Elucidating the Magnetic Anisotropy and Relaxation Dynamics of Low

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Elucidating the Magnetic Anisotropy and Relaxation Dynamics of Low-Coordinate Lanthanide Compounds Peng Zhang,†,‡ Julie Jung,§ Li Zhang,†,‡ Jinkui Tang,*,† and Boris Le Guennic*,§ †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing, 100049, P. R. China § Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, 263 Avenue du General Leclerc, 35042 Rennes Cedex, France S Supporting Information *

ABSTRACT: The magnetic relaxation and anisotropy of 3- and 4-coordinate lanthanide complexes were systematically investigated, and the change of SMM behavior originating from the equatorially coordinating ligand field was successfully elucidated through combined experimental and theoretical studies. Remarkably, a novel approach taking into account the different contributions of atomic charges, dipole moments, and quadrupole moments was used to map the electrostatic potential around the metal center in the DyIII derivatives, revealing the key role played by the ligands as a whole and not just by the coordinating donor atoms as often considered.

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INTRODUCTION Since the beginning of the 1990s, considerable efforts have been devoted to the development of single-molecule magnets (SMMs)1 due to the wide range of exotic functional properties encountered in this class of materials, i.e., magnetic bistability, quantum tunneling of the magnetization (QTM), quantum coherence, etc.2 In more recent years, the extensive use of lanthanide ions has led to some great breakthroughs in the field of molecular magnetism, with the better understanding of magnetic anisotropy and relaxation processes,3 but also with the improvement of the magnetic blocking temperature (TB) as well as the effective energy barrier (Ueff).4 The main reason for this is the tremendous single-ion anisotropy of lanthanide ions that arises from the unquenched orbital angular momentum and the strong spin−orbit coupling, which greatly facilitates the design of SMMs.3b,5However, controlling the magnetic anisotropy of a lanthanide ion under a given ligand environment remains a difficult challenge, mostly due to the intricate electronic structure of such ions.5 Given the large effective energy barriers present in some typical Dy, Tb, and Er SMMs,4c,6 a highly axial molecular symmetry is expected as the transversal components of the magnetic anisotropy which facilitate QTM within magnetic doublets are reduced.7 Additionally, recent ab initio studies of lanthanide-based SMMs revealed that the nature and the orientation of the magnetic anisotropy in lanthanide systems are also strongly influenced by the charge distribution in the first coordination sphere of the lanthanide ion.3h,8 The latter is indeed expected © XXXX American Chemical Society

to shape the 4f electron density due to the predominantly electrostatic character of the chemical bond on the metal site.3b,c,f,g,9 Unfortunately, the high coordination number of lanthanide ions generally results in low-symmetry complexes,5b thus hindering the fine-tuning of the magnetic properties. In this respect, low-coordinate complexes represent new opportunities for a better control of the environment’s and molecule’s symmetry, thus making the relationship between the chemical environment and the magnetic properties easier to understand. Since the recent discovery of three-coordinate Er SMM, which behaves as the first equatorially coordinated lanthanide SMM,10 the road has been open for further investigation. In addition, an eye-catching near-linear two-coordinate Sm complex was synthesized by Winpenny et al., and importantly theoretical predictions reveal an unprecedented SMM property with an effective barrier of more than 1000 cm−1 in the Dy analogue.9b,11 Herein we present a detailed analysis of the magnetic behavior of two four-coordinate lanthanide complexes with a trigonal-pyramidal geometry, Ln[N(SiMe3)2]3ClLi(THF)3 (Figure 1, Ln = DyIII, 1-Dy, and ErIII, 1-Er), through both experimental and ab initio investigations. A mapping of the electrostatic potential in 3- and 4-coordinate DyIII derivatives clearly reveals the key role of molecular symmetry in defining the nature and the orientation of the magnetic anisotropy. Received: December 1, 2015

