научная статья по теме MAGNETIC PROPERTIES OF A LINEAR TRINUCLEAR MANGANESE COMPOUND [MN3(PHCO2)6(BIPY)2] · H2O Химия

КООРДИНАЦИОННАЯ ХИМИЯ, 2009, том 35, № 11, с. 822-826

УДК 541.49

MAGNETIC PROPERTIES OF A LINEAR TRINUCLEAR MANGANESE COMPOUND [Mn3(PhCO2)6(Bipy)2] • H2O

© 2009 H. C. Yao1*, N. Wang1' 2, S. L. Xie1, and Z. J. Li1

department of Chemistry, Zhengzhou University, Zhengzhou 450052, PR. China 2Department of Chemistry & Chemical Engineering, Pingdingshan Institute of Technology, Pingdingshan 467044, PR. China

*E-mail: yaohongchang@zzu.edu.cn Received January 14, 2009

The reaction of the trinuclear oxo-centered mixed-valance complex [Mn3O(O2CPh)6(Py)2(H2O)] with 2,2'-bipyri-dyne (Bipy) and another potential tripodal ligand affords the title compound [Mn3(PhCO2)6(Bipy)2] ■ H2O in good yield. The X-ray crystallographic diffraction study reveals that three mangenese ions are arranged in a linear mode with Mncenter-Mnterminal and Mnterminal-Mnterminal diatances of 3.588 and 7.176 A, respectively. Molar magnetic susceptibility of the compound gradually decreases from 12.23 (300 K) to 4.45 cm3 K mol-1 (2 K). Taking into account the structure of this compound, the data in the 2.0-300 K range were fit to the appropriate theoretical expression to give J = -2.73 cm-1, p = 2.07%, Na = -0.0004 cm3 mol-1, g = 1.992, and R2 = 0.99996. The magnetization versus external magnetic field measurements at 2 K shows that the ground state is ST = 5/2.

INTRODUCTION

Polynuclear mangenese complexes are now receiving increased attention in various scientific areas, including chemistry, physics, and biology. Representative target molecules fascinating inorganic chemists are tetranuclear aggregates as models for the photosynthetic water oxidation centers [1] and "single-molecule magnets" potentially applicable to future molecular device [2, 3]. Most commonly, these complexes are synthesized by the self- or "serendipitous" assembly method [4]. Our approach to Mn-aggregated systems is using oxo-centered manganese triangles of general formula [Mn3O(O2CR)6(L)3] as precursors by choosing suitable chelate ligands through the conventional synthetic method [5, 6]. This route toward the synthesis of high nuclearity clusters has been reported in the literature [7-9].

The work presented here continues this approach, and a new linear complex [Mn3(PhCO2)6(Bipy)2] ■ H2O (I), where Bipy - 2,2'-bipyridyne, has been obtained by using [Mn3O(O2CPh)6(Py)2(H2O)] as the starting materials. The crystal structure of this compound has been communicated recently in preliminary form [10]. In this paper, the magnetic behavior of the title compound was investigated and variable-temperature solid-state magnetic susceptibility studies reveal that the data are consistent with the ST = 5/2 ground state. The conclusion was confirmed by magnetization vs. field study in 0-70 kOe range (2.0 K). Fitting of the data to the appropriate theoretical expression gives J = -2.73 cm-1. The obtained J value is consistent with that of previously published isostructural compounds.

EXPERIMENTAL

The precursor [Mn3O(O2CPh)6(Py)2(H2O)] was synthesized according to the literature method [11]. All the other starting materials were of reagent grade and obtained from commercial sources without further purification.

The elemental analysis was performed with a Flash EA 1112 elemental analyzer. IR spectra were recorded on a VECTOR 22 spectrometer in the spectral range 4000-400 cm1 using the KBr disk method. Thermal analysis was performed in nitrogen with a heating rate of 10°C/min on a NETZSCH STA 409 PC instrument. Variable temperature magnetic susceptibility data were obtained on a polycrystalline sample (16.7 mg) from 2.0 to 300 K in a magnetic field of 2 kOe, using a Quantum Design MPMS-XL7 SQUID magnetometer. The data were corrected for the diamagnetic contributions of the compound obtained from Pascal's constants [12].

A suitable single crystal was chosen from the bulk sample and used for data collection on a Bruker SMART APEX CCD diffractometer using graphite monochromated Mo^a radiation (k = 0.71073 A) at room temperature. Crystallographic and refinement details have been reported [10].

The atomic coordinates and other parameters of structure I have been deposited with the Cambridge Crystallographic Data Center (no. 690822; depos-it@ ccdc .cam. ac .uk).

Synthesis of [Mn3(PhCO2)6(Bipy)2] ■ H2O was carried out as described elesewhere [10].

BVS calculated assuming different oxidation state of manganese ions in the studied complex

Bonds Bond length Mn(II) Mn(III) Mn(IV)

Mn(1)-O(1) 2.251(2) 0.26526 0.24593 0.25819

Mn(1)-O(3) 2.139(2) 0.36392 0.33287 0.34946

Mn(1)-O(6) 2.166(2) 0.33831 0.30944 0.32487

Mn(1)-O(1a) 2.251(2) 0.26526 0.24593 0.25819

Mn(1)-O(3a) 2.139(2) 0.36392 0.33287 0.34946

Mn(1)-O(6a) 2.166(2) 0.33831 0.30944 0.32487

1.93498 1.77648 1.85246

Mn(2)-O(1) 2.300(2) 0.23552 0.21542 0.22616

Mn(2)-O(2) 2.282(2) 0.24726 0.22616 0.23744

Mn(2)-O(4) 2.101(2) 0.40328 0.36887 0.38726

Mn(2)-O(5) 2.075(2) 0.43264 0.39572 0.41545

Mn(2)-N(1) 2.260(4) 0.32929 0.31878 0.30611

Mn(2)-N(2) 2.271(4) 0.31964 0.30944 0.29715

1.96763 1.83439 1.86957

Oxidation state

Mn(II)

