Lanthanide dinuclear complexes constructed by 8-hydroxyquinoline Schiff base showing magnetic refrigeration and slow magnetic relaxation

Accepted Manuscript Research paper Lanthanide dinuclear complexes constructed by 8-hydroxyquinoline Schiff base showing magnetic refrigeration and slow magnetic relaxation Wen-Min Wang, Yue-Hong Ren, Song Wang, Cai-Feng Zhang, Zhi-Lei Wu, Heng Zhang, Ming Fang PII: DOI: Reference:

S0020-1693(16)30506-0 http://dx.doi.org/10.1016/j.ica.2016.09.002 ICA 17247

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

23 July 2016 2 September 2016 3 September 2016

Please cite this article as: W-M. Wang, Y-H. Ren, S. Wang, C-F. Zhang, Z-L. Wu, H. Zhang, M. Fang, Lanthanide dinuclear complexes constructed by 8-hydroxyquinoline Schiff base showing magnetic refrigeration and slow magnetic relaxation, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.09.002

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Lanthanide dinuclear complexes constructed by 8-hydroxyquinoline Schiff base showing magnetic refrigeration and slow magnetic relaxation Wen-Min Wang a, Yue-Hong Ren a, Song Wang a, Cai-Feng Zhang b , Zhi-Lei Wu∗c, Heng Zhang d and Ming Fang e a

Department of Chemistry, Taiyuan Normal University, Jinzhong, 030619, P. R. China

b

Shanxi Engineering Research Center of Humic Acid, Jinzhong, 030619, P. R. China

c

College of Chemistry & Environmental Science, Hebei University, Baoding, 071002, P. R. China

d

Instrumental Analysis Center, Hebei Normal University, Shijiazhuang, 050024, P. R. China

e

Department of Chemistry, Hebei Normal University of Science & Technology, Qinhuangdao, 066004, P. R. China

Abstract Two dinuclear lanthanide(III) complexes based on 8-hydroxyquinoline Schiff base derivatives, [Ln2(hfac)6L2]·0.5C7H16 ( Ln = Gd (1) and Dy (2), HL = 5-(4-methylbenzylidene)-8-hydroxylquinoline and hfac = hexafluoroacetylacetonate), have been synthesized, structurally and magnetically characterized. Both 1 and 2 are phenoxo-O

bridged

eight-coordinated

dinuclear

with

three

complexes, bidentate

and hfac

center and

Ln(III)

two

µ2-O

ions

are

bridging

8-hydroxyquinoline Schiff base ligands. Magnetic studies reveal that 1 exhibits antiferromagnetic exchange interaction between nearby Gd(III) centers and shows magnetic refrigeration (-∆Sm = 14.9 J kg−1 K−1 for ∆H = 7 T at 2 K). Ac susceptibility measurements reveal that 2 exhibits slow magnetic relaxation behavior with the anisotropic barriers (∆E/kB) of 3.0 K. Keywords: dinuclear lanthanide(III) complexes; magnetic refrigeration; slow magnetic relaxation

1. Introduction During the past two decades, the design and synthesis of lanthanide complexes ∗

Corresponding author. E-mail: [email protected] (Z.-L. Wu). 1

have attracted increasing interest because of their fascinating architectures and physical properties with widespread applications in sensors, labels for biomolecules, gas storage, catalysis and molecular magnets [1-3]. Among the recent investigation of molecular magnets, cryogenic magnetic refrigerant and single molecule magnet (SMM) were most noteworthy [4-6]. Magnetic refrigeration depends on the magnetocaloric effect (MCE), which represents the change of isothermal magnetic entropy (∆S m) and adiabatic temperature (∆Tad) in change of applied magnetic field [7]. It has been claimed that magnetic refrigerant materials can potentially replace conventional compressor-based refrigerants for ultralow-temperature applications due to their environment friendliness and economic advantages [8]. Gadolinium ions are widely employed to design efficient magnetic refrigerant materials in that it possesses large-spin ground state, zero orbital momentum, and weak superexchange interactions. To date, several such exciting materials have been constructed and reported by a few groups [9]. However, achieving a molecular material for practical application still remains a challenging task. On the other hand, because of the presence of significant magnetic anisotropy, the lanthanide ions, especially the Dy3+ ion, have been much more used to design and construct SMMs. With an improved knowledge of the magneto-chemical properties of SMMs, a lot of lanthanide SMMs have been synthesized and researched [10]. Towards this goal the challenge for the application of SMMs is the improvement of the reversal energy barrier (∆E/kB) and blocking temperature (TB) [11]. In this contribution, two new dinuclear lanthanide(III) complexes based on 8-hydroxyquinoline Schiff base derivatives (HL) (Scheme 1) and β-diketonate, [Ln2(hfac)6L2]·0.5C7H16

