Homo fullerenes

Systematic investigations of twofold additions of malonates to C70 revealed that the second addition takes place at one of the five a-bonds of the unfunctionalized pole [17, 26], With achiral, C2v-symmerical malonate addends, three constitutionally isomeric bisadducts are formed An achiral one (C2v-symmetrical 1), and two chiral ones (C2-symmetrical 2 and 3), which are obtained as pairs of enantiomers with an inherently chiral addition pattern (Figure 13.5). Twofold addition of chiral malonates leads to the formation of five optically active isomers, two constitutionally isomeric pairs of C2-symmetrical diastereomers and a third constitutional C2-symmetrical isomer (Figure 13.5). Twofold additions of azides to C70 lead to diazabis[70]homo-fullerenes, which served as starting material for the synthesis of bis-(aza[70]-fullerenyl) (Cg9N)2 (Chapter 12) [27]. As further bisadditions, addition reaction to C70 [2+2]cycloaddition of electron-rich bis(diethylamino)ethyne and 1-alkylthio-2-(diethylamino)ethynes [28] and the addition of transition metal fragments have been reported [29-32],... 

In light of oxidative processes, the high degree of resonance stabilization that arises from the maximally occupied HOMO (10 electrons), makes it an extremely difficult task to remove an electron from the HOMO level [31], Thus, [60]fullerene can be considered mostly an electronegative entity which is much more easily reduced than oxidized. 

The most important classes of functionalized [60]fullerene derivatives, e.g. methanofullerenes [341, pyrrolidinofullerenes [35], Diels-Alder adducts [34i] and aziridinofullerene [36], all give rise to a cancellation of the fivefold degeneration of their HOMO and tlireefold degeneration of their LUMO levels (figure Cl.2.5). This stems in a first order approximation from a perturbation of the fullerene s 7i-electron system in combination with a partial loss of the delocalization. 

I he results of their calculations were summarised in two rules. The first rule states that at least one isomer C with a properly closed p shell (i.e. bonding HOMO, antibonding I. U.MO) exists for all n = 60 - - 6k (k = 0,2,3,..., but not 1). Thus Qg, C72, Cyg, etc., are in lhi-< group. The second rule is for carbon cylinders and states that a closed-shell structure is lound for n = 2p(7 - - 3fc) (for all k). C70 is the parent of this family. The calculations Were extended to cover different types of structure and fullerenes doped with metals. 

The most extensive calculations of the electronic structure of fullerenes so far have been done for Ceo- Representative results for the energy levels of the free Ceo molecule are shown in Fig. 5(a) [60]. Because of the molecular nature of solid C o, the electronic structure for the solid phase is expected to be closely related to that of the free molecule [61]. An LDA calculation for the crystalline phase is shown in Fig. 5(b) for the energy bands derived from the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for Cgo, and the band gap between the LUMO and HOMO-derived energy bands is shown on the figure. The LDA calculations are one-electron treatments which tend to underestimate the actual bandgap. Nevertheless, such calculations are widely used in the fullerene literature to provide physical insights about many of the physical properties. 

Calculations for Ceo in the LDA approximation [62, 60] yield a narrow band (- 0.4 0.6 eV bandwidth) solid, with a HOMO-LUMO-derived direct band gap of - 1.5 eV at the X point of the fee Brillouin zone. The narrow energy bands and the molecular nature of the electronic structure of fullerenes are indicative of a highly correlated electron system. Since the HOMO and LUMO levels both have the same odd parity, electric dipole transitions between these levels are symmetry forbidden in the free Ceo moleeule. In the crystalline solid, transitions between the direct bandgap states at the T and X points in the cubic Brillouin zone arc also forbidden, but are allowed at the lower symmetry points in the Brillouin zone. The allowed electric dipole... 

For C70, molecular orbital calculations [60] reveal a large number of closely-spaced orbitals both above and below the HOMO-LUMO gap [60]. The large number of orbitals makes it difficult to assign particular groups of transitions to structure observed in the solution spectra of C70. UV-visible solution spectra for higher fullerenes (C n = 76,78,82,84,90,96) have also been reported [37, 39, 72]. 

Azadienes 89, generated in situ by thermolysis of the corresponding o-aminobenzylalcohols, have been used for the derivatization of [60]-fullerene through C-N bond formation leading to tetrahydropyrido [60]-fullerenes [93]. Theoretical calculations predicted these cycloadditions to be HOMO azadiene-controlled (Equation 2.25). 

The nature of the electronic states for fullerene molecules depends sensitively on the number of 7r-electrons in the fullerene. The number of 7r-electrons on the Cgo molecule is 60 (i.e., one w electron per carbon atom), which is exactly the correct number to fully occupy the highest occupied molecular orbital (HOMO) level with hu icosahedral symmetry. In relating the levels of an icosahedral molecule to those of a free electron on a thin spherical shell (full rotational symmetry), 50 electrons fully occupy the angular momentum states of the shell through l = 4, and the remaining 10 electrons are available... 

Krakovjak MG, Anufrieva EV, Anan eva TD, Nekrasova TN (2005a) Water-soluble fullerene complexes with n-vinylcaprolactam homo- and copolymers and a method for preparation of these complexes. Russian patent RU 2 264 415 10.02.2005... 

The Sc -promoted photoinduced electron transfer can be generally applied for formation of the radical cations of a variety of fullerene derivatives, which would otherwise be difficult to oxidize [135]. It has been shown that the electron-transfer oxidation reactivities of the triplet excited states of fullerenes are largely determined by the HOMO (highest occupied molecular orbital) energies of the fullerenes, whereas the triplet energies remain virtually the same among the fullerenes [135]. 

