inorganics Article Switching the Local Symmetry from D5h to D4h for Single-Molecule Magnets by Non-Coordinating Solvents Xia-Li Ding 1, Qian-Cheng Luo 1 , Yuan-Qi Zhai 1, Qian Zhang 2,* , Lei Tian 3, Xinliang Zhang 4, Chao Ke 5, Xu-Feng Zhang 6, Yi Lv 6 and Yan-Zhen Zheng 1,* 1 Frontier Institute of Science and Technology (FIST), Xi’an 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 Tong 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 Zhan 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