Construction And Assembly Mechanism Of 4f And 3d-4f Single Molecule Magnets Based On Polyhydroxy Schiff Base Ligands

Rare earth metal complexes show broad application prospects in the construction of high-performance single-molecule magnets（SMMs）,catalysis,electronic conduction,and molecular devices because of their structural characteristics and unique electronic arrangement.It is not easy to catch the assembly processes of rare earth metal complexes due to the great uncertainties,complexity and contingency in their synthetic process,which leads to great difficulties for their syntheses under control.Therefore,designing and synthesizing rare earth complexes with special structures has become a major problem to be solved in the modern scientific research for the study of their magnetic properties and assembly mechanisms.The main work of this paper is to design and synthesize rare earth metal single-molecule magnets or single-ion magnets for magnetic studies and magnetic adjustments,as well as to explore their assembly mechanisms.Twenty-six rare earth metal complexes were synthesized using polyhydroxyal Schiff base ligands.Their structures,magnetic properties and assembly mechanisms were fully studied through IR,powder X-ray diffraction analysis,single crystal X-ray diffraction analysis,high-resolution spectrometry and magnetic measurements,as well as theoretical calculations.This paper is divided into seven chapters as follows.Chapter 1 is the introduction part which presents the research status for single molecule magnets and assembly processes,as well as the challenges and opportunities in this field.The related work and research basis of our research group on magnetic regulation and assembly mechanisms of rare earth metal complexes are introduced in detail.The main difficulties and key points in this research field are analyzed,and the research targets fro this work was determined.In Chapter 2,a study is presented for the coordination environments and magnetic properties of a series of six-coordinated Dy（III）complexes tuned by the coordination atoms of O and Cl.Four bis-Schiff bases bearing different spacers and one mono-Schiff base were designed,namely bis（2-hydroxynaphthylmethylene）-hydrazine（H₂L¹）,bis（2-hydroxylnaphthylmethylene）ethylenediamine（H₂L²）,bis（2-Hydroxynaphthylmethylene）-propanediamine（H₂L³）,bis（2-hydroxynaphthyl-methylene）-2-methylpropanediamine（H₂L⁴）and 1-（3-pyridinylmethylimino-methyl）-2-naphthol（HL⁵）.Their reactions with Dy Cl₃·6H₂O under solvothermal conditions gave five six-coordinated Dy（III）complexes[Dy（H₂L¹）₂（H₂O）Cl₃]（1）,[Dy（H₂L²）_3/2Cl₃]_n（2）,[Dy₂（H₂L³）₂Cl₆]·2CH₃CN（3）,[Dy（H₂L⁴）₂Cl₂]Cl·3Me OH（4）and[Dy（HL⁵）（H₂L⁵）Cl₄]（5）with octahedral Dy（III）configuration.1,4 and 5 exhibit mononuclear structures.The Dy（III）ions in 2 are connected through H₂L²ligands to build a two-dimensional sheet with a well separation of the Dy（III）ions.A dinuclear structure is built for 3 with its two Dy（III）ions doubly bridged by two Cl^-ligands.It revealed that the designed ligands can well tune the coordination environments for these six-coordinated Dy（III）ions in these metal complexes.Obvious differences were found in the static magnetic properties for titled metal complexes.3 is a typical zero-field SMM and 1,4 and 5 are field-induced SMMs.2 just presents slow magnetic relaxation behaviour.A further exploration for their slow magnetic relaxation showed the existence of multiple relaxation processes involving in their relaxation mechanism.Their energy barriers(U_eff/k _B)values were estimated to be172.5（4）,83.4（8）,23.85（1）and 138.2（3）K for 1 and 3-5,respectively.The magnetic properties of the Schiff base ligands can be effectively tuned by adjusting the spacers in the Schiff base ligands,thereby affecting the magnetic easy axis and magnetic anisotropy of Dy（III）ions.Theoretical calculations further verified the effects of quantum tunneling efficiency of magnetization,g-tensor,crystal field parameters and magnetic easy axis on the performance of the five complex SMMs.The synthesis,structure and magnetic studies of a series of dinuclear Dy（III）complexes were presented in Chapter 3.A series of different Schiff base ligands of 