Synthesis and Magnetic Properties of a Fe-Mn-Cr Multinuclear Complex with 4-Amino-1,2,4-triazole and Oxalate Ligands Iis Siti Jahro1), Djulia Onggo1), Ismunandar1), Susanto Imam Rahayu1), Esther Jacoba Martha Vertelman2), Petra van Koningsbruggen2) 1) Inorganic and Physical Chemistry Group, Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, Bandung 2) Stratingh Institute of Chemistry and Chemical Engineering,University of Groningen, Groningen, The Netherlands e-mail:
[email protected] Received May 2006, accepted for publication September 2006 Abstract A multinuclear Fe-Mn-Cr complex with 4-amino-1,2,4-triazole (NH2trz) and oxalate (ox) ligands has been synthesized successfully. The formula of the [Fe(NH2trz)3][ClO4][MnCr(ox)3].4H2O complex has been obtained based on the metal and C, H, N contents. The presence of water molecules, metal-ligand bonding and bridge ligand in the multinuclear complex has been confirmed by its infrared spectrum. The compound crystallizes in the hexagonal system with cell parameters of a = b = 18.695 Å and c = 57.351 Å. The compound shows a gradual spin crossover for iron(II) in the [Fe(NH2trz)3]2+ with transition temperature (T1/2) of 205 K. The antiferromagnetic interaction between Cr(III) and Mn(II) ions in the [MnCr(ox)3]nn- network is observed from the Weiss constant (θ) of –2.3 K. Keywords: [Fe(NH2trz)3]2+, [MnCr(ox)3]nn-, Magnetic properties, Spin crossover, Antiferromagnetic Abstrak Senyawa kompleks multi-inti Fe-Mn-Cr dengan ligan 4-amino-1,2,4-triazol (NH2trz) dan oksalat (ox) telah berhasil disintesis. Rumus kimia senyawa kompleks [Fe(NH2trz)3][ClO4][MnCr(ox)3].4H2O diperoleh atas dasar kandungan ion logam dan unsur C, H, dan N. Keberadaan molekul air, ikatan logam-ligan dan gugus ligan jembatan dibuktikan dari spektrum inframerah. Senyawa ini termasuk pada sistim kristal heksagonal dengan parameter sel a = b = 18.695 Å dan c = 57.351 Å. Senyawa ini menunjukkan transisi spin pada ion besi(II) dalam kromofor [Fe(NH2trz)3]2+ dengan temperatur transisi (T1/2) 205 K. Adanya interaksi antiferomagnetik antara ion Cr(III) dan Mn(II) dalam jaringan [MnCr(ox)3]nn- teramati dari nilai konstanta Weiss (θ) sebesar –2,3 K. Kata kunci: [Fe(NH2trz)3]2+, [MnCr(ox)3]nn-, Sifat magnet, Spin crossover, Antiferomagnetik The synthesis, structure, and magnetic properties of the series of molecular magnets formulated as [Z(bpy)3][ClO4][MCr(ox)3]; Z = Ru, Fe, Co and Ni; M = Mn, Fe, Co, Ni, Cu, and Zn; bpy = 2,2’-bipyridine; ox = oxalate dianion, have also been reported (Coronado et al., 2001). The magnetic susceptibility behaviors of the series [Fe(bpy)3][ClO4] [MCr(ox)3] in the temperature range 5–300K were essentially similar. In all compounds, the Weiss constants (θ) were positive, indicating ferromagnetic interactions between neighboring Cr(III) and M(II) ions, whereas the [Fe(bpy)3]2+ entity is a diamagnetic cation containing divalent iron in the low spin state. Octahedral complexes of Fe(II) ions may exist in either the low spin (LS, S = 0) or high spin (HS, S = 2) state, depending on the nature of the ligand field of the metal ion. A transition or crossover between a low spin and a high spin state may occur due to a variation of temperature, pressure, or by light irradiation (Decurtin et al., 1985; Grandjean et al., 1989; Garcia et al., 1998; Gütlich et al., 2000; Floquet
