A new CuII–MnIII heterobinuclear complex exhibiting magnetization relaxation

Inorganica Chimica Acta 362 (2009) 3381–3384

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A new CuII–MnIII heterobinuclear complex exhibiting magnetization relaxation Zong-Wei Li a, Pei-Pei Yang a, Li-Cun Li a,b,*, Dai-Zheng Liao a a b

Department of Chemistry, Nankai University, Tianjin 300071, PR China State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China

a r t i c l e

i n f o

Article history: Received 27 November 2008 Received in revised form 20 February 2009 Accepted 28 February 2009 Available online 10 March 2009 Keywords: Heterobinuclear Schiff base Magnetic properties Slow magnetic relaxation

a b s t r a c t A new heterometallic complex [CuMn(5-Brsap)2(MeOH)(Ac)]  CH3OH (1) (5-Brsap = 5-bromo-2-salicylideneamino-1-propanol) has been synthesized and characterized structurally as well as magnetically. Complex 1 has an alkoxo-bridged Cu(II) and Mn(III) heterobinuclear core, where the Mn(III) and Cu(II) ions have elongated octahedral and square-pyramidal geometries, respectively. In dc magnetic susceptibility measurements reveal that there is strong ferromagnetic interaction between the Mn(III) and Cu(II) ions with an exchange coupling constant J = 67.64 cm1. The ac magnetic susceptibility measurements, frequency dependence in both the real and imaginary signals is observed, which indicates slow relaxation of magnetization. An Arrhenius plot gave the effective anisotropy barrier D/kB = 11.58 K and the preexponential factor i0 = 1.28  106 s. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

In the field of molecular magnetism, there is a growing interest in single-molecule magnets (SMMs), molecules that function as nanomagnets [1–3]. SMMs have been proposed as candidates for high-density information storage, in which each bit of information is stored as the magnetization orientation of an individual molecule, and quantum computation, in which the molecules can serve as qubits [4–6]. Many different directions are being pursued in the study of SMMs [7–9]. One of them focuses on how to obtain the molecules with the large-spin ground state (ST) so that increasing the energy barrier (D) that can be expressed as D ¼ jDST jS2T (for integer spin) or D ¼ jDST jðS2T  1=4Þ (for half-integer spin) [10]. On the other hand, the minimization of SMM nuclearity is also an important issue. Such small SMMs offer appealing simple model systems with a small number of quantum energy levels as well as interesting anisotropic building blocks to construct new magnetic materials [11–13]. To the best of our knowledge, the binuclear complexes with SMMs behavior are very rare. We herein report a new Cu(II)–Mn(III) heterobinuclear complex [CuMn(5Brsap)2(MeOH)(Ac)]  CH3OH (5-Brsap = 5-bromo-2-salicylideneamino-1-propanol), which exhibits slow relaxation of magnetization.

2.1. General All reagents and solvents were purchased from commercial sources and used as received. Elemental analysis for carbon, hydrogen and nitrogen were carried out on a Perkin–Elmer elemental analyzer model 240. Magnetic measurements were performed on a Quantum Design SQUID magnetometer. The SQUID outputs were corrected for the contribution of the sample holder and the magnetic susceptibilities were corrected for the diamagnetism of the constituent atoms (Pascal’s tables). 2.2. Synthesis of [CuMnC(5-Brsap)2(MeOH)(Ac)]  CH3OH (1) Mn(Ac)3  2H2O (0.134 g, 0.5 mmol) solid was added to a solution of 5-Brsap (0.125 g, 0.5 mmol) in 20 mL of methanol and stirred the mixture for 30 min, and then added a solution of Cu (ClO4)2  6H2O (0.092 g, 0.25 mmol) in 5 mL methanol. Further stirring for another 1 h gave rise to a dark brown solution. Dark brown single crystals were obtained by slow evaporation at room temperature after two weeks. Anal. Calc. for C24H31Br2 N2O8CuMn: C, 38.24; H, 4.14; N, 3.98. Found: C, 37.76; H, 3.83; N, 3.61%. 2.3. X-ray crystal structure determinations

* Corresponding author. Address: Department of Chemistry, Nankai University, Tianjin 300071, PR China. Tel.: +86 22 23505465. E-mail address: [email protected] (L.-C. Li). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.02.047

