding in our laboratory to determine whether we can chemically abstract C 1200 units from our C600-derived films. Since fullerene-based polymers may have eventual utility as coatings, sensors, or battery components (if they are conducting), we have sought additional ways to prepare these films. In particular, we have sought means of avoiding the chemical processing that is involved in the preparation of the fullerene epoxide, C6oO. Thus we were pleased to discover that electrochemical reduction of Coo itself in the presence of controlled quantities of dioxygen also leads to film formation. Fig. 18 shows multicycle (25 scans) cyclic voltammograms for C60 in the presence of dioxygen (Trace b) and in the absence of dioxygen (Trace a). Trace B shows the expected two one-electron reduction processes that are expected within this potential
Fig. 14. The two forms of (~2-C700)Ir(CO)CI(PPh3)2present at a common site in the crystalline solid. Pentagons at the ends of the (former) C5 axis of the parent fullerene are shown as solid lines (from Ref. [27]). window. There is no build up of film, no increase in current, and no additional electrochemical features. However, with dioxygen present, (Trace b) there are clear changes in the electrochemistry that are indicative of film formation and an orange brown film is visually observed to coat the electrode surface. Like the film grown from C600, this film is insoluble in organic solvents and water. It adheres to the electrosurfaces and has infrared spectral features similar to the film grown from C600. However, electron microscopy shows a different film morphology from that seen in Fig. 16. Further work to characterize this new film and to compare it to the C600-derived film is in progress. In conclusion, this short article shows that efforts to use transition metal complexes to produce ordered solids for structural study by X-ray crystallography have been successful and have led our group into the study of the fullerene oxides. From there we have found a new means to polymerize fullerenes into a redox active film that coats electrodes. But here we are left with a more difficult structural question than the one we set out to examine. What do these fuilerene films look like at the molecular level? And we are left with the opportunity to explore the utility of these films.
1642
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Fig. 15. Multiscan cyclic voltammograms of C6oO in toluene/ acetonitrile (4:1) at a gold electrode with tetra(n-butyl)ammonium perchlorate as supporting electrolyte. Forty scans are involved in each of the four recordings with the switching potential changed in the four recordings (from Ref. [30]).
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Fig. 17. (a) Cyclic voltammogram of 0.35 mM C6o in o-dichlorobenzene with 0.1 M tetra(n-butyl)ammonium perchlorate as supporting electrolyte at a gold electrode (0.75-mm radius). (b) Cyclic voltammogram (solid line) and differential pulse voltammogram (broken line) of C ,200 in o-dichlorobenzene with 0.1 M tetra(n-butyl)ammonium perchlorate as supporting electrolyte at a gold electrode (0.75-mm radius). Potentials are referenced to ferroccne/ferrocenium couple (from Ref. [36]).
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Fig. 16. Scanning electron micrograph of an electrode which has been coated by electrochemical reduction of C600. Notice the uniform size of the spherical structures which grow out from the electrode surface.
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Fig. 18. Cyclic voltammograms obtained at 100 mV s -~ in the potential region -I-0.25 to - 1.4 V (against a silver/silver perchlorate (0.01 M) reference electrode which gives the ferrocene/ ferrocenium couple at 214 mV) for the reduction of C6o at a gold electrode in a toluene-acetonitrile mixture containing 0.1 M tetra(n-butyl)ammonium perchlorate. Trace a, 0.35 mM C60 in the absence of dioxygen. Trace b, 0.35 mM C6o in the presence of dioxygen. Data are shown for 25 cycles in each case (from Ref. [37]).
Transition metal fullerene chemistry Acknowledgements--We thank the National ScienceFoundation (CHE9022909, CHE9321257, and CHE93921257) for support of this research and Johnson Matthey for a loan of iridium salts.
