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Jul 27, 2009 - afford single-domain magnets,1-6 which may display hyster- ...... S4), the plots of M vs H for 1 and 2 (Figure S5-S6) and the plots of ...
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DOI: 10.1021/cg9002884

Syntheses, Crystal Structures, and Magnetic Properties of Two Cyclic Clusters Comprising Six Iron(III)/Manganese(III) Ions and Entrapping Sodium Ions

2009, Vol. 9 4064–4069

Pei-Pei Yang, Hai-Bin Song, Xi-Feng Gao, Li-Cun Li,* and Dai-Zheng Liao Department of Chemistry, Nankai University, 94 Weijing Road, Tianjin 300071, P. R. China Received March 11, 2009; Revised Manuscript Received July 7, 2009

ABSTRACT: The reaction of iron or manganese salt with sodium 2-{[bis(2-hydroxyethyl)amino]methyl}-4-methylphenate (NaH2L) in CH3OH/CH3CN solution represents a good synthetic route to the cation molecular wheels [NaFe(III)6L6]OH 3 1.5CH3CN (1) and [NaMn(III)6L6]OH 3 CH3OH 3 0.75H2O (2), respectively. Single-crystal X-ray diffractions show that six octahedrally coordinated iron(III) ions in 1 and manganese(III) ions in 2 define a ring and are linked by 12 bridging oxygen atoms from alkoxo groups. The resulting [Fe6(OCH2)12] or [Mn6(OCH2)12] skeleton has the remarkable property of acting as a host for a octahedrally coordinated sodium ion in the center of the ring. DC magnetic susceptibility measurements on complexes 1 and 2 reveal the presence of exchange interactions resulting in a ST =0 or ST =12 ground spin state, respectively. The magnetic behavior of the two clusters indicates antiferromagnetic coupling between the iron(III) centers in 1 and ferromagnetic coupling between the manganese(III) centers in 2.

1. Introduction In recent years, iron and manganese clusters with oxygen atoms acting as bridging ligands have become the focus of intensive research activity since this class of inorganic complexes possesses a variety of structures and is of current interest in materials and bioinorganic chemistry.1-15 For example, nanosized molecules with a high-spin ground state afford single-domain magnets,1-6 which may display hysteresis,7-9 slow magnetic relaxation,10-12 and spin-quantum tunneling characteristics of a purely molecular origin.13-15 One of the most successful synthetic approaches to new polynuclear clusters involves the use of chelates containing alcohol groups because alkoxides are good bridging groups for their versatile bridging modes (μ2 and μ3) and thus favor the formation of polynuclear products.16-21 Subtle changes in reaction conditions have resulted in the formation of unexpected beautiful structures, such as ring-shaped iron or manganese clusters.22-26 Ring-shaped structures occupy a special position among polynuclear complexes for many reasons, not the least of which are their pleasing structural aesthetics. Several recent theoretical studies have indicated that transition metal wheel complexes could be the basis for quantum computation.27 In 2005, Satoshi et al. described an antiferromagnetic FeIII6 ring and a single-molecule magnet MnII3MnIII4 wheel by reactions of quadridentate ligands, N-(2-hydroxy-5-nitrobenzyl)iminodiethanol with FeCl3 and MnCl2 3 4H2O.28 In order to explore the versatility of such quadridentate ligands, we synthesized a new similar quadridentate ligand, sodium 2-{[bis(2-hydroxyethyl)amino]methyl}-4-methyl phenate (NaH2L) (Scheme 1). Two new rings, namely, [NaFe(III)6L6]OH 3 1.5CH3CN (1) and [NaMn(III)6L6]OH 3 CH3OH 3 0.75H2O (2) were obtained by reactions of NaH2L with iron or manganese salt in CH3OH/CH3CN solution. One interesting feature of the two complexes is the presence of a sodium ion trapped in the hollow core of every cluster, which acts as an inorganic *To whom correspondence should be addressed. pubs.acs.org/crystal

