CAO Hong-Yu

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Synthesis, Crystal Structure, and Magnetic Properties of a New Dinuclear Iron Complex with Schiff Base Ligand①

CAO Hong-Yu②

(024000)

A new dinuclear iron(III) complex has been synthesized and structurally charac- terized by X-ray crystallography: [FeIII2()(C6H5COO)(SO4)(CH3OH)2]·CH3CN·CH3OH (1, H3=?-bis(salicylidene)-1,3-diamino-2-propanol). Complex 1 belongs to orthorhombic space group21with= 11.4400(8),= 22.9705(2),= 12.5712(9) ?,= 3303.5(4) ?3,= 4,(000) = 1576,D= 1.531 g·cm–3,M= 761.36,= 1.007 mm–1,= 1.014, the final= 0.0505 and= 0.1018.The crystal packing is stabilized by intermolecular O–H···O hydrogen bonds, forming an extended one-dimensional chain structure. The temperature dependence of magnetic susceptibility measurement shows that antiferromagnetic interaction is propagated between the metal centers. Fit as dinuclear arrangement gave parameters of= 19.7 cm-1,= 1.89 and2= 0.9999.

iron complex, crystal structure, magnetic properties;

1 INTRODUCTION

Iron complexes are attracting attention not only in the fields of bioinorganic chemistry but also for their ability to function as progenitors of novel magnetic molecular materials. In the first hand, these com- plexes are investigated for their interesting redox properties[1]and oxidation catalysts[2, 3]. Many non- heme iron metalloproteins have diverse active site geometries with iron centers which are biologically important[4-6]. On the other hand, it is now well recognized that the synthesis of iron complexes is mainly due to their wide applications in molecular magnetism. Some iron complexes with appropriate topologies sometimes possess large ground-state spin values and large negative magnetic anisotropies and occasionally exhibit single-molecule magnet (SMM) behavior[7, 8].

The chemistry of iron(III) complexes is domina- ted by oxygen-bridged species because of the high oxygen affinity of iron(III) ion[9-11]. Dinuclear iron(III) complexes have been extensively used to molecular magnetism with different structural ar- rangements. Oxygen-bridged dinuclear iron(III) complexes can be divided into three classes: hy- droxido-bridged[12], oxido-bridged[13]and alkoxido or phenoxido bridged species[14, 15]. It has been noted that the antiferromagnetic interaction mediatedoxido-bridge is much stronger than that mediatedother bridges[11, 16].

Proper selection of polydentate ligands and metal ions is the key issue in the design and self-assembly of iron clusters with geometries and special pro- perties. Such complexes are expected to be both soluble and stable in solution as well as having an interesting magnetic behaviour. Organic ligand formed by reaction between the salicylaldehyde and 1,3-diaminopropan-2-ol is a good candidate to cons-truct iron clusters. The Schiff base?-bis(salicy- lidene)-1,3-diamino-2-propanol (H3) has O- or N- donors,and these donors with suitable relative positions in the ligand can coordinate to metal cen- ters. Several works have been devoted tothe exploration of alkoxy-bridged dinuclear iron com- plexes of Schiff-base H3ligand[14, 17, 18].Herein, we describe the synthesis, crystal structure and magnetic properties of a new dinuclear iron complex with the formula [FeIII2()(C6H5COO)(SO4)(CH3OH)2]·CH3CN·CH3OH (1).

2 EXPERIMENTAL

2. 1 Materials

All the reagents and solvents employed were commercially available and used as received without further purification. The Schiff base ligand H3is prepared in a similar manner according to the repor- ted procedures[19, 20].