REFERENCES 1. Taylor, R. (Ed.), The Chemistry of Fullerenes. World Scientific Publishing Co. Pte. Ltd., River Edge, NJ, 1995. 2. Hirsh, A., The Chemistry of Fullerenes. Georg Thieme Verlag, Stuttgart, 1994. 3. Balch, A.L. In: Taylor, R. (Ed.), The Chemistry of Fullerenes. World Scientific Publishing Co. Pte. Ltd., River Edge, NJ, 1995, p, 220. 4. Fagan, P.F., Calabrese, J.C. and Malone, B., Accounts Chem. Res., 1992, 25, 134. 5. Hawkins, J.M., Accounts Chem. Rec., 1992, 25, 150. 6. Westmeyer, M.D., Galloway, C.P. and Rauchfuss, T.B., Inorg. Chem., 1994, 33, 4615. 7. Roth, G., Adermann, P. and Knitter, R., Mater. Lett., 1993, 16, 357. 8. Mavunkal, I.P., Chi, Y., Peng, S.-M. and Lee, G.-H., Oganometallics, 1995, 14, 4454. 9, Rasinkangas, M., Pakkanen, T.T., Pakkanen, T.A., Ahlgren, M. and Rouvinen, J., J. Am. Chem. Soc., 1993, 115, 4901. 10, Arce, M.-J., Viado, A.L., An, Y.-Z., Khan, S.I. and Rubin, Y., J. Am. Chem. Soc., 1996, 118, 3775. 11. Burgi, H.B., Blanc, E., Schwarzenbach, D., Liu, S., Lu, Y., Kappes, M.M. and Ibers, J.A., Angew. Chem. Int. Ed. Engl., 1992, 31, 640. 12. Balch, A.L., Catalano, J.V. and Lee, J.W., lnorg. Chem., 1991, 30, 3980. 13. Balch, A.L., Catalano, V.J., Lee, J.W., Olmstead, M.M. and Parkin, S., J. Am. Chem. Soc., 1991, 113, 8953. 14. Balch, A.L., Ginwalla, A.S., Lee, J.W., Noll, B.C. and Olmstead, M.M., J. Am. Chem. Soc., 1994, 116, 2227. 15. Balch, A.L., Lee, J.W. and Olmstead, M.M., Angew. Chem. Int. Ed. Engl., 1992, 31, 1356. 16. Balch, A.L., Lee, J.W., Noll, B.C. and Olmstead, M.M., J. Am. Chem. Soc., 1992, 114, 10984. 17. Balch, A.L., Lee, J.W., Noll, B.C. and Olmstead, M.M., Inorg. Chem., 1994, 33, 5238. 18. Manolopoulos, D.E. and Fowler, P.W., J. Phys. Chem., 1992, 96, 7603. 19. Manolopoulos, D.E., Fowler, P.W., Taylor, R., Kroto, H.W. and Walton, D.R.M., J. Chem. Soc. Faraday Trans., 1992, 88, 3117.
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20. Balch, A.L., Costa, D.A., Olmstead, M.M., VanCalcar, P.M. To be submitted for publication. 21. Xiao, J., Savina, M.R., Martin, G.B., Francis, A.H. and Meyerhoff, M.E., J. Am. Chem. Soc., 1994, 116, 9341. 22. Martin, G.B., Xiao, J., Savina, M.R., Wilks, M., Francis, A.H., Meyerhoff, M.E., In: Kadish, K.M., Rouff, P.S. (Eds.), Fullerenes. Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, vols. 94-24. Electrochemical Society, Pennington, NJ, 1994, p. 178. 23. Creegan, K.M., Robbins, J.L., Robbins, W.K., Millar, J.M., Sherwood, R.D., Tindall, P.J., Cox, D.M., Smith III, A.B., McCauley, J.P. Jr., Jones, D.R. and Gallagher, R.T., J. Am. Chem. Soc., 1992, 114, 1103. 24. Elemes, Y., Silverman, S.K., Shen, C., Kao, M., Foote, C.S., Alvarez, M.M. and Whetten, R.L., Angew. Chem. Int. Ed. Engl., 1992, 31, 351. 25. Balch, A.L., Costa, D.A., Noll, B.C. and Olmstead, M.M., J. Am. Chem. Soc., 1995, 117, 8926. 26. Smith, A.B., III Strongin, R.M., Brard, L., Furst, G.T., Atkins, J.H., Romanow, W.J., Saunders, M., JimenezVazquez, H.A., Owens, K.G. and Goldschmidt, R.J., J. Org. Chem., 1996, 61, 1904. 27. Balch. A.L., Costa. D.A., Olmstead, M.M. Submitted for publication. 28. Lebedkin, S., Ballenweg, S., Gross, J., Taylor, R. and Kratschmer, W., Tetrahedron Lett., 1995, 36, 4971. 29. Smith III, A.B., Toyuyama, H., Strongin, R.M., Furst, G.T., Romanow, W.J., Chait, B.T., Mirza, U.A. and Haller, I., J. Am. Chem. Soc., 1995, 117, 9359. 30. Balch, A.L., Costa, D.A., Fawcett, W.R. and Winkler, K., J. Phys. Chem., 1996, 100, 4823. 31. Vaughan, G.B.M., Heiney, P.A., Cox, D.E., McGhie, A.R., Jones, D.R., Strongin, R.M., Cichy, M.A. and Smith III, A.B., Chem. Phys., 1992, 168, 185. 32. Balch, A.L., Costa, D.A., Lee, J.W., Noll, B.C. and Olmstead, M.M., lnorg. Chem., 1994, 33, 2071. 33. Balch, A.L., Costa, D.A., Noll, B.C. and Olmstead, M.M., lnorg. Chem., 1996, 35, 458. 34. Raghavachari, K. and Rohlfing, C.M., Chem. Phys. Lett., 1992, 197, 495. 35. Fedurco, M., Costa, D., Balch, A.L. and Fawcett, W.R., Angew. Chem. Int. Ed. Engl., 1995, 34, 194. 36. Winkler, K., Costa, D.A., Balch, A.L. and Fawcett, W.R., J. Phys. Chem., 1995, 99, 17431. 37. Balch, A.L., Costa, D.A., Fawcett, W.R., Winkler, K., Adv. Mater., 1997, 9, 153.
