Redox-active metal–metal bonds between lanthanides in dimetallofullerenes

Abstract

The empty space inside a fullerene cage can be filled with a variety of species, including metal dimers. Encapsulation of Sc₂, Y₂, or lanthanide dimers leads to dimetallofullerenes featuring metal–metal bonding molecular orbital. Such an orbital can be either HOMO or LUMO of the dimetallofullerene molecule. In certain cases, single-occupied metal–metal bonding orbital can be also stabilized. This review is focused on redox processes involving variation of the electron population of metal–metal bonding orbitals in dimetallofullerenes.

Introduction

The encapsulation of metal atoms by carbon cages in endohedral metallofullerenes (EMFs) leads to a plethora of interesting chemical and physical phenomena [1–5]. High chemical and thermal stability of fullerene cage protects endohedral entities from the environment and can stabilize unusual species, which cannot exist otherwise. Metal atoms enclosed inside a fullerene transfer their valence electrons to the carbon cage, resulting in “salts” with cationic metals and anionic fullerene cages. Electrochemistry has been traditionally used as a relatively simple and yet very powerful technique to study electronic structures of EMFs [1,6•].

Empty fullerenes are good electron acceptors and undergo multiple single-electron redox steps in solutions [7]. Encapsulation of metal atoms and clusters results in more complex redox behavior of EMFs since both the carbon cage and the endohedral cluster can exhibit redox activity. Especially interesting are endohedral (in cavea) electron transfer processes, in which the endohedral cluster is redox-active, whereas the carbon cage acts as an inert container transparent to electrons [8,9•]. An obvious prerequisite for the endohedral redox activity in EMF molecules is a localization of frontier molecular orbitals (HOMO or LUMO) on endohedral species. Experimentally, the endohedral redox processes can be identified via unexpected redox behavior (e.g., shifted potential when compared to analogous molecules) or via spectroscopic characterization of the charged species. Electron paramagnetic resonance (EPR) spectroscopy is an especially powerful tool, since EMFs with endohedral redox activity often exhibit rich hyperfine structure with large coupling constants in their ion radicals [10•].

This review is focused on the electrochemistry of EMFs featuring redox-active metal–metal bonds, and in particular on dimetallofullerenes (di-EMFs hereafter). First, we describe the electronic structure of di-EMFs from the molecular orbital (MO) point of view. This description forms a basis for the understanding of the redox behavior of three types of di-EMF: di-EMFs without metal–metal bonds, but with metal-based LUMO; di-EMFs with two-electron metal–metal bonds; and di-EMF with single-electron metal–metal bonds. Discussion of electrochemical properties of di-EMF is accompanied by the results of EPR spectroscopic measurements of their radical species.

Metal–metal bonding in dimetallofullerenes: theoretical description

Computational studies of di-EMF with Sc, Y, or lanthanides (metal is designated as M hereafter) show that these molecules feature metal–metal bonding molecular orbital, whose energy is close to the energy of the frontier cage-based MOs [11,12]. Whether the M–M bonding MO in a given di-EMF is the HOMO or the LUMO depends on the energy match between the metal-based and fullerene-based orbitals.

Figure 1a shows MO energy levels of two fullerene cages typical for di-EMFs, C₈₀-I_h(7) and C₈₂-C_3v(8) (fullerene isomers are designated by their point group symmetry and the number in accordance with Fowler–Manolopoulos spiral algorithm [13]). Characteristic feature of C₈₀-I_h(7) is the 4-fold degenerate orbital occupied by only two electrons. Jahn–Teller distortion reduces the symmetry and introduces a small gap between the HOMO and the 3-fold degenerate LUMO. The electronic structure of the molecule is very unstable, and C₈₀-I_h(7) has never been obtained as an empty fullerene. However, if the LUMO is filled with six electrons, a stable structure with large band gap is obtained [14]. C₈₀-I_h(7) is thus an archetypical cage for EMFs with 6-fold electron transfer from endohedral species to the fullerene [3].

Figure 1.

