inorganics

Article Switching the Local Symmetry from D5h to D4h for Single-Molecule Magnets by Non-Coordinating Solvents

Xia- Ding 1, Qian- Luo 1 , Yuan- 1, Qian 2,* , Tian 3, Xinliang Zhang 4, Chao 5, -Feng Zhang 6, Lv 6 and -Zhen Zheng 1,*

1 Frontier Institute of Science and Technology (FIST), Xi’ Jiaotong University Shenzhen Research School, State Key Laboratory for Mechanical Behavior of Materials, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi’an Key Laboratory of Sustainable Energy and Materials Chemistry, School of Chemistry and School of Physics, 99 Yanxiang Road, Xi’an 710054, China; [email protected] (X.-L.D.); [email protected] (Q.-C.L.); [email protected] (Y.-Q.Z.) 2 State Key Laboratory of Military Stomatology & National Clinical Research Center for Oral Diseases & Shaanxi International Joint Research Center for Oral Diseases, Department of General Dentistry and Emergency, School of Stomatology, Air Force Medical University, 169 West Changle Road, Xi’an 710032, China 3 State Key Laboratory of Military Stomatology, National Clinical Research Center of Oral Diseases, Shaanxi Key Laboratory of Oral Diseases, Department of Cranio-facial Trauma and Orthognathic Surgery, School of Stomatology, The Fourth Military Medical University, 145 West Changle Road, Xi’an 710032, China;   [email protected] 4 Department of Spine Surgery, Honghui Hospital, Xi’an Jiaotong University, 76 Nanguo Road, Citation: Ding, X.-L.; Luo, Q.-C.; Xi’an 710054, China; [email protected] 5 Zhai, Y.-Q.; Zhang, Q.; Tian, L.; Department of Orthopaedic Trauma, Honghui Hospital, College of Medicine, Xi’an Jiaotong University, 28 West Xianning Road, Xi’an 710054, China; [email protected] Zhang, X.; Ke, C.; Zhang, X.-F.; Lv, Y.; 6 Department of Hepatobiliary Surgery, The First Affiliated Hospital of Xi’an Jiaotong University, Zheng, Y.-Z. Switching the Local 277 West Yanta Road, Xi’an 710061, China; [email protected] (X.-F.Z.); [email protected] (Y.L.) Symmetry from D to D for 5h 4h * Correspondence: [email protected] (Q.Z.); [email protected] (Y.-Z.Z.); Single-Molecule Magnets by Tel.: +86-029-833-951-72 (Y.-Z.Z.) Non-Coordinating Solvents. Inorganics 2021, 9, 64. Abstract: A solvent effect towards the performance of two single-molecule magnets (SMMs) was https://doi.org/10.3390/ observed. The tetrahydrofuran and toluene solvents can switch the equatorial coordinated 4- inorganics9080064 t Phenylpyridine (4-PhPy) molecules from five to four, respectively, in [Dy(O Bu)2(4-PhPy)5]BPh4 1 t · · Academic Editors: and Na{[Dy(O Bu)2(4-PhPy)4][BPh4]2} 2thf hex 2. This alternation significantly changes the local Akseli Mansikkamäki and Daniel coordination symmetry of the Dy(III) center from D5h to D4h for 1 and 2, seperately. Magnetic studies B. Leznoff show that the magnetic anisotropy energy barrier of 2 is higher than that of 1, while the relation of blocking temperature is just on the contrary due to the symmetry effect. The calculations of the Received: 2 June 2021 electrostatic potential successfully explained the driving force of solvents for the molecular structure Accepted: 29 July 2021 change, confirming the feasibility of adjusting the performance of SMMs via diverse solvents. Published: 16 August 2021 Keywords: dysprosium; local symmetry; single-molecule magnets; magnetic relaxation; solvent effect Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction There are considerable high performance single-molecule magnets (SMMs) containing dysprosium(III) reported in recent years [1–6], ranging from the cyclopentadienyl (Cp) based system [7–10] to the pentagonal-bipyramidal (PB) family [11–14]. Among these Copyright: © 2021 by the authors. dysprosium(III)-based complexes, highly axial crystal field [14,15] is essential to achieve Licensee MDPI, Basel, Switzerland. good performance of SMMs [16,17]. In this regard, other ancillary components, while This article is an open access article unnecessary to this target, play, however, a key role to stabilize the Dy(III) complexes distributed under the terms and because the radii of the lanthanides are very large [3,4,18]. This is well demonstrated by conditions of the Creative Commons et al. who reported a text-book example of solvent induced symmetry transformation Attribution (CC BY) license (https:// between quasi-D and quasi-O by losing its one coordinated methanol molecule when creativecommons.org/licenses/by/ 5h h exposed to the dry air and then recovered the structure by soaking in methanol for one 4.0/).