(Ln

=

Gd

(1)

and

Dy

(2),

HL

=

5-(4-methylbenzylidene)-8-hydroxylquinoline, hfac = hexafluoroacetylacetonate), have been successfully obtained and further studied by infrared spectra, elemental analyses (EA), powder X-ray diffraction (PXRD), luminescence spectra, and single-crystal

X-ray diffraction.

Magnetic

studies

reveal

that

1

exhibits

antiferromagnetic exchange interaction between adjacent Gd(III) centers and shows magnetic refrigeration (-∆S m = 14.9 J kg−1 K−1 for ∆H = 7 T at 2 K). For complex 2, it 2

exhibits ferromagnetic interaction between Dy(III) ions and displays slow magnetic relaxation behavior with the anisotropic barriers (∆E/kB) of 3.0 K and τ0≈ 3.26×10 -5 s.

Scheme 1 The structure of HL.

2. Experimental section 2.1. General materials and physical measurements All chemicals and solvents used for the syntheses were reagent grade without further purification. The Ln(hfac)3·2H2O (Ln= Gd and Dy) were synthesized according to methods in the literature [12]. The 8-Hydroxyquinoline Schiff base ligand HL was prepared by the already reported methods [13]. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240 CHN elemental analyzer. IR spectra were recorded in the range of 400-4000 cm-1 with a Bruker TENOR 27 spectrophotometer using a KBr pellet. PXRD data were examined on a Rigaku Ultima IV instrument with Cu Kα radiation (λ = 1.54056 Å), with a scan speed of 5° min−1 in the range 2θ = 5−50°. The magnetic measurements were carried out with a Quantum Design MPMS-XL7 and a PPMS-9 ACMS magnetometer. The diamagnetic corrections for the complexes were estimated using Pascal’s constants, and magnetic data were corrected for diamagnetic contributions of the sample holder [14]. 2.2. X-ray Crystallography Single crystal X-ray diffraction data of 1 and 2 were collected on a computer-controlled Rigaku Saturn CCD area detector diffractometer, equipped with confocal monochromatized Mo Kα radiation with a radiation wavelength of 0.71073 Å using the ω-ϕ scan technique. The structures were solved by direct methods and refined with a full-matrix least-squares technique based on F2 using the SHELXS-97 and SHELXL-97 programs [15]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The crystals were measured as soon as they were isolated from 3

the solution, thus there were some disordered solvent molecules. The free solvent molecules of 2 were removed via “SQUEEZE” due to their extreme disorder which could not be solved. Crystallographic data and structural refinement parameters for 1 and 2 are listed in Table 1. Selected bond lengths and angles of complex 1 and 2 are given in Tables S1 and S2, respectively. Crystallographic data of 1 and 2 are in CIF files. CCDC (1493446-1 and 1493445-2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre. Table 1 Crystallographic data and structure refinement summary for 1 and 2 Complexes

1

2

Empirical formula Mr T (K) Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z Dcalcd/g cm−3 µ/mm−1 θ/° F(000) Reflections collected Unique reflns Rint GOF (F2)

C67.50H32F36Gd2N4O14 2121.47 113(2) Trigonal R-3 43.040(2) 43.040(2) 12.0739(9) 90.00 90.00 120.00 19369.7(19) 9 1.646 1.765 3.21 to 27.53 9351 83360 9900 0.0279 1.001 0.0287, 0.0919 0.0327, 0.0948

C64H32Dy2F36N4O14 2089.94 113(2) Trigonal R-3 42.933(7) 42.933(7) 12.073(3) 90.00 90.00 120.00 19272(6) 9 1.621 1.865 3.22 to 25.02 9108 70540 7500 0.0459 1.269 0.0314, 0.1139 0.0340, 0.1151

R1, wR2 (I > 2σ(I)) R1, wR2 (all data)