(a) Molecular orbital energy level of empty fullerene C₈₀-I_h(7) and C₈₂-C_3v(8) compared to those of the metal dimers La₂ and Lu₂ (DFT calculations at the PBE/TZ2P level). Occupied MO levels of fullerenes are shown as black lines, unoccupied levels–as pink lines. Gray arrows indicate donation of six or four electrons from metal dimer to fullerene in corresponding dimetallofullerenes. (b) Frontier molecular orbitals (HOMO and LUMO) of La₂@C₈₀-I_h(7) and Lu₂@C₈₂-C_3v(8).

C₈₂-C_3v(8) has small HOMO–LUMO gap, two low-lying unoccupied MOs, and a significant gap between the LUMO+1 and LUMO+2. The electronic structure of this fullerene is stabilized by addition of four electrons [15]. C₈₂-C_3v(8) (along with C₈₂-C_s(6), which has similar electronic structure) is therefore the most abundant fullerene cage for EMFs with 4-fold electron transfer.

Also shown in Figure 1 are the energy levels of the occupied valence MOs in the two lanthanide dimers, La₂ and Lu₂. La₂ has closed-shell electronic structure with six electrons occupying three MOs (hence (6s)σ_g²(5d)π_u⁴ configuration) [16]. The energies of these MOs are considerably higher than the energy of the LUMO in C₈₀-I_h(7), so when the La₂ dimer is encapsulated inside this cage, a complete transfer of all six valence electrons to the fullerene occurs. The formal charge distribution in the resulting di-EMF molecule is then (La³⁺)₂@C₈₀⁶⁻, the HOMO is localized on the fullerene, whereas the LUMO resembles the (6s)σ_g² orbital of the pristine La₂ dimer (Figure 1b). Thus, there is no La–La bonding in the non-charged La₂@C₈₀, but the LUMO of the molecule has the La–La bonding character, and the bond between metal atoms can be formed if the LUMO is populated by a surplus electron.

The lanthanide contraction results in a substantially different electronic structure of Lu₂ when compared to that of La₂. The ground state of Lu₂ is a triplet, (6s)σ_g²(6s)σ_u²(5d)π_u² [16], with a significant splitting of the spin-up and spin-down orbitals (Figure 1a). These orbitals span a broader energy range than in La₂. In particular, the (6s)σ_g² level in Lu₂ is ca. 2 eV lower in energy than in La₂ and, even more importantly, it has lower energy than the LUMO of C₈₀-I_h(7). As a result, the hypothetical Lu₂@C₈₀-I_h(7) has an open-shell electronic structure with five electrons transferred from Lu₂ to the C₈₀-I_h cage [17•]. C₈₂-C_3v(8) is a more suitable host for the Lu₂ dimer than C₈₀-I_h(7). In Lu₂@C₈₂-C_3v(8), four electrons from the (6s)σ_u²(5d)π_u² levels of Lu₂ are donated to the fullerene cage, whereas the (6s)σ_g² orbital of Lu₂ remains occupied. The formal charge distribution in the di-EMF is then (Lu²⁺)₂@C₈₂⁴⁻. The Lu–Lu bonding orbital resembling the (6s)σ_g² MO of Lu₂ is the HOMO of Lu₂@C₈₂, whereas the LUMO is localized on the fullerene cage (Figure 1b).

Redox-active metal–metal bonds in dimetallofullerenes

Dimetallofullerenes with the metal-based LUMO

Early lanthanides, such as La, Ce, and less studied Pr and Nd, form di-EMFs with the transfer of all six valence electrons to the carbon cage. In addition to the C₈₀-I_h(7) cage, several other fullerenes can act as acceptors of six electrons: La and/or Ce di-EMFs were reported for C₇₂-D₂(10611) [18,19], C₇₆-C_s(17490) [20], C₇₈-D_3h(5) [21,22], C₈₀-D_5h(6) [23], and C₁₀₀-D₅(450) [24]. In all these di-EMFs, the M–M bonding MO is the LUMO, and hence metal–metal bonds are expected to be formed in the anionic state(s).