Published on Web 07/27/2009

crown ether with the [12]crown-6 structure. Another important feature is that product 2 contains six manganese(III) ions instead of Mn(II)Mn(III) mixed-valent. It should be mentioned here that although molecule manganese rings have been reported successively in recent years,25,28-31 very few cyclic complexes containing only Mn(III) ions have been found. As far as we know, a similar ring is only observed in the complex [NaMn6(OMe)12(dbm)6]BPh4 3 2CHCl3 (dbm = dibenzoylmethane) reported by Gian et al. in 1998.26a In this text, we report the syntheses and magnetic properties of a hexanuclear iron(III) ring and a hexanuclear manganese(III) ring. 2. Experimental Section 2.1. Synthesis. 2.1.1. NaH2L. To an aqueous solution (60 mL) containing p-cresol (10.82 g, 100.0 mmol), NaOH (4.00 g, 100.0 mmol), and bis(2-hydroxyethyl)amine (21.10 g, 200 mmol) were added formaldehyde (37%) (20 mL) in ethanol (20 mL), and the resulting solution was stirred at room temperature for 8 h. Removing the solvent under reduced pressure yielded a yellow-brown oil. A large amount of ether was treated with the oil under violent stirring, and NaH2L was obtained as a yellow powder. Purification from ethanol and ether gave the product as yellow needle crystals. Yield: 18.6 g (59%). Selected IR data (ν/cm-1) using KBr disks: 3412vs, br, 2893m, 1611s, 1489vs, 1381m, 1279vs, 1158m, 1084s, 1041m, 986m, 874m, 820m, 791m, 753m, 619m, 519m, 489m (br, broad; vs, very strong; s, strong; m, medium). 1H NMR (DMSO: δ, ppm): 2.07 (s, 3H, CH3), 2.50 (t, 4H, NCH2R), 3.42 (t, 4H, OCH2), 3.65 (s, 2H, ArCH2N), 6.61 (s, 3H, ArH). Mp 202-205 °C. Anal. Calcd for (C12H18NaNO3): C, 58.51; H, 7.58; N, 5.43%. Found: C, 58.30; H, 7.29; N, 5.67%. 2.1.2. [NaFe(III)6L6]OH 3 1.5CH3CN (1). Method A. An orangered solution of [Fe3O(O2CMe)6(H2O)3](NO3) (0.0669 g, 0.1 mmol) in MeCN (10 mL) was treated with NaH2L (0.0798 g, 0.3 mmol) in 5 mL of methanol, and the solution was stirred for 3 h at room temperature. It was then filtered, and the filtrate was allowed to stand undisturbed at room temperature. Dark red crystals of the product formed over 5 days in 35% yield. Anal. Calcd for (C75H101.5Fe6N7.5NaO19): C, 50.71; H, 5.53; N, 6.02%. Found: C, 50.84; H, 5.73; N, 5.93%. Selected IR data (ν/cm-1) using KBr disks: 3422vs, br, 2865m, 1636s, 1485s, 1385m, 1268s, 1128m, 1082m, 1022m, 968m, 874m, 819s, 740m, 636m, 510m, 451m (br, broad; vs, very strong; s, strong; m, medium). r 2009 American Chemical Society

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Crystal Growth and Design, Vol. 9, No. 9, 2009 Scheme 1