2.3. Synthesis 2.3.1. Synthesis of [Gd2(hfac)6L2] ·0.5C7H16(1) A solution of Gd(hfac)3·2H2O (0.05 mmol) in 30 mL boiling n-heptane was heated 4

to reflux for 3 h. Then the solution was cooled to 75°C, and a dry CH2Cl2 (5mL) solution of HL (0.05 mmol) was added. The resulting mixture was stirred for 1 h at this temperature, then cooled it to room temperature. The mixture was filtrated and the filtrate was kept in the dark and concentrated slowly by evaporation at 4 °C. After about two days, block-shaped and red-colored crystals were obtained. Yield 45% based on Gd. Elemental analysis (%) calcd for C67.50H32F36Gd 2N4O14: C, 38.22; H, 1.52; N, 2.64. Found: C, 38.43; H, 1.37; N, 2.75. IR (cm-1): 3733(w), 3150(w), 2927(w),1658(s), 1507(m), 1461(m), 1415(w), 1299(s), 1207(s), 1145(s), 980(w), 801(m), 661(m), 587(m), 508(w). 2.3.2. Synthesis of [Dy2(hfac)6L2]·0.5C7H16 (2) The preparation of 2 was similar to that of 1, except that Dy(hfac)3·2H2O (0.05 mmol) replaced Gd(hfac)3·2H2O. Yield 56% based on Dy. Elemental analysis (%) calcd for C67.50H32F36Dy2N4O14: C, 38.59; H, 1.52; N, 2.67. Found: C, 38.45; H, 1.41; N, 2.82. IR (cm-1): 3732(w), 3151(w), 2927(w),1657(s), 1507(m), 1460(m), 1416(w), 1298(s), 1207(s), 1146(s), 981(w), 800(m), 662(m), 589(m), 509(w).

3. Results and discussion 3.1. Description of the structures of 1 and 2 Single-crystal X-ray diffraction analyses reveal that both complexes 1 and 2 are dinuclear structures. 1 and 2 are isomorphous, and 2 is selected as a representative to describe the structure in detail. As shown in Fig. 1, 2 crystallizes in thetrigonal R-3 space group, with Z = 9, the asymmetric unit contains two eight-coordinated DyIII cations, two L- and six hfac- ligands. The center 8-coordinated DyIII ion is connected by two µ2-O atoms (O1 and O1a) from different bridging L- ligands and six oxygen atoms (O2, O3, O4, O5, O6, O7) from three hfac- coligands, respectively; and the Dy(III) ion is in a slightly distorted dodecahedral geometry. Dy1 and Dy1a are bridged by two phenoxide oxygen atoms (O1 and O1a) of two ligands, leading to a four-membered Dy2O2 core with a Dy–O–Dy angle of 111.71(11)° and a Dy┄Dy distance of 3.8667(7) Å. Finally, the two 8-coordinated Dy(III) ions (Dy1 and Dy1a) andtwo µ2-O atoms (O1 and O1a) form a approximate parallelogram. In 2, the lengths 5

of the Dy–O bonds are in the range of 2.327(3)-2.413(3) Å, which are comparable to those reported phenoxo-bridged lanthanide complexes previously [16].

Fig.1 Molecular structure for 2 (hydrogen atoms are omitted for clarity)

3.2. Powder X-ray Diffraction To confirm the phase purity of 1 and 2, the crystalline products of 1 and 2 were characterized by X-ray powder diffraction (PXRD) at room temperature (Fig. S1, see ESI†). The diffraction peaks of bulk sample are consistent with the simulated pattern in terms of the single crystal data, indicating the presence of mainly one crystalline phase in the corresponding sample of 1 and 2. The differences in intensity may be due to the preferred orientation of the crystalline powder samples. 3.3. Luminescence Property The solid state luminescence property of 2 was measured at room temperature. The excitation wavelength for emission spectra of 2 is 295 nm (Fig.S2, see ESI†), three characteristic peaks of DyIII can be observed in the emission spectra, which belong to the transitions of 4F9/2→6H15/2 (481 nm), 4F9/2→6H13/2 (575 nm) and 4F9/2→6H11/2(622 nm) [17]. 3.4. Magnetic Properties The magnetic properties of 1 and 2 were investigated by solid-state magnetic susceptibility measurements in the temperature range of 2.0−300 K on a Quantum Design MPMS-7 SQUID magnetometer under an applied magnetic field of 1000 Oe (all the measurements were carried out on polycrystalline samples). The plot of χMT versus T, where χM is the molar magnetic susceptibility, is shown in Fig 2. For 1 and 2, χMT values at 300 K are 15.92(1) and 28.30 (2) cm3 K mol−1, respectively, which are in good agreement with the expected values for two isolated 6

