Polyhedron 27 (2008) 2751–2756

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Polyhedron

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Mononuclear and binuclear iron(III) complexes incorporating N4O3

coordinating heptadentate ligand: Synthesis, structure and magnetic properties

Reena Singh a, Coen de Graaf b, Enrique Colacio c, Kajal Krishna Rajak a,*

a Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Raja S. C. Mullick Road, Kolkata, West Bengal 700 032, Indiab Department de Química Física i Inorgànica, Universitat Rovira i Virgili, Maecel-lí Domingo s/n, 43007 Tarrangona, Spainc Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain

a r t i c l e i n f o a b s t r a c t

III

Article history:Received 17 April 2008Accepted 14 May 2008Available online 9 July 2008

Keywords:Iron(III)Heptadentate ligandStructureMagnetic properties

0277-5387/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.poly.2008.05.035

* Corresponding author.E-mail addresses: kajalrajak@hotmail.com, kkraja

Rajak).

The N4O3 coordinating heptadentate ligand afforded the mononuclear [Fe (HL)][BPh4] (1) and binuclear[Fe2

IIIL(OAc)2][BPh4] (2) complexes. In complex 1, the ligand binds in a trianionic N2O3 fashion whereas inthe case of 2 the ligand binds in the trianionic N4O3 form in which the iron ions are held together by l-phenoxo and bis l-acetato bridges. In 1, the Fe(III) center has a trigonal bipyramidal geometry (s = 0.84)whereas in 2 both the Fe(III) centers have a distorted octahedral geometry. Complex 2 shows an intramo-lecular weak antiferromagnetic interaction. Gas phase geometry optimizations have been performedusing density functional theory without any symmetry constraints. The gas phase optimized structuresagree well with the X-ray structure.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Enzymes bearing phenol and/or carboxylate coordinated non-heme iron active sites play an important role in different biologicalprocesses [1]. It has been documented that in certain proteins [2]the function and stabilization of the geometry of the iron activesite is governed by the interaction of the phenol functional groupof the amino acid tyrosine. Moreover, structural studies of someisolated enzymes [3] containing binuclear active sites reveal thatin the coordination environment, the iron centers are connectedby an oxygen containing ligand (e.g. O2

2�, OH� or H2O) and/orthe carboxylate group from a glutamate or aspartate residue.Therefore the study of the chemistry of iron(III)-phenolato as wellas carboxylate bridged diiron(III) species using O,N-coordinatingpolydentate ligands have attracted increasing attention [1b,4]. Inthis context the chemistry of iron(III) with monochelator conform-ationally labile N4O3 coordinating heptadentate ligands are rare[5]. In addition to the chemistry of biologically relevant polynu-clear complexes of iron(III), the study of the variable temperaturemagnetic moment is also important to examine the type of mag-netic interaction involved in such species [6]. The interesting prop-erties exhibited by iron-phenolato and carboxylate bridgedbinuclear Fe(III) complexes provide an opportunity to study thechemistry of mononuclear and binuclear iron complexes using aconformationally labile N4O3 coordinating heptadentate ligand.

ll rights reserved.

k@chemistry.jdvu.ac.in (K.K.

In this paper, we describe the synthesis of Fe(III) mononuclearand l-phenoxo and l-acetate bridged diferric complexes incorpo-rating a trianionic N4O3 coordinating heptadentate ligand. Thecomplexes are characterized spectroscopically and their electro-chemical behaviors are also scrutinized. The variable temperaturemagnetic moment for the dinuclear complex is discussed. The X-ray and gas phase optimized structures for the two complexesare reported.

2. Experimental

Materials. All the starting materials were analytically pure andwere used without further purification, and the solvents were puri-fied by standard procedures. The ligand was synthesized using astandard procedure [7].

Caution! Perchlorate salts are highly explosive and should behandled with care and in small amounts.