Inorganics 2021, 9, 64. https://doi.org/10.3390/inorganics9080064 https://www.mdpi.com/journal/inorganics Inorganics 2021, 9, x FOR PEER REVIEW 2 of 10

Dy(III) complexes because the radii of the lanthanides are very large [3,4,18]. This is well demonstrated by Tong et al. who reported a text-book example of solvent induced symmetry transformation between quasi-D5h and quasi-Oh by losing its one coordinated Inorganics 2021, 9, 64 methanol molecule when exposed to the dry air and then recovered the structure2 of by 10 soaking in methanol for one day [19]. The compounds with identical ligands are provided with completely different magnetic properties, as well as the effective energy barrier (Ueff) dayand [blocking19]. The temperature compounds with(TB), which identical are ligandstwo pivotal are provided indexes evaluating with completely the properties different of SMMs [20–22]. magnetic properties, as well as the effective energy barrier (Ueff) and blocking temperature Furthermore, it has also been discovered that the similar transform in configuration (TB), which are two pivotal indexes evaluating the properties of SMMs [20–22]. triggeredFurthermore, by the itsolvent has also effect been discoveredleads to the that change the similar of related transform properties, in configuration such as triggeredhydrophobicity by the solventand luminescence effect leads to response the change [23–25]. of related Zang’s properties, group such synthetized as hydropho- one bicitydendrimers and luminescence of Ag12@POSS response6 by introducing [23–25]. Zang’spolyhedral group oligomeric synthetized silsesquioxane one dendrimers (POSS) of Agwith12@POSS thiol group6 by introducing modifying polyhedraland observed oligomeric that its silsesquioxanecore cluster went (POSS) through with a thiol structural group modifyingchange within and flattened observed cubo-octahedral that its core cluster and went normal through cubo-octahedral a structural caused change by within different flat- tenedsolvents cubo-octahedral of acetone and and tetrahydrofuran, normal cubo-octahedral making causedthe film by matrix different embellished solvents of by acetone them andpossess tetrahydrofuran, distinctive hydrophobicity making the film [23]. matrix embellishedg et al. prepared by them one possess case distinctiveof dumbbell- hy- drophobicityshaped crystalline [23]. Zhang molecular et al. rotor prepared and onethey case found of dumbbell-shaped that it and its solvated crystalline crystal molecular possess rotora structural and they difference found that and it that and a itsdihedral solvated angle crystal change possess of about a structural 30° exists, difference leading andto a thatluminescent a dihedral change angle of change about of 10 about nm [24]. 30◦ exists, leading to a luminescent change of about 10 nmHowever, [24]. other than the direct coordinating solvent, here, we show that the uncoordinatedHowever, othersolvent than can thealso direct have a coordinating significant impact solvent, on here,the final we coordination show that the number unco- ordinatedof the equatorial solvent canligands also haveof two a significant dysprosium(III) impact SM on theMs. finalUsing coordination the similar number synthetic of theprocedure equatorial with ligands only varying of two the dysprosium(III) reaction solvents, SMMs. we Usingobserved the that similar the synthetictetrahydrofuran proce- dure(THF) with and only toluene varying solvents the reaction can switch solvents, the equatorial we observed coordinated that the tetrahydrofuran 4-Phenylpyridine (THF) (4- andPhPy) toluene molecules solvents from can five switch to four, the equatorial which finally coordinated leads to 4-Phenylpyridine two complexes, (4-PhPy)namely, t molecules[Dy(OtBu)2 from(4-PhPy) five5 to]BPh four,4 which1 and finally Na{[Dy(O leads totBu) two2(4-PhPy) complexes,4][BPh namely,4]2}∙2thf [Dy(O∙hex 2Bu). The2(4- t PhPy)corresponding5]BPh4 1 and point Na{[Dy(O group ofBu) the2 molecule(4-PhPy)4 ][BPhis altered4]2}· 2thffrom·hex local2. D The5h to corresponding D4h, respectively. point It groupshould of be the noted molecule that the is PB altered complexes from are local usuallyD5h to crystallizedD4h, respectively. in THF or It pyridine should be solvents noted thatand thethis PB is the complexes first time are that usually we isolated crystallized the PB in THFcomplex or pyridine in toluene. solvents We have and calculated this is the firstthe electrostatic time that we potentials isolatedthe (ESPs) PB complexof the equatorial in toluene. ligand We and have two calculated classes of the solvents, electrostatic from potentialswhich we (ESPs)could see of the the equatorial configuration ligand transition and two in classes different of solvents, solvent fromwas validated which we due could to seethe thevaried configuration electrostatic transition interaction in different between solvent the solvent was validated and ligand. due to Considering the varied elec- the trostaticdifferent interaction properties between of both theSMMs, solvent this and va ligand.riety of Considering supramolecular the different chemistry properties process ofrealizes both SMMs,the switch this of variety magnetic of supramolecularcontrol of compounds. chemistry process realizes the switch of magnetic control of compounds. 2. Results and Discussion 2. Results and Discussion 2.1.2.1. Synthesis ofof 1–21–2 TheThe synthesissynthesis steps steps of theseof these two complexestwo complexes are shown are shown in Scheme in 1Scheme. The only 1. differenceThe only isdifference the treatments is the treatments of the powder of the after powder being after dried being from dried the previous from the reaction previous using reaction the THFusing solvent. the THF solvent.

SchemeScheme 1.1. TheThe syntheticsynthetic routeroute forfor ComplexComplex 11 andand 22..