PIh S0022-3697(97)00043-7
J. Phys. Chem Solids Vol 58, No. 11, pp. 1633-1643, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/97 $17.00 + 0.00
TRANSITION METAL FULLERENE CHEMISTRY: THE ROUTE FROM STRUCTURAL STUDIES OF EXOHEDRAL ADDUCTS TO THE FORMATION OF REDOX ACTIVE FILMS ALAN L. BALCH*, V.J. CATALANO, D.A. COSTA, W.R. FAWCETT, M. FEDERCO, A.S. GINWALLA, J.W. LEE, M.M. OLMSTEAD, B.C. NOLL and K. W I N K L E R Contributionfrom the Departmentof ChemistryUniversityof California,Davis,CA 95616, USA Abstract--This article encompasses:(1) an update on recentdevelopmentson transitionmetal/fullereneadduct formation, (2) the use of metal complexesto form ordered crystallineadducts of fullerenesfor study by X-ray diffraction, (3) the characterizationof fullereneoxides, and (4) the formationof redox active filmsfrom fullerene oxides and from fullerenes.© 1997 ElsevierScience Ltd.
1. INTRODUCTION This paper will focus on recent research at the University of California, Davis on selected aspects of fullerene chemistry. This work began with a fascination with the chemical opportunities afforded by the novel exterior surface of C60 with its set of 30 olefinic C-C bonds at 6:6 ring functions [1, 2]. It has proceeded from the formation of organometallic adducts of C6o and higher fuilerenes onto the study of fullerene oxidation and fullerene oxidation products. In the study of the structures of higher fullerenes and fullerene oxides, the organometallic chemistry has been useful for the formation of crystalline adducts that can be studied by single-crystal X-ray diffraction. During our study of the fullerene epoxide, C6oO, we discovered that it formed a redox active film that coated and adhered to a variety of electrode surfaces. Thus the article will conclude with some information on these films. However, at the outset a brief update on the general area of transition metal/fullerene chemistry will be provided.
2. TRANSITIONMETAL/FULLERENECHEMISTRY: AN UPDATE A comprehensive overview of the chemistry of fullerenes with inorganic and organometallic complexes has recently been published and the reader is referred there for detailed information on this area [3]. Some of that work is covered in Fig. 1 which highlights routes to the formation of metal complexes bound to the outer surface
*Correspondingauthor.
of C6o. A significant number of metal complexes are now known to bond to C60 through an ~2-olefinic like bond as shown in Reaction (1) [3, 4]. The work of the Hawkins' laboratory has effectively studied the osmylation of C60 and higher fullerenes as set out in Reaction (3) [5]. Recently Rauchfuss and co-workers have shown that the S-S bond of $2Fe2(C0)6 adds across a 6:6 ring junction of C60 to form the new binuclear assembly shown in Reaction (4) [6]. This development may open up new organosulfur chemistry for the fullerenes. In this context it is interesting to note that the fullerenes have not yet been found to react with elemental sulfur although they do form co-crystals C6o.nSs [7]. In terms of multiple additions to C60, recent work has shown that binuclear metal complexes with small bridging ligands (CI-, H-) can bind to two adjacent olefinic sites on the fullerene surface [8]. (This complements earlier work on the addition of halo-bridged iridium complexes C60 [9].) Fig. 2 shows crystallographically determined structures for two such complexes, one with and the other without a direct metal-metal bond [8]. It should be possible to build up more complex organometallic structures on the fullerene surface which can be viewed as presenting an unusual convex surface upon which the synthetic chemist can work. Certainly the most remarkable recent development in transition metal fullerene chemistry has come from the laboratory of Y. Rubin who has been able to use an organometallic reaction to break fullerene C-C bonds in a C6o organic adduct [10]. The relevant chemistry is shown in Fig. 3. Part A shows the chemistry involved in the organic and then the organometallic functionalization of C60. Part B shows the crystallographically determined structure of the product, 0/5-CsH5)Co(C64H4). In
1633
1634
A.L. BALCH et al.
(1)
~
+ MLnL'--)
~ ML n =
MI~ +L'
Pt(PR3)2, Pd(PR3)2, RhH(CO)(PPh3)2, Fe(CO)4, etc. 0
(2)
~
(3)
+
Ir(CO)CI(PR3)2- ~
7-
+ OsO4 + 2.2 py --)
(4)
o/ Iio\py
25oc
ST--'~e(CO)~
Fig. 1. Routes to the formation of transition metal complex/fullereneadducts. For (1) and (2) see Ref. [3]. For (3) see Ref. [5]. For (3) see Ref. [6]. the latter the C(1)-C(59) bond has opened up to a distance of 2.41 .~, whereas the comparable distance in C60 is 1.36 ,A,.This result is likely to spark interest in the organometallic reactivity of chemically modified
Rul•
fullerenes which offer even more sites for chemical reaction than the highly symmetric fullerenes themselves and the opportunity to markedly alter the geometry within the core fullerene.
H
Fig. 2. Adducts of C6o with two metal centers bound to adjacent olefinic sites on a six-memberedface (from Ref. [8]).
Transition metal fullerene chemistry
,E
1635
[4+4] ,.