Mn(II)

RESULTS AND DISCUSSION

Treatment of a solution of [Mn3O(O2CPh)6(Py)2(H2O)] in CH3CN with a mixture solution of 1,1,1-iro(hydroxymethyl)methylamine (L) and Bipy in CH2Cl2 gave a brown solution from which compound I was obtained in 46% yield by evaporating slowly at room temperature. The transformation is summarized as in eq. 1:

2 [ Mn3O ( O2CPh)6 ( Py)2 ( H2O )] + 4Bipy

CH3CN CH2Cl2

CH3CN CH2Cl2'

2 [ Mn3 ( PhCO2 )6 ( Bipy) 2 ] ■ H2O + 4Py + O2

(1)

Equation 1 is a short cut to represent the overall reaction occurred. This is reasonably deduced from that

the [Mn^IIMnIIO]6+ unit in the precursor has been reduced to a [ Mn" ]6+ species in the product. The oxidation states of manganase ions are assigned on the basis of charge balance. Bond valence sum (BVS) calculations [13] further confirm the assignment (table).

The ligand L, which is expected to display different bridging modes as other tripodal alcohols [l4], was not included in the compound. This can clearly be seen from its crystallographic structure (Fig. 1). The results of elemental and thermal analyses further confirm the conclusion.

The structure consists of a [Mn3(PhCO2)6(Bipy)2] species and one water molecule. The view of the structure is provided in Fig. 1. It shows that the linear complex includes a central octahedral Mn(1) ion located on a crystallographic inversion center. Its coordination sphere is composed of six oxygen atoms from six different benzoates. The central manganase ion is flanked

by two octahedrally distorted (Mn(2) and Mn(2A)) ions. For the Mn(2) ion, four of its six coordination positions are occupied by the oxygen atoms O(1), O(2), O(4), and O(5) from the benzoate ligands. The remaining two positions are filled with the nitrogen atoms from the Bipy ligand. The Mn-N and Mn-O bond lengths are in a range of 2.075-2.300 Á, which could be compared with that in the similar complexes,

[Mn^1 (CH3CO2)6(Bipy)2] [15], Mn^1 (AcO)6(Pybim)2

[16], and [ Mn" (ClCH2COO)6(Bipy)2] [17]. The Mn(1)-Mn(2) distance of 3.588 Á. The Mn(1) and Mn(2) atoms are bridged by six benzoate ligands, four of which show the common : n1 coordination mode, while the other two adopt the ^3-n2 : n1 coordination mode (Fig. 1).

Assignments manganase ions as Mn(II) are based on the following observations. The trinuclear molecules is neutral and composed of six monoanionic ben-zoate donors (e.g., benzoates) and two neutral Bipy ligands. Additional support for this oxidation level is provided by the almost equal distances of the Mn-O and Mn-N bond lengths, which is typical of a d5 system [15, 17]. BVS further confirm this assignments (table).

There exists intramolecular hydrogen bonds in the trinuclear unit (dashed line in Fig. 1) and intermolecular hydrogen bonds between the units (dashed line in Fig. 2). These intra- and intermolecular hydrogen bond with C-O distance of 3.149(5)-3.458(5) Á and C-H-O angle of >115° are typical of C-H-O hydrogen bonds [18]. The compound then packed together via intermolecular C-H-O hydrogen bond, as well as van der Waals attractions (Fig. 2).

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YAO et al.

Fig. 1. View of the structure of the compound I with atom labeling scheme. The dashed lines represent the intramolecular hydrogen bonds.

The temperature-dependent dc magnetic susceptibility measurement was performed on the crystalline sample in the temperature range 2.0-300 K in a field of 2 kOe. The XmT value gradually decreases from 12.23 (300 K) to 10.72 cm3 K mol-1 (80 K) before falling rapidly to 4.45 cm3 K mol-1 at 2 K ( Fig. 3). The xmT value at room temperature is smaller than the expected value of

13.125 cm3 K mol-1 for three uncoupled Mn(II) ions with Si = 5/2 (gj = 2). This is indicative of an antiferro-magnetic coupling, as confirmed by the decrease in %mT when T decreases. At low temperature the %mT value is indicative of an ST = 5/2 spin ground state.

Taking into account the structure of this compound, two exchange pathways are considered. One pathway is between the central metal ion and the terminal metal ones

Fig. 2. Packing diagram of the title compound, showing one layer of molecules connected by intermolecular C-H—O hydrogen bonds (dashed lines). All H atoms not included in the intermolecular hydrogen bond were omitted for clarity.

0 50 100 150 200 250 300 Temperature, K

Fig. 3. Molar magnetic susceptibility xm and %mT vs. temperature.

with coupling constant 2J. The other is between two terminal ions with the coupling constant 2J (Scheme).

i—

Mn(2)-

2J ■

— 2/ — -Mn(1)-Scheme.

2J

—H

-Mn(2A)

We used the following Hamiltonian (eq. 2) to describe the low-lying electronic states:

H = -2J(SS + SjS2a) - 2J S2S2A, (2)

where S is the spin of the manganase ion. The Kambe method gives the eigenvalue expression in eq. 3:

E(S, S) = - JS(S + 1) - (J - J)S'(S' + 1), (3)

where S is the total spin of the molecule and S is the spin quantum nu

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