2.2.2.2. Single CrystalCrystal StructureStructure TheThe X-rayX-ray single single crystal crystal structure structure analysis analysis reveals thatreveals the crystallographicthat the crystallographic asymmetric unit of 1 is composed of a seven-coordinate mononuclear cation [Dy(OtBu) (4-PhPy) ]+ asymmetric unit of 1 is composed of a seven-coordinate mononuclear cation [Dy(O2 tBu)25(4- and a charge-balancing counteranion BPh − (Table S1). The central Dy(III) center in the 5 + 4 4− PhPy) ] and a charge-balancing counteranion BPh (Table S1).t The central Dy(III)+ center asymmetric unit of 2 is six-coordinate with the formula of [Dy(O Bu)2(4-PhPy)t 4] (Figure1 ).+ in the asymmetric unit of 2 is six-coordinate with the formula of [Dy(O Bu)2(4-PhPy)4] The structure of 1 is very similar to previously reported dysprosium(III) complexes belong- (Figure 1). The structure of 1 is very similar to previously reported dysprosium(III) ing to the family of PB geometry. The average Dy-O distance of 2.123(2) Å for 1 is a little longer than that of 2 (2.066(8) Å); the five equatorial Dy-N bond lengths are ranging from 2.55 to 2.59 Å for 1, which are obviously longer than those of 2 (averaged at 2.468(8) Å, see Table S2 for detail). Indeed, the PB polyhedron is quite regular with an axial O-Dy-O angle of 175.14(7)◦ and 180◦ for 1 and 2, respectively, and N-Dy-N angles ranging from 69.44(12)◦ to 73.41(11)◦. These geometries lead to the continuous shape measurement (CShM) cal- III culations [26] for the Dy ions of 0.889 for local D5h symmetry of 1 and 0.693 for the Inorganics 2021, 9, x FOR PEER REVIEW 3 of 10

complexes belonging to the family of PB geometry. The average Dy-O distance of 2.123(2) Å for 1 is a little longer than that of 2 (2.066(8) Å); the five equatorial Dy-N bond lengths are ranging from 2.55 to 2.59 Å for 1, which are obviously longer than those of 2 (averaged Inorganics 2021, 9, 64 at 2.468(8) Å, see Table S2 for detail). Indeed, the PB polyhedron is quite regular with3 of an 10 axial O-Dy-O angle of 175.14(7)° and 180° for 1 and 2, respectively, and N-Dy-N angles ranging from 69.44(12)° to 73.41(11)°. These geometries lead to the continuous shape measurement (CShM) calculations [26] for the DyIII ions of 0.889 for local D5h symmetry of compressed octahedron of 2, namely with tetragonal–bipyramidal (D ) local-symmetry 1 and 0.693 for the compressed octahedron of 2, namely with tetragonal–bipyramidal4h (D4h) (Tablelocal-symmetry S3). (Table S3).

t + t Figure 1. The molecular structures of the [Dy(OtBu)2(4-PhPy)5] + cation in 1 (a) and [Dy(O tBu)2(4- Figure 1.The molecular structures of the [Dy(O Bu)2(4-PhPy)5] cation in 1 (a) and [Dy(O Bu)2(4- PhPy)4]+ cation in 2 (b); other atoms and disordered two THF molecules, one hexane molecule and PhPy) ]+ cation in 2 (b); other atoms and disordered two THF molecules, one hexane molecule and one Na4 + cation per formula unit in the channels are omitted for clarity in 2. The polyhedron of the + oneDyIIINa ionscation center perviewed formula respectively unit in the from channels the front are (left) omitted and forthe claritytop (right). in 2. The polyhedron of the DyIII ions center viewed respectively from the front (left) and the top (right). In the molecular packing, the shortest intermolecular Dy∙∙∙Dy distance is 12.57(3) Å In the molecular packing, the shortest intermolecular Dy···Dy distance is 12.57(3) for 1 and 15.63(2) Å for 2 (Figures S2 and S3), respectively. More interestingly, the Å for 1 and 15.63(2) Å for 2 (Figures S2 and S3), respectively. More interestingly, the disordered organic THF/hexane molecules are easily lost in the 3D extended channels disordered organic THF/hexane molecules are easily lost in the 3D extended channels structure formed by the intermolecular π-π stacking interactions (Figure 2a–c), with a 2/3 structure formed by the intermolecular π-π stacking interactions (Figure2a–c), with a voids existing in the 3D extended channels structure of compound 2 according to the 2/3 voids existing in the 3D extended channels structure of compound 2 according to the ‘SQUEEZE’ routine. Further characterizations indicate that there are two disordered THF ‘SQUEEZE’ routine. Further characterizations indicate that there are two disordered THF molecules, one disordered hexane molecule and one Na+ cation per formula unit in the molecules, one disordered hexane molecule and one Na+ cation per formula unit in the channels as confirmed by ICP, TGA, elemental analysis, and charge balance (details can channels as confirmed by ICP, TGA, elemental analysis, and charge balance (details can be seen in the previous report [5]). There is no such special supermolecular structure for be seen in the previous report [5]). There is no such special supermolecular structure for 11 (Figure(Figure2 2d).d). However,However, thethe solventsolvent isis readilyreadily lostlost inin 22,, at at a a temperature temperature in in THF THF with with no no structural changes,changes, only only losing losing the guestthe guest molecules molecules while thewhile host structurethe host wasstructure maintained. was Wemaintained. did not findWe did the not structure find the of structure compound of 2compoundto revert to2 to compound revert to compound1 in the process 1 in the of process of measurement. It is possible that the solubility of Na[BPh4] in THF vs. toluene is measurement. It is possible that the solubility of Na[BPh4] in THF vs. toluene is a large drivera large of driver the change of the inchange structure. in structure. This also This manifests also manifests that solvation that hassolvation an important has an influenceimportant oninfluence the coordination on the coordination geometry andgeom spatialetry and packing spatial structure packing of structure the complexes, of the whichcomplexes, probably which lead probably to having lead different to having effects different on the effects magnetic on propertiesthe magnetic of theproperties complexes. of the complexes.