Method B. To a solution of NaH2L (0.0798 g, 0.3 mmol) in methanol (10 mL) was added Fe(NO3)3 3 6H2O (0.0620 g, 0.15 mmol) in 5 mL of acetonitrile. When Et3N (0.3 mmol) was added into the resulting dark blue solution, a dark red solution was obtained immediately. After the solution was stirred at room temperature for 3 h, the mixture was filtered to remove a red powder. The dark-red crystals were obtained by slow evaporation of the filtrate after one week. The product was collected and washed with cool acetonitrile and air-dried. Yield: 32%. The product was identified by IR spectral and element analytical comparison with material from method A. 2.1.3. [NaMn(III)6L6]OH 3 CH3OH 3 0.75H2O (2). Method A. A solution of NaH2L (0.0400 g, 0.15 mmol) in methanol (10 mL) was added into Mn(acac)3 (0.0355 g, 0.1 mmol) in 10 mL of acetonitrile solution. The resulting dark black solution was filtered after being stirred for 2 h. Dark-black crystals suitable for X-ray crystallographic analysis were obtained by slow evaporation of the filtrate after 5 days. The product was collected and washed with cool methanol and air-dried. Yield: 40%. Anal. calcd. for (C73H102.5Mn6N6NaO20.75): C, 50.28; H, 5.63; N, 4.56%. Found: C, 50.09; H, 5.86; N, 4.80%. Selected IR data (ν/cm-1) using KBr disks: 3428vs, br, 2862m, 1635s, 1486s, 1340, 1263m, 1129s, 1059m, 1018m, 937m, 889s, 818m, 738m, 623m, 580m, 518m (br, broad; vs, very strong; s, strong; m, medium). Method B. To a stirred red-brown solution of Mn(O2CPh)2 (0.1512 g, 0.50 mmol) in CH3CN (10 mL) was added a solution of NaH2L (0.1339 g, 0.5 mmol) in methanol (10 mL). The mixture was stirred for 3 h and the resulting dark black brown solution was filtered. The filtrate was allowed to stand undisturbed at room temperature. Dark black crystals of the product formed over 5 days in 36% yield. The product was identified by IR spectral and X-ray crystallographic comparison with material from method A. 2.2. Materials and Measurements. Unless otherwise stated, all reagents were obtained commercially at the best grade available, and were used as received, without further purification. [Fe3O(O2CMe)6(H2O)3](NO3) was prepared as previously described in the literature.32 All reactions were carried out under aerobic conditions. The infrared spectra of the complexes in KBr pellets were obtained on a Bruker Tensor 27 IR spectrometer in the range of 4000400 cm-1. The magnetic susceptibility data were recorded using a Quantum Design MPMS-7 SQUID magnetometer in the temperature range from 2 to 300 K at an applied magnetic field of 2 KG. Elemental analyses of carbon, hydrogen, and nitrogen were carried out with a Perkin-Elmer 240 analyzer. NMR spectra were recorded on Bruker Avance DPX-250 (1H NMR 250 MHz) in pure deuterated solvents with dimethylsulfoxide (DMSO) as internal standards. X-ray powder diffraction (XRD) patterns were measured using a Bruker D4 powder diffractometer at 40 kV, 100 mA for Cu KR radiation, with a scan speed of 0.2 s/step and a step size of 0.02° (2θ). 2.3. X-ray Crystal Structure Determinations. Single-crystal X-ray studies for complexes 1 and 2 at 113(2) K were performed on a Rigaku Saturn CCD diffractometer equipped with a graphite crystal monochromator situated in the incident beam for data collection. Suitable crystals of 1 and 2 were attached to glass fibers using silicone grease for data collection. The determinations of unit cell parameters and data collections were performed with Mo-KR radiation with radiation wavelength of 0.71073 A˚ by using the ω-scan technique. Lorentz polarization and absorption corrections

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were applied by using the multiscan program.33 The structures were solved by direct methods and refined with the full-matrix leastsquares technique using the SHELXS-97 and SHELXL-97 programs.34 The non-hydrogen atoms were treated anisotropically. Hydrogen atoms on carbon atoms of L3- ligands were placed in calculated, ideal positions and refined as riding on their respective carbon atoms, while hydrogen atoms on solvent molecules of crystallization were not located. The hydrogen bonds are not revealed because of the absence of these hydrogen atoms. For complex 1, a total of 1042 parameters were refined in the final cycle of refinement using 14 498 reflections with I > 2σ(I). For complex 2, a total of 1045 parameters were refined in the final cycle of refinement using 15 854 reflections with I > 2σ(I). Further details about crystal data and structure refinement for 1 and 2 are summarized in Table 1. CCDC (715106 and 715105 for complexes 1 and 2 respectively) contains the supplementary crystallographic data for this paper. Selected bond distances and angles are given in Tables 2 and 3. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (þ44) 1223-336-033; or e-mail: [email protected].

