Understanding the Magnetic Anisotropy toward Single-Ion Magnets

Article pubs.acs.org/accounts

Understanding the Magnetic Anisotropy toward Single-Ion Magnets Yin-Shan Meng, Shang-Da Jiang,* Bing-Wu Wang,* and Song Gao* Beijing National Laboratory of Molecular Science, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China CONSPECTUS: Single-molecule magnets (SMMs) can retain their magnetization status preferentially after removal of the magnetic field below a certain temperature. The unique property, magnetic bistable status, enables the molecule-scale SMM to become the next-generation high-density information storage medium. SMMs’ new applications are also involved in high-speed quantum computation and molecular spintronics. The development of coordination chemistry, especially in transition metal (3d) and lanthanide (4f) complexes, diversifies SMMs by introducing new ones. In both 3d and 4f SMMs, the ligands play a fundamental role in determining the SMMs’ magnetic properties. The strategies for rationally designing and synthesizing highperformance SMMs require a comprehensive understanding of the effects of a crystal field. In this Account, we focus mainly on the magneto-structural correlations of 4f or 3d single-ion magnets (SIMs), within which there is only one spin carrier. These one-spin carrier complexes benefit from getting rid of exchange interactions and relatively large distances of magnetic centers in the lattice, providing the ease to construct high-performance SIMs from the crystal field perspective. We will briefly introduce the crystal field approach for 4f or 3d complexes and then the magnetic anisotropy analysis via the displaced-charge electrostatic model. This idea has been proposed for years, and the related work is also highlighted. The angular-resolved magnetometry method, predominating in determining the magnetic anisotropic axes direction, is discussed. We also give a brief introduction of the quantum chemistry ab initio method, which has shown to be powerful in understanding the magnetic anisotropy and lowlying states. In the constructing and characterizing part, we give an overview of the SIMs based on lanthanide and transition ions, reported by our group in the past 5 years. In the 4f-SIMs survey, we discuss how β-diketonates and cyclomultienes, and their combination, as ligands to influence magnetic anisotropy and provide some suggestion on designing SIMs based on different lanthanide ions. In the 3d-SIMs survey, we fully discuss the correlation between zero-field-splitting parameter D and molecular geometrical angle parameters. Finally, we lay out the challenges and further development of SIMs. We hope the understanding we provide about single-ion magnetic properties will be helpful to design high-performance SMMs.

1. INTRODUCTION The research on the widely known single-molecule magnets (SMMs) has been well developed in the past 20 years since the discovery of the famous Mn12 molecule and the study of the mesoscopic quantum effect in spin physics and molecular magnetism.1 Inspired by SMMs’ intrinsic interest and by their own interdisciplinary specialties, researchers of various backgrounds, synthetic and theoretical chemistry, physics, and materials science, have devoted great effort to this field. The unique properties of SMMs at the molecular scale render them to be the next-generation high-density information storage materials and a potential candidate for molecular spintronics devices.2,3 To make SMMs applicable, one needs to enhance their working temperature to exhibit the magnetic bistability property. It was at first believed that the most efficient way was to enhance the spin value of the ground state. Unfortunately, since the single-ion magnetic anisotropy is often canceled out within one molecule, the overall magnetic anisotropy is usually small, creating a small energy barrier for the whole molecule. On the other hand, the mononuclear © 2016 American Chemical Society

SMMs, also called single-ion magnets (SIMs), containing only one spin carrier (d-block or f-block ions), allow us to understand and control the single-ion magnetic anisotropy. Up to now, scientists have successfully synthesized many mononuclear SMMs by controlling molecular symmetry and designing crystal fields.4−7 Many fruitful results have been presented: some SIMs exhibit energy barriers up to 1000 K, with blocking temperatures as high as 20 K.8,9 Meanwhile, the theoretical analysis from an electrostatic perspective also provides a more intuitive way to understand single-ion magnetic anisotropy.10−15 To this hot topic, our group has also paid much attention in the past 5 years. We have developed the molecular-symmetrycontrolling approach in designing 4f and 3d SIMs and discussed the substituent effects. In this Account, we summarize our efforts in this field and provide some commentary on the crystal field approach. Received: May 11, 2016 Published: October 21, 2016 2381