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Figure 2. (aFigure) The packing2. (a) The diagram packing of diagram the main of skeleton the main structure skeleton forstructure2 in one for asymmetric 2 in one asymmetric unit cell. (unitb) The cell. 3D extended  (b) The 3D extended channels structure for 2; the BPh4 and disordered components in channels are channels structure for 2; the BPh4 and disordered components in channels are omitted for clarity. (c) The packing diagram for 2 alongomitted the a direction. for clarity. (d) ( Thec) The packing packing diagram diagram for for1 along 2 along the the a direction. a direction. (d) The packing diagram for 1 along the a direction. 2.3. Magnetic Properties 2.3. Magnetic Properties From the temperature-dependent direct current (dc) magnetic susceptibilities, com- From the temperature-dependent direct current (dc) magnetic susceptibilities, plexes 1 and 2 show similar paramagnetism. The temperature dependence of the magnetic complexes 1 and 2 show similar paramagnetism. The temperature dependence of the susceptibilities were carried out under 1 kOe dc field in the temperature range of 300–2 K magnetic susceptibilities were carried out under 1 kOe dc field in the temperature range (Figure S4), which gave the χT products (in the unit of cm3 K mol−1) of 14.08 for 1 at 300 K, of 300–2 K (Figure S4), which gave the χT products (in the unit of cm3 K mol−1) of 14.08 for which are very close to the expected value of 14.17 cm3 K mol−1 for free Dy3+ ion. Upon 3 −1 3+ 1 at 300 K, whichcooling, are veryχT valuesclose to in the two expected complexes value keep of essentially 14.17 cm constant.K mol for At lowerfree Dy temperature about ion. Upon cooling,20 K, χ theT values sudden in drop two of complexesχT product keep indicates essentially the onset constant. of magnetic At lower blocking (Figure S4). temperature about 20The K,field the sudden (H) dependence drop of χ ofT the product magnetization indicates ( Mthe) foronset1 (Figure of magnetic3a) were measured at blocking (Figurea 10S4). Oe/s sweeping rate. For 1, the magnetic hysteresis loops were not closed up to 13 K, The field (Hwhich) dependence are in substantial of the magnetization agreement with (M) thefor zero-field-cooled1 (Figure 3a) were and measured field-cooled (ZFC-FC) at a 10 Oe/s sweepingmagnetizations rate. For 1 peak, the atmagnetic about 11 hysteresis K for 1 (Figureloops were3c). not On closed the other up hand,to 13 the butterfly K, which are inmagnetic substantial hysteresis agreement loops with were the observed zero-field-cooled only up to and 5 K andfield-cooled did not have (ZFC- a peak in ZFC-FC FC) magnetizationsmagnetizations peak at about for 112 asK for shown 1 (Figure previously 3c). On [5 ].the Moreover, other hand, we the could butterfly see a smaller step in magnetic hysteresiszero dcloops field were regime observed and even only a widerup to 5 hysteresis K and did loop not at have 2 K fora peak1 compared in ZFC- to 2 (Figure3b), FC magnetizations for 2 as shown previously [5]. Moreover, we could see a smaller step which indicates a blocking temperature of SMM with D5h local symmetry clearly higher in zero dc field regimethan the and one even with a Dwider4h local hysteresis symmetry. loop We at 2 reasoned K for 1 compared that this isto because2 (Figure the pseudo-D5h 3b), which indicatessymmetric a blocking SMM cantemperature better restrain of SMM the quantum with D5h tunneling local symmetry of magnetization clearly (QTM) effect higher than the atone zero with field D4h and local hence, symmetry.TB is enhanced.We reasoned that this is because the pseudo- D5h symmetric SMM can better restrain the quantum tunneling of magnetization (QTM) effect at zero field and hence, TB is enhanced.

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a) 2.0 K b) 1 4 4.0 K 4 2 7.0 K T = 2 K 10 K 10 Oe/s 2 13 K 2 B B μ μ / /

0 0 M M

-2 -2

-4 -4

-2-1012-2 -1 0 1 2 H / T H / T c) d) 104 1.8 ZFC Exp FC Fit 1.5 103 -1 11 K 1 1.2 - mol 3 s 2 /