PhCI
2
1
3a
3_
Fig. 3. Organometallicchemistry leading to rupture ofa C-C bond in the fullerene core. (Bottom)Structure of 075-CsHs)Co(C64H4). (Top) Chemical reactions leading to the formation of (~5-CsHs)Co(C64H4)(from Ref. [10]). 3. METAL COMPLEXES FOR THE FORMATION OF ORDERED CRYSTALLINE SOLIDS SUITABLE FOR SINGLE-CRYSTAL X-RAY DIFFRACTION STUDIES
The uniformity of the exterior surface of the high symmetrical fullerenes frequently introduces orientational disorder into solid fullerenes [1 1]. By appending groups to the surface of the fullerenes which reduce the intrinsic fullerene symmetry, it is possible to obtain ordered solids which are amenable to study by singlecrystal X-ray diffraction. We have employed the adduct formation with Vaska's complex, Ir(CO)CI(PR3)2, as shown in eqn (2) to prepare such adducts [12-17]. C6o + Ir(CO)CI(PR3)2 ~- (~/2_ C6o)Ir(CO)CI(PR3)2 (2)
The reaction is reversible, and dissolution of such adducts generally results in their dissociation into the free fullerene and the parent iridium complex. Fig. 4 compares the structures of three such adducts: (r/2-C6o)Ir(CO)Cl(PPh3)2 [12], (r/2-C70)Ir(CO)Cl(PPh3)2 [13] and (7/2-C84)Ir(CO)CI(PPh3)2 [14]. The relative size differences between these fullerenes are readily apparent. Crystallographic characterization of these adducts not only determines important metric parameters but also gives insight into the sites of chemical reactivity within the fullerenes. Thus, in C6o the situation is fairly simple, there are two bond types, those at 6:6-ring junctions and those at 6:5-ring junctions. The iridium binds to a 6:6-ring junction. In C7o there are eight types of C - C bonds, four of which involve 6:6-ring junctions. In this
1636
A.L. BALCH et al.
Fig. 4. A comparison of the structures of (r/2-C60)Ir(CO)Cl(PPh 3)2 (Ref [ 12]), (T/2-C701r(CO)CI(PPh 3)2 (Ref. [ 13]) and (~/2-C84)Ir(CO)CI(PPh3)2 (Ref. [14]), as determined by X-ray crystallography. case the adduct formation occurs at one of the poles of the fullerene where the carbon atoms are most pyramidalized [13]. In C84 there are even more choices for reactivity and it appears that bond localization is responsible for creating sites to which the iridium complex preferentially adds [14]. In fact, the situation with C84 is complex. There are 24 individual isomeric forms of C84, each of which obey the isolated pentagon rule [18]. Spectroscopic data supported by theoretical calculations of stability indicate that the Ca4 obtained from vaporization of graphite consists predominantly of two isomers of Dzd and D2 symmetry. The structures of these two are shown in Fig. 5. The crystallographic data from the black crystals of (~/2-Cg4)Ir(CO)CI(PPh3)2 reveal the structure shown at the bottom of Fig. 5 [14]. The D2d isomer of Cs4 is bound to the iridium through a 6:6-ring junction that lies at the end of the principle C2 axis of the fullerene. In this case, crystallization has lead to a partial separation of the C84 isomers, and it is the D2d, rather than the more prevalent D2 isomer, which is found in the solid adduct. This is particularly significant since the Ca4 isomers so far have resisted chromatographic separation. Due to the weak fullerene binding that Vaska's complex exhibits, only single addition products with 1/1 fullerene/Ir ratio have been found to form [12-14]. However, alteration in the substituents on the phosphine ligands can increase the reactivity of the iridium atoms in this sort of complex, and with Ir(CO)CI(PMe2Ph)2 it has been possible to obtain double addition products of C6o and C70 as shown in Fig. 6 [15, 16]. For C60, double addition leads to the formation of the statistically least probable "para" addition product. Symmetry considerations, crystal packing, and solubility play important roles in determining which of the eight possible double addition products crystallizes. In the four examples of
double addition of this type so far examined, all have the "para" arrangement [16, 17]. The process of crystallization selects this one compact geometric arrangement from an array of addition products that exist in solution. For C70, as for C6o, steric factors preclude placing the two added iridium complexes in close proximity. Thus, in (CT0){Ir(CO)CI(PMe2Ph)2}2 the two iridium centers are bound to identical sites at opposite poles of the fullerene [15]. The reversible nature of the fullerene/Ir(CO)Cl(PR3)2 binding is nicely demonstrated by the 31p {IH} NMR spectra shown in Fig. 7 for solutions obtained from Cro{Ir(CO)CI(PEt3)2}2 [17]. At room temperature, the broad spectrum (A) shows two resonances in dynamic exchange with one another. Upon cooling, the resonances sharpen as shown in trace B, In trace B the resonance at 21 ppm is that of free Ir(CO)CI(PEt 3)2 while the complex set of resonances at - 1 5 to - 2 0 ppm result from the single addition product and several isomers of double addition products. Addition of C 60to this solution produces the spectrum shown in trace C. The equilibrium has shifted so that all of the iridium complex now is present as the single addition product, (~/2-C60)Ir(CO)CI(PEt3)2. The binding of Vaska-type complexes to fullerenes produces relatively small distortions of the fullerene geometry. Those distortions that do occur are largely confined to the region in the immediate vicinity of the iridium center. Fig. 8 shows a line drawing of the structure of C60{Ir(CO)CI(PPhMe2)2}2 onto which is superimposed an ideal C60 molecule. The observable deviations occur at the opposite ends. Binding of the Vaska-type complex results in a radical displacement of the iridium-bound carbon atoms away from the fullerene core by ca. 0.2 ,~. As a consequence, the distance between the mid-points of the "para" C - C
