Air-stable redox-active nanomagnets with lanthanide spins radical-bridged by a metal-metal bond

Abstract

Engineering intramolecular exchange interactions between magnetic metal atoms is a ubiquitous strategy for designing molecular magnets. For lanthanides, the localized nature of 4f electrons usually results in weak exchange coupling. Mediating magnetic interactions between lanthanide ions via radical bridges is a fruitful strategy towards stronger coupling. In this work we explore the limiting case when the role of a radical bridge is played by a single unpaired electron. We synthesize an array of air-stable Ln₂@C₈₀(CH₂Ph) dimetallofullerenes (Ln₂ = Y₂, Gd₂, Tb₂, Dy₂, Ho₂, Er₂, TbY, TbGd) featuring a covalent lanthanide-lanthanide bond. The lanthanide spins are glued together by very strong exchange interactions between 4f moments and a single electron residing on the metal-metal bonding orbital. Tb₂@C₈₀(CH₂Ph) shows a gigantic coercivity of 8.2 Tesla at 5 K and a high 100-s blocking temperature of magnetization of 25.2 K. The Ln-Ln bonding orbital in Ln₂@C₈₀(CH₂Ph) is redox active, enabling electrochemical tuning of the magnetism.

Fig. 1

Molecular structure of Ln₂@C₈₀(CH₂Ph). Single-occupied Ln–Ln bonding molecular orbital (left; carbons are gray, hydrogens are white, lanthanides are green), and schematic depiction of the molecule (right; the arrow indicates an unpaired electron residing on the Ln–Ln bonding orbital)

Fig. 2

Structure and dynamics of Ln₂@C₈₀(CH₂Ph). a Molecular structure of {Dy₂} at 100 and 290 K and atomic displacement parameters as a function of temperature between 100 and 290 K (to guide the eye, vertical lines separate displacement parameters of the C₈₀ cage, CH₂Ph group, and Dy atoms); Dy atoms are shown as spheres with radii proportional to the site occupancies; b Raman spectra of {Ln₂} compounds in the low-frequency range measured at 77 K; two metal-cage stretching modes are indicated by red dotted lines, the most prominent fullerene cage squashing mode is indicated by a black dotted line, atomic displacements for the metal-cage stretching modes near 150 and 165 cm⁻¹ are also shown on the right; c ¹H NMR spectra of {Ln₂} measured in CS₂ solution at room temperature; {Tb₂} signals in the spectrum of the {TbGd} sample are denoted by asterisks

Fig. 3

Electron paramagnetic resonance (EPR) spectroscopy of {Gd₂}. a X-band and Q-band EPR spectra of frozen {Gd₂} solution in toluene near 100 K together with the spectra simulated for spin S = 15/2 with g_iso = 1.987 and zero field splitting (ZFS) parameters D = 1.00 GHz and E = 0.22 GHz (inhomogeneous broadening is accounted for by ZFS strain StrD = 0.029 GHz and StrE = 0.027 GHz); asterisks mark unidentified signals (presumably of low spin states or organic impurities), the inset shows the spin-density distribution in {Gd₂}; b Zeeman splitting for spin S = 15/2 with the above ZFS parameters (magnetic field is parallel to z-axis of the ZFS tensor); also shown are energies of the X-band (9.4 GHz) and Q-band (34 GHz) microwave photons, EPR-active transitions (ovals and small arrows), and the resonance fields corresponding to the g-factor of 1.987 (vertical dotted lines)

Fig. 4

Magnetic properties of {Ln₂} molecules. a Blocking temperature of magnetization in {Tb₂}, {TbGd} and {TbY}; dotted lines are measurements of magnetic susceptibility χ during cooling in the field of 0.2 T, solid lines are measurements during heating in the field of 0.2 T of zero-field cooled samples (sweep rate 5 K min⁻¹, arrows indicate direction of the measurement for each curve), vertical red dotted lines denote T_B values; b magnetic hysteresis curves for {Tb₂}, sweep rate 9.5 mT s⁻¹; c alignment of Ln magnetic moments in {Ln₂} according to ab initio calculations: collinear in {Tb₂} and {Dy₂}, tilted in {Ho₂} (the arrows indicate directions of the single-ion quantization axis for each Ho), easy-plane in {Er₂}, in the latter the Ln spins are visualized as ellipsoids; d low-temperature magnetization curves for {TbY}, sweep rate 2.9 mT s⁻¹; the inset shows enhancement of the field range, in which magnetic hysteresis is observed

Fig. 5

Relaxation of magnetization in {Ln₂} molecules. a magnetic relaxation times of {Tb₂}, full dots are zero-field data, open dots are in-field data, red line denotes Orbach processes, solid horizontal line denotes QTM; the inset shows schematically two main relaxation pathways in {Tb₂}: QTM and Orbach relaxation via an exchange-excited state, red arrows are Ln spin, blue arrow is a free electron spin; b magnetic relaxation times of {Ho₂} at zero field (full dots) and in different fields between 0.1 and 0.4 T, red and purple solid lines are Orbach processes, blue line is a possible Raman contribution (~T^10.1); the inset shows magnetic field dependence of relaxation times at 1.8 and 5 K; c magnetic relaxation times of {TbY}, dashed horizontal line is a QTM contribution to zero-field relaxation, magenta line is a low-power process (~T^1.70), dark blue line is a combination of both, light blue line is a Raman process (~T ^4.64)

Fig. 6

Electron transfer properties of {Ln₂} molecules. a Cyclic voltammogram of {Er₂} in o-dichlorobenzene solution as a representative example of the {Ln₂} series. b Schematic description of the single-electron reduction and oxidation of {Ln₂} compounds showing addition of one electron to the Ln–Ln bond and removal of one electron from the fullerene cage. c The first oxidation (red dots) and reduction (blue dots) potentials of {Ln₂} in o-dichlorobenzene/TBABF₄ solution as a function of ionic radius of Ln (for {TbY}, the average radius of Tb³⁺ and Y³⁺ is used; lines are shown to guide the eye); d ¹H NMR spectra of {Tb₂}^–, {Ho₂}^– and {Er₂}^– anions in d₄-o-dichlorobenzene (colored lines) in comparison to the spectra of neutral compounds (light gray lines). e Schematic description of the spin-valve effect of the {Ln₂} molecule: in a certain bias range limiting the current to the metal-based LUMO, only the electrons with their spin antiparallel to the spin of the molecule can pass through