1 10 -

cm 0.9

/ τ χ 0.6 101

0.3 100 0 5 10 15 20 25 30 35 40 40 50 60 70 80 90 100 T / K T / K FigureFigure 3.3. ((a)) MagneticMagnetic hysteresishysteresis loopsloops forfor1 1.(. b(b)) The The comparison comparison of of the the magnetic magnetic hysteresis hysteresis at at 2 K2 K for 1forand 1 and2.( 2c). ( Field-cooledc) Field-cooled (FC) (FC) and and zero-field-cooled zero-field-cooled (ZFC) (ZFC) magneticmagnetic susceptibilities of of 11 measuredmeasured under a dc field of 1000 Oe. (d) Temperature dependence of the relaxation time τ in a zero dc field under a dc field of 1000 Oe. (d) Temperature dependence of the relaxation time⁄ τ in a zero dc field for 1, the solid lines are the best fits according to the equation = + . −1 −1 −Ue f f /T n for 1, the solid lines are the best fits according to the equation τ = τ0 e + CT . Alternating current (ac) susceptibilities under zero dc field were measured for 1 Alternating current (ac) susceptibilities under zero dc field were measured for 1 (Figure S5 and S6) at a relatively high temperature range, and for 2 in details, they can be (Figure S5 and S6) at a relatively high temperature range, and for 2 in details, they can be seen in the previous report [5]. The maxima of both the in-phase (χ′) and out-of-phase (χ″) seen in the previous report [5]. The maxima of both the in-phase (χ0) and out-of-phase components showed a clear temperature dependence (Figure S5). The frequency- (χ00) components showed a clear temperature dependence (Figure S5). The frequency- dependent ac susceptibilities were also measured and the peaks of χ″ can be all observed dependent ac susceptibilities were also measured and the peaks of χ00 can be all observed from 40–97 K for 1 (Figure S6a) at the frequency ranging from 1 to 1218 Hz. Moreover, the from 40–97 K for 1 (Figure S6a) at the frequency ranging from 1 to 1218 Hz. Moreover, the frequency-dependent data can be by a modified Debye function to obtain the Cole−Cole frequency-dependent data can be by a modified Debye function to obtain the Cole−Cole plots for 1 (Figure S7). The α value is less than 0.01 (Table S4) and only has a peak, plots for 1 (Figure S7). The α value is less than 0.01 (Table S4) and only has a peak, demonstrating narrow distributions of relaxation times and one relaxation mechanism for demonstrating narrow distributions of relaxation times and one relaxation mechanism this sample. Figure 3d for 1 shows the plots of τ−1 versus T for the temperature-dependent for this sample. Figure3d for 1 shows the plots of τ−1 versus T for the temperature- relaxation rate. The values are obtained by fitting the ac data with the equation = dependent ⁄ relaxation rate. The τ values are obtained by fitting the ac data with the + eff 0 −1 −, 1giving−Ue f f / theT followingn parameters: for 1, U = 1785(6) K, τ = 1.96(4) × equation τ = τ0 e + CT , giving the following parameters: for 1, Ueff = 1785(6) 10–12 s, C = 5.3(6) × 10–5 s–1 K–n, n = 3.02(5). These parameters are very similar to other D5h K, τ = 1.96(4) × 10−12 s, C = 5.3(6) × 10−5 s−1 K−n, n = 3.02(5). These parameters are SMMs0 due to the fact that they share the same coordination geometry [27]. For 2, the very similar to other D5h SMMs due to the fact that they share the same coordination equation has been modified with an additional QTM term, which gives Ueff = 2075(11) K, geometry [27]. For 2, the equation has been modified with an additional QTM term, which τ0 = 5.61(2) × 10–13 s, C = 5.60(4) × 10–3 s–1 K–n, n = 2.85(4) and τQTM = 0.46 s. From these results, gives U = 2075(11) K, τ = 5.61(2) × 10−13 s, C = 5.60(4) × 10−3 s−1 K−n, n = 2.85(4) we couldeff see that in the higher0 temperature the relaxation mechanisms for both complexes and τ = 0.46 s. From these results, we could see that in the higher temperature the are similar,QTM whereas in the low temperature region where QTM is effective, the relaxation relaxation mechanisms for both complexes are similar, whereas in the low temperature behavior is disparate for two complexes. Due to its local D4h symmetry effect, 2 has region where QTM is effective, the relaxation behavior is disparate for two complexes. Due stronger QTM effect compared to 1. For 1, due to the nearly perfect D5h local symmetry, to its local D symmetry effect, 2 has stronger QTM effect compared to 1. For 1, due to QTM is much4h suppressed. This can be also seen in the hysteresis loops (Figure 3). At 2 K the nearly perfect D local symmetry, QTM is much suppressed. This can be also seen in (Figure 3b) at the zero5h field region, the hysteresis is much wider for 1 than that of 2, the hysteresis loops (Figure3). At 2 K (Figure3b) at the zero field region, the hysteresis is indicating that QTM is well mitigated [28]. much wider for 1 than that of 2, indicating that QTM is well mitigated [28].

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2.4.2.4. AbAb initioinitio CalculationCalculation ToTo gaingain insight insight into into the the magnetic magnetic properties properti andes electronicand electronic structure, structure, ab initio ab calcula- initio tioncalculation towards towards1 was performed 1 was performed at SA-CASSCF/RASSI at SA-CASSCF/RASSI level by using level OPEN by MOLCAS using OPEN (see SupportingMOLCAS (see Information Supporting for details).Information The for highly details). axial The g-tensor highly of groundaxial g-tensor Kramer of Doublet ground Kramer Doublet (KD) could be observed (gx = gy = 0, gz = 19.89) as well as local principal (KD) could be observed (gx = gy = 0, gz = 19.89) as well as local principal magnetizations ofmagnetizations that almost along of that the almost O-Dy-O along direction the O-Dy-O (Figure direction4, insert), (Figure and 4, its insert), wave and function its wave is ratherfunction pure is rather with 99.9% pure with |±15/2> 99.9% (Table |±15/2> S5). (Table Meanwhile, S5). Meanwhile, the calculated the calculated LoProp charges LoProp alsocharges prove also the prove existence the existence of a strong of a axialstrong crystal axial fieldcrystal with field more with negative more negative charges charges on O atomson O thanatoms equatorial than equatorial N atoms N (Table atoms S7). (Table Moreover, S7). Moreover, the excited the KDs excited contain KDs relatively contain purerelatively wave pure function wave until function the fourth until KD:the fourth 99.8% |KD:±13/2> 99.8% (KD |±13/2>2), 99.4% (KD |2),± 99.4%11/2> |±11/2> (KD2), 87.1%(KD2), | 87.1%±9/2> |±9/2> (KD3 )(KDand3)70.1% and 70.1% |±1/2> |±1/2> (KD (KD4) lying4) lying at 781at 781 K, 1300K, 1300 K, 1576K, 1576 K and K and 1636 1636 K, respectively.K, respectively. The The third third excited excited KD KD with with highly highly mixed mixed characteristic characteristic and and its its g zgzangle angleof of 86.8686.86°◦ indicateindicate that the Orbach Orbach relaxation relaxation passes passes through through this this state, state, making making the the theoretical theoreti- calvalue value of ofUeffU effaroundaround 1736 1736 K, K, which which is is closely closely consistent consistent with thethe experimentalexperimental valuevalue (Figure(Figure4 ).4).