3. Results and Discussion 3.1. Synthetic Aspects and General Properties. The synthesis of 2-{[bis(2-hydroxyethyl)amino]methyl}-4-methylphenol has been reported in the literature.35 However, the synthetic protocols vary in the use of solvent, reagents, and reaction temperature in forming its sodium salt. The reaction of 2-{[bis(2-hydroxyethyl)amino]methyl}-4-methylphenate (NaH2L) with [Fe3O(O2CMe)6(H2O)3](NO3) in acetonitrile and methanol solution yielded a dark-red solution from which after slow evaporation of the solvent large single crystals of [NaFe(III)6L6]OH 3 1.5CH3CN (1) were separated. We have found that the carboxylate groups are uncoordinated to the central ions and every Fe(III) ion is occupied by three trivalent L3- ligands. The same product can be obtained from the reaction between Fe(NO3)3 3 6H2O and NaH2L in the presence of triethylamine as a base in CH3OH/CH3CN. Reaction of NaH2L with Mn(acac)3 or Mn(O2CPh)2 in MeCN/MeOH leads to the formation of a dark black solution and a light brown precipitate. Removal of the precipitate and subsequent evaporation of the brown/ black solutions leads to the formation of crystalline [NaMn(III)6L6]OH 3 CH3OH 3 0.75H2O (2) after approximately 5 days. The product contains six MnIII ions and a similar phenomenon is only discovered in complex [NaMn6(OMe)12(dbm)6]BPh4 3 2CHCl3 (dbm = dibenzoylmethane) reported by Gian et al. in 1998. The purity of complexes 1 and 2 is confirmed by a comparison of experimental and simulated powder X-ray diffraction (Figures S3 and S4, Supporting Information). The experimental peaks are in good agreement with those calculated from X-ray singlecrystal diffraction data. 3.2. Description of the Structures. 3.2.1. [NaFe(III)6L6]OH 3 1.5CH3CN (1). Complex 1 crystallizes in the orthorhombic space group Pbca. The crystal structure of 1 comprises cationic hexanuclear units [NaFe(III)6L6]þ (Figure 1), hydroxide anions and crystallization solvent molecules (CH3CN). In the wheel, six μ2-alkoxo groups (O3, O5, O8, O11, O14, and O17) bridge the iron ions on the rim, which themselves are linked to the central sodium ion through six μ3-bridging alkoxo groups (O2, O6, O9, O12, O15, and O18) that act as spokes to form the wheel structure. The six iron(III) ions are arranged a slightly distorted hexagon with Fe 3 3 3 Fe from 3.2066(10) to 3.2498(10) A˚ and Fe1 3 3 3 Fe2 3 3 3 Fe3 from

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Crystal Growth and Design, Vol. 9, No. 9, 2009 Table 1. Data Collection and Processing Parameters for Complexes 1 and 2

empirical formula formula weight temperature (K) wavelength crystal system space group unit cell dimensions

volume (A˚3) Z calculated density (Mg/m3) absorption coefficient (mm-1) F(000) crystal size θ range for data collection limiting indices reflections collected independent reflections absorption correction max. and min transmission refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole

1

2

C75H101.5Fe6N7.5NaO19 1770.23 113(2) K 0.71073 A˚ orthorhombic Pbca a = 23.178(5) A˚ b = 24.882(5) A˚ c = 28.509(6) A˚ R = 90° β = 90° γ = 90° 16442(6) 8 1.430 1.107 7384 0.12  0.04  0.02 mm 1.99-25.02° -27 e h e 27, -29 e k e 29, -33 e l e 33 139660 14498 [R(int) = 0.0963] semiempirical from equivalents 0.9782 and 0.8787 full-matrix least-squares on F2 14498/75/1042 1.078 R1 = 0.0557, wR2 = 0.1624 R1 = 0.0744, wR2 = 0.1768 1.354 and -0.607 e A˚-3

