Fluorescence and Magnet Relaxation of 3d-4f Tetranuclear Cluster Complexes Based on Zn（Ⅱ）Schiff Base Building Block and 6-(Hydroxymethyl)pyridine-2-carboxylic Acid

LIU Cai-Ming HAO Xiang

(Beijing National Laboratory for Molecular Sciences,Center for Molecular Science,Institute of Chemistry,Chinese Academy of Sciences,Beijing 100190,China)

Abstract:Two new 3d-4f tetranuclear cluster complexes，[Zn2Ln2(salen)2(CO2PyCH2O)2(MeOH)2](ClO4)2·2MeOH(Ln=Dy(1)，Tb(2);H2 salen=N，N′-bis(3-methoxysalicylidene)-1，3-diaminopropane;HCO2PyCH2OH=6-(hydroxymethyl)pyridine-2-carboxylic acid)，have been prepared solvothermally using Zn(salen)as the building block and CO2PyCH2O2-as the co-ligand.They are Zn-Ln2-Zn cluster complexes.Magnetic measurements indicate ferromagnetic interactions between lanthanide（Ⅲ）ions.The Zn2Dy2 complex 1 shows magnet relaxation under a 2 000 Oe dc field，while the Zn2Tb2 complex 2 displays both field-induced magnet relaxation and fluorescence properties.CCDC:1968639，1;1968640，2.

Keywords:3d-4f cluster complex;crystal structure;fluorescence;magnetic properties

0 Introduction

Recently，Zn（Ⅱ） Schiff base complexes and their analogs have played a special role in assembling highperformance single-molecule magnets (SMMs)[1-3]and fluorescent SMM multifunctional molecular materials[4-6]，mainly because they can be used as complex-type ligands to chelate lanthanide（Ⅲ） ions，especially the Dy（Ⅲ） ion，forming a coordination configuration in favor of SMM properties together with other co-ligands[1-3];in addition，they can provide antenna effects to promote the emission of lanthanide（Ⅲ） ions[4-8].Notably，luminescent SMMs have potential applications in the fields of optoelectronics and multimode sensing[9].However，it is still a great challenge to obtain fluorescent SMMs due to the difficulty of luminous energy matching in lanthanide（Ⅲ）-based SMM systems，in which other two prerequisites(a large spin ground state(S)and a negative magnetic anisotropy parameter(D))must be met.

We have also been studying 3d-4f heterometallic SMMs[10-17]，and looking forward to obtaining some fluorescent SMMs.Notably，we found that Zn(salen)(H2salen=N，N′-bis(3-methoxysalicylidene)-1，3-diaminopropane)can be used as a good building block to construct 3d-4f heterometallic SMMs under solvothermal conditions by virtue of the coordination of bridging ligands or co-ligands[16-17].The choice of coligands is important，since these co-ligands determine whether the 3d-4f heterometallic cluster product can be formed and determine the structure of the product.Recently，we chose 6-(hydroxymethyl)pyridine-2-carboxylic acid (HCO2PyCH2OH)as the co-ligand to assembly such 3d-4f heterometallic cluster complexes for exploring new fluorescent SMMs.Herein we report solvothermal syntheses，crystal structures，fluorescence and magnet properties of two new 3d-4f tetranuclear cluster complexes based on the Zn(salen)building block，[Zn2Ln2(salen)2(CO2PyCH2O)2(MeOH)2](ClO4)2·2MeOH(Ln=Dy(1)，Tb(2))，which are isostructural compounds with a Zn-Ln2-Zn core.Complex 1 exhibits magnet relaxation under a dc field，while complex 2 shows both field-induced magnetrelaxation and fluorescence properties.

1 Experimental

1.1 Materials and methods

All reagents were purchased and used without further purification.Zn(salen)(H2O)was prepared by reference method[18].Elemental analyses for C，H and N were performed on a Heraeus Chn-Rapid instrument.The fluorescence spectra in the solid state were measured on a HITACHIF-4500 luminescence spectrophotometer at room temperature.The magnetic susceptibility measurements were performed on a Quantum Design MPMS-XL5 SQUID magnetometer.Diamagnetic corrections were estimated from Pascal′s constants.

Caution!Perchlorate compounds are potentially explosive and should be handled with care and used only in small amounts.