Diffraction intensity data of the single crystals of complex 1 were collected on a Bruker SMART 1000 CCD diffractometer employing graphite-monochromated Mo Ka radiation (k = 0.71073 Å). The structure was solved by direct methods by using the program SHELXS-97 [14] and refined by full matrix least-squares

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methods on F2 with the use of the SHELXL-97 [15] program package. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were set in calculated positions and refined as riding atoms with a common fixed isotropic thermal parameter. Crystal data for 1: C24H31Br2CuMnN2O8, Mr = 753.81. Monoclinic, space group P21/c with a = 11.6584(14) Å, b = 18.673(2) Å, c = 13.2221(16) Å, b = 100.300(2)°, V = 2832.0(6) Å3, Z = 4, Dcald = 1.768 g/cm3, T = 294(2) K, l(Mo Ka) = 4.067 mm1, GOF = 1.041, F(0 0 0) = 1508, R1 = 0.0336, wR2 = 0.0800 for 4625 observed reflections with I > 2r(I). 3. Results and discussion

Fig. 2. Packing diagram of structure 1 viewed along the c-axis.

4.5

-1

4.0

3.5

3

χΜT/cm Kmol

Single crystal X-ray diffraction analysis reveals the complex consists of discrete Cu(II)–Mn(III) binuclear units and uncoordinated methanol molecules. The ORTEP drawing of complex 1 is shown in Fig. 1. In the binuclear unit, copper(II) and manganese(III) ions are triply-bridged by two alkoxo and one acetate groups. The Mn(III) ion possesses a distorted octahedral geometry, whereas the Cu(II) ion has a distorted square–pyramidal coordination environment. The equatorial plane of each metal ion is formed by two alcoholic oxygen, imino nitrogen and phenolic oxygen atom of 5Br-sap ligands. The Mn(II) and Cu(II) ions are displaced by 0.0064 and 0.1446 Å from their basal planes, respectively, toward to acetate group. The in-plane Mn–O and Cu–O bond lengths are in the range of 1.877(2)–1.981(2) Å. The Mn–N(1) and Cu–N(2) bond distances are 2.013(3) and 1.965(3) Å, respectively. The axial position of the Mn(III) are occupied by two oxygen atoms of O(7) and O(5) from the coordinated methanol molecule and acetate group, respectively. The apical position of the Cu(II) is occupied by another oxygen atom (O(6)) from the acetate. Thus the acetate group links two metal ions in syn–syn axial–axial positions. These positions exhibit Jahn–Teller elongation with bond lengths of 2.289(2) Å (Mn–O(5)), 2.344(3) Å (Mn–O(7)) and 2.308(2) Å (Cu–O(6)). The Cu–Mn separation is 2.9537(6) Å, which is slightly shorter than that found in [CuMnCl(5-Brsap)2(MeOH)] (3.000(2) Å) [16]. The Cu–O(2)–Mn and Cu–O(4)–Mn angles are 99.05(9)° and 99.14(9)°, respectively. There are intermolecular hydrogen bond interactions (Fig. 2) involving the oxygen atoms (O(5), O(6)) of the acetate group and the OH groups of the coordinated and non-coordinated methanol molecules (O(7)–H(7)–O(6) (x + 1/2, y + 3/2, z + 1/2): 2.716 Å/137.32°; O(8)–H(8)–O(5) (x, y, z + 1): 2.744 Å/170.88°). The variable-temperature magnetic susceptibility of 1 was measured from 300 to 2.0 K in an applied field of 1000Oe. The vMT versus T plot is shown in Fig. 3. At 300 K, the vMT value is 3.89 cm3 K mol1, which is higher than the expected value of 3.276 cm3 K mol1 for the isolated Mn(III) and Cu(II) ions assuming g = 2.0. With a decrease in the temperature, the vMT value gradu-

3.0

2.5

2.0 0

50

100

150

200

250

300

T/K Fig. 3. vMT vs. T plot for complex 1 and the solid line represents the best-fit.

ally increases to reach a plateau value of 4.36 cm3 K mol1 at 50.0 K, followed by a decrease below 30 K. The plateau value is

Fig. 1. Molecular structure of complex 1 with atom-labeling (30% probability) and hydrogen atoms and uncoordinated methanol molecule are omitted.