C73H102.5Mn6N6NaO20.75 1748.74 113(2) K 0.71073 A˚ monoclinic P21/n a = 19.373(4) A˚ b = 22.202(4) A˚ c = 21.804(4) A˚ R = 90° β = 105.66(3)° γ = 90° 9030(3) 4 1.286 0.883 3638 0.24  0.20  0.06 mm 1.94-25.02° -19 e h e 23, -20 e k e 26, -25 e l e 25 51078 15854 [R(int) = 0.1128] semiempirical from equivalents 0.9489 and 0.8160 full-matrix least-squares on F2 15854/93/1045 1.049 R1 = 0.1026, wR2 = 0.2589 R1 = 0.1459, wR2 = 0.2944 1.384 and -0.624 e A˚-3

Table 2. Selected Bond Lengths (A˚) and Angles (°) for 1 Fe(1)-O(1) Fe(1)-O(18) Fe(1)-O(2) Fe(2)-N(6) Fe(2)-O(3) Fe(2)-O(15) Fe(3)-O(13) Fe(3)-O(12) Fe(3)-O(15) Fe(4)-N(4) Fe(4)-O(14) Fe(4)-O(9) Fe(5)-O(7) Fe(5)-O(6) Fe(5)-O(9) Fe(6)-N(2) Fe(6)-O(4) Fe(6)-O(2) Fe(1)-O(2)-Fe(6) Fe(1)-O(5)-Fe(6) Fe(6)-O(8)-Fe(5) Fe(5)-O(11)-Fe(4) Fe(4)-O(14)-Fe(3) Fe(3)-O(17)-Fe(2))

1.884(3) 2.027(3) 2.076(3) 2.212(4) 1.981(3) 2.028(3) 1.881(3) 2.008(3) 2.097(3) 2.197(4) 1.983(3) 2.024(3) 1.886(3) 2.006(3) 2.081(3) 2.177(4) 1.883(3) 2.022(3) 103.72(13)) 107.57(15) 106.97(15) 107.33(14) 106.17(14) 107.49(15)

Fe(1)-O(5) Fe(1)-O(3) Fe(1)-N(1) Fe(2)-O(16) Fe(2)-O(17) Fe(2)-O(18) Fe(3)-O(17) Fe(3)-O(14) Fe(3)-N(5) Fe(4)-O(10) Fe(4)-O(11) Fe(4)-O(12) Fe(5)-O(11) Fe(5)-O(8) Fe(5)-N(3) Fe(6)-O(5) Fe(6)-O(8) Fe(6)-O(6) Fe(2)-O(3)-Fe(1) Fe(5)-O(6)-Fe(6) Fe(4)-O(9)-Fe(5) Fe(3)-O(12)-Fe(4) Fe(2)-O(15)-Fe(3) Fe(1)-O(18)-Fe(2)

1.973(3) 2.031(3) 2.192(4) 1.894(3) 2.017(3) 2.089(3) 1.959(3) 2.032(3) 2.164(4) 1.883(3) 2.020(3) 2.073(3) 1.979(3) 2.029(3) 2.190(4) 2.022(3) 1.984(3) 2.066(3) 108.17(14) 104.74(15) 103.37(14) 103.73(13) 102.02(14) 104.27(13)

118.57(27) to 120.95(24)°. The average Fe-O-Fe bond angle formed by μ2-alkoxo or μ3-alkoxo with six FeIII ions in complex 1 (from 102.02(14) and 108.17(14)°) is 105.4°. The octahedral coordination geometry of the Fe(III) ions is considerably distorted, and the coordination spheres of every Fe(III) ion are exclusively occupied by a nitrogen atom and five oxygen donors from three different L3ligands. Twelve oxygen atoms from alkoxo groups act as bridges between adjacent Fe(III) centers in μ2 and μ3 fashions and support the cyclic skeleton of the cluster, which acts as a host for a octahedrally coordinated sodium ion in the center. Coordination bond lengths with oxygen atoms are in the range of 1.884(3)-2.089(3) A˚ and are shorter than that involving the nitrogen atom in the range