DOI: 10.1021/acs.accounts.6b00222 Acc. Chem. Res. 2016, 49, 2381−2389

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2. THEORETICAL AND EXPERIMENTAL ASPECTS

Ĥ CF =

2.1. Crystal Field Approach toward 4f-SIMs

∑ [B0k Ĉ0k(i) + ∑ (Bqk (Ĉ−kq(i) + (−1)q Cq̂ k(i)) k=0

Lanthanide ions possess the magnetic properties different from 3d ions due to the intrinsic nature of 4f orbitals. The characters of lanthanide ions are summarized as follows: (1) The 4f electrons are screened by 5d and 6s orbitals so that the orbital momentum is largely conserved in the crystal field other than EuIII(7F0), GdIII(8S7/2), and LuIII. (2) The spin−orbit coupling of lanthanide ions is much stronger than that of the transition metal ions. (3) The 2S+1LJ multiplets pattern of LnIII ions is derived from Pauli Principle and Hund’s Rule by taken the Coulombic interaction and the spin−orbit coupling into consideration. The crystal field effect acts as a much weaker interaction on 4f ions that further splits the 2S+1LJ multiplets (see Figure 1). This

+

k B′kq i(Ĉ −q(i)

q=1 k

− ( −1)q Cq̂ (i)))]

(1)

where Bkq and B′kq are the crystal field parameters and Ckq(i) is the Racah operator which is proportional to the spherical Harmonics Cqk(i) = 4π /(2k + 1) Y kq(i).16 The key to understand the magnetic properties and the spectroscopic information is to identify the crystal field parameters.17,18 However, the difficulty lies in that the lanthanide ions normally crystallizes in a low molecular symmetry, affording the overparameterization problem. 2.2. Understanding the Magnetic Anisotropy via Electrostatic Model

Understanding of magnetic anisotropy is vital for rational design of Ln-SIMs, which can be viewed from the asphericity of the 4f electron density.19 The 4f shell’s asphericity with respect to the Ising limit state can be expanded as a linear combination of the spherical harmonics Y0k . According to this idea, Rinehart and Long have proposed to rationally arrange the negative charges so as to stabilize the Ising limit state of various lanthanide ions.10 As was also addressed by Rinehart and Long, the different ground states correspond to various shape of electron density, we note here that one should state the relative ground state of the 4f ion, before ascribing lanthanide ions to the oblate (axial pressed) or the prolate (axial elongated) type. As what we can see in Figure 2c, the shape of the 4f shell for DyIII varies from prolate to oblate when the ground state differs from |±1/2⟩ to |±15/2⟩.10,15 The 4f-shell electron cloud shape of the Ising limit states are also plotted in the Figure 2a and b. Soncini et al. have proposed an intuitive method: minimizing electrostatic potential energy so as to find the quantized axis direction of DyIII with respect to its Ising limit state.14 This method that has been coded in MAGELLEN program has proved efficient for finding the easy axis direction of the DySIMs; The drawbacks, however, are that this method is limited in Dy-SIMs cases with non-neutral ligands. Meanwhile, Ruiz and Aravena also revealed the relation between different electrostatic potential and magnetic anisotropy.13 On the other hand, Coronado and co-workers proposed two different models by taking the charge-shifting into consideration and coded them as SIMPRE program to obtain the crystal field parameters.11,12 On the basis of SIMPRE program’s central idea, we include the negative charge displacement in the electrostatic model to predict the quantized axis direction.15 The displacement direction depends on the coordination

Figure 1. Energy splitting schematic diagram of the electronic structures of lanthanide ions in logarithmic energy scale.

crystal field splitting varies with the coordination environments and determines the Ln-SIMs’ magnetic anisotropy and spin− lattice relaxation behavior. It is therefore essential to gain insight into this fine electronic structure, which can be well described by the crystal field theory. The terms of the crystal field Hamiltonian depend mainly on the symmetry of the central lanthanide ion. A lanthanide ion located in the C1 symmetry corresponds to 27 terms for describing the crystal field’s splitting (eq 1).

