Single-Electron Lanthanide-Lanthanide Bonds Inside Fullerenes toward Robust Redox-Active Molecular Magnets

Abstract

A characteristic phenomenon of lanthanide-fullerene interactions is the transfer of metal valence electrons to the carbon cage. With early lanthanides such as La, a complete transfer of six valence electrons takes place for the metal dimers encapsulated in the fullerene cage. However, the low energy of the σ-type Ln-Ln bonding orbital in the second half of the lanthanide row limits the Ln₂ → fullerene transfer to only five electrons. One electron remains in the Ln-Ln bonding orbital, whereas the fullerene cage with a formal charge of -5 is left electron-deficient. Such Ln₂@C₈₀ molecules are unstable in the neutral form but can be stabilized by substitution of one carbon atom by nitrogen to give azafullerenes Ln₂@C₇₉N or by quenching the unpaired electron on the fullerene cage by reacting it with a chemical such as benzyl bromide, transforming one sp² carbon into an sp³ carbon and yielding the monoadduct Ln₂@C₈₀(CH₂Ph). Because of the presence of the Ln-Ln bonding molecular orbital with one electron, the Ln₂@C₇₉N and Ln₂@C₈₀(R) molecules feature a unique single-electron Ln-Ln bond and an unconventional +2.5 oxidation state of the lanthanides. In this Account, which brings together metallofullerenes, molecular magnets, and lanthanides in unconventional valence states, we review the progress in the studies of dimetallofullerenes with single-electron Ln-Ln bonds and highlight the consequences of the unpaired electron residing in the Ln-Ln bonding orbital for the magnetic interactions between Ln ions. Usually, Ln···Ln exchange coupling in polynuclear lanthanide compounds is weak because of the core nature of 4f electrons. However, when interactions between Ln centers are mediated by a radical bridge, stronger coupling may be achieved because of the diffuse nature of radical-based orbitals. Ultimately, when the role of a radical bridge is played by a single unpaired electron in the Ln-Ln bonding orbital, the strength of the exchange coupling is increased dramatically. Giant exchange coupling in endohedral Ln₂ dimers is combined with a rather strong axial ligand field exerted on the lanthanide ions by the fullerene cage and the excess electron density localized between two Ln ions. As a result, Ln₂@C₇₉N and Ln₂@C₈₀(CH₂Ph) compounds exhibit slow relaxation of magnetization and exceptionally high blocking temperatures for Ln = Dy and Tb. At low temperatures, the [Ln³⁺-e-Ln³⁺] fragment behaves as a single giant spin. Furthermore, the Ln-Ln bonding orbital in dimetallofullerenes is redox-active, which allows its population to be changed by electrochemical reactions, thus changing the magnetic properties because the change in the number of electrons residing in the Ln-Ln orbital affects the magnetic structure of the molecule.

Figure 1

(a) Third ionization potentials (IP₃, blue dots) and energies of the 4fⁿ5d¹6s² → 4fⁿ5d²6s¹ excitations (green dots) of lanthanides. The red dashed horizontal line marks the border between divalent and trivalent lanthanides in monometallofullerenes. (b) MO levels in C₈₀-I_h and C₈₂-C_3v cages (black, occupied MOs; pink, vacant MOs) as well as La₂ and Lu₂ dimers. Reproduced with permission from ref (35). Copyright 2018 Elsevier. (c) Molecules of La₂@C₈₀-I_h with the La–La bonding LUMO and Lu₂@C₈₂-C_3v with the Lu–Lu bonding HOMO.

Figure 2

(a) Oxidation states of lanthanides in dimetallofullerenes; the color code for different states (light brown for Ln³⁺, light green for Ln^2.5+, and light blue for Ln²⁺) is used in subsequent figures. (b) Stabilization of di-EMFs with a single-electron Ln–Ln bond in the form of Ln₂@C₈₀(CH₂Ph), [Ln₂@C₈₀]⁻, and Ln₂@C₇₉N. (c) Spin density distributions in Gd₂@C₈₀(CH₂Ph), [Gd₂@C₈₀]⁻, and Gd₂@C₇₉N (“+”, green; “–”, red; transparent and solid isosurfaces have isovalues of 0.0012 and 0.014, respectively).