Electrochemical studies of La₂@C_2n (2n = 72, 78, 80) showed that these di-EMFs exhibit 2–3 reversible single-electron reduction steps. The first reduction of La₂@C₈₀-I_h occurs at −0.31 V (all redox potentials discussed hereafter are measured in o-dichlorobenzene and are referred to the Fe(Cp)₂^+/0 redox couple) [25]. Likewise, the first reductions of La₂@C₇₂ (−0.68 V) [26], La₂@C₇₈ (−0.40 V) [22], and La₂@C₈₀-D_5h (−0.36 V) [23] are significantly more positive than for the EMFs with fullerene-based reductions (usually more negative than −1 V [1]). The first reduction potentials of analogous Ce di-EMFs are cathodicaly shifted by 0.04–0.13 V versus isostructural La di-EMFs (Table 1) [21,23,27,28].

Table 1. Redox potentials of di-EMFs featuring the M–M bonding HOMO or LUMO in comparison to selected clusterfullerenes^a.

EMF	E+2/+1	E+1/0	E0/−1	E−1/−2	E−2/−3	gapECb	Ref.
Metal-based LUMO
La2@C72-D2(10611)	0.75	0.24	−0.68	−1.92	−	0.92	[26]
Ce2@C72-D2(10611)	0.82	0.18	−0.81	−1.86	−	0.99	[27]
La2@C76-Cs(17490)	0.65	0.21	−0.63	−1.83	−2.40	0.84	[20]
La2@C78-D3h(5)	0.62	0.26	−0.40	−1.84	−2.28	0.66	[22]
Ce2@C78-D3h(5)	0.79	0.25	−0.52	−1.86	−2.23	0.77	[21]
La2@C80-D5h(6)	0.78	0.22	−0.36	−1.72	−	0.58	[23]
Ce2@C80-D5h(6)	0.66	0.20	−0.40	−1.76	−2.16	0.60	[23]
La2@C80-Ih(7)	0.95	0.56	−0.31	−1.72	−	0.87	[25]
Ce2@C80-Ih(7)	0.95	0.57	−0.39	−1.71	−	0.96	[28]
Metal-based HOMO
Er2@C82-Cs(6)	0.65	0.02	−1.01	−1.31	−	1.03	[32••]
Lu2@C82-Cs(6)	0.74	0.34	−1.00	−1.32	−1.77	1.34	[32••]
Er2S@C82-Cs(6)c	−	0.39	−1.01	−1.85	−2.21	1.40	[32••]
Sc2S@C82-Cs(6)c	0.65	0.39	−0.98	−1.12	−1.73	1.37	[50]
Sc2@C82-C3v(8)	−	0.02	−1.16	−1.53	−1.73	1.18	[32••]
ErSc@C82-C3v(8)	−	0.08	−1.11	−1.49	−1.72	1.19	[32••]
Er2@C82-C3v(8)	−	0.13	−1.14	−1.41	−1.83	1.27	[32••]
YLu@C82-C3v(8)	−	0.23	−1.13	−	−	1.36	[32••]
Lu2@C82-C3v(8)	0.95	0.50	−1.16	−1.46	−1.77	1.66	[32••]
Er2S@C82-C3v(8)c	0.88	0.51	−0.98	−1.21	−1.70	1.49	[32••]
Sc2S@C82-C3v(8)c	0.96	0.52	−1.04	−1.19	−1.63	1.56	[50]
Lu2@C86-C2v(9)	−	0.31	−1.01	−1.34	−	1.35	[31]
Sc2C2@C86-C2v(9)c	−	0.47	−0.84	−1.11	−1.63	1.31	[51]
Single-electron M–M bonding MO
Gd2@C79N-Ih(7)	−	0.51	−0.96	−1.98	−	1.45	[43]
La2@C80-CH2Ph	−	0.15	−0.92	−1.34	−1.64	0.97	[45••]
Y2@C80-CH2Ph	0.98	0.52	−0.52	−1.29	−1.60	1.04	[42••]
Gd2@C80-CH2Ph		0.52	−0.86	−1.35		1.38	[47]
Tb2@C80-CH2Ph		0.51	−0.79	−1.36	−1.71	1.30	[47]
Dy2@C80-CH2Ph	0.98	0.52	−0.60	−1.28	−1.58	1.12	[42••]
Ho2@C80-CH2Ph		0.51	−0.54	−1.33		1.05	[47]

^aAll potentials are measured in o-dichlorobenzene solution and referenced versus Fe(Cp)₂^+/0 redox pair; redox processes involving M–M bonding orbitals are highlighted in bold;

^bgap_EC is defined as E^+1/0 − E^0/−1 ;

^cclusterfullerenes with cage-based first oxidation steps, listed here for comparison to di-EMFs with the same fullerene cages.