Figure 2. (a, b) 4f electron density distribution of corresponding LnIII ions in their Ising limit state; (c) electron density distribution of DyIII ion in corresponding MJ state; (d) schematic of displacement in σ-bond and π-bond types. 2382


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Figure 3. Experimental Cartesian system (left) and rotating experiments along corresponding experimental axis (right).

bonding type (Figure 2d): for σ-bond, the negative charge is displaced closer to the lanthanide ion along the linear connection of the two nuclei; for π-bond, it is displaced closer along the normal of the aromatic plane. The electrostatic model is on the foundation of the crystal field theory and is approximation of the latter, therefore there exist some limitations of this method. First of all, this approach is based on the assumption that the ground state of the 4f ion is of Ising limit, while this is not always the case. This method would fail in the prediction of a system deviating largely from a strong easy axial anisotropy; second, the value of the charges on the ligands are very rough. For instance, those neutral coordination molecules, such as water or tetrahydrofuran, are considered having no contribution to the crystal field, which is obviously not true; third, the crystal field theory is based on the electrostatic interaction without considering the covalent components in the bond. In the case that the ligands could not be treated as the good approximation of point charge, i.e., the non-negligible covalent metal−ligand bond exists, this approach does not work either.

matrix for transferring the crystal unit cell frame to the experimental system. In the low-field approximation, the magnetization is associated with the susceptibility tensor under an experimental Cartesian system by ⎛ sin θ cos φ ⎞T ⎛ χxx χxy χxz ⎞⎛ sin θ cos φ ⎞ ⎟⎜ ⎟ ⎜ ⎟ ⎜ M = H0⎜ sin θ sin φ ⎟ ⎜ χyx χyy χyz ⎟⎜ sin θ sin φ ⎟ ⎟⎜ ⎟ ⎜ ⎟ ⎜ ⎝ cos θ ⎠ ⎜⎝ χzx χzy χzz ⎟⎠⎝ cos θ ⎠

(2)

The rotating experiments can be done with the help of a horizontal rotator whose rotation axis is perpendicular to the static magnetic-field direction (Figure 3). By fitting the angular-resolved data to eq 2, one can determine the crystal’s χ tensor with respect to the experimental Cartesian system. The molecular magneticsusceptibility tensor can thereby be determined from the crystal one when the two are identical. But certain elements of molecular tensor vanish when the molecular symmetry is lower than the lattice, leading to the restriction that the molecular susceptibility tensor can be obtained only when the symmetry of the crystal lattice is not higher than the molecule.

2.3. Understanding the Magnetic Anisotropy through Ab Initio Calculations

2.5. Effective Spin Hamiltonian toward 3d-SIMs

The recent progress of ab initio calculations considering relativistic effects provides, for understanding of electronic structures, a promising alternative to the crystal field Hamiltonian approach. The calculations are handled by complete active space self-consistent field method (CASSCF), and/or complete active space second order perturbation method (CASPT2) and then the restricted active space state interaction calculations (RASSI) by taken the spin−orbit coupling into consideration that implanted in MOLCAS package.20 The magnetic anisotropy, barriers, relaxation paths, magnetic couplings are deduced by applying SINGLE_ANISO or POLY_ANISO program.21,22 It has proved rather powerful both in the noninteracted Ln-SIMs and the exchange-coupled Ln-SMMs, though it is not a routine work for synthetic chemists without the help of theoretical chemists.17

The phenomenal understanding of the magnetic anisotropy of 3d-SIMs is the effective spin Hamiltonian approach. The occurrence of zero field splitting (ZFS) of the spin system of S > 1/2 is mainly due to the second-order spin−orbit coupling recovered by state-mixing. A recent publication from the Ruiz’s group claims that large magnetic anisotropy of 3d transition metal ions originates from the effective mixing of low-lying spin−orbit free excited states with the ground states as well as from the contribution of the first-order spin−orbit coupling.26 The axial anisotropy parameter D for high-spin mononuclear transition metal complexes of various electronic configurations and coordination modes were estimated at extended Hückel calculation levels. It is reviewed that the low-coordinate complexes of d6, d7, and d8 configurations are more likely to have large negative D values. Accordingly, low-coordinate transition metal complexes are preferable for designing 3d SIMs.

