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Organometallic Chemistry of Fullerenes:2 5 ( )h - and h - p Complexes

R. C. HADDONDepartments of Chemistry and Physics, University of Kentucky,Lexington, Kentucky 40506-0055

Received 5 June 1997; accepted 2 July 1997

ABSTRACT: The reactivity of the fullerenes is primarily a function of theirstrain, as measured by the pyramidalization angle or curvature of the conjugatedcarbon atoms. A consideration of the orientation of the p-orbitals shows thath 2-complexation reactions lead to reaction products with the fullerenes that arevery similar to those obtained from unstrained alkenes. Furthermore, a largeamount of strain energy is released in this reaction, so it is clear just why thisreaction is important in fullerene chemistry. On the other hand, it is shown thatthe p-orbitals of C are poorly oriented for overlap with an exohedral metal60atom centered over the five- or six-membered rings, but well disposed foroverlap with an endohedral metal atom centered under the five- or six-memberedrings. Q 1998 John Wiley & Sons, Inc. J Comput Chem 19: 139]143, 1998

Introduction

t is well known that the chemistry of theI fullerenes is characterized by addition reac-tions.1 ] 4 The fullerenes undergo such reactionswith relative ease because the conversion of trigo-nal carbon atoms to tetrahedral carbon atomsserves to release the tremendous strain present inthe spheroidal geometry.5

It is straightforward to understand how s-bondformation will be assisted by the geometry of thecarbon atoms by a consideration of the POAV1

Ž .pyramidalization angle u . In POAV1 theory thePp-orbital axis vector is defined to be that vector

Correspondence to: R. C. Haddon

that makes equal angles to the s-bonds at a conju-gated carbon atom. As can be seen in Figure 1, theC POAV pyramidalization angle of u s 11.64860 P

is actually closer to the ideal tetrahedral angle thanto the planar geometry required for the ideal trigo-nal angle.

Thus, it is clear that a chemical reaction thatconverts trigonal carbon atoms to tetrahedral car-bon atoms will receive a very large steric assis-tance due to the relaxation of the strain in the cage.In fact, in an idealized analysis it was concludedthat at least 20 carbon atoms would have to beconverted to tetrahedral bonding in order to ab-sorb the curvature and strain in C .5 Thus it is60

clear that the organic functionalization reactionsthat lead to s-bond formation with the fullerenecarbon atoms will receive a very large steric accel-

( )Journal of Computational Chemistry, Vol. 19, No. 2, 139]143 1998Q 1998 John Wiley & Sons, Inc. CCC 0192-8651 / 98 / 020139-05

HADDON

FIGURE 1. Schematic representation ofpyramidalization angle.

eration in comparison with unstrained substrates.In this way the stability associated with the aro-matic nature of the fullerene topology is maskedby the countervailing drive of the fullerenes torelieve strain.5

In this article, we focus on the organometallicchemistry of the fullerenes. It has been knownsince the earliest work on fullerene chemistry thath 2-complexes are readily formed and isolated.6 ] 12

Ž .Given the presence of five-membered rings 5-MRsin the fullerenes, why is there not a well-devel-oped class of pentahapto h 5-complexes? We firstbegin with a review of the steric assistance re-ceived by the h 2-complexation reaction.5 The strain

Ž . Ž .energy analysis of the formation of Ph P Pt C ,3 2 60

using the structure reported by Fagan and cowork-ers,6 is shown in Figure 2. For comparison pur-

Ž .poses we make use of the structure of Ph P Pt3 213Ž .Cl C CCl , as a representative example of2 2

h 2-complexation of an unstrained alkene. ThePOAV1 strain energy analysis of this process isdiagrammed in Figure 2.5

