synthesis and characterization of two new oxo-centered trinuclear complexes of manganese and iron

7/15/2019 Synthesis and Characterization of Two New Oxo-centered Trinuclear Complexes of Manganese and Iron

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I nternational Journal of Scientifi c Research i n Knowledge (I JSRK), 1(7), pp. 182-188, 2013 Available online at http://www.ijsrpub.com/ijsrk

ISSN: 2322-4541; ©2013 IJSRPUB

http://dx.doi.org/10.12983/ijsrk-2013-p182-188

182

Full Length Research Paper

Synthesis and Characterization of Two New Oxo-centered Trinuclear Complexes of

Manganese and Iron

Haman Tavakkoli*, Arezoo Ghaemi, Fouad Mohave

Department of Chemistry, Science and Research Branch, Islamic Azad University, Khouzestan, Iran

*Corresponding author: [email protected]

Received 1 May 2013; Accepted 3 June 2013

Abstract. Two new oxo-centered trinuclear complexes, one of them a mixed-valence complex [Mn2MnO(CCl3CO2)6(Py)3] (1)and the other, mixed-metal complex of [Fe2MnO(CCl3CO2)6(H2O)3].NO3 (2) were synthesized by the direct reaction between

metal nitrates and trichloroacetic acid. These complexes were characterized by elemental analyses (CHN), atomic absorption

spectroscopy and spectral (IR, electronic) studies. These are new types of oxo-bridged mixed-metal complexes in which the

carboxylate ligand is trichloroacetic acid. The UV spectra of the complexes exhibited strong bands in the region 213 and 257

nm which are related to the (π → π*) and (n → π*) transitions of the pyridine and H2O ligands, respectively. The IR spectra of

these compounds showed two strong stretching vibrations bands, indicating a bridging coordination mode of the carboxylic

group of the ligand in the complexes.

Keywords: oxo-centered; trinuclear complexes; carboxylic ligand; IR spectra.

1. INTRODUCTION

Transition-metal carboxylate chemistry has played a

key role in the conceptual development of moderninorganic chemistry (Kim and Cho, 2004). The

current interest in the trinuclear, oxo-centered metalcarboxylate assemblies of the general composition[M3O(OOCR)6L3]

+(where M= trivalent 3-d metal,

RCOO=carboxylic acid (R = CH3, Ph, etc) L=monodentate ligand) is due to these complexes haveserved as important models to test theories of magnetic and electronic coupling between metal ions(Beattie et al., 2003 and Gavrilenko et al., 2002). Theyare particularly valuable as frame works for systematically studying metal-metal interactions inclusters and constitute an important class of

compounds in transition metal chemistry. They have been characterized with a wide variety of first-row

and heavier transition metals, with mixed-metal(Blake et al., 1998) and mixed-valency combinations(Wu et al., 1998).

Electron transfer interactions, and subtledistortions from regular structures, can be detected by

their effects on molecular vibrations. These arevaluable precursors for the synthesis of higher nuclearity clusters exhibiting interesting magnetic properties. Surprisingly, little attention has been paidto the consideration of substitution properties, with

only a few reports of comparative studies regardingthe redox properties when the terminal ligand L

changes. There is interest in establishing howsubstitution in the Carboxylate Bridge affects theliability of the terminal ligand and the redox potential

of the metal centers. These carboxylate complexes areof additional interest when the carboxylate isunsaturated, because there is a potential scope for

further polymerization in the solid state by cross-linking of the substituents (Chavan et al., 2001 and

Maragh et al., 2005). Trinuclear μ3,-oxo-bridged ironcarboxylate complexes have been studied in greatdetail (Sorai et al., 1986 and Woehler et al., 1986),whereas analogous mixed-valence manganese-carboxylate complexes have been the object of onlylimited investigation to date. Each carboxylate anionspans two metal centers at the periphery of the[M3(μ3-O)]

6+core, while the neutral monodentate

ligands occupy the remaining coordination sites oneach metal center, and as a result the coordinationaround the metal center is approximately octahedral

(Fig.1)In previous studies, we reported fabrication of

some trinuclear oxo-centered complexes withsaturated and unsaturated carboxylate ligands(Yazdanbakhsh et al., 2010 and Tavakkoli et al., 2011

and Yazdanbakhsh et al., 2009). In this paper, thesyntheses and characterization of mixed-valence and

mixed-metal clusters are reported. These are,however, limited to clusters coordinated by relativelysmall organic ligands.



Tavakkoli et al.

Synthesis and Characterization of Two New Oxo-centered Trinuclear Complexes of Manganese and Iron

183

2. EXPERIMENTAL

2.1. Materials

All starting materials, except (N-n-Bu4MnO4), used inthis study were analytical grade and purchased fromMerck.