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vM ¼

-8.8 -8.9

0.38

2 2

vtotal ¼ vM =½1  vM ðzJ =Ng b Þ The best-fit parameters obtained are J = 67.64 cm1, gCu = 2.11, gMn = 2.0, zJ0 = 0.21 cm1 and R = 4.53  104 (R is defined as [(vM)obs  (vM)calc]2/[(vM)obs]2). The fitting results show there is strong ferromagnetic coupling between the Mn(III) and Cu(II) ions. In complex 1, the coordination around the copper(II) ion is 4+1, thus the magnetic orbital of the copper(II) ion is mainly of dx2  y2(b1) type lying in the basal plane, with a small contribution from dz2(a1), while the Mn(III) ion has elongated octahedral geometry, the magnetic orbitals of the Mn(III) ion are dxz, dyz (eg), dxy(b2g) and dz2(a1g) and they are orthogonal to the magnetic orbital dx2  y2(b1) of the Cu(II) ion, which results in the ferromagnetic interactions. It is worth noting that the observed ferromagnetic coupling in complex 1 is weaker than that of the similar complex [CuMnCl(5-Brsap)2(MeOH)] [16]. This can be attributed to the overlap between two magnetic orbitals (dz2) of the Mn(III) and Cu(II) ions through the acetate bridge group. The ac magnetic susceptibility data for 1 were shown in Fig. 4, showing a frequency dependence in both the real (v0 ) and imaginary (v00 ) parts. The profile of these curves indicates complex 1

5

800Hz 1200Hz 1400Hz

4

-1

0.42

0.43

0.44

0.45

-1

maybe be a SMM. Note that we applied a static field of 0.3 T in order to avoid tunneling at zero applied field. It well known that the tunneling mechanism can be suppressed by applying a dc field removing the degeneracy of the states connected by the tunneling mechanism. The magnetic relaxation time can be slowed down by applied a static dc field [18,19]. The maxima of v00 were used to determine the relaxation time i. An Arrhenius plot of ln(i) versus 1/T (Fig. 5) allowed us to determine the effective anisotropy barrier D/kB = 11.58 K and the pre-exponential factor i0 = 1.28  106 s, which are in agreement with single-molecule magnet behavior [20,21]. 4. Conclusion In summary, we report a new heterobinuclear complex [CuMn(5-Brsap)2(MeOH)(Ac)]  CH3OH (1) with Cu(II)–Mn(III) core, which shows strong ferromagnetic interaction between the Mn(III) and Cu(II) ions and slow magnetic relaxation at low temperature. It is a new example of binuclear complex with probable SMMs behavior. Acknowledgment This work was supported by the National Science Foundation of China (Nos. 50672037, 20471032) and the NSF of Tianjin (No. 09JCYBJC05600). References

3

3

0.41

Fig. 5. The natural logarithm of the relaxation time (i) versus the inverse of temperature (T1) plot for 1.

4 1 g þ g 5 Mn 5 Cu

1

0.40

-1

6 1 g  g 5 Mn 5 Cu

2

0.39

T /K

0

χ'',χ'/cm mol

-8.7

-9.1

with

g 5=2 ¼

-8.6

-9.0

2 2 Nb2 10g 3=2 þ 35g 5=2 expð5J=2kTÞ 2 þ 3 expð5J=2kTÞ 4kT

g 3=2 ¼

-8.5

ln( τ /s)

consistent to the value of Curie constant of 4.375 cm3 K mol1 with g = 2.0 for S = 5/2 state. These results indicate there is substantial ferromagnetic interaction between the Mn(III) and Cu(II) ions and the spin ground state ST equals 5/2. The decrease in the vMT values below 20 K is due to zero-field splitting and/or intermolecular antiferromagnetic interactions. On the basis of the crystal structure, the magnetic susceptibility data of complex 1 can be analyzed by the Cu(II)–Mn(III) binuclear model. Therefore, the magnetic data were fitted by the following expression derived ^Cu . Considering the intermolec^Mn S ^ ¼ JS from spin Hamiltonian H ular interaction, the molecule field correction was introduced [17]

0 1.5 1.0 0.5 0.0 1

2

3

4

5

6

7

8

T/K Fig. 4. AC susceptibilities at different frequencies for 1 in a dc field of 3 KOe with an oscillating field of 3 Oe.

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