Transition metal fullerene chemistry
1637
I
D 2 d (23)
D2 (22)
cs 83 C6 C65
c"
c,3
C73
'"
/
s
" \
/
Fig. 5. Top ideal structures of the D2d (23) and D2 (22) isomers of C84 that are the most prevalent forms in the generated fullerene preparations. The numbers in parenthesis refer to the tabulation of C s4 isomers of Manolopoulos and Fowler [ 18, 19]. Bottom: the crystallographically determined structure of (r/2-Cs4)Ir(CO)CI(PPh3)2.The three two-fold axes pass through the centers of bonds between the circles that are hatched (from Ref. [14]). bonds that are attached to the iridium centers is 7.33 .A, which can be compared to the corresponding distance of 6.94 ,~ in C6o itself. Additionally, the C - C bonds that are involved in binding the iridium center are elongated by ca. 0.14 ,~. Recent work in our laboratory suggests that cocrystallization of fullerenes with Co(OEP) (OEP is the dianion of octaethylporphyrin) offers another way of obtaining fullerenes in ordered crystalline forms [20]. Fig. 9 shows a view of the structure of C60{Co(OEP)}2 [20]. Several features are noteworthy. Two of the metalloporphyrins cradle the fullerene, but there is some asymmetry in the positioning of the fullerene between these two porphyrins. The approaches of the fullerenes to the cobalt atoms are two long for covalent bond formation where C o - C distances in the range 19-2.2 ,~ would be expected. However, the fullerene/
porphyrin contacts are much shorter than normal van der Waals contacts, which should be in the 3.0-3.3-,~ range. Finally despite the lack of covalent bonding to anchor the fullerene, the C60 moiety is fully ordered in this structure. The formation of this fullerene/porphyrin aggregate is interesting not only for its structural information, but also because of its relation to the use of porphyrins in the chromatographic separation of fullerenes [21, 22]. Silica-based chromatographic supports which have been functionalized through covalent binding of porphyrins and metalloporphyrins have utility in fullerene separation and purification. Specific porphyrin/fullerene interactions have been postulated to be responsible for the chromatographic utility. Fig. 10 shows a conceptual drawing that illustrates the proposed fullerene/porphyrin interaction [22]. Examination of
1638
A.L. BALCH et al.
S
Fig. 8. A superposition of the structure of C60{Ir(CO) CI(PPhMe2)2} 2(solid lines) and an ideal C60 molecule (dashed lines). Note that the distortion of the fullerene is confined to the region near the iridium atoms.
II (
~-~
01a
C4
•
c,, .j.
cdc,
4. FULLERENE OXIDATION AND FULLERENE OXIDE STRUCTURES
C37
Fig. 6. The structures of C6o{Ir(CO)CI(PMe2Ph)2}2 (from Ref. [16]) and CT0{Ir(CO)CI(PMe2Ph)}2 (from Ref. [15]).
particle size effects on the chromatographic efficiency led to the suggestion, illustrated by this drawing, that the fullerene could interact with a pair of closely spaced porphyrin appendages [22]. The C60{Co(OEP) }2 structure in Fig. 9 is an interesting model for this interaction.
k
_~ ...........
C, -80°C
I u~ . . . .
I
30
. . . .
I
20
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30
. . . .
I
tO
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20
. . . .
0
C
10
I
. . . .
-10
I
Any technological application of the fullerenes is likely to be effected or limited by oxidative degradation by atmospheric oxygen. We and others have been exalaining oxidation of C6o under a variety of conditions [23-25]. As a result, the oxides C600 [23, 24], C6oO2 [25], C700 [26, 27] and Ci2oO [28-30] have all received serious structural study by spectroscopic and crystallographic investigation. Structures in the form of line drawings of each of these are shown in Fig. 11. Initial work in this laboratory focused on crystallographic characterization through the use of adducts with Vaska's complex. This was done since C6oO itself, like C6o, shows orientational disorder [31]. The crystallographic work agrees with earlier spectroscopic studies which show that C600 has an epoxide structure with the oxygen atom lying over a 6:6-ring junction [32, 33]. Fig. 12 shows the structure of (~/2-C6oO)Ir(CO)CI(PPh3)2 [32] and (7/2-C6o)Ir(CO)Cl(AsPh3)2 [33]. Both show some disorder in the oxygen atom position with two partially
[Co(OEP)]2C6o
. . . .
-20
-30
,
,. . . .
0
-10
-20
-30
Col2)
I ' 30
'
'
'
I ' 20
'
'
'
I ' 10
'
'
'
I 0
'
'
'
' I -10
. . . .
I -20
'
'
' -30
Fig. 7. The 31p { IH} NMR spectra of C6o{Ir(CO)CI(PEt3)2}2 in 6:4 o-dichlorobenzene/toluene at (A) 20, (B) -80 and (C) -80°C in the presence of an excess of added C60 (from Ref. [171).