1.2 Preparation of[Zn2Dy2(salen)2(CO2PyCH2O)2(MeOH)2](ClO4)2·2MeOH(1)

6-(Hydroxymethyl)pyridine-2-carboxylic acid(0.1 mmol)，Dy(ClO4)3·6H2O(0.05 mmol)and Zn(salen)(H2O)(0.1 mmol)in 10 mL methanol were stirred at room temperature for 10 min，and this mixture was then transferred into 25 mL Teflon-lined stainless steel vessel and kept at 100℃for 3 days under autogenous pressure.After the autoclave had cooled to room temperature at rate of 0.15 ℃·min-1，yellow plate crystals of 1 were harvested，washed with methanol and then dried at ambient temperature(Yield:18% based on Dy).Anal.Calcd.for C56H66Cl2Dy2N6O26Zn2(%):C，38.09;H，3.77;N，4.76.Found(%):C，38.12;H，3.61;N，4.73.IR(KBr，cm-1):3 593m，3 436s，3 095w，3 044w，2 953w，2 939w，2 920w，2 849w，2 825w，1 626vs，1 603m，1 564w，1 469s，1 443w，1 367m，1 300m，1 277w，1 225s，1 198w，1 167w，1 094s，1 063s，1 021w，1 004w，983w，946w，927w，807w，774w，741m，714w，685w，651w，634w，623w，600w，555w，530w，465w，434w.

1.3 Preparation of[Zn2Tb2(salen)2(CO2PyCH2O)2(MeOH)2](ClO4)2·2MeOH(2)

The preparation procedure for 2 was the same as that for 1 but using Tb(ClO4)3·6H2O(0.05 mmol)instead of Dy(ClO4)3·6H2O(0.05 mmol).Yellow block crystals of 2 were harvested，washed with methanol and then dried at ambient temperature (Yield 16%based on Tb).Anal.Calcd.for C56H66Cl2N6O26Tb2Zn2(%):C，38.24;H，3.78;N，4.78.Found(%):C，38.29;H，3.81;N，4.75.IR (KBr，cm-1):3 595m，3 431s，3 089w，3 042w，2 953w，2 941w，2 921w，2 848w，2 824w，1 627vs，1 602m，1 564w，1 469s，1 416m，1 366m，1 299s，1 276w，1 224s，1 199w，1 176w，1 094s，1 063s，1 021w，1 004w，980w，946w，927w，850w，806w，774w，741m，714w，685w，650w，624m，600w，556w，531w，506w，464w，432w.

1.4 Crystal structure determination

Single-crystal diffraction data′s collection for complexes 1 and 2 was carried out on a Bruker SMART APEX-CCD with MoK α radiation(λ=0.071073 nm)at 170 K.The data were corrected for Lorentzpolarization effects，and absorption corrections were applied.The structures of complexes 1 and 2 were solved by direct methods.The structures were refined using SHELXL-2015[19].All non-hydrogen atoms were refined anisotropically;the hydroxyl hydrogen atoms in the methanol molecules were located in difference Fourier maps，allother hydrogen atoms were generated geometrically and allowed to ride on their parent carbon atoms.The crystallographic data and refinement parameters are listed in Table 1.Selected bond lengths and angles for the two complexes are given in Table 2.

CCDC:1968639，1;1968640，2.

Table 1 Crystallographic data for 1 and 2

Table 2 Selected bond lengths(nm)and angles(°)for 1 and 2

Continued Table 2

2 Results and discussion

2.1 Crystal structures

Complexes 1 and 2 are isomorphic，so we use compound 1 as an example to describe the crystal structure.The crystal space group of compound 1 is P21/c.As shown in Fig.1a，this complex consists of[Zn2Ln2(salen)2(CO2PyCH2O)2(MeOH)2]2+cluster cation，perchlorate anions and solvent methanol molecules.Two CO2PyCH2O2-ligands provide two O atoms on the η2-CH2O-arms to bridge two Dy atoms，while the pyridine N atom and the carboxyl O atom in each CO2PyCH2O2-ligand chelate the respective Dy atom，generatingthe crystallographically centrosymmetric[Dy2(CO2PyCH2O)2]2+core，withthe Dy…Dy separation of 0.3759nm.The[Zn2Ln2(salen)2(CO2PyCH2O)2(MeOH)2]2+cluster cation is thus formed by two Zn(salen)(MeOH)building blocks chelating the[Dy2(CO2PyCH2O)2]2+core from opposite directions.