2.4. Determining the Magnetic Anisotropy via Experiments

The weak correlation between the quantized axis direction and pseudoaxial symmetry could lead to the prediction failure. The distribution of the negative charges from the ligands is the major factor that affects the magnetic anisotropy. This is also reflected in the displaced charge model analysis and recent examples (Dy3+/DOTA).11,15,23 Hence, the validation of the above theoretical analysis relies on the determination of magnetic anisotropy via experiment. The angular-resolved magnetometry method is a convenient and powerful tool.24,25 With the crystal face indexing information, one can deduce a

3. CONSTRUCTING AND CHARACTERIZING SIMs 3.1. Survey of 4f-SIMs

In the very beginning of our research, a DC field dependent slow magnetic relaxation was observed in the NdIII containing 2D coordination polymer.27 The slow magnetic relaxation was at first attributed to the presence of frustration which is unveiled by the static DC field.28 This DC field induced slow 2383


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Figure 4. (a) Experimental (red) and calculated easy axis with (blue) and without (green) consideration of charge displacement of compound [Dy(tBu-acac)3(bpy)]. (b) Experimental and simulated angular-resolved susceptibility.

Figure 5. Central DyIII ion geometry of (a) [Zn2(L1)2DyCl3]·2H2O and (b) [Zn2(L1)2Dy(H2O)Br2]·[ZnBr4]0.5; panels (c) and (d) represent corresponding hysteresis.

methyl-1,5-naphthyridine-3-carbonitrile) SIMs. The photoluminescence measurement under a pulsed magnetic field (0−36 T) combined with ab initio calculations unambiguously demonstrated the magnetic relaxation barriers and paths.43 Taking the advantage of the P1̅ space group of [Dy(tBuacac)3(bpy)] crystal (tBu-acac = 2,2,6,6-tetramethylheptane3,5-dionate, bpy = bipyridine), one can determine the molecular magnetic anisotropic axes.38 The experimentally determined easy axis lies nearly in the plane defined by the two opposite β-diketonates and DyIII ion, consistent with recent predictions of the electrostatic model approach (Figure 4). 3.1.2. Electrostatic Potential Design: High-Performance Zn−Dy−Zn-Type SIMs. As mentioned in section 2.4, the charge distribution symmetry of the ligands dominates the magnetic anisotropy. Compared with the highly symmetrical structure found in high-performance lanthanide SIMs, the very recent reported Zn−Dy−Zn type SIM [Zn2(L1)2DyCl3]·2H2O (H2L1: N,N-bis(3-methoxysalicylidene)phenylene-1,2-diamine) has only a C2 axis through the Cl and DyIII center but shows a relatively high Ueff of 430 K and a hysteresis temperature of 8 K (Figure 5a and c).44 The following electron density analysis elucidates that four oxygen atoms and one chlorine atom with a

magnetic relaxation was later found to be rather general in lanthanide containing complexes with or without 3d spin carriers.29−31 Now we tend to assign this phenomenon to the single ion anisotropy origin. In fact, the magnetic interactions between lanthanide ions which functions in the magnetic relaxation process were observed in recent work.32−35 Along with these observations, we gradually developed the 4f singleion magnets. 3.1.1. β-Diketonate-Coordination: Eight-Coordinate Dy-SIMs. In 2010, we reported a mononuclear [Dy(acac)3(H2O)2] (acac: acetylacetonate) molecule crystallized with an uncoordinated water and ethanol molecule, forming a pseudolocal symmetry of D4d.36 The simple idea originates from the clue that previously reported SIMs are found with a high-fold rotation axis. Dynamic study clearly shows the thermal magnetic relaxation and QTM process below 8 K. Later, plenty of analogs of this SIM, with a different type of βdiketonates, were reported.37−41 At the same time, the mixed (phthalocyaninato) (porphyrinato) dysprosium double-decker SIMs which are cocrystallized with fullerene, were also synthesized and studied.42 More recently, we obtained series of [ADyL4]·[solvent] (A = Na, K, Rb, Cs; L = 4-hydroxy-82384


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Figure 6. (a) Molecular structure of [Cp*Er(COT)]; (b) temperature dependence study; (inset) Arrhenius fitting; (c) frequency dependence study.