Fig. 9. A view of the structure of C6o{Co(OEP)}2 (from Ref. [20]).
Transition metal fullerene chemistry
1639
O e
B
Fig. 10. Proposed porphyrin/fullerene interactions in chromatographic supports with porphyrin units appended to the surface (from Ref. [22]). occupied sites for the triphenylphosphine complex [32] and four such sites for the triphenylarsine complex [33]. Notice that the epoxide functionality provides an electronic effect which influences the site of binding of the iridium complex and which generally places that group on a 6:6-ring junction that is immediately adjacent to the epoxide. Oxidation of C60 with m-chloroperoxybenzoic acid yields at least six oxidation products but some control can be exercised to produce predominantly C6oO and a single isomeric form of C6002 [25]. 13C NMR spectroscopy reveals that this isomer has C~ symmetry. Crystallographic investigation through the formation of an adduct with Vaska's complex (Fig. 13) shows that this isomer of C6002 has a bis-epoxide structure in which the two oxygen atoms lie over two 6:6-ring junctions that are part of a common six-membered ring. Thus, the epoxide groups are as close together as possible. As Fig. 13 shows (part A), there are seven partially occupied oxygen atom sites in the crystal with all lying over 6:6-ring junctions. Careful analysis of the fractional occupancies of these sites shows that the pattern can only be explained if one isomer with the two epoxide groups immediately adjacent to each other is present. Part B of Fig. 13 shows the predominant adduct that is present in this solid. Oxidation of C7o with m-chloroperoxybenzoic acid or by photooxygenation with rubrene as photosensitizer yields C700 [26, 27]. JaC NMR and 3He NMR studies (the latter of the endohedral He@C700) indicate that two isomers of C700 are formed and that these have the epoxide groups attached to the C~-Cb and Co-Co bonds of the fullerene as shown in Fig. 11 [26]. Crystallographic examination of the adduct, (7/2-C 700)Ir(CO)CI(PPh3) 2, reveals a complex structure which involves disorder in the orientation of the C70 unit and in the oxygen atom site [27]. Fig. 14 shows the two orientations that are observed [27]. These occupy a common site and differ in location of the
OO
d
//~
• '~
d
b
a
I
1
•
2
0___.
d
4
Fig. 11. Line drawings of C6oO, C6oO2 (the known isolated isomer), C7oO(four isomers), and C 1200. long axis (originally the C5 axis) of the fullerene. Additionally, there are two sites for the epoxide oxygen atom. The structural work indicates that the oxygen atom does not reside in the equatorial belt of C70, where theoretical work indicates that the molecule would have the greatest stability [34]. Rather, the crystallographic work is consistent with the presence of the two isomeric forms (1 and 2) of C700 shown in Fig. 11. Recently a chromatographic separation of these isomers has been achieved [27].
5. ELECTROCHEMICAL STUDIES OF FULLERENE OXIDES: THE FORMATION OF REDOX ACTIVE
FILMS Electrochemical reduction with repeated cycling of C600, C6002 and C700 at the second reduction step results in the coating the electrode with orange brown
1640
A.L. BALCH et al.
C50~ C 5 3 ~
C6
C54C ~ 4 '
Fig. 12. The structures of (~2-C~O)Ir(CO)Cl(PPh3)2(from Ref. [321) and (~2-C~O)Ir(CO)CI(AsPh3)2(from Ref. [331).
films that strongly adhere to the electrode surface [35, 36]. Representative electrochemical data for the reduction of C600 are shown in Fig. 15 [36]. Repeated cycling as shown in Trace D leads to an increase in current during the reductive processes and the development of new features at ca. - 0 . 2 V range upon re-oxidation. In order to observe film formation it is necessary to reach the reduction potential for the second reduction process for C6oO. When the switching potential is set so that only the first one-electron reduction of C6oO occurs, no film formation occurs as seen in Trace A of Fig. 15. However, as the switching potential reaches more negative values (as seen in Traces B, C, and D) increases in current that accompany film growth during repeated cycling are observed. The second reduction, to form C 6002-, appears to involve a reaction that involves the epoxide unit, possibly opens that three-membered ring, and induces polymerization. The film that coats the electrode surface is insoluble in common organic solvents and in water. The infrared spectrum of the film shows features due to tetra(n-butyl ammonium) ions, and the fullerene units. Images of the film itself obtained by scanning and transmission electron microscopy show that the film involves the formation of spherical units of surprisingly uniform size. Fig. 16 shows a scanning electron micrograph of the surface of a gold electrode that has been