Fig.1 Crystal structures of 1(a)and 2(b)with ellipsoids at 30%probability

The Zn1 atom is chelated by two phenol O atoms and two imine N atoms from the Schiff base ligand to form the bottom of the square pyramid，and the apex of the square pyramid is provided by the terminal methanol oxygen atom.The average bond distance of Zn-N (0.204 6 nm)is a little shorter than the average bond distance of Zn-Obase(0.205 6 nm)，and the latter is also slightly longer than the Zn-Oapexbond (0.204 6 nm)(Table 2);the N1-Zn1-N2 bond angle(97.90°)is larger than the O2-Zn1-O3 bond angle(75.85°)(Table 2).The Dy1 atom is eight-coordinated by two phenol O atoms and two methoxy O atoms from the Schiff base ligand，one pyridine N atom，one carboxyl O atom and one η2-CH2O-arm O atom from the CO2PyCH2O2-ligand as well as another η2-CH2O-arm O atom from the other CO2PyCH2O2-ligand.The mean bond length of Dy-O is 0.236 4 nm，which is shorter than that of Dy-N(0.246 5 nm)(Table 2).The Dy1-O5-Dy1i(Symmetry code:i1-x，1-y，1-z)bond angle(110.12°，Table 2)is comparable with the Dy-O-Dy bond angle(110.88°)in Dy2(pdmH)2(DBM)4·2EtOH(H2pdm=2，6-pyridinedi-methanol，DBM=dibenzoylmethanato)[20].The SHAPE software[21]analysis reveals that the coordination configuration of Dy1 is the triangular dodecahedron，with an offset value of 2.210 from the D2dsymmetry.The Zn1 atom is connected to the Dy1 atom through the bridging effect of the phenol O atom from the Schiff base ligand，with the Zn1… Dy1 separation of 0.353 4 nm;these Zn1-Dy1 heterometallic atoms together with the crystallographically symmetrical Zn1i-Dy1i(Symmetry code:i1-x，1-y，1-z)heterometallic atoms form a Zn-Ln2-Zn topology structure.Such a cluster core is reminiscent of some Zn2Dy2SMMs with carbonate bridges formed by fixing carbon dioxide in the air，which all have similar Zn-Dy2-Zn core structures[22-24].In addition，the perchlorate anion acts as a balance charge，and solvent methanol molecules are present in the crystal lattice.There is a strong hydrogen bond between the methanol molecule and the uncoordinated carboxyl oxygen atom from the CO2PyCH2O2-ligand with the O13…O6 distance of 0.275 1 nm，which helps to stabilize the crystal structure.

Compound 2 has the same structure (Fig.1b)as compound 1(Fig.1a)，except that the average bond length of Tb-O(0.237 3 nm)in 2(Table 2)is slightly longer than the average bond distance of Dy-O(0.236 4 nm)in 1 due to the lanthanum contraction effect.

2.2 Magnetic properties

The dc susceptibility of compounds 1 and 2 was measured in a range of 2～300 K.As shown in Fig.2，the room temperature χT values of compounds 1 and 2 were 28.33 and 23.68 cm3·K·mol-1，respectively，which are consistent with the theoretical values of two isolated Dy（Ⅲ） ions (28.34 cm3·K·mol-1)and two isolated Tb（Ⅲ） ions(23.64 cm3·K·mol-1)，respectively.From 300 K to 50 K，the χT values of 1 and 2 were almost unchanged;when the temperature continued to decrease，their χT values decreased slightly;after less than 9 K for 1 and 8 K for 2，their χT values suddenly increased，reaching the maximum values at 2 K(33.59 and 28.05 cm3·K·mol-1for 1 and 2，respectively).Such magnetic behaviors are the results of the combined effect of the thermal depopulation of mjlevels of the Ln（Ⅲ）ion and the ferromagnetic coupling between the Ln（Ⅲ） ions[17，24-26].