Besides the value of the first reduction potential and its metal-dependence, another indication of the metal-based reduction in La and Ce di-EMFs is the difference between the first and the second reduction potentials, which amounts to 1.23–1.44 V (Table 1). For a fullerene redox process based on the same MO, the difference between the first and second reduction steps is usually within 0.4–0.5 V. The metal-based redox process results in a much larger potential difference for the consequent redox steps, because these steps are either based on the M–M bonding MO (with a much higher on-site Coulomb interaction than in the fullerene) or affect different MOs (one metal-based, and one delocalized over the carbon cage). Thus, both the high potential of the first reduction step and the large gap between the first and the second reduction potentials point to the population of the M–M bonding MO and hence formation of the single-electron M–M bond at the first reduction step.

Formation of the single-occupied La–La bonding MO in the [La₂@C₈₀-I_h]^•− anion radical is further confirmed by EPR spectroscopy [29]. The M–M bonding orbitals in di-EMFs have hybrid spd character with large s-contribution, and population of such MOs by a single electron is expected to give paramagnetic species with large metal-based hyperfine constants [10•,11]. Indeed, huge isotropic ¹³⁹La coupling constant of 364 G was reported in the radical anion [La₂@C₈₀-I_h]^•− [29,30].

Dimetallofullerenes with metal-based HOMO

Due to the lanthanide contraction, the metals close to the end of the lanthanide row exhibit more covalent character in their compounds. In di-EMFs, Er and Lu give away only two electrons to the fullerene cage (the formal charge of the fullerene cage is thus −4). The remaining metal-based valence electrons then form the M–M bond via the sigma-type spd-hybrid MO. The most abundantly produced di-EMFs with four-fold charged fullerene cages are the two isomers of C₈₂, C_s(6) and C_3v(8) [31–34]. Structural characterization was also reported for several other Lu di-EMFs, including Lu₂@C₇₆-T_d(1) [35], Lu₂@C₈₄-D_2d(23), and Lu₂@C₈₆-C_2v(9) [31]. If the M–M bonding MO is the HOMO (as predicted by theory), these di-EMFs should feature a metal–metal bond already in the pristine non-charged state, and this bond should be electrochemically active in the oxidation processes.

The experimental confirmation of the metal–metal bonding in di-EMFs is not very straightforward. The formal charge of the fullerene cage in M₂@C₈₂ can be deduced from Vis-NIR spectroscopic measurements. UV-vis-NIR absorption spectra of the di-EMFs with the same fullerene cage and different metals are virtually identical. Presumably, the excitation originating from the metal-based HOMO have very low intensity and cannot be observed, resulting in the dominance of the π →π* transitions in the fullerene cages. Similar spectra are also observed for sulfide clusterfullerenes M₂S@C₈₂ [32••,36] or carbide clusterfullerenes M₂C2@C₈₂ [12,37]. In cluster-fullerenes, the non-metal endohedral entity bears a negative charge (C₂²⁻, O²⁻, or S²⁻), the metal atoms are in their 3+ state, whereas the cage has the negative charge of −4. Such clusterfullerenes do not feature metal–metal bonds, and their frontier MOs are usually localized on the fullerene cage [6•]. The close resemblance of the absorption spectra of di-EMF and clusterfullerenes proves that the carbon cage in these EMFs has the same formal charge, −4. Thus, +2 oxidation state of metal atoms appears natural. However, the presence of the M–M bond does not automatically follow from the oxidation state. X-ray absorption spectra at the M_4,5 edge (3d→4f excitations commonly used in the studies of lanthanides) did not show substantial difference between Er₂@C₈₂ or Er₂C₂@C₈₂ [38]. However, both Er²⁺ and Er³⁺ states in EMFs feature the same 4f¹² occupation, and therefore absorption at the M_4,5 edge may be not sensitive enough to the difference in the valence orbital populations.