coated with film grown from C6oO. Piles of spherical units extend out from both edges of the electrode and meet at the center. These spherical units have a diameter that corresponds to a linear row of ca. 1000 buckyballs. However, at this time we do not know whether these spherical units are solid or hollow. The film is sufficiently stable that repeated electron microscopic views of the same structural features can be obtained. C12oO, which has recently been prepared by the thermal reaction between C6oO and C60 [28-30], is an interesting structural model for a linking group that may be present within these fuilerene films. A line drawing of the structure of C 12oO,which involves the formation of a tetrahydrofuran-like linkage between the two fullerenes, is shown in Fig. 11. Under electrochemical reduction, C 1200 does not act as a film-forming precursor [30]. This is not entirely surprising. The linkage between the two fullerene units is relatively unstrained and is hidden from the environment by the surrounding fullerene cages. Fig. 17 compares the electrochemical reduction of C~o with that of C 1200 [30]. In the potential range utilized here, C6o shows three reversible, one-electron reductions as can be seen in Trace a of Fig. 17. Trace b shows the reductive behavior for C 120O. Waves corresponding to those of C60 show broadening and resolution into pairs of reduction processes. Further examination has shown that there are three
Transition metal fullerene chemistry
f
1641
°
~20B
Fig. 13. The structures of (~t2-C6oO2)Ir(CO)CI(PPh3)2as determined by X-ray crystallography. (A) The location of the seven oxygen atom sites. (B) The principle form of the molecules in the solid (from Ref. [25]). pairs of one-electron reductions in the electrochemical cycles shown in Trace b of Fig. 17. These pairs correspond to the single-electron reductions of C60, but in the case of C l20 we see successive additions of single electrons to each ball within the fullerene "dimer". The resolution observed indicates that some degree of communication between the two fullerene units exists in C 1200. Work is proceeding in our laboratory to determine whether we can chemically abstract C 1200 units from our C600-derived films. Since fullerene-based polymers may have eventual utility as coatings, sensors, or battery components (if they are conducting), we have sought additional ways to prepare these films. In particular, we have sought means of avoiding the chemical processing that is involved in the preparation of the fullerene epoxide, C6oO. Thus we were pleased to discover that electrochemical reduction of Coo itself in the presence of controlled quantities of dioxygen also leads to film formation. Fig. 18 shows multicycle (25 scans) cyclic voltammograms for C60 in the presence of dioxygen (Trace b) and in the absence of dioxygen (Trace a). Trace B shows the expected two one-electron reduction processes that are expected within this potential
Fig. 14. The two forms of (~2-C700)Ir(CO)CI(PPh3)2present at a common site in the crystalline solid. Pentagons at the ends of the (former) C5 axis of the parent fullerene are shown as solid lines (from Ref. [27]). window. There is no build up of film, no increase in current, and no additional electrochemical features. However, with dioxygen present, (Trace b) there are clear changes in the electrochemistry that are indicative of film formation and an orange brown film is visually observed to coat the electrode surface. Like the film grown from C600, this film is insoluble in organic solvents and water. It adheres to the electrosurfaces and has infrared spectral features similar to the film grown from C600. However, electron microscopy shows a different film morphology from that seen in Fig. 16. Further work to characterize this new film and to compare it to the C600-derived film is in progress. In conclusion, this short article shows that efforts to use transition metal complexes to produce ordered solids for structural study by X-ray crystallography have been successful and have led our group into the study of the fullerene oxides. From there we have found a new means to polymerize fullerenes into a redox active film that coats electrodes. But here we are left with a more difficult structural question than the one we set out to examine. What do these fuilerene films look like at the molecular level? And we are left with the opportunity to explore the utility of these films.
1642
A.L. BALCH et al. I
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Fig. 17. (a) Cyclic voltammogram of 0.35 mM C6o in o-dichlorobenzene with 0.1 M tetra(n-butyl)ammonium perchlorate as supporting electrolyte at a gold electrode (0.75-mm radius). (b) Cyclic voltammogram (solid line) and differential pulse voltammogram (broken line) of C ,200 in o-dichlorobenzene with 0.1 M tetra(n-butyl)ammonium perchlorate as supporting electrolyte at a gold electrode (0.75-mm radius). Potentials are referenced to ferroccne/ferrocenium couple (from Ref. [36]).
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Transition metal fullerene chemistry Acknowledgements--We thank the National ScienceFoundation (CHE9022909, CHE9321257, and CHE93921257) for support of this research and Johnson Matthey for a loan of iridium salts.