Fig. 1: Schematic representation of the oxo-centered trinuclear clusters structure [M3O(RCOO)6(L)3]z

N-n-Bu4MnO4; This material was prepared, asoutlined in the literature (Yazdanbakhsh et al., 2007), by mixing aqueous solutions of KMnO4, (5.00 g, 31.6

mmol) and N-n-Bu4Br (12.00 g, 37.2 mmol) with

vigorous stirring to give a total volume of ca. 200 ml.The immediate purple precipitate was collected byfiltration, washed thoroughly with distilled water anddiethyl ether and dried in vacuum at ambienttemperature: yield > 90%.The C, H and N analyses were realized on a Thermo

Finnigan Flash model EA1112 elemental analyzer.The atomic absorption analyses were performed on aShimadzu model AA-670 atomic absorptionspectrometer. The IR spectra of KBr discs (600 – 4000cm

-1) were recorded on a Buck 500 spectrometer. The

electronic spectra were registered in the range of 200 –

600 nm on a Perkin-Elmer 1600 spectrometer.

Preparation of [Mn2MnO(CCl3CO2)6(Py)3]

N-n-Bu4Br (12 g, 37.2 mmol) was added to anaqueous solution of KMnO4 (5 g, 31.6 mmol) under vigorous stirring. The immediately formed purple

precipitate was collected by filtration, washedthoroughly with distilled water and diethyl ether anddried in vacuo at room temperature. ThenMn(OOCMe)2.4H2O (10 mmol, 2.5 g) andtrichloroacetic acid (54 mmol, 8.83 g) were dissolved

in a solvent mixture comprising pyridine (7 ml) andabsolute ethanol (15 ml). The resulting solution was

stirred while solid N-n-Bu4MnO4 (3.5 mmol, 1.25 g)was added in small portions and stirred to give a dark

brown homogeneous solution. This solution wasallowed to stand undisturbed for 48 h and the resultinglarge octahedral-shaped crystals were collected by

filtration, washed with pyridine and dried in vacuum.

(Yield: 72%), m.p.: 185

˚

C, Anal. Calc. for C27H15Cl18Mn3 N3O13: C, 23.26; H, 1.07; N, 3.01; Mn,11.84%. Found: C, 22.96; H, 1.15; N, 3.11; Mn,11.65%.

Preparation of [Fe2MnO(CCl3CO2)6(H2O)3].NO3

A mixture of Fe(NO3)3.9H2O (1.08 g, 2.68 mmol) andMn(NO3)2.4H2O (0.34 g, 1.34 mmol) was dissolved in25 ml deionezed water and was refluxed for 10 min.Then, NaCCl3CO2 (2.22 g, 12 mmol) was added andthe reflux continued for 5 h. The resulting brown

solution was allowed to cool and stored for 2 days at20

°C. The black crystals were filtered off, washed

copiously with Et2O and dried in vacuum. (Yield:85%), m.p.: 275

˚C, Anal. Calc. for

C12H6Cl18Fe2MnNO22: C, 10.89; H, 0.45; N, 1.06; Fe,8.47; Mn, 4.16%. Found: C, 10.64; H, 0.62; N, 1.14;Fe, 8.85; Mn, 4.39%.

3. RESULTS AND DISCUSSION

3.1. IR Spectroscopy

The IR spectra of these complexs are shown in Fig. 2that indicate the presence of carboxylate, H2O and

{M2MO} groups. The observed vibrationalfrequencies υasym(COO) and υsym(COO) for the



International Journal of Scientific Research in Knowledge (IJSRK), 1(7), pp. 182-188, 2013

184

carboxylate ligand support the presence of bridgingcoordinated carboxylates in all the complexes. For themixed-metal complexes, it appears from thecarboxylate stretching frequencies that all six ligandsare approximately equivalent and they are best

represented as bidentate bridges. For a new series of trinuclear mixed metal complexes, Cannon assignedthe IR spectra and identified the vibrational modes of the central M3O core (Baranwal et al., 2009). Hefound that the reduction in site geometry from D3h to

C2v lifted the degeneracy of the asymmetric M3Ostretches and two bands were seen. These spectra

indicate a single pair of carboxylate stretchingvibrations at 1601 and 1425 cm-1 for (1), 1615 and1430 cm

-1 for (2) assigned to υasym(COO) and

υsym(COO), respectively. The difference (Δυ =

υasym(COO) –υsym(COO)) is 176 and 185 cm – 1

. In theIR spectrum of complex 2, the characteristic vibrationfrequencies for H2O groups appear at ~3500 – 3600 cm-

1. For identification of the metal-oxygen bonds of

M3O group, IR spectra in the range of 800-400 cm-1

were used (Boudalis et al., 2002). The band observedfor asymmetric vibration associated with the M2M'Ounit splits into two components, A1 and B2 (Cannonand White, 1988) .These spectra show thecharacteristic bands for the valence oscillations

υas(Fe2MnO) in the region 565 cm-1

(A1) and 420 cm-1

(B2) and υas(Mn3O) is observed at 580 cm

-1. All data

of IR spectroscopy for these compounds are given inTable 1.