The removal of the radical electron corresponds to the first oxidation process. The resulting cation should be diamagnetic. The first reduction is relatively easy, because filling of the HOMO leads to the closed shell species La Cg2T Theoretical calculations predicted that the location of the lanthanum within the cage is off-center, which allows a stronger interaction with carbon atoms of the fullerene sphere [81- 3]. 

As pointed out by Diederich [77] it is important to investigate fullerene properties as a fimction of addition pattern and the nature of the addend. Various pronoimced correlations and trends can be deduced. In general, with increasing reduchon in the conjugated fullerene re-chromophore, (1) the HOMO-LUMO gap increases ... 

Lull and co-workers showed that the tethered bisazides such as 2,2-dibenzyl-l,3-diazidopropane (124) undergo [3-t2] cycloadditions to adjacent [6 6]-bonds (cis-l addition). Thermal extrusion of N2 afforded a mixture of the corresponding cis-1-1,2,3,4-bisimino[60]fullerenes and twofold cluster-opened 2,3,4,5-bis-aza-homo[60]-fullerenes [36, 37, 111]. The reaction of with optically active bisazides yielded enantiomerically pure bis-azafullerenes with the same addition pattern [111]. 

Figure 14.8 Lowest energy VB structures, PM3 calculated lengths of [5,6 - double bonds, HOMO coefficients and energies of different dihydro[60 fullerenes.

Fig. 5 (a) Electrochemistry of Cgo fullerene, cyclic voltammetry (top) and differential pulse voltammetry (bottom) (Reprinted with permission from [54]). (b) Schematic representation of HOMO and LUMO orbitals after addition of six electrons (red arrows) to the fullerene... 

The electrochemical properties of TNT-EMFs, M3N C2n n > 39) differ from those of the empty cage fullerenes (see Fig. 6) due to the interaction of the metal cluster with the carbon cage and because the structure of these carbon cages are generally different. As a consequence, the reductive processes are electrochemically irreversible but chemically reversible. The oxidative processes occur at lower potentials because the HOMO orbital is mainly localized on the trimetallic nitride clusters and the HOMO-LUMO gaps in solution are smaller [25,58]. The endohedral metallo-fullerenes M C2n show similar behavior but even smaller HOMO-LUMO gaps [59]. 

The fabrication of diodes on silicon substrates was demonstrated using the supramolecular interactions between a 5,10,15,20-tetra(3-fluorophenyl)porphyrin and Ceo fullerene with a rectification ratio of 1,500 (see Fig. 11). The rectifying behavior is explained by theoretical calculations which show that the LUMO orbital is located mainly on the fullerene whereas the HOMO orbital is located on the porphyrin moiety [99]. 

Diazo compounds also undergo cycloaddition with fullerenes [for reviews, see (104),(105)]. These reactions are HOMO(dipole)-LUMO(fullerene) controlled. The initial A -pyrazoline 42 can only be isolated from the reaction of diazomethane with [60]fullerene (106) (Scheme 8.12) or higher substituted derivatives of Ceo (107). Loss of N2 from the thermally labile 42 resulted in the formation of the 6,5-open 1,2-methanofullerene (43) (106). On the other hand, photolysis produced a 4 3 mixture of 43 and the 6,6-closed methanofullerene (44) (108). The three isomeric pyrazolines obtained from the reaction of [70]fullerene and diazomethane behaved analogously (109). With all other diazo compounds so far explored, no pyrazoline ring was isolated and instead the methanofullerenes were obtained directly. As a typical example, the reaction of Cgo with ethyl diazoacetate yielded a mixture of two 6,5-open diastereoisomers 45 and 46 as well as the 6,6-closed adduct 47 (110). In contrast to the parent compound 43, the ester-substituted structures 45 and 46, which are formed under kinetic control, could be thermally isomerized into 47. The fomation of multiple CPh2 adducts from the reaction of Ceo and diazodiphenylmethane was also observed (111). The mechanistic pathway that involves the extrusion of N2 from pyrazolino-fused [60]fullerenes has been investigated using theoretical methods (112). 

Recently, it has been shown that there are two distinct classes of fullerenes. The first class, which includes Geo, C70. and several stable isomers of the higher fullerenes, have large HOMO-LUMO gaps and are soluble in many organic... 

The best solvent to obtain reversible oxidation waves of fullerenes has been, undoubtedly, TCE [60, 78]. One reversible oxidation has been observed for all isomers of Cg4 studied so far. Oxidations are irreversible in pyridine and in PhCN [53,54,78]. Oxidation potentials along with HOMO-LUMO gaps in solution for all the higher fullerenes are presented in Table 9. Note that the higher fullerenes have a smaller experimental HOMO-LUMO gap than Cgo and Cyo, and these experimental values correlate well with theoretical predictions [60, 78]. 

The small HOMO-LUMO band gap and presence of other close-in-energy MOs results in fullerenes being easily polarized. They all give very intense Raman scattering lines and have relatively large x values useful for NLO applications (11). Indeed, C60 is one of the best materials known to date for optical limiting. 

In terms of the Dewar-Chatt model of bonding, for v metal complexation one double bond is effectively removed from the fullerene conjugation system due to extensive interaction between metal d orbitals and the fullerene HOMO and LUMO (7). The remaining 29 double bonds then behave almost identically to uncomplexed C60 with their IR, Raman, UV-vis, and 13C NMR spectra showing only slight perturbations, mainly as a result of diminution of symmetry effects. Nevertheless, it is important to state that the fullerene metal interaction is not confined purely to the former s HOMO and LUMO, and that other molecular orbitals are energetically suitable for interaction 89,90). The spectroscopic evidence cited for the preceding statement is as follows ... 

---