REFERENCES 1. Taylor, R. (Ed.), The Chemistry of Fullerenes. World Scientific Publishing Co. Pte. Ltd., River Edge, NJ, 1995. 2. Hirsh, A., The Chemistry of Fullerenes. Georg Thieme Verlag, Stuttgart, 1994. 3. Balch, A.L. In: Taylor, R. (Ed.), The Chemistry of Fullerenes. World Scientific Publishing Co. Pte. Ltd., River Edge, NJ, 1995, p, 220. 4. Fagan, P.F., Calabrese, J.C. and Malone, B., Accounts Chem. Res., 1992, 25, 134. 5. Hawkins, J.M., Accounts Chem. Rec., 1992, 25, 150. 6. Westmeyer, M.D., Galloway, C.P. and Rauchfuss, T.B., Inorg. Chem., 1994, 33, 4615. 7. Roth, G., Adermann, P. and Knitter, R., Mater. Lett., 1993, 16, 357. 8. Mavunkal, I.P., Chi, Y., Peng, S.-M. and Lee, G.-H., Oganometallics, 1995, 14, 4454. 9, Rasinkangas, M., Pakkanen, T.T., Pakkanen, T.A., Ahlgren, M. and Rouvinen, J., J. Am. Chem. Soc., 1993, 115, 4901. 10, Arce, M.-J., Viado, A.L., An, Y.-Z., Khan, S.I. and Rubin, Y., J. Am. Chem. Soc., 1996, 118, 3775. 11. Burgi, H.B., Blanc, E., Schwarzenbach, D., Liu, S., Lu, Y., Kappes, M.M. and Ibers, J.A., Angew. Chem. Int. Ed. Engl., 1992, 31, 640. 12. Balch, A.L., Catalano, J.V. and Lee, J.W., lnorg. Chem., 1991, 30, 3980. 13. Balch, A.L., Catalano, V.J., Lee, J.W., Olmstead, M.M. and Parkin, S., J. Am. Chem. Soc., 1991, 113, 8953. 14. Balch, A.L., Ginwalla, A.S., Lee, J.W., Noll, B.C. and Olmstead, M.M., J. Am. Chem. Soc., 1994, 116, 2227. 15. Balch, A.L., Lee, J.W. and Olmstead, M.M., Angew. Chem. Int. Ed. Engl., 1992, 31, 1356. 16. Balch, A.L., Lee, J.W., Noll, B.C. and Olmstead, M.M., J. Am. Chem. Soc., 1992, 114, 10984. 17. Balch, A.L., Lee, J.W., Noll, B.C. and Olmstead, M.M., Inorg. Chem., 1994, 33, 5238. 18. Manolopoulos, D.E. and Fowler, P.W., J. Phys. Chem., 1992, 96, 7603. 19. Manolopoulos, D.E., Fowler, P.W., Taylor, R., Kroto, H.W. and Walton, D.R.M., J. Chem. Soc. Faraday Trans., 1992, 88, 3117.
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20. Balch, A.L., Costa, D.A., Olmstead, M.M., VanCalcar, P.M. To be submitted for publication. 21. Xiao, J., Savina, M.R., Martin, G.B., Francis, A.H. and Meyerhoff, M.E., J. Am. Chem. Soc., 1994, 116, 9341. 22. Martin, G.B., Xiao, J., Savina, M.R., Wilks, M., Francis, A.H., Meyerhoff, M.E., In: Kadish, K.M., Rouff, P.S. (Eds.), Fullerenes. Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, vols. 94-24. Electrochemical Society, Pennington, NJ, 1994, p. 178. 23. Creegan, K.M., Robbins, J.L., Robbins, W.K., Millar, J.M., Sherwood, R.D., Tindall, P.J., Cox, D.M., Smith III, A.B., McCauley, J.P. Jr., Jones, D.R. and Gallagher, R.T., J. Am. Chem. Soc., 1992, 114, 1103. 24. Elemes, Y., Silverman, S.K., Shen, C., Kao, M., Foote, C.S., Alvarez, M.M. and Whetten, R.L., Angew. Chem. Int. Ed. Engl., 1992, 31, 351. 25. Balch, A.L., Costa, D.A., Noll, B.C. and Olmstead, M.M., J. Am. Chem. Soc., 1995, 117, 8926. 26. Smith, A.B., III Strongin, R.M., Brard, L., Furst, G.T., Atkins, J.H., Romanow, W.J., Saunders, M., JimenezVazquez, H.A., Owens, K.G. and Goldschmidt, R.J., J. Org. Chem., 1996, 61, 1904. 27. Balch. A.L., Costa. D.A., Olmstead, M.M. Submitted for publication. 28. Lebedkin, S., Ballenweg, S., Gross, J., Taylor, R. and Kratschmer, W., Tetrahedron Lett., 1995, 36, 4971. 29. Smith III, A.B., Toyuyama, H., Strongin, R.M., Furst, G.T., Romanow, W.J., Chait, B.T., Mirza, U.A. and Haller, I., J. Am. Chem. Soc., 1995, 117, 9359. 30. Balch, A.L., Costa, D.A., Fawcett, W.R. and Winkler, K., J. Phys. Chem., 1996, 100, 4823. 31. Vaughan, G.B.M., Heiney, P.A., Cox, D.E., McGhie, A.R., Jones, D.R., Strongin, R.M., Cichy, M.A. and Smith III, A.B., Chem. Phys., 1992, 168, 185. 32. Balch, A.L., Costa, D.A., Lee, J.W., Noll, B.C. and Olmstead, M.M., lnorg. Chem., 1994, 33, 2071. 33. Balch, A.L., Costa, D.A., Noll, B.C. and Olmstead, M.M., lnorg. Chem., 1996, 35, 458. 34. Raghavachari, K. and Rohlfing, C.M., Chem. Phys. Lett., 1992, 197, 495. 35. Fedurco, M., Costa, D., Balch, A.L. and Fawcett, W.R., Angew. Chem. Int. Ed. Engl., 1995, 34, 194. 36. Winkler, K., Costa, D.A., Balch, A.L. and Fawcett, W.R., J. Phys. Chem., 1995, 99, 17431. 37. Balch, A.L., Costa, D.A., Fawcett, W.R., Winkler, K., Adv. Mater., 1997, 9, 153.