1. Introduction The synthesis and study of magnetic properties in di- and multinuclear metal complexes have been an active field of research in the past few years (Decurtin et al., 1994; Min et al., 2005). The multinuclear complexes could be synthesized using bridging ligand such as the oxalate anion. The oxalate (ox = C2O42-) ion can act as a mediator for magnetic exchange interaction between the transition-metal centers in multidimensional networks. Tremendous progress in molecule-based magnets has been shown in oxalate multinuclear complexes since the discovery of new polymeric bimetallic phases of variable dimensionality (Tamaki et al., 1992). Two-dimensional bimetallic phases {[A][MM’(ox)3]}n, A = [N(n-C3H7)4], [N(n-C4H9)4], [N(n-C5H9)4], [P(n-C4H9)4], [P(n-C6H5)4], M = Mn, Fe, Cr, Ni, Cu; M’ = Cr, Fe have attracted a lot of attentions due to their ferro- (Mathonière et al., 1996), ferri- (Pellaux et al., 1997) or canted antiferromagnets (Decurtins et al., 1996) with critical temperature ranging from 5 up to 44 K. 95
Jahro et al., Synthesis and Magnetic Properties of a Fe-Mn-Cr Multinuclear 96
et al., 2003). The spin crossover from one spin state to the other in Fe(II) (3d6) complexes is accompanied by a modification of the magnetic behavior from the diamagnetic (S = 0) to the paramagnetic (S = 2) state. The series of Fe(II) complex compounds of general formula [Fe(NH2trz)3]X2.XH2O show thermal spin crossover. Variation of the non-coordinated anion (X) in [Fe(NH2trz)3]X2.nH2O leads to compounds with signifcantly different spin crossover characteristic (van Koningsbruggen et al., 1997). It has been reported that the pure [Fe(NH2trz)3][ClO4]2 material shows a smooth transition around T1/2 = 130 K (Kröber et al., 1993). Recently, Sieber et al. (2000) found a difference in magnetic properties for [Co(bpy)3]2+ entities embedded in various [MCr(ox)3]n2n- networks, where M(I) is either sodium or lithium ions. In [Co(bpy)3][NaCr (ox)3], the Co(II) complex is in a high spin ground state, while in [Co(bpy)3][LiCr(ox)3], the Co(II) exhibits a thermal spin crossover, with a transition temperature of 161 K (Sieber et al., 2000). Our group has explored the incorporation of potential Fe(II) spin crossover chromophores in a related bimetallic oxalate framework. In this paper, a multinuclear Fe-Mn-Cr complex containing 4-amino1,2,4-triazole (NH2trz) and oxalate is reported. The synthesis, characterization and magnetic properties of [Fe(NH2trz)3][ClO4] [MnCr(ox)3].4H2O compound are described. 2. Method The research included preliminary work on the solubility of the starting materials, synthesis, elemental analysis, IR spectroscopic characterization and magnetic measurements of the complex compound. 2.1 Materials Mn(NO3)2.4H2O, C2H4N4 (4-amino-1,2,4-triazole = NH2trz), methanol, and distilled water were used as obtained without further purification. The K3[Cr (ox)3].3H2O and [Fe(NH2trz)3](ClO4)2 have been prepared based on a published paper (Jahro, 2005). 2.2 Instruments The instruments used were atomic absorption spectrophotometer (AAS) Shimadzu AA8801S, infrared spectrophotometer Shimadzu FTIR-8400 and X-ray Diffractometer Philips PW 1835. The C, H, N, elemental analyses by CHNSO Mikrounsure Analyzer model Fison EA 1108 have been done in Universiti Kebangsaan Malaysia (UKM). Variable-temperature magnetic susceptibility measurements were performed on a Quantum Design magnetometer with a superconducting quantum interference device. The field was kept constant at 0.1 T, while the temperature was varied from 5–340 K.