A

DOI: 10.1021/acs.inorgchem.5b02792 Inorg. Chem. XXXX, XXX, XXX−XXX

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(%) calcd for C30H78ClDyLiN3O3Si6 (1-Dy): C, 39.93, H, 8.71, N, 4.66. Found: C, 39.86, H, 8.65, N, 4.69. Elemental analysis (%) calcd for C30H78ClErLiN3O3Si6 (1-Er): C, 39.72, H, 8.67, N, 4.63. Found: C, 39.88, H, 8.60, N, 4.72. X-ray Crystallography. Single crystal X-ray data of the complexes were collected on a Bruker Apex II CCD diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data processing was completed with the SAINT processing program. The structure was solved by direct methods and refined by full matrix leastsquares methods on F2 using SHELXTL-97.14 The locations of the lanthanide atoms were easily determined, and C, O, and N atoms were determined from the difference Fourier maps. The non-hydrogen atoms were refined anisotropically. All hydrogen atoms were introduced in calculated positions and refined with fixed geometry with respect to their carrier atoms. The crystallographic data for 1-Dy and 1-Er are available as Supporting Information. CCDC 1039488 (1Dy) and 1039489 (1-Er) also contain the crystallographic data for this paper and can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Crystal data, structural refinement parameters, and selected bond distances and angles are listed in Tables S1 and S2. Magnetic Measurements. Magnetic susceptibility measurements were obtained in the temperature range 2−300 K, using a Quantum Design MPMS XL-7 SQUID magnetometer equipped with a 7 T magnet. The experimental magnetic data are corrected for the diamagnetism estimated from Pascal’s constants15 and sample holder calibration. Ab Initio Calculations. CASSCF/SI-SO calculations have been carried out on the X-ray structures. See the Supporting Information for computational details. Mapping of the Electrostatic Potential. The electrostatic potential generated in the close environment of a lanthanide ion by its ligands is modeled using the following multipolar expansion:

Figure 1. Structure of complex 1 with violet, pink, green, blue, red, and gray spheres representing Ln, Li, Cl, N, O, and C atoms, respectively. Hydrogen atoms are omitted for clarity.

Remarkably, K. R. Dunbar and co-workers experimentally reported a similar four-coordinate Er SMM, showing an effective barrier of 44 cm−1 and magnetic hysteresis up to 3 K,12 but a detailed theoretical study is still lacking.

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EXPERIMENTAL SECTION

General Considerations. All reactions were carried out under dry and oxygen-free argon atmosphere by using standard Schlenk or glovebox techniques. Glassware was oven-dried before use. Solvents were distilled from sodium benzophenone ketyl or CaH2 under pure argon atmosphere prior to use. The LiN(SiMe3)2 ligand was purchased from Acros Organics and used without further purification. Elemental analysis for C, H, and N was performed on a PerkinElmer 2400 analyzer. Synthesis of Ln[N(SiMe3)2]3ClLi(THF)3 (Ln = DyIII, 1-Dy; ErIII, 1Er). 1-Dy and 1-Er were prepared in a similar fashion to the Eu analogue which was reported previously.13 A 1.5 mL solution of LiN(SiMe3)2 (1 mol/L) was added at 0 °C to a stirred suspension of LnCl3 (0.5 mmol) in 10 mL of tetrahydrofuran. Stirring was continued overnight, and the solvent was removed under reduced pressure. The remaining solid was treated with n-hexane. The resultant solution was filtered and allowed to stand undisturbed at room temperature for several days to yield the products 1-Dy and 1-Er. Elemental analysis

N

V (M ) =

∑ i=1

qi ri ⃗

+

pi ⃗ · ri ⃗ ri ⃗

3

+

ri ⃗·(Q̅ i × ri )⃗ ri ⃗

5

⎛ 1 ⎞ ⎟ + O⎜ 9 ⎝ ri ⃗ ⎠

From a fundamental point of view, this formula is used to describe the electrostatic potential V(M) generated by a finite distribution of charges (located each at point i such as i = 1, ..., N, with N the total number of charges present) in a given point of space, here called M,

Figure 2. Highlights of the symmetry differences between model 1-Dy′ (Figure S1) and complex 2-Dy. The reference structures of 1-Dy′ (top) and 2-Dy (bottom) are rotated around the Z-axis that goes through the Dy atom and that is perpendicular to the XY plane (plane of the sheet), by 120° and 240°. B