lower electron density constitute a hard plane containing the DyIII center, and phenoxyl oxygen atoms with a higher electron density locate on the two sides of this hard plane. This special electron density distribution around the oblate DyIII ion leads to significant SIM behaviors. The different SIMs’ behaviors of its substituent analog [Zn2(L1)2Dy(H2O)Br2]·[ZnBr4]0.5 can be attributed to the large deviation from the equatorial plane of the five oxygen atoms, which reduces the magnetic anisotropy (Figure 5b and d). 3.1.3. π-Coordination: Organometallic Er-SIMs. In 2011, an organometallic molecule, [Cp*Er(COT)] (Cp* = C5Me5; COT = C8H82−), which behaves as a single-ion magnet, has open a new access in constructing SMMs.45 Interestingly, this sandwich-type SIM exhibits two thermally activated relaxation processes caused by the two conformers of the disordered COT ring (Figure 6). Magnetic hysteresis can be recorded up to 5 K with a coercive field of 1.3 T at a fieldsweeping rate of 700 Oe/s on a magnetically diluted sample.46 Sessoli and co-workers have measured an angular-resolved magnetization on the single crystal of this compound.47,48 The quantized axis determination fails due to the space group restriction. Nevertheless, the theoretical investigation has elucidated that the orientation of easy axis is along the pseudoaxial direction. Because of the specificity of the two ligands’ π-coordination nature, the charges felt by the ErIII ion are not on the carbon nuclei, but show obvious delocalization. We have performed the electrostatic potential simulation on it. If charge displacement is ignored, however, the on-nuclei charges can stabilize the quantized axis only on the equator plane (Figure 7).15 This result supports the investigation by Baldovi ́ et al. using LPEC (lone pair effective charge) model, where the pure REC (radial effective charge) model does not work.49 These results once again demonstrate the importance of the charge displacement. Subsequently we put forward the π-coordination strategy by using boratabenzene ligands and obtained three new Er-SIMs ([(C 5 H 5 BR)Er(COT)] (R = H; CH 3 ; NEt 2 ). 50 The bortabenzene ligands show poorer electron donor character than the Cp*. Magnetic studies show that the complex [(C5H5BCH3)Er(COT)] exhibits the highest energy barrier (300 cm−1) among all reported Er-SIMs. We also find that their

Figure 7. (a) Displaced charges of the COT2− and Cp* ligands; electrostatic analysis with (b) and without (c) consideration of the charge displacement.

magnetic properties are strongly influenced by the substituent on the boron atom. For the ErIII ion, the use of poorer axial electron donor ligands would decrease the electronic interaction along the uniaxial direction, and enhance the uniaxial magnetic anisotropy. 3.1.4. Hybridization Strategy: Half-Sandwich-Type Organometallic Dy-SIMs. Inspired by the outstanding magnetic properties of β-diketonate- and cyclomultienesupported SIMs, we designed and synthesized three halfsandwiched DyIII complexes [(CpR)Dy(DBM)2(THF)]·(CpR = C5Me5; C5Pr4Ph; C5Me4TMS) with a Janus structural motif.51 More specifically, the easy axis lies in the direction of two DBM ligands, much as in a paddle-wheel-shaped DyIII/β-diketonate system (Figure 8a). In addition, we designed and synthesized a series of bis-pentamethylcyclopentadienyl-supported dysprosium SIMs containing different equatorial ancillary ligands very recently.52 We found that the ancillary ligands can largely affect the SIMs’ performance; weak-field ancillary ligand will enhance these type Dy-SIMs performance as smaller transverse anisotropy is introduced (Figure 8b). 3.1.5. Neutral Is Not “Neutral”: C6Me6-Coordinate DySIM. Instead of employing negative-charge π-coordination ligands, we investigated the effect of neutral π-bonded arene ligand in complex [(C6Me6)Dy(AlCl4)3].53 The local geometry of the DyIII ion can be described as a distorted pentagonal bipyramid (Figure 9). Dynamic magnetic study indicates the SIMs property with Ueff of 101 K. If formal charges of arene are 2385


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consists with the experimental result only when the charge displacement of electrons is taken into consideration (Figure 10). 3.2. Survey of 3d-SIMs