Table 3. Selected Bond Lengths (A˚) and Angles (°) for 2a Mn(1)-O(1) 1.868(6) Mn(1)-O(5) 1.959(5) Mn(1)-N(1) 2.241(8) Mn(2)-O(4) 1.851(6) Mn(2)-O(8) 1.932(6) Mn(2)-N(2) 2.203(6) Mn(3)-O(7) 1.851(6) Mn(3)-O(2)#1 1.931(6) Mn(3)-N(3) 2.206(7) O(1)-Mn(1)-O(3) 162.5(3) O(3)-Mn(1)-O(5) 79.8(2) O(1)-Mn(1)-N(1) 92.5(3) O(4)-Mn(2)-O(8) 92.6(3) O(4)-Mn(2)-O(5) 96.3(3) O(8)-Mn(2)-O(5) 163.5(2) 83.8(2) O(6)-Mn(2)-N(2) O(5)-Mn(2)-N(2) 78.1(2) O(6)-Mn(2)-O(3) 91.6(2) O(7)-Mn(3)-O(2)#1 90.7(3) O(7)-Mn(3)-O(8) 96.3(3) O(2)#1-Mn(3)-O(8) 163.7(3)

Mn(1)-O(3) 1.931(5) Mn(1)-O(2) 2.035(6) Mn(1)-O(9)#1 2.252(6) Mn(2)-O(6) 1.932(5) Mn(2)-O(5) 2.023(5) Mn(2)-O(3) 2.239(5) Mn(3)-O(9) 1.928(5) Mn(3)-O(8) 2.024(6) Mn(3)-O(6) 2.229(5) O(1)-Mn(1)-O(5) 89.3(2) O(1)-Mn(1)-O(2) 96.5(3) O(3)-Mn(1)-N(1) 82.2(3) O(6)-Mn(2)-O(8) 79.2(2) O(6)-Mn(2)-O(5) 93.8(2) O(4)-Mn(2)-N(2) 92.1(2) O(8)-Mn(2)-N(2) 115.5(3) O(4)-Mn(2)-O(3) 97.5(2) O(8)-Mn(2)-O(3) 93.7(2) O(9)-Mn(3)-O(2)#1 78.8(2) O(9)-Mn(3)-O(8) 96.5(2) O(2)#1-Mn(3)-Mn(2) 129.6(2)

a Symmetry transformations used to generate equivalent atoms: #1 x þ 1, -y þ 2, -z þ 1.

of 2.164(4)-2.212(4) A˚, which is in good agreement with the published literature.28 3.2.2. [NaMn(III)6L6]OH 3 CH3OH 3 0.75H2O (2). X-ray crystallography shows that complex 2 crystallizes in the monoclinic space group P21/n, and the molecule lies on a crystallographic center of symmetry (Figure 2). The symmetric unit of 2 contains two independent [NaMn(III)6L6]þ molecules, two hydroxide anions, one CH3OH, and threequarters of H2O solvent molecules. Here, we discuss one of the two similarly independent parts, namely, the cationic hexanuclear units [NaMn(III)6L6]þ. In the wheel, six μ2alkoxo groups (O2, O5, and O8) bridge the manganese ions on the rim, which themselves are linked to the central sodium ion through six μ3-alkoxo groups (O3, O6, and O9) acting as

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Figure 3. χmT versus T and χm versus T plots for 1. The solid line results from the theoretical calculation using the value in the text.

Figure 1. Molecule structure of complex 1 (hydrogen atoms, hydroxide anion, and solvent molecules have been omitted for clarity).