Figure 3

Molecular structures of di-EMFs with a single-electron Ln–Ln bond from single-crystal X-ray diffraction: (a) Dy₂@C₈₀(CH₂Ph); (b) La₂@C₈₀(C₃N₃Ph₂); (c) Tb₂@C₇₉N·Ni(OEP); (d) Gd₂@C₇₉N·Ni(OEP). Large colored spheres show the Ln sites with the highest occupancies. The Ln···Ln distances are 3.896(1) Å (Dy), 3.784(2) Å (La), 3.902(1) Å (Tb), and 3.835(9) Å (Gd).

Figure 4

(a, b) X-band EPR spectra of {Y₂} in toluene at (a) room temperature and (b) 150 K. (c, d) Q-band (34 GHz) and X-band (9.4 GHz) EPR spectra of {Gd₂} in frozen toluene. The inset in (d) shows the room-temperature spectrum. (e) ¹H NMR spectra of {Ln₂} compounds in CS₂ at room temperature. Asterisks in the spectrum of {TbGd} denote the signals of {Tb₂}. Adapted with permission from (a, b) ref (23) and (c–e) ref (22). Copyright 2017 and 2019, respectively, Springer Nature.

Figure 5

(a) Four types of di-EMF redox behavior. (b) Cyclic voltammograms of {Ln₂} (Ln = Er, Ho, Dy) in o-dichlorobenzene (o-DCB). Vertical dashed lines denote the reduction (E_1/2(0/−)) and oxidation (E_1/2(+/0)) potentials of {Ho₂}. (c) Redox potentials in the {Ln₂} series. Horizontal dotted lines denote potentials of the fullerene-based redox processes, and dashed green lines show the variation of the potential of the redox-active Ln–Ln orbital. (d) Schematic description of the MO levels in {Er₂}, {Gd₂}, and {La₂}. Cage MOs are shown in gray and metal-based MOs in green.

Figure 6

(a) Reaction of {Ln₂} with cobaltocene. (b) Room-temperature ¹H NMR spectra of {Er₂}⁻, {Tb₂}⁻, and {Ho₂}⁻ anions in o-DCB-d₄ solution (colored lines) compared with the spectra of pristine {Ln₂} compounds (gray lines). Reproduced with permission from ref (22). Copyright 2019 Springer Nature.

Figure 7

(a–c) Magnetization blocking temperatures of (a) {Dy₂} (b), {Tb₂}, and (c) Tb₂@C₇₉N. The magnetic field was 0.2–0.3 T, and the temperature sweep rate was 5 K min^–1. (d–f) Magnetic hysteresis of (d) {Dy₂}, (e) {Tb₂}, and (f) Tb₂@C₇₉N. The magnetic field sweep rate was 3 mT s^–1 in (d) and (f) and 9.5 mT s^–1 in (e). (g–i) Magnetization relaxation times of (g) {Dy₂}, (h) {Tb₂}, and (i) Tb₂@C₇₉N. Adapted with permission from (a, d, g) ref (23), (b, e, h) ref (22), and (c, f, i) ref (19). Copyright 2017 and 2019 Springer Nature and 2019 Wiley-VCH, respectively.

Figure 8

(a) Schematic representation of di-EMFs with a single-electron Ln–Ln bond as a three-center [Ln³⁺–e–Ln³⁺] spin system. (b) Alignment of the Tb spins (green arrows) and the unpaired electron spin (red arrow) in Tb₂@C₇₉N. (c) Alignment of the Ln and unpaired electron spins (red arrows) in {Ln₂} molecules. The spins of Er ions are visualized as ellipsoids built upon the g tensors of their single-ion ground states. (d) Low-energy spectrum of the spin Hamiltonian (eq 6) for {Tb₂} with K^eff = 55 cm^–1. Red lines visualize transition probabilities. Tb and unpaired spins are shown with green and red arrows, respectively. Dashed arrows show the QTM and Orbach mechanisms.

Figure 9

Comparison of experimental χT values (dots, arbitrary units) measured for (a) Gd₂@C₇₉N and (b) Tb₂@C₇₉N in a field of 1 T to the results of calculations for different values of the coupling constant K^eff. Adapted with permission from (a) ref (20) and (b) ref (19). Copyright 2018 Royal Society of Chemistry and 2019 Wiley-VCH, respectively.

Figure 10

(a) Magnetization curves for {TbY} below 5 K. Reproduced with permission from ref (22). Copyright 2019 Springer Nature. (b) DFT-computed spin density distribution and Gd electron exchange coupling constants in {GdLa}, {GdY}, and {GdLu} (PBE0/DKH-TZVP level of theory; the isosurface visualization parameters are the same as in Figure 2).