Gd(III) ions (8S7/2, g = 2 and C = 7.88 cm3 K mol−1) and two Dy(III) ions (6H15/2, g = 4/3 and C = 14.17 cm3 K mol−1). Upon cooling, χMT value of 1 almost stay constant in the temperature range 300−20 K and then drops rapidly to a minimum value of 9.12 cm3 K mol−1 at 2 K, which indicates the presence of a weak antiferromagnetic interaction between the Gd(III) ions [18]. For 2, as the temperature is decreased, the χMT products gradually increase between 300−50 K and then increase rapidly to a maximum value of 43.90 cm3 K mol−1 at 2 K. Such behavior suggests significant ferromagnetic interactions between adjacent Dy(III) ions within the complex 2 [19]. In addition, the magnetic susceptibilities of 1 and 2 can be fitted to the Curie−Weiss law, giving the parameters θ = −0.89 K, C = 15.98 cm3 K mol−1 for 1 and θ = 1.68 K, C = 27.85 cm3 K mol−1 for 2. The negative parameters of θ further prove that a weak antiferromagnetic coupling between Gd(III) ions existed in 1 and the positive θ value of 2 also suggests week ferromagnetic interactions between the Dy(III) ions in 2 (Fig S3, see ESI†) [20]. As the dinuclear units in 1 are well separated from each other, the antiferromagnetic coupling observed most likely originates from intramolecular interactions, the exchange pathway being provided by the double µ-phenol bridges. Consequently, the magnetic data was analyzed by means of a simple dimer law for two interacting spin octets derived through the isotropic spin Hamiltonian Ĥ = −JŜ1·Ŝ2 (where J is the exchange coupling parameter, and Ŝ1 and Ŝ2 are the spin operators of the local spins (Ŝ1 = Ŝ2 = 7/2)). The best-fit parameters obtained from Van Vleck’s equation are: J = − 0.12 cm−1, g = 2.02 and R = 2.16 × 10−6 ( R is the agreement factor defined as Σ[(χMT)obs(i)－(χMT)cacl(i)]2/ Σ[(χM T)obs(i)]2 ). The calculated curve matches well with the magnetic data in the whole temperature range investigated and the parameters are comparable to those reported Gd2 complexes previously (Fig. S4, see ESI†) [21].

7

Fig. 2 Plots of χMT versus T for 1 and 2 at an applied field of 1000 Oe between 2.0 and 300 K.

The magnetization data of 1 are carried out in the range of 0−8.0 T at 2.0-10.0 K (Fig. 3 (left)). The M versus H curves display a gradual increase with the increasing field and saturation value of 14.01 Nβ for 1 at 8.0 T and 2.0 K, being extremely approximate with the theoretical value of 14 Nβ for two individual Gd(III) (S = 7/2, g = 2) ions. Magnetic entropy changes ∆S m of 1 are calculated from the M versus H data to evaluate the MCE. ∆S m can be calculated by using the Maxwell equation: ∆Sm(T) = ∫ [∂M(T, H)/∂T]H dH

(1)

According to eq 1 [22], the −∆Sm values of 1 can be obtained, and the plots of −∆Sm versus T are shown in Fig. 3 (right). For 1, the maximum value of −∆Sm is 14.9 J K−1 kg−1 (calculated as 2Rln(2S+1)), expected maximum of which is 16.3 J K−1 kg−1 for a field change ∆H = 7 T at 2.0 K. The difference of −∆Sm between the experimental and theoretical values for 1 is mainly due to the antiferromagnetic interaction between Gd(III) ions in 1 [23a]. The observed maximum −∆S m value of 1 is smaller than those of reported Gd(III)-based molecular systems, such as the antiferromagnetic {Gd2} complex (17.25 J kg−1 K−1, ∆H = 7 T at 3 K), phenoxo-O bridged {Gd 2} complexes (Gd2(hfac)4(L)2, −∆Sm = 20.69 J K−1 kg−1, ∆H = 8 T at 2 K) and (Gd 2(tfac)4(L)2, −∆S m = 17.05 J K−1 kg−1, ∆H = 8 T at 3.0 K) [23].