Figure 2. Molecule structure of complex 2 (hydrogen atoms, hydroxide anion, and solvent molecules have been omitted for clarity).

spokes to form the wheel structure. The [Mn6(OCH2)12] ring displays a 12-metallacrown-6 structure and acts as a host for a sodium ion. On the bases of bond valence sum calculations36 and the presence of Jahn-Teller distortion in MnIII ions, the six manganese ions are MnIII ions. Every MnIII ion adopts an elongated octahedral geometry coordinated by a nitrogen atom and five oxygen atoms from three different L3- ligands. A tetragonal elongation of the coordination

polyhedra is evident along the N1-Mn1-O9, N2-Mn2O3, and N3-Mn3-O6 directions, which are roughly perpendicular to each other as a consequence of edgesharing between MnNO5 octahedron. Axial bonds involve μ2-O atoms (alkoxo) and nitrogen atoms from L3- ligands and have unequal lengths with Mn-O from 2.229(5) to 2.252(6) A˚ and Mn-N from 2.203(6) to 2.241(8) A˚. The equatorial sites are occupied by μ2-O from alkoxo and oxygen atoms from phenoxy with Mn-O from 1.851(6) to 2.035(6) A˚. 3.3. Magnetic Properties of Complexes 1 and 2. DC magnetic susceptibility measurements were carried out on crystalline samples of 1 and 2 in the 2-300 K temperature range and an external magnetic field of 2 KG. The temperature dependence of the magnetic susceptibilities for 1 is shown in Figure 3. For 1, the value of χmT at 300 K was 15.38 cm3 mol-1 K, which is much reduced from that of six noninteracting metal centers of Fe3þ with S=5/2, g=2 (26.28 cm3 mol-1 K). There is a steady decrease in χmT with decreasing temperature from 15.38 cm3 mol-1 K at 300 K to 0.025 cm3 mol-1 K at 2 K. This behavior is consistent with dominant antiferromagnetic (AF) interactions and indicates that the complex has an ST = 0 spin ground state. The molar magnetic susceptibility for 1 increased as the temperature was lowered, reaching a maximum at 130 K, and decreased rapidly at 20 K. The susceptibility data for complex 1 could be well simulated, using the program MAGPACK.37,38 The exchange coupling constant (J) between FeIII ions was estimated to be -9.85 cm-1 with a gFe value of 2.03. The antiferromagnetic interaction in complex 1 is also supported by the field dependence of the magnetization (0-5 T) measured at 2.0 K (Figure S5, Supporting Information). As seen, the magnetization increases slowly with field, and the magnetization value of 0.0372 Nβ achieved at the highest field (5 T) is far below the saturation value (30 Nβ) expected for six isolated high-spin Fe(III) species, clearly suggesting the presence of antiferromagnetic coupling in 1 with ST =0 ground state. The magnetic behavior of 1 is very similar to those reported six-numbered Fe(III) rings. The magnitude of magnetic exchange coupling between the Fe(III) ions in 1 is compared with those of known hexanuclear Fe(III) rings.28,39,40 As known, the important parameter in magneto-structural correlations for the alkoxo-bridged Fe(III) complexes is the Fe-O-Fe angle: bigger Fe-O-Fe angles lead to stronger exchange coupling interactions. In complex 1, the average Fe-O-Fe angle is 105.4°, bigger than 102.5° in the

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Figure 4. The χmT versus T plots for 2. The solid line results from the theoretical calculation using the value in the text.