The strain energy analysis is divided into twoparts: the local and global terms.5 The local termoriginates from the atoms that undergo changes ingeometry as a result of direct coordination orbonding to the substituent—in this case the twoatoms coordinated to the platinum atom. Theglobal term reflects the changes in geometry ofthe remaining atoms in the fullerene—in this casethe 58 atoms not coordinated to platinum. Al-though it might be expected that the only atomsundergoing a change in geometry would be thoseat the point of attachment this is not the case,because curvature, pyramidalization, and hy-bridization are approximately conserved in thefullerenes.5, 14 ] 16 Given the fact that the two atoms

5 ( ) ( )6FIGURE 2. POAV1 strain energy analysis of the formation of Ph P Pt C . For comparison purposes, the same3 2 6013 2( ) ( )analysis is applied to the structure of Ph P Pt Cl C CCl , as a representative example of h -complexation of an3 2 2 2

( ) 2unstrained alkene. The strain energy SE is obtained from the relationship SE = 196 = u kcal / mol, where u isP Pexpressed in radians.17 The local strain relief pertains to C1 and C2, the points of attachment of the metal. The globalstrain relief refers to strain relaxation that occurs in the other 58 carbon atoms due to the fact that complexationincreases the pyramidalization at C1 and C2, and thereby reduces the strain at the remaining carbon atoms.5

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at the point of attachment become more pyrami-Ž .dalized u increases from 11.648 to 15.428 , theP

amount of pyramidalization required at the otherwcarbon atoms is reduced u decreases from 11.648P

Ž .xto 11.518 average . Although the reduction inpyramidalization is small, because it is spreadover 58 carbon atoms, it can have a large effect.5

Note that it is this term that is responsible forfullerene selectivity. Because the average carbonatom is less pyramidalized the substitutedfullerene is less reactive. However, due to the lossof symmetry, some carbon atoms may becomemore pyramidalized and the selectivity issue re-quires an examination of the individual carbonatom pyramidalization angles.

Using the relationship SE s 196 = u 2 kcalrmolPŽ . 17u radians , the analysis presented in Figure 2P

clearly shows that the formation of fullerene h 2-complexes receives a considerable steric accelera-tion. In the present case this amounts to almost 23kcalrmol, most of which will be available in theearly stages of the reaction and serve to lower thetransition state energy. We now turn to a consider-ation of the h 5- and h6-complexation reactions andtheir geometric demands. In this case, the curva-ture of the fullerenes works against the coordina-tion chemistry, basically because the orbitals areskewed outward from the centers of the 5- and6-MRs, and the simultaneous overlap of the metalwith all 5 or 6 of the ring p-hybrids is ineffective.5, 6

In fact it is possible to solve for the orientation ofthe p-hybrids with respect to the normals to the

Ž .planes of the 5- and 6-MRs Table I . The directionof the p-orbital axis vector according to POAV1theory is given above; the POAV2 p-orbital axisvector is obtained by construction of the hybridorbital that is locally orthogonal to the s-orbitals ata conjugated carbon atom.18 Due to the symmetry,in the case of the 5-MR, the POAV2 makes anangle of 106.38 to a vector from the center of thering to the carbon atom, such that the POAV isdirected radially away from the ring center. Thusthe p-orbitals are poorly oriented for h 5- and h6-complexation reactions, and improvement of theoverlap would increase the strain. Recent experi-mental work supports this general picture.

A 6-MR face-capping arene-like complex of C60has recently been isolated and characterized:

Ž . Ž 2 2 2 . 19Ru CO m ]h , h , h ]C . In this complex3 9 3 60all three of the partial double bonds in a single6-MR are complexed in dihapto fashion. Thus,with the spread in p-orbitals it is possible to com-plex three metal atoms to the 6-MR.

TABLE I.( )Orientation of C p-Orbital Axis Vectors POAVs .60

POAV1 POAV2( ) ( )Angle of POAV to: deg. deg.

a5 ]6 Bond 101.6 99.5b6 ]6 Bond 101.6 105.5

Normal to 5-MR 20.1 16.3Normal to 6-MR 23.8 25.8

aBond that separates 5- and 6-MRs.bBond that separates two 6-MRs.