In 2003, we reported the first homospin single-chain magnet by employing the CoII as spin carriers.55 The slow relaxation of this 1D chain originates from both the magnetic coupling and single ion anisotropy of the CoII ions. Due to its relatively large spin−orbit coupling, the CoII ion is able to display a rather prominent large zero-field splitting. Following this idea, we have constructed a few CoII-based single-ion magnets in the past years. 3.2.1. CoIICoIII3 Clusters: Zero-Field Six-Coordinate CoII-SIMs. In 2013, we reported a six-coordinate cobalt complex [(HNEt3) (CoIICoIII3L6)] (L = R-4-bromo-2-((2hydroxy-1-phenylethylimino)methyl)phenol), exhibiting slow magnetic relaxation without the static magnetic field.56 The center CoII ion, the only spin carrier within the molecule, is suited in a slightly distorted triangular prism of D3 symmetry (Figures 11(a) and (b)). DC magnetic measurement analysis reveal a very large zero-field splitting of uniaxial type (D = −115 cm−1, E = 2.8 cm−1). This is in accordance with the prediction of d7 configuration featuring trigonal prism coordination mode.26 Dynamic susceptibility study confirms its SIMs’ property with the Ueff of 109 K (Figure 11(c)). The deviation from linearity in ln τ vs T−1 plot in the absence or under applied dc field in low temperature range may attribute to the existence of Raman process. The subsequent study was made by manipulation of the substituent on Schiff base ligand H2L and counteractions.57 Magnetic research has demonstrated that the structural modification can finely tune the Ueff in a wide range. We found that the ZFS parameters depend strongly on angular parameters θ, φ, and γ (Figure 11, θ, φ, and γ describe the deviation from the ideal D3h symmetry). Small γ pitch, large φ angle, and departure of θ angle from 54.74° cause the distortion of D3h symmetry and introduce the transverse anisotropy. 3.2.2. d8 Configuration CoI−NHC Complex: Field Induced Two-Coordinate SIMs. It has been expected that NiII complexes may be very promising candidates of d8 SIMs,

Figure 8. (a) Crystal structures of [(C5Me5)Dy(DBM)2(THF)]. Red and blue line represents the ab initio calculated and electrostatic analysis easy axis, respectively. (b) Schematic of [Cp*2Dy]+. Z axis is the quantized easy axis (green dashed line).

Figure 9. Ab initio calculated easy axis of [(C6Me6)Dy(AlCl4)3] compound.

treated as zero, the electrostatic analysis predicts the easy axis in the equatorial plane, contradicting ab initio calculated results. It is because that the delocalized charges of the arene ligand are non-negligible. 3.1.6. Non-Kramers TmIII: A Less Common SIM. We recently reported two half-sandwiched-type SIMs [(Tp)Tm(COT)] and [(Tp*)Tm(COT)].54 Magnetic study shows that they slow magnetic relaxation. The loss of 3-fold axis symmetry in [(Tp*)Tm(COT)] introduces rhombic transverse terms that mix smaller |MJ| states into Ising limit ground states. The angular-resolved magnetometry measurement on a [(Tp*)Tm(COT)] single crystal shows that the easy axis is close to the Tm-centroid (COT) direction. The electrostatic analysis

Figure 10. (a) Experimental (red), ab initio calculated (green) and the electrostatic analytical (blue) easy axis of [(Tp*)Tm(COT)]; (b) ac measurements of diluted [(Tp)Tm(COT)]; (c) angular-dependent susceptibility plots of [(Tp*)Tm(COT)]. 2386


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Figure 11. (a) Structure for [CoIICoIII3]− part; (b) definition of the angular parameters (θ, φ, and γ); (c) temperature dependence plots. Inset: Arrhenius fitting of the relaxation process.

since some NiII compounds show very large magnetic anisotropy.6,7 Examples of d8 SIMs, however, are still rare.58 The cobalt(I)-N-heterocyclic carbene complexes, [Co(IMes)2][BPh4] (IMes: 1,3-dimesitylimidazol-2-ylidene), represent a less reported d8 SIM.59 It bears nearly linear C(carbene)−CoC(carbene) alignment mode. The SIM behavior was exhibited under a 2000 Oe DC field. Magnetic anisotropy analysis indicates the easy-plane anisotropy nature and the Raman process may predominate instead of the Orbach process. Surprisingly, its analogue, [Co(IAd)2][BArF4] (IAd: 1,3dimesitylimidazol-2-ylidene; BArF4: tetra(3,5ditrifluoromethylphenyl)borate), shows no SIMs behavior. This divergence is probably due to the specificity of NHCmetal interactions. Unlike the reported series of two-coordinate FeII complexes, the d orbitals of CoI are strongly mixed because of the pronounced d−π interactions. The CASPT2/RASSI calculations show that D parameter strongly depends on the dihedral angle of the two NHC planes. Especially, the case of a 90° dihedral angle would largely quench the spin−orbit coupling, as in the case of [Co(IAd)2][BArF4]. These results imply that, in NHC-coordinate 3d complexes, d−π interactions would be the key to understanding the magnetic properties.