Electrochemistry provides more straightforward approach to the problem. If the M–M bonding MO is indeed the redox-active HOMO of di-EMFs, the first oxidation potential should exhibit pronounced metal-dependence in contrast to the first reduction potential, which corresponds to the cage-based LUMO and is not expected to vary much from metal to metal. Indeed, electrochemistry reveals pronounced differences in the electronic structure of Er₂@C₈₂ and Lu₂@C₈₂ [32••]. Their reduction potentials are rather similar (Figure 2a; note that C₈₂-C_3v isomer exhibits irreversible reduction steps, whereas reduction steps of C₈₂-C_s isomers are fully reversible). Such a similarity of the potentials points to the fullerene-based nature of the underlying redox steps, in agreement with DFT prediction. On the contrary, oxidation potentials of Er₂@C₈₂ and Lu₂@C₈₂ are strongly metal-dependent. Er₂@C₈₂ isomers have their first oxidation step at ca. 0.3–0.4 V lower potentials than Lu-counterparts (Table 1). The same trend was observed for M₂@C₈₂-C_3v structures with other metals, including Sc₂@C₈₂ and mixed-metal ErSc@C₈₂ and YLu@C₈₂ di-EMFs [32••]. With almost identical reduction potential, they exhibit large variability of the first oxidation potentials (Figure 2b). Metal-dependence of the first oxidation potential in di-EMFs confirms the computationally predicted metal– metal bonding HOMO in these molecules (Figure 2c). Lu has the lowest energy of the M–M bonding HOMO, and hence Lu di-EMFs exhibit the highest oxidation potentials when compared to other metals. In fact, oxidation potentials of Lu-di-EMFs are close to the cage-based oxidation potentials of sulfide clusterfullerenes M₂S@C₈₂ (Table 1), and it is hard to distinguish if the first oxidation step of Lu₂@C₈₂ isomers is metal- or fullerene-based. Lower oxidation potentials of di-EMFs with other metals unequivocally point of the metal-based processes.

Figure 2.

(a) Cyclic voltammetry of Er₂@C₈₂ and Lu₂@C₈₂ dimetallofullerenes with C_3v (8) and C_s(6) cage isomers in o-dichlorobenzene/TBAPF₆ solution at 100 mV s⁻¹; whereas the first reduction potentials of Lu₂@C₈₂ and Er₂@C₈₂ with the same fullerene cage are virtually identical (denoted by blue dashed line), the first oxidation potentials are different by more than 0.3 V (red lines); (b) square wave voltammetry of several M₂@C₈₂-C_{3 v}(8) at the first oxidation step (M₂ = Lu₂, YLu, Er₂, ErSc, Sc₂); (c) HOMO orbitals for Lu₂@C₈₂, Y₂@C₈₂, and YLu@C₈₂ ; (d) EPR spectrum of Sc₂@C₈₂⁺ cation in o-dichlorobenzene at room temperature, a(⁴⁵Sc) = 199.2 G, g = 1.994; the lines show assignment of the peaks in terms of |I, m_I) nuclear spin quantum numbers of the Sc₂ dimer. Reproduced with permission from the Ref. [32••].

Single-electron oxidation of di-EMF with M–M bonding HOMO produces a single-occupied metal-based orbital with unprecedented spin properties. Large contribution of metal s-atomic orbital to the M–M HOMO of M₂@C₈₂ yields a large isotropic hyperfine coupling constant for metals with non-zero nuclear spin in [M₂@C₈₂]^•+ cation radicals. A striking example is the cation radical of Sc₂@C₈₂, which at room temperature in o-dichlorobenzene solution exhibits well-resolved EPR spectrum with the hyperfine structure spanning 2800 G (Figure 2d). Instead of 15 lines expected for two equivalent Sc with nuclear spin of 7/2, experimental spectrum comprises 64 lines caused by additional splitting due to the large ⁴⁵Sc hyperfine constant, a(⁴⁵Sc) = 199.2 G [32••]. Formation of the single-electron Er–Er bond in [Er₂@C₈₂-C_3v]^•+ was supported by SQUID magnetometry. The oxidation of Er₂@C₈₂ strongly modified the spin state of the endohedral Er₂ unit, presumably creating a three-center [Er³⁺–e–Er³⁺] system with stronger exchange interactions than in the pristine Er₂@C₈₂ [32••].