3-（（5-bromo-2-hydroxybenzylidene）amino）propane-1,2-diol（H₃L⁶）,2-（2-（2-hydroxy-3-methoxybenzylidene）-1l2-diazaneyl）acetimidamide（H₃L⁷）and 2-（（（2-hydroxynaphthalen-1-yl）methylene）amino）benzoic acid（H₂L⁸）and 2-（（（5-bromo-2-hydroxycyclohexa-1,3-dien-1-yl）methylene）amino）benzoic acid（H₂L⁹）were designed and reacted with different types of rare earth metal salts,giving six dinuclear Dy（III）complexes[Dy₂（HL⁶）₂（NO₃）₂]₂（6）,[Dy₂（HL⁶）₂Cl₂]（7）,[Dy₂（L⁷）₂（H₂O）₆]·4H₂O（8）,[Dy₂（HL⁷）₂（OAc）₆]·Et OH（9）,[Dy₂（H₂L⁸）₄（NO₃）₂]·2CH₃CN（10）and[Dy₂（H₂L⁹）4（NO₃）₂]·2CH₃CN（11）.The Dy（III）ions in complexes 6 and 7 are eight-coordinated in bicapped trigonal prismatic geometry and seven-coordinated in pentagonal bipyramid geometry,respectively.The magnetic behaviors of 6 and 7 are regulated by adjusting the coordination number of metal ions to further change the coordination configuration and symetries.The two Dy（III）ions are bridged by two hydroxyl groups from two Schiff base ligands for complex 8,and by four OAc^-ligands in two different bridging modes for complex 9.Complexes 8 and 9 utilize the same ligand to change the linkage between two Dy（III）ions,thereby changing the magnetic exchange between the two Dy（III）ions.The two Dy（III）ions in both 10and 11 are bridged by four carboxylate groups with the same coordination numbers and similar coordination geometries of Dy（III）ions.Their differences lie in the non-coordinating groups on their ligands,which lead to their different magnetic properties through changing their electronic densities.In this study,the structures and magnetic properties of a series of dinuclear complexes were tuned,achieving the appearance and disappearance of single molecule magnet with single or double magnetic relaxations.The studies of structures,magnetic properties and assembling mechanisms for a series of planar polynuclear Dy（III）single molecule magnets are presented in chapter4.A series of different Schiff base ligands 2-（（（2-hydroxynaphthalen-1-yl）methylene）amino）-2-methylpropane-1,3-diol(H₃L¹⁰),2-（Hydroxymethyl）-2-（（（2-hydroxynaphthalen-1-yl）methylene）amino）propane-1,3-diol(H₄L¹¹),2,2’-（（1E,1’E）-（（2-Hydroxypropane-1,3-diyl）bis（azamethylene））bis（methylene））bis（4-bromophenol）(H₃L¹²)and 2-（（（5-Bromo-2-hydroxycyclohexa-1,3-dien-1-yl）methylene）amino）-2-ethylpropane-1,3-diol(H₄L¹³)were designed and synthesized.Their reactions with different types of rare earth metal salts gave four Dy（III）complexes[Dy₄(L¹⁰)₂（L′）₄（Me OH）₂（CH₃CN）₂（H₂O）₂]Cl₂（12）,[Dy₄(HL¹¹)₂（L′）)₄（Me OH）₆]Cl₂·-3Me OH·2CH₃CN（13）,[Dy₄(L¹²)₄（HL′′）₂（OH）₂]·5CH₃CN（14）and[Dy₆（μ₃-OH）₂(HL¹³))₂(H₂L¹³)₂(H₃L¹³)₂Cl₂（Et OH）₂]Cl₂·2CH₃CN（15）with planar skeletons.Complexes 12 and 13 both have planar tetranuclear skeletons with different main terminal ligands.Complex 14 also presents planar skeleton,which differs from complexes 12 and 13 in the way of bridging among the four Dy（III）ions.Complex15 was synthesized by changing the ligands to form a hexanuclear double triangular skeleton.The magnetic regulation in these complexes is achieved through changing terminal coordination group and coordination numbers.The synthesis and design of double-relaxed SMMs are realized by changing the bridging oxygen atoms with different ligands,and thus changing the magnetic exchange between the four-nuclear plane Dy（III）.In this chapter,the structure and magnetic properties of dinuclear Dy（III）-based complexes were studied and regulated,and the behavior of single-molecule magnets was realized from single to double magnetic relaxation.The synthesis and assembly mechanism of the four rare earth complexes was studied in detail,and the assembly mechanisms of the four complexes were proposed according to high-resolution mass spectrometry.In chapter 5,we designed and synthesized 1-[[(2-hydroxyethylimino]methyl]-2-naphthol(H₂L¹⁴)and 4-bromo-2-[（2-hydroxypropylimino）methyl]phenol(H₂L¹⁵),which have good solubilities in conventional solvents.H₂L¹⁴was reacted with the corresponding metal salts to obtain two new dodecanuclear complexes[Dy₄Co₈（μ₃-OH）₈(L¹⁴)₈（OAc）₄（H₂O）₄]·3Et OH·3CH₃CN·H₂O（16）and[Dy₄Ni₈（μ₃-OH）₈(L¹⁴)₈（OAc）₄（H₂O）₄]·3.5Et OH·0.5CH₃CN·5H₂O（17）.H₂L¹⁵was reacted with the corresponding metal ions to obtain two new