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5 Metalation of C(60) with Metal-centered Radicals

5 Metalation of C(60) with Metal-centered Radicals

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228



6 Radical Additions



Scheme 6.15



center of the spectrum, which is apparently related to the products of multiple

addition of the carbon-centered radicals resulting from the homolytic cleavage of

Hg-CF(CF3)2 [10, 72].

Refluxing benzene solutions of C60 in the presence of a 20-fold excess of Bu3SnH

leads to hydrostannylation (Scheme 6.15) [73]. Multiple additions can also take place.

To maximize the yield of the monoadduct C60HSnBu3 (24), the time dependence

of the reaction was followed quantitatively by HPLC. After about 4 h, the concentration of the monoadduct 24 reaches its maximum. Compound 24 can be isolated by

preparative HPLC on a C18-reversed-phase stationary phase with CHCl3–CH3CN

(60 : 40, v/v) as eluent. The structure of C60HSnBu3 (24) was determined by 1H NMR

spectroscopy and other methods, showing that a 1,2-addition takes place regioselectively (Scheme 6.15) [73].



6.6

Addition of bis(Trifluoromethyl)nitroxide



The treatment of C60 with an excess of bis(trifluoromethyl)nitroxide [(CF3)2NO•

radicals] leads to multiple additions with an average of 18 (CF3)2NO groups bound

to the fullerene sphere (Scheme 6.16) [74]. The reaction has been carried out in a

glass tube equipped with a Teflon valve. The resulting light brown solid decomposes

above 158 °C.



Scheme 6.16



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231



7

Transition Metal Complex Formation

7.1

Introduction



The electron deficiency of the fullerenes C60 and C70, established by calculations,

electrochemistry and the reaction behavior towards nucleophiles, has been

independently demonstrated by investigations of fullerene transition metal

complexes. Various single-crystal structures and spectroscopic studies show that

the complexation of transition metals to the fullerene core proceeds similar to wellestablished reactions of electron-deficient olefins in a dihapto manner to one or

more π-bonds or as hydrometalation reactions. Some of these complexation

reactions are more or less reversible. The resulting thermodynamic control explains

the remarkable regioselectivity observed at the formation of higher addition

products, for example bisadducts or hexakisadducts. Packing effects in the solidstate structure can also play an important role for the exclusive formation of a

specific regioisomer. One driving force for the formation of a certain solid-state

structure of fullerene derivatives could be due to π–π interactions of ligand-bound

electron-rich arene moieties and the electron-poor fullerene core itself. This is

demonstrated by impressive examples for supramolecular arrangements in single

crystals of fullerene transition metal complexes.



7.2

η2-C60) Transition Metal Complexes





The question as to whether C60 behaves like an aromatic or an electron-deficient

alkene was answered elegantly by Fagan [1–3]. The complex [Cp*Ru(CH3CN)3]+O3SCF3− (Cp* = η5-C5(CH3)5) reacts with electron-rich planar arenes upon displacement of the three coordinated acetonitrile ligands, resulting in a strong η6-binding

of ruthenium to the six-membered rings of the arenes [4]. Conversely, the reaction

of this ruthenium complex with an electron-deficient alkene leads to the displacement of one acetonitrile ligand and the formation of a η2-olefin complex. Upon the

reaction with a ligand containing both an alkene functionality and an arene ring,

the ruthenium exclusively binds to the arene ring [4]. This remarkable selectivity

makes the ruthenium complex a good candidate for testing the transition metal



232



7 Transition Metal Complex Formation



complex formation behavior of C60. Allowing C60 to react with a 10-fold excess of

[Cp*Ru(CH3CN)3]+ O3SCF3− in CH2Cl2 at 25 °C for five days results in a brown

precipitate of [[Cp*Ru(CH3CN)2]3(C60)]3+ (O3SCF3−)3, in which two acetonitrile

ligands are retained on each ruthenium [2]. This suggests that each ruthenium is

bound to one double bond of the C60 sphere. Therefore, the reactivity of C60 is also

shown from this point of view to be that of an electron-deficient olefin.

Considering the geometry of the p-orbitals within a hexagon of the C60 framework,

which are tilted away from the center of the ring, it is already obvious that a binding

of C60 to a metal in a hexahapto fashion is not favorable because the orbital overlap

will be weakened. This was confirmed in some theoretical calculations [5–8]. Using

PM3(tm) or combined PM3(tm)-density functional theory studies the exchange of

a η6-bound benzene for C60 was calculated. With most transition metal complex

fragments this exchange was found to be endothermal.

Coordination to two or three of the double bonds of a hexagon was found in

various osmium, rhenium, iridium or ruthenium complexes, but these complexes

do not exhibit a η6-complexation. In fact metal clusters, not single metal atoms,

bind to C60 such that the metals are η2-bound to adjacent bonds of a C6 face

(Section 7.3).

The binding of C60 to transition metals in a η2-fashion becomes clearly evident

by the formation of various complexes [9] (Scheme 7.1) of platinum, palladium

and nickel [1–3, 10–16], iridium, cobalt and rhodium [9, 17–30] iron, ruthenium

and osmium [25, 31, 32], manganese [29, 33], titanium [34], rhenium and tantalum

[25, 29] and molybdenum and tungsten [35–44]. Low-valent complexes of these

metals easily undergo complexation with electron-deficient olefins [45, 46]. A typical

structural aspect is the loss of planarity of the olefin coordinated to the transition

metal, because the four groups bound to the olefin bend back away from the metal

(Figure 7.1). In substituted ethylenes, C2X4, this deformation increases upon

increasing electronegativity of X. In C60, the arrangement around [6,6] double bonds

is already preorganized. Therefore, the combination of both strain and electron

deficiency of the [6,6] double bonds is an important driving force of η2-binding of

C60 with low-valent transition metals. The reaction of equimolar amounts of

(Ph3P)2Pt(η2-C2H4) with C60 leads to a dark emerald green solution of (Ph3P)2Pt(η2C60) (4, Scheme 7.1). Single-crystal X-ray structure analysis confirmed the η2-binding

of the platinum to a [6,6] fullerene double bond [1].



Figure 7.1 Coordination of an olefin to the transition metal fragment

M(PPh3)2 (M = e.g. Ni, Pd, Pt) leads to a deviation from planarity of

the olefin system. The angle θ is a measure of the deformation of the

groups attached to the carbon–carbon double bonds from planarity.



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