8

Fig. 3 (left): M versus H plots for 1 (a) at T = 2.0−10.0 K and H = 0−8.0 T; (right): Temperature dependence of magnetic entropy change (−∆S m) as calculated from the magnetization data of 1 at T = 2.0−10.0 K and 0−7.0 T.

In order to study the magnetic relaxation dynamics of complex 2, the alternating-current (ac) magnetic susceptibility measurements were performed as a function of temperature and frequency on polycrystalline samples under zero dc magnetic field (Fig. 4).There is no obvious frequency dependence below 20 K in the in-of-phase component susceptibility (χ′) for 2, however, the out-of-phase susceptibility (χ″) clearly display frequency-dependent signals below 6 K but no well-defined peaks are observed due to the quantum tunneling of the magnetization (QTM) [24].

Fig. 4 Temperature dependence of the in-phase χ′(left) and out-of-phase χ″(right) components of the ac magnetic susceptibility for 2in zero dc fields with an oscillation of 3.0 Oe.

Because no well-defined peaks were observed in the out-of-phase component susceptibility (χ″) for 2, thus, the energy barrier (∆E/kB) of 2 could not be gained by Arrhenius fitting. Assuming that only one relaxation process exists in 2, the energy 9

barrier (∆E/kB) could be roughly estimated with the following equation (eq 2) [25]： ln(χ''/χ')=ln(ωτ0)+∆E/kBT

(2)

As depicted in Fig. 5, by fitting the experimental χ''/χ' data to eq 2, we obtained an estimation of the energy barrier ∆E/kB ≈ 3.0 K and τ0 ≈ 3.26×10-5 s for 2, the τ0 agrees with the expected value of 10 -5-10-12 s for SMMs [26].

Fig. 5 Plots of natural logarithm of χ″/χ′ versus 1/T for 2, the solid lines represent the fitting results over the range of 111−2311 Hz and in the temperature range 2.0-2.8K.

4. Conclusion In summary, the structures and magnetic properties of two dinuclear lanthanide (III) complexes based on 8-hydroxyquinoline Schiff base ligand have been reported. Both1and 2 are phenoxo-O bridged Ln2 complexes, each center LnIII ion is eight-coordinated with three bidentate hfac coligands and two µ2-O bridging ligands. Magnetic studies reveal that 1 exhibits antiferromagnetic exchange interaction between adjacent Gd(III) centers and shows magnetic refrigeration (-∆Sm = 14.9 J kg−1 K−1 for ∆H = 7 T at 2 K), while 2 exhibits ferromagnetic interaction between Dy(III) ions and displays slow magnetic relaxation behavior with the anisotropic barriers (∆E/kB) of 3.0 K.

Acknowledgment We gratefully acknowledge the NSFC (21501043 and 51174275) and the Research Project Supported by Shanxi Scholarship Council of China.

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Graphical abstract

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Graphical abstract Both 1 and 2 are phenoxo-O bridged dinuclear complexes with similar structures. Magnetic studies reveal that1exhibits antiferromagnetic exchange interaction between adjacent Gd(III) centers and shows magnetic refrigeration (-ΔSm = 14.9 J kg−1 K−1 for ΔH = 7 T at 2 K). Ac susceptibility measurements of 2 reveal that it exhibits slow magnetic relaxation behavior with the anisotropic barriers (ΔE/kB) of 3.0 K.

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(1) Two

phenoxo-O bridged dinuclear lanthanide(III) compounds, [Gd2(hfac)6L2]·0.5C7H16(1) and [Dy2(hfac)6L2 ]·0.5C7H16 (2), have been synthesized and structurally characterized. −1 (2) Magnetic studies reveal that 1 shows magnetic refrigeration (-∆S m = 14.9 J kg K−1 for ∆H = 7 T at 2 K). (3) Ac susceptibility measurements of 2 reveal that it exhibits slow magnetic relaxation behavior with the anisotropic barrier (∆E/kB) of 3.0 K.

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