Fig.2 Plots of the dependence of χT on temperature for complexes 1 and 2

For1 taking997 Hzasan example，the imaginary part( χ″)of the ac susceptibility did not appear a response signal at a 0 dc field(Fig.3a)，which may be caused by the existence of the quantum tunneling effect.Therefore，in order to investigate the change in the χ″，a dc field was applied to suppress the quantum tunneling effect[27-28].As shown in Fig.3b，at less than 6 K，the χ″shows a response signal in the frequency range of 10～997 Hz at a 2 000 Oe dc field and displays frequency dependence，indicating that the quantum tunneling effect does exist in 1，which may be suppressed by a dc field.However，no peaks appears in the χ″-T curves of all frequencies at above 2 K，indicating that the energy barrier value of this SMM is very small.This phenomenon is not uncommon.In comparison，many Dy（Ⅲ） complexes will not show a χ″signal even if a dc field is applied，because they are not SMMs at all[20].

Fig.3 (a)Ac susceptibilities measured in a 2.5 Oe ac magnetic field for 1(Hdc=0 Oe);(b)Temperature dependence of χ″for 1 under a 2 000 Oe dc field

Fig.4 (a)Ac susceptibilities measured in a 2.5 Oe ac magnetic field for 2(Hdc=0 Oe);(b)Temperature dependence of χ″for 2 under a 2 000 Oe dc field

The ac susceptibility of complex 2 was similar to that of complex 1.Under a 0 dc field，no response signal appears at χ″of complex 2 at 997 Hz(Fig.4a)due to the quantum tunneling effect too.However，under a 2 000 Oe dc field，in a range of 10～1 399 Hz，the χ″of complex 2 appears a response signal at less than 4 K，and exhibits frequency dependent(Fig.4b)，indicating that it is also a field-induced SMM.The response temperature region of the χ″signal of complex 2 was lower than that of complex 1，and its radiance was smaller than that of complex 1，which implies that the energy barrier value of complex 2 is even smaller than that of complex 1.

2.3 Luminescent properties

We studied the solid-state excitation spectrum and solid-state emission spectrum of complex 2.As shown in Fig.5，the solid-state excitation spectrum of 2，which is monitored by the emission of5D4→7F5transition of the Tb（Ⅲ） ion at 541 nm，displayed an intense broad band from about 300 nm to about 400 nm，suggesting the strong charge transfer (CT)from ligands to the Tb（Ⅲ） ion[29].However，in this range，the typical excitation bands of Tb（Ⅲ）ions caused by the7F5→5L6，7F5→5L9，7F5→5L10and7F5→5G6transitions[30]were completely blocked.The solid-state emission spectrum of 2 was excited at 350 nm and showed a very strong emission band at 541 nm，which made 2 emit green light;besides this emission band caused by the5D4→7F5transition，the other three emission bands caused by the5D4→7F6，5D4→7F4and5D4→7F3transitions could be observed clearly，which had peaks at 486，582 and 624 nm，respectively.

Fig.5 Solid-state excitation spectrum and solid-state emission spectrum for complex 2

Notably，Schiff bases are often used as ligands to construct luminescent Zn-Ln heterometallic SMMs[2，31-32]，but the Schiff base ligands themselves tend to have weaker antenna effects on the fluorescence of Dy（Ⅲ）or Tb（Ⅲ） ions.Therefore，it is necessary to resort to other co-ligands.This work shows that the Zn(salen)building block together with the CO2PyCH2O2-ligand can have a good antenna effect on the fluorescence of Tb（Ⅲ）ions.As a comparison，our recent research indicates that the Zn(salen)building block together with the 2，2′-bipyridine-6，6′-dicarboxylate ligand only show a weaker antenna effect on the fluorescence of Dy（Ⅲ）ions，in which only two emission bands caused by the7F9/2→6H15/2and7F9/2→6H13/2transitions are observed[17].These results imply that the introduction of a suitable aromatic co-ligand can enhance the antenna effect on the fluorescence of Tb（Ⅲ） ions or Dy（Ⅲ） ions.

3 Conclusions

In summary，two Zn2Ln2heterometallic cluster complexes derived from salen-type Schiff base ligand and 6-(hydroxymethyl)pyridine-2-carboxylic acid were prepared by solvothermal method.Both complexes display field-induced slow magnet relaxation，furthermore，the Zn2Tb2complex exhibits additional fluorescence properties due to ligands′effective antenna effect on the fluorescence of the Tb（Ⅲ）ion.This work demonstrates that the Zn(salen)complex fragment is not only a good building block to construct 3d-4f heterometallic SMMs，but also can play an antenna effect together with other suitable ligands，triggering Ln（Ⅲ） ions to emit light，which is a feasible way to obtain magnetic-optical multifunctional molecular materials.