perspective. The comprehensive understanding of SIMs’ magnetic anisotropy enables us to study more complicated SMMs whose magnetic interaction can be detailed and studied. Furthermore, rational design of new SMMs, in which the magnetic easy axis was purposefully oriented, would be more promising. We are optimiztic that this goal is not far off.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Yin-Shan Meng is currently a Ph.D. candidate in the College of Chemistry and Molecular Engineering, Peking University, under the supervision of Prof. Song Gao. He received his B.S. degree in 2012 from the College of Chemistry, Beijing Normal University. His research focus on organometallic single-molecule magnets. Shang-Da Jiang is a Research Scientist in the College of Chemistry and Molecular Engineering in Peking University since 2015. He obtained his Ph.D. in chemistry in 2011 with the supervision of Prof. Song Gao in Peking University. Dr. Jiang worked as a postdoctor in Physikalisches Institut in Universität Stuttgart between 2011 and 2014 supported by Alexander von Humboldt-Foundation. From 2014 to 2015, he worked in the LNCMI-CNRS in Grenoble as a postdoctor. His research interesting focus on the magnetic anisotropy and quantum coherence of molecular magnets.

4. OUTLOOK Up to now, fruitful SIMs based on 3d and 4f elements have been discovered. The understanding of single-ion magnetic anisotropy indeed provides us new clues for designing new single-molecule magnets. Some challenges are to be solved: (1) Reaching the limit of the effective energy barrier. Some exciting results have been reported, such as the Ueff of pentagonal bipyramidal DyIII complex reaching 1000 K and the Ueff of modeled two-coordinate DyIII complex being predicted to surpass 1000 cm−1.9,60 In fact, aside from its remarkable magnetic property, the unique chemical structure is also fascinating and challenging for synthetic chemists. (2) Reducing the QTM rate and other relaxation processes. It also argues that high Ueff is not directly correlated with high TB, as some reported SIMs show very large energy barrier but with fast QTM; while some SIMs exhibit high TB but with relative smaller energy barrier. Therefore, evaluating the SIM performance with the Ueff parameter may not be suitable in some cases. Finally, we identify two further steps of SIMs. One is identifying the new properties or physics under critical or extreme conditions such as high pressure and high magnetic field. The other is “turning back” to SMMs from a new

Bing-Wu Wang obtained his Ph.D. in 2004 from Peking University under the supervision of Prof. Guang-Xian Xu. After a postdoctoral stay with Prof. Zhi-Da Chen (2004−2006), he joined the College of Chemistry and Molecular Engineering in Peking University and currently is an Associate Professor. His current research interests are in the areas of molecular magnetism and related phenomenon in materials and biological science. Song Gao got his B.S. and Ph.D. in chemistry at Peking University (PKU) in 1985 and 1991, respectively, and then worked at PKU until now. He was a Humboldt Research Fellow in TH Aachen in 1995− 1997. He has been Cheung Kong Professor in College of Chemistry and Molecular Engineering at PKU since 2002. He was elected as member of Chinese Academy of Sciences in 2007. In 2013, he was elected as TWAS Fellow. He is member of Editorial Advisory Board 2387


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for Chem. Soc. Rev., Chem. Sci. etc., Editor-in-Chief of Inorg. Chem. Front., and Associate Editor of Nat. Sci. Rev. His research interests are coordination chemistry; molecular nanomagnets; molecular and crystal engineering; multifunctional molecular materials.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21321001, 21290171, 21571008, and 91422302) and National Key Basic Research Program of China (2013CB933401).

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DEDICATION In memory of Prof. Heiko Lueken from RWTH Aachen. REFERENCES

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Understanding the Magnetic Anisotropy toward Single-Ion Magnets

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