FigureFigure 4.4. AbAb initioinitio calculatedcalculated electronicelectronic statesstates ofof 11.. TheThe numbersnumbers besidebeside thethe arrowsarrows expressexpress thethe relativerelative transitiontransition propensity. propensity. The The local local principal principal magnetizations magnetizations of theof the ground ground Kramer’s Kramer’s doublet doublet of 1of. For 1. For clarity, clarity, all hydrogenall hydrogen atoms atoms are are omitted omitted (insert). (insert).

TheThe interrelatedinterrelated complex complex2 2possessing possessing local localD D4h4h symmetrysymmetry hashas alreadyalready beenbeen discusseddiscussed viavia crystalcrystal field field parameters parameters (CFPs) (CFPs) in the in previous the previous literature, literature, indicating indicating that such geometrythat such geometry adverts the emergence of significant non-axial crystal field termsq (, q ≠ 0) and adverts the emergence of significant non-axial crystal field terms (Bk , q 6= 0) and can becan one be one of the of the candidates candidates to designto design high high performance performance SMMs. SMMs. Notwithstanding Notwithstanding strongstrong uniaxialuniaxial crystalcrystal fieldsfields realizedrealized in in both both complexes, complexes, the theU Ueffeff valuevalue forfor1 1is is lowerlower thanthan that that of of 22,, whichwhich can can be be understood understood by by means means of of the the following following aspects. aspects. Firstly, Firstly, the incrementthe increment of the of coordinationthe coordination number number in the equatorin the equator circle partly circle causes partly the causes reduction the reduction of the local of symmetric the local environmentsymmetric environment around Dy 3+aroundions, whichDy3+ ions, proceeds which toproceeds changing to CFPs.changing We CFPs. found We that found the 0 absolutethat the valueabsolute of axialvalue parameterof axial parameterB2 in 1 ( −6.53) in 1 is ( less−6.53) than is less that than in 2 (that−9.50), in 2hinting (−9.50), identicalhinting identical relationship relationship in respect in of respect respective of axialityrespective of groundaxiality KD. of ground Moreover, KD. the Moreover, gz value ofthe ground gz value KD inof 1ground(19.89) isKD also in smaller 1 (19.89) compared is also with smaller2 (19.98). compared Furthermore, with 2 from (19.98). the perspectiveFurthermore, of from magneto-structural the perspective correlation, of magneto-structural the length ofcorr Dy-Oelation, in 1 theis larger length than of Dy-O in 2 whilein 1 is the larger relationship than in 2 reverseswhile the for relationship Dy-N bonds reverses (LDy-O(avg) for Dy-N= 2.123 bonds Å, L (LDy-N(avg)Dy-O(avg) == 2.5772.123 ÅÅ, forLDy-N(avg)1,LDy-O(avg) = 2.577 =Å 2.064 for 1, Å, LDy-O(avg) LDy-N(avg) = 2.064= 2.648 Å, L ÅDy-N(avg) for 2). = Evidently,2.648 Å for the 2). shorter Evidently, Dy-O the distance shorter playsDy-O a distance leading plays role in a suchleading competitive role in such effect competitive between both effect different between types both of different bond length. types Meanwhile,of bond length. the angle Meanwhile, of O-Dy-O the inangle the latter of O-Dy-O is 179.95 in◦ ,the more latter linear is 179.95°, than 1 (175.14 more ◦linear), making than this1 (175.14°), variety ofmaking coordination this variety mode of more coordination suitable for mode ions more with suitable oblate electron for ions density. with oblate electronIn addition, density. electrostatic potentials (ESPs) of distinctive solvents and transversal ligandsIn wereaddition, calculated electrostatic to explain potentials configuration (ESPs) of transformation distinctive solvents in distinctive and transversal solvents (Figureligands5 ).were It was calculated found that to THFexplain and configuration 4-PhPy possess transformation similar electrostatic in distinctive potential solvents distri-

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(Figure 5). It was found that THF and 4-PhPy possess similar electrostatic potential bution,distribution, while while there wasthere no was or onlyno or a only little a negative little negative potential potential region regi on theon surfaceon the surface of toluene. of Duetoluene. to pure Due electrostatic to pure electrostatic attractions betweenattractions Dy andbetween N, in Dy THF and solvent N, environment,in THF solvent the moleculesenvironment, of THF the andmolecules 4-PhPy of are THF mutually and 4-Ph exclusivePy are mutually in electrostatic exclusive interaction, in electrostatic leading tointeraction, the formation leading of ato complex the formation with localof a complexD4h symmetry. with local Meanwhile, D4h symmetry. the rotatability Meanwhile, of thethe carbon–carbonrotatability of singlethe carbon–carbon bond between single the two bond rings between provides the favorable two rings conditions provides for thefavorable formation conditions of this configurationfor the formation in terms of this of configuration steric hindrance. in terms Contrarily, of steric a hindrance. compound withContrarily, local D a5h compoundconfiguration with is local formed D5h inconfiguration toluene solvent is formed owing in to toluene the fact solvent that molecules owing tendto the to fact contact that inmolecules a complementary tend to contac wayt to in maximize a complementary the electrostatic way to interaction.maximize the In short,electrostatic from the interaction. nature of bonding,In short, wefrom understand the nature the of phenomenon bonding, we of understand solvent-induced the configurationphenomenon transformation,of solvent-induced which affectsconfigur theation properties transformation, of SMMs andwhich it is theaffects flexibility the ofproperties supramolecular of SMMs chemistry and it is the that flexibility leads to of these supramolecular interesting and chemistry meaningful that leads coordination to these reactionsinteresting emerging. and meaningful coordination reactions emerging.