DOI: 10.1021/acs.inorgchem.5b02792 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (left) Temperature dependence of the static magnetic susceptibility times temperature (χT) for 1-Dy and 1-Er under an applied field of 1000 Oe. (right) Magnetization (M) measurements reveal a butterfly shaped magnetic hysteresis in 1-Er at 1.9 K. located inside the charge distribution. r⃗i stands for the position vector going from point M to point i, while qi, p⃗i and Q̅ i stand respectively for the value of charge, dipole moment, and quadrupole moment of the charge located at point i. Higher-order terms are included in the truncation term O. These multipolar moments describe formally the spatial distribution of the charge at point i. For each considered complex, the sum over i runs over all atomic positions, except that of the lanthanide ion. For each atom, the point charge and the associated multipolar moments are extracted from a LoProp calculation performed on the ground state of the system.16 This potential is calculated in a given number of M points, equally spaced in a sampling cube (1 × 1 × 1 Å3) centered on the position of the lanthanide ion. Since a large set of 3D data is generated, to represent it efficiently, we use 2D maps, cut into the sampling cube along a given meaningf ul direction (i.e., typically, a plane containing the magnetic easy-axis or easy-plane). This approach, that has been already applied on a DyIII SMM,8 differs from the ones already proposed in the literature and implemented in the MAGELLAN and SIMPRE programs,3c,17 since we do not aim at predicting any magnetic behavior. Our approach is rather to be seen as a tool to help understanding and analyzing the magnetic anisotropy of a lanthanide-based complex, structurally and electrostatically speaking.

Dy′. Such subtle deviation from C3 symmetry has a great impact on the magnetic anisotropy of the lanthanide center, as will be revealed by the following comparison of the electrostatic potential. Magnetic Properties. Static (Dc) Measurements. At room temperature and under a 1000 Oe dc field, the χT values are 14.74 and 10.3 cm3 K mol−1 for 1-Dy and 1-Er, respectively (Figure 3). These values are in good agreement with the expected values of 14.17 and 11.48 cm3 K mol−1 for single DyIII and ErIII ions, respectively. As observed in most mononuclear lanthanide complexes reported to date,18 the χT values initially exhibit a slight drop with the decrease of temperature, followed by an obvious decrease in the low-temperature region. At 2 K, the χT products reach 8.80 and 7.46 cm3 K mol−1 for 1-Dy and 1-Er, respectively. The initial decrease at high temperature is due to the depopulation of the Stark sublevels of lanthanide ions, but the low-temperature behavior is probably due to the large inherent magnetic anisotropy of the complexes. In addition, the non-superposition on a single master curve for M vs H/T curves (Figures S2 and S3) at different temperatures reveals the presence of significant magnetic anisotropy and/or low-lying excited states in both 1-Dy and 1-Er. Notably, a butterfly shaped magnetic hysteresis (Figure 3), similar to the one obtained for complex 2-Er,10 was observed at 1.9 K in 1-Er using a sweep rate accessible with a conventional magnetometer, indicating a strongly blocked behavior of magnetization.18a Dynamical (Ac) Measurements. Under zero dc field, the out-of-phase component of the ac susceptibility (χ″) of 1-Er shows a strong frequency dependence in the 1.9−10 K range (Figure 4), indicating a typical SMM behavior, contrary to 1Dy, for which the χ″ component remains small whatever the temperature (Figure S4). These rather different behaviors are correlated with the ligand field being predominantly equatorial in both complexes, while DyIII is oblate and ErIIIproblate. In addition, when compared with the three-coordinate complex 2-Er,10 1-Er demonstrates a clear temperature independence below 5 K (Figure 4), indicating that pure QTM relaxation occurs at higher temperature in 1-Er than in 2Er. The ln(τ) vs T−1 curve exhibits a crossover from a thermally activated to a temperature independent regime. The linear fitting of this curve above 5 K gives an effective barrier (Ueff) of 44 cm−1 with τ0 = 8.88 × 10−8 s based on an Arrhenius law. Fitting the Cole−Cole plots to the generalized Debye model gives a series of α parameters below 0.30, indicating a very narrow distribution of relaxation times in a single relaxation process.19 The fast QTM and small Ueff in 1-Er with respect to 2-Er are expected to result from the small changes in the

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RESULTS AND DISCUSSION Crystal Structure of Complexes 1-Dy and 1-Er. With respect to the three-coordinate complexes, Ln[N(SiMe3)2]3 (Ln = DyIII, 2-Dy; ErIII, 2-Er),10 the Ln[N(SiMe3)2]3ClLi(THF)3 complexes (1) contain a similar Ln[N(SiMe3)2]3 moiety with only slight differences in bond lengths and angles (Table S2). Remarkably, the C3 molecular symmetry observed in 2 is lost in 1 (Figure 2). Most likely, this comes from the additional coordination of the Cl¯ ion to the lanthanide ion in 1, which gives rise to a trigonal-pyramidal geometry around the latter ion. In addition, the lanthanide ion lies closer to the plane formed by the three coordinating N atoms in 1 than in 2, i.e., about 0.40 Å in 1-Dy and 0.43 Å in 1-Er (about 0.53 Å in 2-Dy and 0.57 Å in 2-Er), while the Ln−Cl bond distance appears to be rather long, 2.601 Å in 1-Dy and 2.571 Å in 1-Er. Finally, since the Ln−Cl−Li angle is close to 180°, the three THF molecules which further coordinate the Li+ ion lead thus to the presence of only pseudo-C3 symmetry in 1. From the electrostatic point of view, the Li+ ion coordinated to the Cl¯ ion simply balances the negative charge of the latter. As a result, an equatorially coordinating ligand field still dominates around the lanthanide ion in 1. Removing the ClLi(THF)3 moiety in complex 1-Dy results in model complex 1-Dy′ (Figure S1), which has the same formula as complex 2-Dy. A close comparison between 1-Dy′ and 2-Dy (Figure 2) reveals the above-mentioned critical divergence, that is, the almost perfect C3 symmetry is lost in 1C