Dimetallofullerenes with single-electron metal–metal bond

Whereas early and late lanthanides tends to form di-EMFs with tri- and di-valent state of metals, respectively, yttrium and lanthanides in the middle part of the lanthanide row (Gd–Ho) give di-EMF with even more peculiar electronic structure. Computational studies of M₂@C₈₀-I_h (M = Y, Lu) showed that the ground electronic state for these di-EMF is a triplet [17•]. The M–M bonding MO is occupied by a single electron, and another unpaired spin is delocalized over the fullerene cage. The formal charge distribution is then (M₂)⁵⁺@C₈₀⁵⁻. During the extraction of fullerenes from the arc-discharge soot by standard fullerene solvents (such as CS₂ or toluene) these molecules remain insoluble, presumably due to polymerization or aggregation with the soot particles.

Electronic structure of such M₂@C₈₀-I_h di-EMFs can be stabilized by addition of an electron, which yields to closed-shell electronic structure of the fullerene cage, (M₂)⁵⁺@C₈₀⁶⁻ [39]. Indeed, the synthesis of M₂@C₈₀ derivatives was accomplished when EMFs were extracted from the soot with N,N-dimethylformamide, which is known to form fullerene anions during extraction (Figure 3a) [40,41]. Chemical derivatization with a single radical group R (R = CF₃ or benzyl CH₂Ph) is another way to quench the cage-based radical in M₂@C₈₀-I_h [17•,42••]. Finally, addition of an electron is equivalent to substitution of one carbon atom by nitrogen. C₇₉N⁵⁻ is isoelectronic to C₈₀⁶⁻, and stable M₂@C₇₉N compounds were obtained in Dorn’s group for M = Y, Gd, and Tb [43,44]. The common feature of M₂@C₈₀−, M₂@C₇₉N, or M₂@C₈₀(R) is the single-electron M–M bond stabilized inside the fullerene. For M = Y, localization of the spin density on the Y–Y bonding MO can be confirmed by EPR spectroscopy, which revealed similar spectra in all three types of EMFs with large isotropic ⁸⁹Y hyperfine coupling constants near of 80 G (Figure 3b,c) [17•,42••,44]. Formation of the single-electron La–La bond in La₂@C₈₀(CH₂Ph) and similar radical monoadducts of La₂@C₈₀-I_h was also confirmed by EPR spectroscopy and single-crystal X-ray diffraction [45••,46].

Figure 3.

(a) Schematic description of the electron distribution between M–M bonding MO and fullerene cage in dimetallofullerenes M₂@C₈₀-I_h (M = Y, Tb, Dy, etc.) and a chemical route to stabilize these structures via reduction and subsequent nucleophilic substitution yielding air-stable M₂@C₈₀(CH₂Ph) monoadduct. (b) EPR spectra of the toluene solution of Y₂@C₈₀(CH₂Ph) at room temperature and at 150 K (below the freezing point of the solvent); the isotropic RT spectrum has g-factor of 1.9733 and the a_iso(⁸⁹Y) value of 223.8 MHz; the axial spectral pattern in frozen solution is reproduced by g_⊥ = 1.9620, g_ǁ = 1.9982, a_⊥(⁸⁹Y) = 208.0 MHz, a_ǁ(⁸⁹Y) = 245.9 MHz; (c) spin density distribution in Y₂@C₈₀(CH₂Ph) computed at the PBE0/TZVP level; (d) square wave voltammetry of M₂@C₈₀(CH₂Ph) (M = Y, Dy, and Tb), black vertical bars denote redox potentials of La₂@C₈₀(CH₂Ph) from Ref. [45••], dotted lines denote the first oxidation (cyan) and the first reduction (red) potentials. Based on the data from Ref. [42••].