FigureFigure 5.5. Electrostatic potentials potentials of of THF, THF, toluene, toluene, an andd 4-PhPy 4-PhPy from from left left to to right, right, respectively. respectively. (isovalue = 0.001). (isovalue = 0.001).

3.3. MaterialsMaterials andand Methods 3.1.3.1. SynthesisSynthesis

t t ForFor 11,, inin anan argon glovebox, a mixture of DyCl DyCl33 (0.5(0.5 mmol, mmol, 134 134 mg), mg), NaO NaOBuBu (1 (1 mmol, mmol, 9696 mg)mg) and NaBPh NaBPh4 4(0.5(0.5 mmol, mmol, 171 171 mg) mg) was was added added with with about about 10 mL 10 THF mL in THF Schlenk in Schlenk tube, tube,which which gave a gave cloudy a cloudy solution. solution. After stirring After stirringfor 24 h, for the 24 solution h, the solutionwas filtered was and filtered the andsolvent the was solvent removed was removedby vacuum by to vacuum get a white to get powder a white of the powder products; of the 2 mL products; toluene 2and mL toluene4-PhPy and(3 mmol, 4-PhPy 465 (3 mg) mmol, were 465 then mg) added were thento th addede powder to thewith powder further with stirring. further Colorless stirring. Colorlesscrystals suitable crystals for suitable X-ray fordiffraction X-ray diffraction were grown were by grown slow bydiffusion slow diffusion of hexane of hexaneto the tosolution the solution at room at temperature room temperature after three after days three: Yield days: 278 Yieldmg, 36% 278 (based mg, 36% on Dy); (based Elemental on Dy); Elementalanalysis calcd analysis (%) for calcd C98 (%)H95BDyN for C985OH295: CBDyN 76.03,5O H2 :6.19, C 76.03, N 4.52; H 6.19, found: N 4.52; C 76.05, found: H 6.20, C 76.05, N H4.53. 6.20, IR N was 4.53. as IRshowed was as in showed Figure S1 in (see Figure Supplementary S1 (see Supplementary Materials). Materials). TheThe synthesissynthesis processprocess of 2 is the same as as 1 with THF THF used used instead instead [5]. [5].

3.2.3.2. X-rayX-ray CrystallographyCrystallography Data AllAll datadata werewere recorded recorded on on a Brukera Bruker SMART SMART CCD CCD diffractometer diffractometer with with MoK MoKα radi-α 2 ationradiation (λ = ( 0.71073λ = 0.71073 Å). Å). The The structures structures were were solved solved by by direct directmethods methods andand refined refined on on FF2 usingusing Olex2.Olex2. CCDC 2086927 ( 1) contains the the supplementary supplementary crystallographic crystallographic data data for for this this paperpaper includingincluding Tables S1–S4.S1–S4. Th Thee CIF CIF and and the the checkCIF checkCIF files files for for 11 cancan be be found found in in the the SupplementarySupplementary Materials.Materials.

3.3.3.3. MagneticMagnetic Properties MagneticMagnetic susceptibilitysusceptibility measurements were were carried carried out out with with a a Quantum Quantum Design Design MPMS-XL7MPMS-XL7 SQUIDSQUID magnetometer (Quantum (Quantum Design Design Company, Company, San San Diego, Diego, CA, CA, USA) USA) uponupon coolingcooling fromfrom 300300 toto 22 KK inin variablevariable appliedapplied fields.fields. Ac suscep susceptibilitytibility measurements measurements werewere performedperformed atat frequencies frequencies of of between between 1 1 and and 1500 1500 Hz Hz with with an an oscillating oscillating field field of of 3.5 3.5 Oe andOe and with with variable variable dc applied dc applied field. field. Powder Powder samples samples were were embedded embedded in eicosane in eicosane to avoid to anyavoid field any induced field induced crystal reorientation.crystal reorientation. Crystalline Crystalline powders powders were fixed were with fixed eicosane, with wrappedeicosane, wrapped with film, with and film, placed and inplaced the center in the center of a straw. of a straw. A diamagnetic A diamagnetic correction correction has beenhas been calculated calculated from from Pascal Pascal constants constants and and embedding embedding eicosane eicosane has has been been applied applied to theto observed magnetic susceptibility. The results are included in Table S4 and Figures S4–S7 (see Supplementary Materials).

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3.4. Electronic Structure Calculations Complete Active Space Self-Consistent Field (CASSCF) calculation was performed to understand the magnetic properties of 1 via its electronic structure using OPEN MOL- CAS [29] and its geometry structure was obtained straightly from X-Ray single crystal structure without optimization. The basis sets from the ANO-RCC library [30] were em- ployed for all atoms: VTZP quality for Dy, VDZP quality for O and N atoms, as well as VDZ quality for others; 21 sextets, 224 quartets, and 490 doublets were calculated in the RASSCF module to acquire the state-averaged CASSCF orbitals. Then, a spin-orbit (SO) coupling Hamiltonian was constructed and diagonalized in the RASSI module [31] through the chosen 21 sextets, 128 quartets, and 130 doublets. Ultimately, all magnetic properties of Dy(III) ion, such as g-tensors, crystal field parameters, transition magnetic moment matrix, magnetic susceptibility, and magnetization plot, were computed and the output was obtained via the SINGLE_ANISO program [32]. For assuring calculation accuracy, we also considered employing the Cholesky decomposition for two-election integrals. Including Tables S5–S8 (see Supplementary Materials).