A pentahapto metal complex of a fullerenederivative was recently isolated and characterized:Ž 5 . w x 20Tl h ]C Ph ? 2.5 THF . We analyze the struc-60 5

ture of this compound in some detail in whatfollows; further consideration of h6-complexationreactions is deferred to a later study, although it isof interest to note that the directionality of thep-orbital axis vectors favors coordination of the5-MRs over the 6-MRs.

In Figure 3 we show the pyramidalization an-gles of the pentaphenyl-fullerene anion in thiscomplex, as calculated from the structure suppliedby the investigators.20 The phenyl groups areomitted from the structure, but their point of sub-stitution on the fullerene is indicated by solidcircles. It is immediately apparent that these car-bon atoms absorb a large fraction of the curvatureor pyramidalization. In fact these carbon atoms arepyramidalized considerably beyond the tetrahe-dral pyramidalization angle of u s 19.478, andP

this is the most pyramidalized derivative to beexamined. The pyramidalization angles in the bot-tom half of the molecule are changed little fromthose in the free C molecule. The net result is to60

flatten the upper carbon atoms in thefullerene—those toward the Tlq atom and particu-larly those atoms in the 5-MR that are actuallycomplexed by the metal atom. This strongly sug-gests that the pyramidalization in free C is the60determining factor in inhibiting h 5-complexation.

This point is further reinforced by a considera-tion of the inclination angles made by the p-orbitalsto the radial vector in the plane of the 5-MR asshown in Figure 4. Using POAV2 theory, the angleof 106.38 in free C , drops to 98.58 in the thallium60

complex. Thus, it is clear that the pentaphenylsubstitution greatly improves the orbital overlapbetween 5-MR p-orbitals and the metal atom, tothe point that the h 5-complex is now stable.

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( 5 ) [ ] 20FIGURE 3. POAV1 pyramidalization angles of the Tl h ]C Ph ? 2.5 THF structure, averaged over the pseudo-60 5fivefold axis. The numbers in parentheses refer to the number of carbon atoms.

Hirsch and coworkers have shown that s-bondformation with the concave face of the C60molecule is strongly inhibited due to the strainthat would be present in the product.21 The onlyapparent exception to this behavior is the sugges-tion that steric crowding on the convex surface ofC H leads to a preference for some fraction60 60of the hydrogen atoms to bond to the concavesurface.22 It is known that s-bond formation andh 2-complexation reactions readily occur on theconvex face.1 ] 4, 23 This is shown schematically inFigure 5, along with the present results whichsuggest that h 5- and h6-complexation reactionsprefer the opposite face—that is, the concave facedue to considerations of orbital overlap.

An alternative viewpoint of this process isshown using the sense of the p-orbital axis vector.The positive sense of the POAV is defined to lietoward that lobe of the p-orbital that has the samesign as the s-orbital.24 Thus, reactants that have alow nuclearity on the fullerene and form strongbonds to one or two atoms will be favored by apositive sense of the POAV due to the divergenceof the orbital hybrids. On the other hand, reactants

Ž .with a high nuclearity involving five or six atomswill be favored by a negative sense of the POAV,which leads to convergence of the orbitals in theinterior of the fullerene.

This behavior may not hold at small pyramidal-ization angles, such as those found in the outer6-MRs of corannulene, as a recent study providescomputational evidence for complexation of an RuŽ U .qCp fragment on the concave face of the coran-

FIGURE 4. Inclination of the POAV2 to the radial vector( 5 ) [ ]of the 5-MR in C and in the Tl h ]C Ph ? 2.5 THF60 60 5

structure.20 With POAV1 theory the values are: 111.18( ) ( )C , 101.18 complex .60

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FIGURE 5. Fullerene reactivity with respect to chemistryon the outside and inside of the fullerenes.

Žnulene molecule although the analysis is compli-. 25cated by steric congestion .

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