Potentially, the half-occupied M–M bonding orbital can be redox active both in reduction and oxidation processes. However, DFT calculations predict large energy difference between occupied and unoccupied counterparts of the MO [42••]. As a result, the occupied component of the M–M bonding MO in La₂@C₈₀(CH₂Ph) is predicted to be the HOMO, whereas the LUMO is localized on the fullerene cage. In Y₂@C₈₀(CH₂Ph), analogous calculations showed metal-based LUMO and fullerene-based HOMO.

Y and Gd–Ho M₂@C₈₀(CH₂Ph) derivatives exhibit virtually identical oxidation potential at +0.51–0.52 V (Table 1, Figure 3d) [32••,47]. The lack of the metal dependence is an indication of the fullerene-based oxidation in these di-EMFs, in agreement with DFT prediction for Y₂@C₈₀(CH₂Ph). With the first oxidation potential at +0.15 V [45••], La₂@C₈₀(CH₂Ph) is an obvious outlier exhibiting the metal-based oxidation. Thus, the metal–metal bonding MO of La₂@C₈₀(CH₂Ph) is depopulated in the oxidation process, whereas for other metals the single-electron M–M bond is not affected.

The first reduction potentials of M₂@C₈₀(CH₂Ph) derivatives are different and span the range from −0.52 V in Y₂@C₈₀(CH₂Ph) to −0.92 V in La₂@C₈₀(CH₂Ph). In the Gd–Ho row, the potential is changing gradually with the size of the lanthanide, more negative values corresponding to larger ionic radii. This behavior is consistent with the metal-based reduction for Y and medium-size lanthanides. Hence, the single-electron M–M bond turns to a two-electron bond in their monoanions. In La₂@C₈₀(CH₂Ph), the process switches to the fullerene-based reduction.

Electrochemical studies of other di-EMFs with single-electron M–M bond were reported so far only for Gd₂@C₇₉N [43]. Its first oxidation potential at +0.51 V is very close to that of M₂@C80(CH₂Ph) derivatives with fullerene-based oxidation (Table 1). The first reduction at −0.96 V is more negative than in any M₂@C₈₀(CH₂Ph), including La₂@C₈₀(CH₂Ph). Computational studies showed that Gd₂@C₇₉N has two low-energy unoccupied MOs, one Gd-based and one delocalized over the fullerene, thus making it hard to distinguish between the fullerene- and metal-based reductions. However, the large difference between the first and the second reduction potentials of almost 1 V (Table 1) indicates that the first reduction of Gd₂@C₇₉N may indeed involve the Gd–Gd bonding MO.

Outlook

The unique environment of endohedral fullerene provides a possibility to stabilize exotic species with unconventional bonding situation, such as lanthanide dimers with metal–metal bonds. Whereas many lanthanide complexes with low oxidation states have been synthesized [48••], no other molecular compounds with lanthanide–lanthanide bonds have been reported so far [49••]. Furthermore, the M–M bonding MOs in dimetallofullerenes are redox active and undergo single-electron reduction or oxidation, which leads to radical species with single-electron M–M bonding MOs. Electrochemistry is thus found to be a convenient technique to study metal–metal bonds in fullerenes. Redox variability of the population of the lanthanide–lanthanide bonding MOs in di-EMFs is very useful for tuning their magnetic properties. The presence of the unpaired valence electron in lanthanide-based di-EMFs results in giant exchange interactions and coupling of local 4f-derived spins and unpaired spin in the M–M bonding MO into a larger “superspin”. If lanthanides with large magnetic anisotropy (such as Dy or Tb) are coupled this way, single molecule magnets with high blocking temperature of magnetization can be obtained [42••]. Semi-occupied M–M bonding MO is also essential for the spin-polarized electronic transport through single fullerene molecules, which can lead to single-molecule electronic and spintronic devices.

Abbreviations

EMF

endohedral metallofullerene

di-EMF

dimetallofullerene

MO

molecular orbital

HOMO

highest occupied molecular orbital

LUMO

lowest unoccupied molecular orbital

EPR

electron paramagnetic resonance

M–M bond

metal–metal bond

Vis-NIR

visible and near-infrared

DFT

density functional theory.