2.3 Synthesis of the Fe-Mn-Cr Multinuclear Complex A solution of [Fe(NH2trz)3](ClO4)2 (0.015 mmol) in aqueous methanol 70% (15 mL) was added to a mixed solution containing K3[Cr(C2O4)3]3H2O (0.015 mmol) and Mn(NO3)2.4H2O (0.015 mmol) in aqueous methanol 30% (9 mL). The mixture was stirred at room temperature for 12 hours. The purple-grayish precipitate was filtered off and dried over P4O10. 3. Results and Discussion 3.1 Synthesis The [Fe(NH2trz)3][ClO4] [MnCr(ox)3].4H2O compound was isolated from aqueous methanol with 48% yield. The synthesis method had been chosen based on the solubility of [Fe(NH2trz)3]2+ which was mainly soluble in methanol, while the [MnCr(ox)3]complex was soluble in water. The Fe-Mn-Cr complex was obtained as powder material with purple-grayish colour. It was hardly soluble in water and organic solvents like acetone, diethyl ether, methanol, ethanol, acetonitrile, dimethylformamide (DMF) and dimethylsulfoxide (DMSO). The formula of this compound has been obtained based on the metal ions and the C, H, N analyses. The elemental contents are listed in Table 1. Table 1. The metal ion and C, H, N contents of [Fe(NH2trz)3][ClO4][MnCr(ox)3].4H2O The elemental contents (%) Fe
Mn
6.26 (6.57)
5.98 (6.46)
Cr
C
H
N
7.04 17.72 2.86 19.30 (6.11) (16.95) (2.37) (19.76)
(Calculated values) Attempts to obtain single crystals suitable for single-crystal X-ray diffraction analysis so far had been unsuccessful. The compound was obtained as a powder form material. The material was characterized by X-ray powder diffraction profile and indexed in P6/MMM to derive unit cell parameters by Le Bail refinement in Rietica program. This compound crystallized in the hexagonal system with the cell parameters a = b = 18.695 Å and c = 57.351 Å. The structure of this compound resembled that found by X-ray single-crystal diffraction for [A][MnCr(ox)3] with A = [P(C6H5)4]+ (Decurtin et al., 1994). The structure consisted of two-dimensional bimetallic network. This network was formed by bisbidentate oxalate ligands connecting Mn(II) and Cr(III) ions in such a way that each Mn(II) ion was surrounded by three Cr(III) ions and vice versa, leading to polymeric nets with all metal ions maintaining the different chirality. The anionic network sublattices left
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some holes where the cationic Fe(II) complex and perchlorate counterion were located. The presence of the various molecular constituents could be confirmed by its infrared (IR) spectrum.
The spectrum of this complex was confined to the most important vibrations in the 350–4000 cm-1 region as shown in Figure 1.
3.2 Magnetic property The spin crossover behaviour of [Fe(NH2trz)3] (ClO4)2 is normally observed at lower than room temperature. However, when this compound was combined with the [MnCr(ox)3]n- complex, the spin transition behaviour shifted to higher temperature. The magnetic properties of the [Fe(NH2trz)3][ClO4] [MnCr(ox)3].4H2O compound had been measured between 5–340K, and the plot of the product of the magnetic susceptibility and the temperature, as well as the magnetic moments vs the temperatures are shown in Figure 2.
µ eff (BM)
From the IR spectrum, the characteristic absorptions of the oxalato-bridged group, NH2trz ligands and O–M–O bonding (M = Cr, Mn) had been observed. For the oxalate-bridged group, the wide νas O–C band at 1624 cm-1, a sharp νs O–C located at 1384 cm-1 and a doublet δ O–C band at 800 cm-1 had been obtained. The peak at 414 cm-1 clearly supports O–M– O (M = Cr, Mn) bonds. For the NH2trz ligand, the wide band in the 3433 cm-1 was characteristic of – N–H aromatic ring, the characteristic of – C = N – aromatic absorbed in the range 1650–1550 cm-1, the wide band at 1624 cm-1 was due to this. The wide band at 3413 cm-1 also suggested that water molecules were present in this complex.
χMT (cm3ml-1K)
Figure 1. The infrared spectrum of [Fe(NH2trz)3][ClO4] [MnCr(ox)3].4H2O.