DOI: 10.1021/acs.inorgchem.5b02792 Inorg. Chem. XXXX, XXX, XXX−XXX

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the axis associated with the gZ value of the latter anisotropy tensor are represented for all complexes in Figure 5. Finally, the magnetic curves are represented in Figures S9 and S10.

Figure 5. Orientation of the calculated gZ direction associated with the ground Kramers doublet for complexes 1 and 2. Hydrogen atoms were removed for clarity.

Magnetic Anisotropy. As expected, almost opposite results are observed between the Dy and Er derivatives. This is mainly due to the distinct electron density shape in both ions, i.e., oblate for DyIII, while problate for ErIII. Under the dominant equatorial ligand field, both Er complexes, 1-Er and 2-Er, exhibit strongly axial magnetic anisotropy corresponding to the |MJ⟩ eigenstate with the highest possible value of MJ for ErIII, i.e., |MJ⟩ = |±15/2⟩, which explains the excellent SMM properties. These wave functions are in good agreement with the value of the χT product at low temperature (i.e., close to 10 cm3 K mol−1). Nevertheless, the slight increase in the transversal components of the ground state anisotropy tensor in 1-Er with respect to 2-Er (Table S6) is expected to lead to faster QTM in 1-Er than in 2-Er, which is completely consistent with their SMM performance. Conversely, in 2-Dy the ground Kramers doublet is predominantly composed of the |MJ⟩ = |±1/2⟩ eigenstate, which explains the low value of the χT product at low temperature (i.e., close to 7 cm3 K mol−1). The associated magnetic anisotropy is almost perfectly planar (gX ≈ gY ≫ gZ) with a direction for the gZ component that coincides with the C3 axis of the molecule. Such a rare example of nearly perfect equatorial ligand field provides a typical model to study the magnetic anisotropy features in lanthanide SMMs. In contrast, an axial anisotropy is observed in complex 1-Dy with a ground state mainly composed of the |MJ⟩ = |±15/2⟩ eigenstate (Table S6), in agreement with the value of the χT product at low temperature (i.e., close to 10 cm3 K mol−1). Quite surprisingly, the associated magnetic easy-axis is located in the equatorial plane, i.e., 90° away from the pseudo-C3 axis of the complex. Herein, a critical question consists of explaining the distinct anisotropy properties between both Dy complexes, one being easy-plane and the other being easy-axis. Hence more detailed investigations are performed. Let us first start with the 3-coordinate complex 2-Dy. According to Rinehart and Long,3b an equatorial ligand field is expected to be destabilizing for oblate ions like DyIII, leading thus to a ground state mainly composed of the |MJ⟩ eigenstate with the smallest possible value of MJ. This is indeed what is observed since, as already mentioned earlier, the wave function

Figure 4. (top) Frequency-dependent out-of-phase components (χ″) of ac susceptibility at different temperatures in 1-Er. (bottom) Magnetization relaxation time as ln(τ) vs T−1 for 1-Er under zero static field from best fit to the Arrhenius law of the thermally activated regime (solid line). Inset: Cole−Cole plots for 1-Er. The solid lines indicate the fits to the generalized Debye model.