3.5. DFT Calculations To acquire wave function information of both complexes and electrostatic potentials (ESP) of distinctive solvents and transversal ligand, the calculations based on Density Function Theory (DFT) were performed using Gaussian 09 E01 [33]. The PBE density functional [34] was employed in all calculations with Grimme’s D3 dispersion correction considered [35–37]. Primarily, the positions of hydrogen atoms of 1 and 2 were optimized. We replaced Dy(III) ion to Y(III) in light of similar radius between them, and set its atomic mass as 162.5 (the same as natural abundance-weighted mass of dysprosium). The Stuttgart RSC 1997 effective core potential (ECP) [38,39] was applied for 28 core electrons of Y(III) and corresponding valence basis set was used for the remaining valence electrons, while the rest of atoms were treated with cc-pVDZ basis set [40,41]. Then the whole molecules of toluene, THF and 4-phenylpyridine were optimized by the same basis set. Harmonic vibrational calculations indicate that there is no imaginary vibration mode and all optimized minimum- energy structures have already been at stationary points on the potential energy surface. The results are included in Tables S9–S13 (see Supplementary Materials).

4. Conclusions To summarize, we observed, for the first time, that a non-coordinating solvent can also significantly impact the coordinate number of the equatorial ligands in the pentagonal– bipyramidal geometry of the dysprosium(III) SMMs. The performances of SMMs are subsequently switched. The effective energy barrier in 2 is higher than 1 while the relation of blocking temperature is just the contrary. The structural transform is explained by the electrostatic interaction between the solvent and the ligands, which confirmed the feasibility of adjusting the performance of SMMs by supramolecular chemistry of solvents effect.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/inorganics9080064/s1, Figure S1: The IR spectrum of complex 1 and 2; Table S1: Crystal- lographic data for complex 1 and 2; Table S2: Selected Bond Lengths (Å) and Bond Angles (deg) for complex 1 and 2; Table S3: The CShM’s values of the first coordination sphere of compound 1; Table S4: The CShM’s values of the first coordination sphere of compound 2; Figure S2: Packing diagram for complex 1; Figure S3: Packing diagram for complex 2; Table S4: Relaxation fitting parameters of a generalized Debye model for 1; Figure S4: The variable-temperature dc magnetic susceptibility (1 kOe) and the field dependence of the magnetization (inset) at 2 K for 1; Figure S5: Temperature-dependence of the in-phase χ0 and out-of-phase χ” ac susceptibility signals under zero dc field for 1; Figure S6: Frequency-dependence of the in-phase χ0 and out-of-phase χ” in a zero dc field for 1 with ac frequencies of 1–1218 Hz; Figure S7: Cole-Cole plots for the ac susceptibilities in a zero dc field for 1; Table S5: Ab initio results for the J = 15/2 multiple of DyIII in 1; Table S6: Ab initio calculated crystal field parameters for 1; Table S7: Ab initio calculated LoProp charges of the atoms near Dy center in 1; Table S8: Average transition magnetic moment elements between the states Inorganics 2021, 9, 64 9 of 10

of 1; Table S9: Geometry coordinates of 1 in population analysis calculations; Table S10: Geometry coordinates of 2 in population analysis calculations; Table S11: Geometry coordinates of toluene molecule in population analysis calculations; Table S12: Geometry coordinates of THF molecule in population analysis calculations; Table S13: Geometry coordinates of 4-phenylpyridine in population analysis calculations. The CIF and the checkCIF files for 1. Author Contributions: Conceptualization, X.-L.D. and Y.-Z.Z.; methodology, X.-L.D., Q.-C.L., Y.-Q.Z. and Y.-Z.Z.; software, X.-L.D., Q.-C.L. and Y.-Q.Z.; validation, X.-L.D., Q.-C.L., Y.-Q.Z., Q.Z., L.T., C.K., X.Z., X.-F.Z., Y.L. and Y.-Z.Z.; formal analysis, X.-L.D.; Y.-Q.Z. and Q.-C.L.; investigation, Y.-Z.Z., Q.Z., L.T., C.K., X.Z., X.-F.Z. and Y.L.; resources, Q.Z., L.T., C.K., X.Z., X.-F.Z. and Y.L.; data curation, X.-L.D., Q.-C.L. and Y.-Q.Z.; writing—original draft preparation, X.-L.D. and Q.-C.L.; writing—review and editing, X.-L.D., Q.-C.L. and Y.-Z.Z.; visualization, Y.-Z.Z.; supervision, Y.-Z.Z.; project administration, Y.-Z.Z.; funding acquisition, Y.-Z.Z., Q.Z., L.T., C.K., X.Z., X.-F.Z. and Y.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (21871219, 21773130 and 21620102002); Key Laboratory Construction Program of Xi’an Municipal Bureau of Science and Technology (201805056ZD7CG40); The Shenzhen Science and Technology Program (JCYJ20180306170859634); The Fundamental Research Funds for Central Universities. Data Availability Statement: All data needed to evaluate the paper are present in the main text or the Supplementary Materials. Additional data related to this paper may be requested from the authors. Acknowledgments: This work was supported by the following funding and authors. We also thank the Instrument Analysis Center of Xi’an Jiaotong University for the assistance from Gang . Conflicts of Interest: The authors declare no conflict of interest.

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