reported complex [LiFe6(OCH3)12(dbm)6]ClO4,39 and thus the exchange coupling in complex 1 (J=-9.85 cm-1) was stronger than that of complex [LiFe6(OCH3)12(dbm)6]ClO4 (J = -7.34 cm-1). Similar average Fe-O-Fe angles in 1 (105.4°) and in the reported complex [NaFe6(OCH3)12(dbm)6]ClO4 (105.5°)40 result in almost equivalent exchange coupling (J=-9.85 cm-1 and -9.95 cm-1, respectively). The temperature dependence of the magnetic susceptibilities for 2 is shown in Figure 4. The χmT value of 21.81 emu mol-1 K at 300 K, which is larger than expected for six uncoupled S=2 spins (18.0 emu mol-1 K with g=2.00), increased as the temperature was lowered, reaching a maximum value of 69.59 emu mol-1 K at 8.0 K. This value is close to the limit of 78.0 emu mol-1 K expected for parallel alignment of the spins to give a ground S = 12 state. The sudden decrease in the χmT value below 8.0 K is due to magnetic anisotropy and/or through-space antiferromagnetic interactions. Comparison of the χmT values measured at low temperature with those expected for the various ground configurations proves that the six coupling constants are ferromagnetic. The susceptibility data for complex 2 were also fitted by using the magnetism package MAGPACK. In complex 2, Mn1 3 3 3 Mn2, Mn2 3 3 3 Mn3 and Mn3 3 3 3 Mn10 separations are equivalent within experimental error [3.2163(19), 3.2201(18), and 3.2191(19) A˚, respectively], so we can get a good agreement between calculated and experimental values of the susceptibility over 8.0 K by fitting the data on the assumption of identical coupling constants in the ring with J = 7.95 cm-1, g = 1.90 (shown in Figure 4 as a solid line). Attempts were made to fit the data on the assumption of different coupling constants in the ring, but this does not account for the observed magnetic behavior. The obtained magnetic coupling value between the Mn(III) ions is comparable to those of known hexanuclear Mn(III) ring.26a The ferromagnetic nature of exchange-coupling interactions in 2 can be explained by assuming prevalent eg-eg contributions. Given the elongated nature of the distortion from octahedral symmetry, the metal dx2-y2 orbitals are empty. Because of the arrangement of local elongation axes in the structure, the dz2 magnetic orbitals have a nonzero overlap with the empty dx2-y2 orbitals through μ3-alkoxo ligands. This dx2-y2/z2 pathway is expected to provide a ferromagnetic contribution to magnetic coupling (shown

Figure 5. Selected distances (A˚) are gathered in the partial view of the neighboring Mn(III) atoms with an indication of the relative orientation of the magnetic orbital dz2 having a nonzero overlap with the empty dx2-y2 orbital.

in Figure 5).41 The magnetization curve at 2 K (Figure S6, Supporting Information) shows a rapid increase with the external field because of the ferromagnetic interactions in hexanuclear Mn(III) rings. At 5.0 T, the magnetization value is 22 Nβ, lower than the theoretical value of 24 Nβ for a ferromagnetically coupled MnIII6 system. Such behavior can be often observed in Mn(III)-containing complexes because of the zero-field splitting of the Mn(III) ions.42 Furthermore, above 2 K, slow paramagnetic relaxation was not observed from alternating current magnetic susceptibility studies oscillating field at frequencies up to 1200 Hz (Figure S7, Supporting Information). 4. Conclusions In summary, the use of sodium 2-{[bis(2-hydroxyethyl)amino]methyl}-4-methyl-phenate (NaH2L) ligand represents a convenient strategy for assembling high-nuclearity iron or manganese clusters and two new cyclic complexes have been synthesized and characterized crystallographically and magnetically. X-ray diffraction analyses reveal that their cyclic skeletons are supported by μ2- and μ3alkoxo bridges and the sodium ions are situated at the center of the hexagonal wheels and stabilize the two complexes. The magnetic behavior of the two clusters indicates antiferromagnetic coupling between the iron(III) centers with ST =0 in 1 and ferromagnetic coupling between the manganese(III) centers with ST =12 in 2. The alternating current (AC) magnetic susceptibility measurement for 2 was carried out in the temperature range from 2 to 20 K oscillating at frequencies up to 1200 Hz; however, complex 2 showed no obvious frequency-dependent in-phase (χm0 ) and out-of-phase (χm00 ) signals. Acknowledgment. This work was financially supported by the National Science Foundation of China (Nos. 20471032, 50672037) and the NSF of Tianjin (No. 09JCYBJC05600).

Article

Crystal Growth and Design, Vol. 9, No. 9, 2009

Supporting Information Available: X-ray crystallographic files in CIF format, topology (Figures S1-S2), XRD patterns (Figure S3S4), the plots of M vs H for 1 and 2 (Figure S5-S6) and the plots of χm0 T and χm00 versus T for complex 2 (Figure S7). This information is available free of charge via the Internet at http://pubs.acs.org.

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