2. 2 Synthesis of the complex 1

Complex 1 was synthesized by adding FeSO4·7H2O (55.6 mg, 0.20 mmol) to a solution of H3(47.7 mg, 0.20 mmol) and triethylamine (0.042 ml, 0.30 mmol) in acetonitrile/methanol (1:1) (20 mL). After stirring for 1 hour, benzoic acid (13.4 mg, 0.10 mmol) was added, and the resulting mixture was stirred for 5 h to afford a black solution, which was left unperturbed to allow slow evaporation of the solvent. Black single crystals, suitable for X-ray diffraction analysis, were formed after about two weeks. Yield: 32 mg (41.8% based on metal salt). Elemental analysis (%) calcd. for C29H35Fe2N3O12S: C, 45.75, H, 4.63, N, 5.52; found C, 45.63; H, 4.65; N, 5.59. IR band (KBr, cm-1): 3359(m), 3039(w), 2923(m), 2859(w), 2819(s), 2759(s), 1715(s), 1676(s), 1635(s), 1610(s), 1524(s), 1470(s), 1452 (s), 1432 (m), 1375(m), 1342(m), 1300(m), 1261(m), 1239(m), 1206(w), 1178(w), 1141(m), 1057(w), 1039(m), 917(w), 851(w), 810(w), 757(s), 608(w), 532(w), 440(w), 413(w).

2. 3 X-ray crystallography

A black crystal of complex 1 having approximate dimensions of 0.31mm × 0.30mm × 0.28mm was used for structure determination. Crystallographic data were collected on a Bruker Apex II CCD diffractometer with graphite monochromated Moradiation (= 0.71073 ?) at 185(2) K for complex 1. A total of 17307 reflections were collected with 6462 independent ones (int= 0.0626), of which 4703 observed reflections with> 2() were used in the succeeding refinements with –14≤≤8, –28≤≤28 and –15≤≤15.The structure was solved by direct methods using SHELXS-97[21]and refined on2by full-matrix least-squares using SHELXL-97[22]. All non-hydrogen atoms were refined with aniso- tropic thermal parameters. All hydrogen atoms were generated geometrically and refined isotropically using the riding model. For the complex, con- vergence was reached at the final0.0505 (2()) and= 0.1018 (for all data), with allowance for the thermal anisotropy for all non-hydrogen atoms.

2. 4 Physical measurements

Elemental analyses for C, H, and N were carried out on a Perkin-Elmer 2400 analyzer. FTIR spectra were recorded with a VERTEX 70 Fourier transform infrared spectrophotometer using the reflectance technique (4000～300 cm-1).Samples were prepared as KBr disks. Magnetic susceptibility measurements were performed in the temperature range 2～300 K, using a Quantum Design MPMS XL-7 SQUID magnetometer equipped with a 7 T magnet. The diamagnetic corrections for the compounds were estimated using Pascal’s constants[23],and magnetic data were corrected for diamagnetic contributions of the sample holder.

3 RESULTS AND DISCUSSION

3. 1 Structure determination

Complex 1 has been structurally characterized by single-crystal X-ray crystallography. Selected bond lengths and bond angles are summarized in Table 1. The drawing of the molecule is shown in Fig. 1. Its structure consists of dinuclear [Fe2]3+unit, one benzoic acid anion and one sulfate anion for charge balance, as well as two coordinated methanol mole- cules, methnoal and acetonitrile molecules of crystallization. The ligand provides two nitrogen atoms, two phenolato oxygen atoms and one-alkoxo oxygen group and chelates two iron ions. Two FeIIIatoms are bridged by a-alkoxo oxygen atom (O(2)) from the ligand, one sulfate group and one benzoic acid group, forming a dinuclear complex. The coordination environment at each FeIIIatom can be described as six-coordinated with a distorted octahedral geometry, with one imine nitrogen atom (N(1)/N(2)), one-alkoxo oxygen atom (O(2)) and one phenolate oxygen (O(1)/O(3)) from the ligand, and one oxygen atom (O(4)/O(5)) of benzoic acid anion forming a NO3equatorial plane. One oxygen atom (O(7)/O(6)) from a methanol molecule and one oxygen atom (O(9)/O(8)) from a sulfate group are at the apical positions. The deviations of Fe(1) from the O(1), O(2), O(4), N(1) planeand Fe(2) from the O(2), O(3), O(5), N(2) plane are 0.066(6) and 0.062(2) ?, respectively. The dihedral angle between the two NO3equatorial planes is 28.1(1)°. In the basal planes, the Fe–O bond lengths are in the 1.882(1)～2.002(4) ? range. The axial Fe–O distances fall in the longer range of 2.009(8)～2.084(4) ?. The longer Fe×××Fe distance and larger Fe–O–Fe angle of complex 1 are 3.499(9) ? and 122.33(1)°, compared with those in the other related similar dinuclear FeIIIcomplexes[11, 17, 24, 25].