equatorially coordinating ligand field upon the coordination of Cl− ion in 1-Er. Furthermore, if the temperature dependence of relaxation times (τ) is fitted by considering QTM, Raman, and Orbach processes (Figure S5),20 a slightly increased effective barrier is obtained, i.e., Ueff = 49 cm−1 (τ0 = 4.48 × 10−8 s), as well as a QTM relaxation time of 10.8 ms. Under dc field, QTM is significantly reduced in 1-Er at 1.9 K while no substantial effect is observed at higher temperature. Indeed the χ″ peak position remains unchanged at 8 K, with only a reduction of amplitude (Figure S6), indicating that the Orbach process is not influenced by the external dc field in the high temperature region. Remarkably, the application of a dc field allows the observation of clear χ″ peaks in complex 1-Dy, indicating a field-induced SMM behavior (Figure S7). In contrast with 2-Dy, the appearance of SMM behavior in 1-Dy can be assigned to the coordination of the Cl− ion to the Dy center, which leads to a symmetry reduction in the environment of the DyIII ion. Extracting relaxation times (τ) from the χ″ versus frequency plots and fitting the ln(τ) vs T−1 curve with an Arrhenius law (Figure S8) gives an effective barrier of 12 cm−1 (τ0 = 1.46 × 10−7 s). Ab Initio Calculations. In order to rationalize the abovementioned magnetic behaviors and to get a better understanding of the relaxation mechanisms in low coordinate lanthanide SMMs, ab initio calculations were performed for all complexes 1 and 2 (see computational details in the Supporting Information). The calculated energies for all Kramers doublets of the ground multiplet of all complexes are shown in Tables S4 and S5. The magnetic anisotropy tensor of the ground spin− orbit state of all complexes and its wave function in term of |MJ⟩ eigenstates are summarized in Table S6. The orientation of D

DOI: 10.1021/acs.inorgchem.5b02792 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Electrostatic potential maps for complex 1-Dy (top), model 1-Dy′ (middle), and complex 2-Dy (bottom). These maps are centered on the DyIII ion and show the potential in the plane containing the DyIIIion which is parallel to the equatorial ligand field plane and perpendicular to the pseudo-C3 axis. These maps, from left to right, represent the individual contributions of the atomic charges, dipole moments, and quadrupole moments, respectively. The black axes stand for the almost symmetry axes found in the different electrostatic potential maps and are shown to guide the eyes.

of the spin−orbit ground state of 2-Dy is predominantly composed of the |MJ⟩ = |±1/2⟩ eigenstate. For this eigenstate, the gZ value is expected to be close to 1.3 and much smaller than the gX and gY values, which is indeed the case here. This justifies both the planar magnetic anisotropy and the resulting poor SMM behavior, with strong quantum tunneling of 2-Dy. From the structural point of view, the 4-coordinate complex 1-Dy is expected to have an even more equatorial ligand field, since the DyIII ion is closer to the plane formed by the three N atoms of the N(SiMe3)2 ligands despite the presence of the Cl− ion coordinated to the lanthanide ion. However, the magnetic anisotropy of 1-Dy is found highly axial while that of 2-Dy is planar. To examine the effective role of the ClLi(THF)3 moiety, similar ab initio calculations were performed on a modified version of 1-Dy, that is, the 1-Dy′ model (Figure S1). The orientation of the magnetic easy-axis associated with the ground state does not change significantly between 1-Dy and 1Dy′; there is only a small weakening of the level of axiality (gX = 0.86, gY = 3.63, gZ = 16.53 for 1-Dy′; gX = 0.58, gY = 2.10, gZ = 17.60 for 1-Dy, Table S6). This indicates that the ClLi(THF)3

moiety, from both electronic and electrostatic points of view, is not responsible for the difference of magnetic behavior between 1-Dy and 2-Dy. The latter may finally be attributed only to structural modifications in the equatorial plane, i.e., some critical deviations from the rigorous C3 symmetry (Figure 2). Hereafter, in order to better understand how the equatorial ligand field of these complexes drives the shape and the orientation of their magnetic anisotropy, we use the electrostatic potential mapping approach presented in the Experimental Section to discuss the case of the DyIII derivatives, i.e., 1-Dy, 1-Dy′ and 2-Dy. The electrostatic potential maps shown in Figure 6 represent the features of the electrostatic potential generated by the ligands in the close environment of the DyIII ion for 1-Dy and 1-Dy′. A symmetry axis is found in all the electrostatic potential maps (black axis in Figure 6). This axis coincides, within less than 6°, with both the direction that minimizes on average the electrostatic potential and the direction of the magnetic easyaxis calculated in the ground state of 1-Dy (Figure S11) and 1Dy′ (Figure S12). The similarities in the potential maps of 1E