Table 1: Selected IR bands (cm-1

) for complexes

Compound asym (COO) sym (COO) (M3O) (C-H) (C-Cl) (O-H)

1 1601 1425 580 3036 824 ─

2 1615 1430 420-565 2970 816 3570

Fig. 2: IR spectra of [Mn2Mn(O)(CCl3COO)6(py)3] (a) and [Fe2Mn(O)(CCl3COO)6(H2O)3].NO3 (b) (KBr pellet)



Tavakkoli et al.


185

3.2. Electronic spectroscopy

The electronic spectra of the trinuclear complexes can be interpreted to a good approximation in terms of theindividual metal ions, together with ligand-metal

charge transfer transitions. The electronic spectra of the complexes were recorded in the range of 200-600nm in dichloromethane solution. The spectra of theoxo-centered complexes 1 and 2 show thecharacteristic bands provided by both metal ions. The

UV spectra of the complex 1 and 2 (Fig.3) exhibited a

strong bands in the region 257 and 213 nm which isrelated to the (π → π*) transitions of the pyridine (Py)ligand (Chen et al., 2005) and (n → π*) transition of the water ligand.

The Vis spectra for complex 1 and 2 are shown in

Fig. 4. As is obvious in this Figure, broad band in theregion 480 nm, which can be assigned to the transitionfrom 5Eg to 5T2g, should be attributed to the existenceof Mn

(III)(d

4) ion in these complexes. Obtained data

can be assigned and characterized based on other

literature (Mizoguchi et al., 1999).

Fig. 3: UV spectra of the [Mn2MnO(CCl3CO2)6(Py)3] (a) and [Fe2MnO(CCl3CO2)6(H2O)3].NO3 (b)

4. CONCLUSION

Two new Oxo-centered trinuclear complexes with thegeneral formula [M2M'O(CCl3COO)6(L)3] where M =

Mn, M' = Mn, L = py (1); M = Fe, M' = Mn, L = H2O(2) were prepared and studied by elemental analysis(CHN), electronic and infrared spectroscopy and

atomic absorption spectroscopy. All compounds havea similar μ3-oxo structure. The IR investigations of

these compounds show intensive absorption bands,which are assigned to υasym(COO) and υsym(COO)

vibrations. Furthermore, the three metal ions are bound to a central oxygen atom and adjacent metal

ions are bridged by two carboxylate ligands. Inaddition, for mixed metal complex 2, the atomicabsorption data show a statistical 2:1 disorder of iron

and manganese atoms, respectively.




186

Fig. 4: Vis spectra of the [Mn2MnO(CCl3CO2)6(Py)3] (a) and [Fe2MnO(CCl3CO2)6(H2O)3].NO3 (b)

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Dr. Haman Tavakkoli is an assistant professor in inorganic chemistry in Islamic Azad University,

Khouzestan Science and Research Branch, Iran. He received his M.Sc and Ph.D degree in inorganic

chemistry from Ferdowsi University of Mashhad, Iran in 2007 and 2012, respectively. He has published14 refereed articles in valid ISI journals and 10 scientific articles in conference proceedings. Dr.

Tavakkoli’s fields of expertise are in coordination chemistry, nanomaterials chemistry and physics and

environmental chemistry.

Dr. Arezoo Ghaemi obtained her Bachelor’s degree from the Ferdowsi University of Mashhad, Iran in

chemistry in 2004. She later bagged her Master’s and Doctorate degrees in Analytical chemistry from

Ferdowsi University of Mashhad, Iran in 2006 and 2012, respectively. She has graduated with first

grade in Ph.D degree. At present, Dr. Ghaemi is an assistant professor in analytical chemistry in Islamic

Azad University, Khuzestan Science and Research Branch, Iran. She has published numerous articles in

ISI journals and conference proceedings.

Fouad Mohave is a MSc student in Organic Chemistry Science and Research Branch, Islamic Azad

University, Khouzestan, Iran.

synthesis and characterization of two new oxo-centered trinuclear complexes of manganese and iron

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