Fig. 1 . A view of complex 1, showing the atomic numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. The hydrogen atoms and solvent molecules are omitted for clarity

Table 1. Selected Bond Lengths (?) and Bond Angles (o) for Complex 1

In the crystal structure, two adjacent molecules are connected to each other through O(6)–H(6A)···O(10)#2 and O(7)–H(7A)···O(10)#2 (sym- metry code #2:–+2, –+2,+1/2) hydrogen bonds, and the molecule is linked together by a hydrogen bond O(12)–H(12A)···O(11)#1 (symmetry code #1:–+1, –+1,+1/2) with methanol molecule, forming an extended one-dimensional chain structure (Fig. 2 and Table 2).

Table 2. Hydrogen-bond Parameters (? and °) for Complex 1

Symmetry transformations used to generate the equivalent atoms: #1: –+1, –+1,+1/2; #2: –+2, –+2,+1/2

Fig. 2 . 1D chain structure of complex 1 via the c axis. Hydrogen bonds are shown as green dashed lines. The acetonitrile molecules are omitted

3. 2 IR spectrum

IR absorption bands of complex 1 are respectively observed at 3359 and 1610 cm-1, indicating the presence of phenolic OH and C=N groups. In addi- tion, the strong bands at 1452 and 1470 cm–1are assigned to C=C benzene ring and the band at 757 cm–1can be owned to the aromatic C–H out-of-plane bending vibration in complex 1[26].

3. 3 Magnetic property

The magnetic susceptibility of complex 1 has been measured in the region of 2～300 K. A plot of χMT. T is shown in Fig. 3. TheMT value at 300 K is 5.5 cm3mol?1K, which is in line with the values reported for other dinuclear-oxo bridged iron(III) complexes[24, 27]and much lower than the spin-only value of 8.8 cm3mol?1K expected for two non-interacting high-spin Fe3+ions (g = 2).Upon cooling, the χMT product decreases continuously and reaches 0.007 cm3mol?1K at 2 K, indicating the occurrence of antiferromagnetic intramolecular interaction between the two Fe3+ions.

Fig. 3. χMTplot data for complex 1 at 1000 Oe

The experimental susceptibility data were analy- zed by means of the theoretical equation derived for a Heisenberg-van Vleck model (H = ?S1S2) with S1= S2= 5/2[28].The best fit gave= ?19.71 cm?1, g= 1.89 and2= 0.9999. The strength of the Fe–Fe interaction () falls in the expected range[11, 29].

4 CONCLUSION

A new dinuclear iron(III) complex derived from a N2O3-donor Schiff base ligand H3has been synthe- sized and structurally characterized by X-ray crys- tallography. The metal centers of complex 1 exhibit a distorted octahedral geometry, and bridge a- alkoxo oxygen atom (O(2)) from the ligand, one sulfate group and one benzoic acid group. The crystal packing is stabilized by intermolecular O– H···O hydrogen bonds, forming an extended one- dimensional chain structure. Magnetic analysis of complex 1 reveals that antiferromagnetic interaction is propagated between the metal centers.

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29 October 2014; accepted 7 January 2015 (CCDC 1031050)

①Supported by the Natural Science Foundation of Mongolian Autonomous Region, China (No. 2014MS0215)

. Cao Hong-Yu. Tel: +86 0476-8300368, Fax: +86 0476-8300368, E-mail: chy.0211@163.com

10.14102/j.cnki.0254-5861.2011-0550