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initio, and electrostatic potential study and consequently rationalized the change of SMM behavior originating from changes in the molecular symmetry. Indeed, we evidenced that, to understand the magnetic anisotropy of a lanthanide-based complex, the whole molecule has to be considered and not only the first coordinating atoms and the apparent shape of the ligand field they draw. It indeed appeared that the nature and the orientation of the magnetic anisotropy are driven by the features of the electrostatic potential generated by the ligands of the complex, which in turn is directly related to the molecular symmetry. This analysis was made possible by taking advantage of low-coordinate lanthanide SMM systems, which effectively simplify the coordination geometry. Such a deep exploration enhances the strength of the electrostatic model in elucidating the magnetic anisotropy of lanthanide systems but also in designing new lanthanide-based SMMs with high performance. This work suggests that one should take a closer look at the molecular symmetry of the complex, which plays a decisive role, to tune the magnetic properties. Also, the comparison of electrostatic potential maps for the two Dy-based complexes, 1Dy and 2-Dy, revealed that all contributions, i.e., the zerothorder atomic charges but also higher order moments, have to be included in the description to get the best possible description of the potential generated by the environment.

Dy and 1-Dy′ (Figures 6, S11, and S12) suggest that the features of the electrostatic potential in the equatorial plane are driven more by the structural characteristics in this plane than by the ClLi(THF)3 moiety itself. When comparing the maps obtained for 1-Dy to those obtained for 2-Dy (Figure 6), obvious differences are observed. In particular, whatever the contribution, the electrostatic potential in 2-Dy holds a perfect C3 symmetry axis, matching exactly the structural one (Figures 2 and S13). Since there is no unique symmetry element in the maps of 2-Dy, contrary to 1Dy or 1-Dy′, no preferential magnetic direction emerges in the equatorial plane of 2-Dy. Moreover, since the electrostatic potential is less intense in this plane than along the C3 axis, the direction minimizing the electrostatic potential ends up corresponding with the plane that coincides with the planar magnetic anisotropy in 2-Dy. Finally, the differences in the potential features between 1-Dy and 2-Dy are expected to be responsible for the changeover from magnetic anisotropy plane to axis when going from 2-Dy to 1-Dy, and can only result from the structural changes induced by the addition of the ClLi(THF)3 moiety in 1-Dy with respect to 2-Dy. Importantly, it appears that the symmetry of the electrostatic potential generated by the ligands is directly related to the whole molecular symmetry and not only to the first coordinating atoms. Second, going from 2-Dy to 1-Dy′ and then 1-Dy suggests that the more you go away from the C3 symmetry, the more the magnetic anisotropy gets axial. Finally, it seems that the features of the electrostatic potential directly drive the shape and the orientation of the magnetic anisotropy of such complexes. Indeed, by stabilizing a given |MJ⟩ eigenstate, the potential ends up establishing the magnetic behavior of the system, independently of the shape of the ligand field. That is to say, it might be that the magnetic anisotropy and, thus, the magnetic behavior of the system are driven more by the molecular symmetry than by the shape of the ligand field, suggesting that the oblate or prolate character of the lanthanide ions as presented by Rinehart and Long3b should be considered cautiously and that most likely more subtle effects step in. Relaxation Dynamics. Ab initio calculations by M. Shanmugam, G. Rajaraman, and co-workers revealed an unprecedented magnetic relaxation via the fourth excited state in the 3-coordinated ErIII SMM 2-Er.21 Similar calculations were also performed on the 4-coordinated complexes 1-Dy and 1-Er (Figure S14). Herein, a possible relaxation pathway via the fourth excited state similar to that in 2-Er was revealed in 1-Er. However, in contrast to the process in 2-Er, the transition dipole moments connecting the different excited Kramers doublets in 1-Er exhibit critical increases, especially in the third excited states, which could effectively enhance the thermal activated QTM relaxation, and thus shortcut the effective barrier (Ueff). Nevertheless, such results still suggest the great potential of low coordinate complexes, where magnetic relaxation could indeed occur via Kramers doublets lying above the first-excited doublet.22 Finally, comparing 1-Dy to 2Dy, the coordination of the Cl¯ ion results in an obvious change of the ground state nature, leading thus to a change in magnetic anisotropy, which goes from easy-plane to easy-axis type, and to the appearance of field induced SMM behavior in 1-Dy.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02792. Computational details; physical measurements; and structural, magnetic, and calculated tables and figures (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21371166, 21331003, and 21521092) for financial support. B.L.G. and J.J. thank the French GENCI/IDRISCINES center for high-performance computing resources (Project x2015080649).

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REFERENCES

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CONCLUSION We have investigated the magnetic behavior of low-coordinate lanthanide complexes through a combined experimental, ab F

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.5b02792 Inorg. Chem. XXXX, XXX, XXX−XXX