T (K) Figure 2. Plot of the temperature dependence of the product of the magnetic susceptibility and the temperature ( = χMT vs T) and the magnetic moment (● = µeff vs T). There were three potentially paramagnetic transition metal ions present in the [Fe(NH2trz)3] [ClO4][MnCr(ox)3].4H2O complex i.e. the Cr(III), Mn(II) and Fe(II) ions. The spin value (S) of Cr(III) was 3/2, Mn(II) was 5/2 in high spin state and Fe(II) was 0 in low-spin (LS) and 2 in high-spin (HS) state. The magnetic data were expected to be a superposition of those for the paramagnetic Mn-Cr oxalate network and the Fe(II) fragment. The antiferromagnetic interaction between Cr(III) and Mn(II) in the
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1/χM (cm3 mol)
χM (cm3ml-1)
[MnCr(C2O4)3]-n network yielded S = 1, resulting in a χMT value equal to 1 cm3mol-1 K, and µeff = 2.83 BM. From the data at 5 K, the χMT value was observed to be 2.96 cm3 mol-1K. As the temperature increased the value of χMT reached 3.25 cm3mol-1K at 25 K and formed a plateau up to 226 K. The χMT value suggested a superposition of the magnetic contribution of the Mn-Cr oxalate framework together with a fraction of almost 50 % of the Fe(II) ions being in the high-spin state. From 226 K the value of χMT was markedly increased, which was the sign for the occurence of an Fe(II) spin crossover. The maximum value of 4.51 cm3 mol-1 K observed at 337 K was lower than the χMT value of 6.01 cm3mol-1K which would be expected for a system in which all Fe(II) ions in the high-spin state. From the data, it was evident that the spin crossover of Fe(II) was not complete. The unpaired spins on neighboring Cr(III) and Mn(II) ions in [MnCr(C2O4)3]n- networks to be coupled with each other was mediated by the oxalate ion as a bridging ligand, a phenomenon referred as magnetic exchange. The magnetic exchange tended to an antiparallel arrangement of the coupled spins which was called antiferromagnetism. The indication of an antiferromagnetic interaction between Cr(III) and Mn(II) in [Fe(NH2trz)3] [ClO4][MnCr(ox)3].4H2O was clearly shown by the plot of magnetic susceptibilittes (χM) and its reciprocal equivalent (1/χM) versus temperature (T) as given in Figure 3. The 1/χM curve was fitted with a straight line and from this the Curie constant (C) and the CurieWeiss constant (θ) were obtained using the equation χM = C/(T – θ). Fitting the curve in the temperature range 5–245 K resulted in a Weiss constant (θ) of –2.3 K. This negative value indicated an antiferromagnetic interaction between the Cr(III) and Mn(II) ions.
Temperature (K) Figure 3. Plot of the temperature (T) dependence of the reciprocal magnetic susceptibilities and magnetic susceptibilities (• = χM Vs T, = χM Vs T).
4. Conclusion The multinuclear complex of formula [Fe(NH2 rz)3]ClO4][MnCr(ox)3].4H2O had been synthesized. The molecular composition had been supported by metal ion contents, the elemental C, H, N data and the IR spectra in 350–4000 cm-1 range. The temperature dependence of the magnetic (χMT) data recorded in the temperature range 5–340K indicated a characteristic of thermal spin crossover of [Fe(NH2trz)3]2+ and an antiferromagnetic interaction between neighboring Cr(III) and Mn(II) ions in [MnCr(C2O4)3]n- networks. Acknowledgement This research was funded by Direktorat Jendral Pendidikan Tinggi, Departemen Pendidikan Nasional Indonesia, Research Grant No. 322/SP3/PP/ DP2M/II/2006. The authors are thankful to Prof. Ibrahim Baba (UKM) for C, H, N elemental analysis. References Coronado, E., J. R. Galán-Mascarós, C. J. GómezGarcia, and J. M. Martinez-Agudo, 2001, Molecule-based Magnets Formed by Bimetallic Three-dimensional Oxalate Networks and Chiral Tris(bipyridyl) Complex Cations. The Series [ZII(bpy)3][ClO4][MIICrIII(ox)3] (ZII = Ru, Fe, Co, and Ni; MII = Mn, Fe, Co, Ni, Cu, and Zn; ox = Oxalate Dianion), Inorg. Chem., 40, 113-120. Decurtin, S., P. Gütlich, K. M. Hasselbach, A. Hauser, and H. Spiering, 1985, Light-induced Excitedspin-state Trapping in Iron(II) Spin-crossover Systems. Optical Spectroscopic and Magnetic Susceptibility Study, Inorg. Chem., 24, 21742178. Decurtin, S., H. W. Schemalle, and H. R. Oswald, 1994, A Polymeric Two-dimensional Mixedmetal Network. Crystal Structure and Magnetic Properties of {[P(Ph)4][MnCr (ox)3]}n, Inorg. Chem. Acta, 216, 65-73. Decurtin, S., W. H. Schmalle, R. Pellaux, P. Schneuwly, and A. Hauser, 1996, Chiral, Three-dimensional Supramolecular Compounds: Homo-and Bimetallic Oxalate and 1,2Dithiooxalate-bridged Networks. A Struc-tural and Photophysical Study, Inorg. Chem., 35, 1451-1460. Decurtin, S., H. W. Schemalle, P. Schneuwly, J. Ensling, and P. Gütlich, 1994, A Concept for the Synthesis of 3-Dimensional Homo- and Bimetallic Oxalate-bridged Networks [M2(ox) 3]n. Structural, Mössbauer, and Mag-netic Studies in the Field of Molecular-based Magnets, J. Am. Chem. Soc., 116, 9521-9528.
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