heteronuclear nanoclusters[Dy ₄^IIINi ₈^II（μ₃-OH）₈(L¹⁵)₈（OAc）₄（H₂O）₄]·3.5Et OH·4CH₃CN（18）and[Dy ₁₀^IIIMn ₄^IIIMn ₂^IIO₄（OH）₁₂（OAc）₁₆(L¹⁵)4(HL¹⁵)2（Et OH）₂]·2Et OH·2CH₃CN·2H₂O（19）.Complexes 16–18 have similar structures,presenting a cyclic tetracubane skeleton linked through four shared Dy（III）vertexes,in which the four inner Dy（III）ions are wrapped by eight peripheral 3d metal ions.Complex 19 also presents a cyclic skeleton.Magnetic studies showed that the four compounds exhibit slow magnetic relaxation behavior,and complex 19 is a field-induced single molecule magnet.High resolution electrospray mass spectrometry（HRESI-MS）showed that the framework of complexes 16-19 had high stability with increasing energy of ion source.Their possible formation mechanisms were proposed through tracking their intermediates via time-dependent HRESI-MS.In chapter 6,we studied the construction and assembling mechanism of cubane-based 3d-4f heterometallic single molecule magnets.Two heterometallic nanoclusters[Dy₂Co₈（μ₃-OCH₃）₂(L¹⁶)₄(HL¹⁶)₂（OAc）₂（NO₃）₂（CH₃CN）₂]·CH₃CN·H₂O（20）and[Dy₄Co₆(L¹⁶)₃(HL¹⁶)₃（OAc）₆（OCH₂CH₂OH）₃（H₂O）]·9CH₃CN（21）were prepared with the Schiff base ligand of H₃L¹⁶in-situ formed from the condensation of 2-hydroxy-1-naphthaldehyde with 3-amino-1,2-propanediol in the presence of Co（OAc）₂·4H₂O and Dy（NO）₃·6H₂O.In addition,we also designed and synthesized the ligands of 3-（（5-bromo-2-hydroxybenzylidene）amino）propane-1,2-diol（H₃L⁶）and 2-（hydroxymethyl）-2-（（（2-hydroxynaphthalen-1-yl）methylene）amino）propane-1,3-diol(H₄L¹⁷).Their solvothermal reactions with the corresponding metal salts gave[Dy₂Ni₆（L⁶）₂（HL⁶）₄（OAc）₄（CH₃OH）₄]·4CH₃OH（22）and[（C₂H₅）₃NH]₂[Cu₁₂Dy₁₄O（OH）₁₆(H₂L¹¹)8(HL¹¹)₄（HL′′′）₄（OAc）₈]（NO₃）₄（OAc）₂·6CH₃CN·5Et OH（23）.Complexes 20 and 21 have similar semicircle-like tricubane dodecanuclear metal frameworks,but with different ratios of Dy（III）to Co（II）.Complex 22 is a Dy₂Ni₆octanuclear heterometallic cluster having a dicubane skeleton with the two Dy（III）vertexes from the two cubanes bridged.Complex 23features a centipede-shaped structure constructed from eight Dy₃Cu cubanes sharing the Dy（III）vertices.It is worth noting that the skeleton of cluster 20,21 and 22 has a high stability even under a high energy of ion source of high resolution electrospray mass spectrometry（HRESI-MS）.The species changes in the reaction solution were tracked by time-dependent HRESI-MS technique,revealing the possible assembly mechanism of 20-23.Magnetic studies showed that complexes 20,21 and 22 exhibit single-molecule magnet behaviors.In chapter 7,a research was carried out for the constructions and assembly mechanisms of 3d-4f single-molecule magnets with the embedded Dy（III）ions surrounded by Ni（II）ions.Three different Schiff base ligands of 2-（（（2-hydroxynaphthalen-1-yl）methylene）amino）benzoic acid（H₂L⁸）,hexa-1,3-dien-1-yl)methylene)amino)-2-（hydroxymethyl）propane-1,3-diol(H₄L¹³)and 4-bromo-2-（（（3-hydroxypropyl）imino）methyl）phenol(H₂L¹⁷)were designed and synthesized.Their solvothermal reactions with Dy（III）and Ni（II）ions gave three heterometallic complexes[Dy Ni₆（μ₃-OH）₃（L⁸）₆（Et OH）₃]·5Et OH·CH₃CN·H₂O（24）,[Dy₃Ni₆（μ₃-OH）₈(L¹³)₈（OAc）₃（H₂O）₆]·2Me OH（25）and[Dy ₄^IIINi ₈^II（μ₃-OH）₈(L¹⁷)₈（OAc）₄（H₂O）₄]·3Et OH·2CH₃CN·2H₂O（26）with the Dy（III）ions embedded in the cluster core by Ni（II）ions.Complex 24 is an example with one Dy（III）ion surrounded by six Ni（II）ions and six L⁸Schiff base ligands.Complex 25presents a Dy（III）triangle surrounded by six Ni（II）ions and eight L¹³schiff base ligands constructing a Dy₃Ni₆skeleton.Complex 26 presents a cyclic tetracubane skeleton with the inner four Dy（III）ions wrapped by eight peripheral Ni（II）ions.The step-by-step construction of the ligand-driven synthesis of Dy Ni₆to Dy₃Ni₆to Dy₄Ni₈was realized.Magnetic studies revealed no slow magnetic relaxation behavior for complex 24,a typical single-molecule magnet under 0 field for complex25,and only slow magnetic relaxation behavior for complex 26.HRESI-MS showed that the frameworks of the three complexes have high stability under the gradually increasing ion source energy.Their possible formation mechanisms were proposed by tracking the intermediates through time-dependent HRESI-MS.