2.1. Physical measurements

UV–vis spectra were recorded with a Perkin–Elmer LAMBDA 25spectrometer. Electrochemical and IR spectra were measured witha Perkin–Elmer L-0100 spectrometer. Electrochemical measure-ments were performed on acetonitrile solutions of the complexeson a CHI 620A electrochemical analyzer using a platinum elec-trode. Tetraethylammonium perchlorate (TEAP) was used as thesupporting electrolyte and the potentials are referenced to thestandard calomel electrode (SCE) without junction corrections.The electron paramagnetic resonance experiment was performed

Table 1Crystal data and structure refinement parameters for complexes 1 and 2

Complex 1 Complex 2

Formula C55H62BFeN4O3 C60H73BFe2N4O9

Formula weight 893.75 1116.73Crystal system monoclinic triclinicSpace group C12/C1 P�1a (Å) 36.273(7) 10.947(2)b (Å) 10.608(2) 14.656(3)c (Å) 28.959(6) 18.701(4)a (�) 90.00 71.439(4)b (�) 119.08(3) 86.525(4)c (�) 90.00 74.833(4)V (Å3) 9738(3) 2744.5(10)Z 8 2Dcalc (Mg m�3) 1.219 1.351l (mm�1) 0.357 0.590h (�) 1.61 - 25.00 1.52 - 24.00T (K) 293(2) 293(2)R1,a wR2

b [I > 2r(I)] 0.0755, 0.1213 0.1197, 0.2676Goodness-of-fit (GOF) on F2 1.102 1.216

a R1 ¼PjFo j � jFcj=

PjFoj.

b wR2 ¼ ½P

wðF2o � F2

c Þ2=P

wðF2oÞ

2�1=2.

2752 R. Singh et al. / Polyhedron 27 (2008) 2751–2756

on a Varian E-112 (X band, 9.1 GHz) spectrometer at a microwavepower of 1 mW and a modulation amplitude of 0.5 gauss, and thespectra were collected using a quartz dewar. Diph-enylpicrylhydrazyl (dpph, g = 2.0037) was used to calibrate thespectra. Magnetic measurements were carried out on polycrystal-line samples with a Quantum Design MPMS XL SQUID susceptom-eter operating at a magnetic field of 0.1 T between 2 and 300 K. Thediamagnetic corrections were evaluated from Pascal’s constants.Elemental analyses (C, H, N) were performed on a Perkin–Elmer2400 Series II Elemental Analyzer.

2.2. Synthesis of the complexes

[FeIII(HL)][BPh4] (1). H3L (129 mg, 0.25 mmol) was added to asolution of FeCl3 � 6H2O (68 mg, 0.25 mmol) in methanol (10 ml)and the resulting violet solution was stirred at room temperaturefor 1 h. Addition of Et3N (0.104 ml, 0.75 mmol) resulted in a deepbrown colored solution which yielded a deep brown crystallineproduct after addition of NaBPh4 (85 mg, 0.25 mmol) on slow evap-oration. The product was collected by filtration and washed withcold water. Deep brown crystals were obtained by layering theproduct with DCM/hexane. Yield: 0.179 g, 80%. Anal. Calc. forC55H62BFeN4O3: C, 73.39; H, 6.92; N, 6.32. Found: C, 73.85; H,6.99; N, 6.26%. UV–Vis (kmax/nm (e/M�1 cm�1) CH3CN solution):500 (5549), 275 (18373). IR (KBr)/cm�1: m(C–O) 1270; m(Fe–O)690. Epa (FeIII/FeII): �0.191 V (irr.).

[FeIII2 L(OAC)2]BPh4 (2). A solution containing H3L (129 mg,

0.25 mmol) and FeCl3 � 6H2O (135 mg, 0.5 mmol) in 15 ml methanolwas stirred at room temperature. After 15 min sodium acetate(123 mg, 1.5 mmol) was added, followed by NaBPh4 (171 mg,0.5 mmol) after an additional 30 min. Stirring was continued for1 h and a deep blue solid started separating; it was evaporated todryness and the resulting solid was washed thoroughly with water.The deep blue solid was dried in vacuum and recrystallized frommethanol. Yield: 0.213 g, 76%. Anal. Calc. for C60H73BFe2N4O9: C,64.51; H, 6.52; N, 5.05. Found: C, 64.45; H, 6.59; N, 5.01%. UV–Vis(kmax/nm (e/M�1 cm�1) CH3CN solution): 545 (59964), 321(18295), 275 (36565). IR (KBr)/cm�1: m(C–O) 1270; m(Fe–O) 694;m(O2CMe) 1564 and 1452. Epa (FeIII/FeII): �0.131 V.

2.3. Crystallographic studies

Single crystals suitable for X-ray crystallographic analysis of thecomplex [FeIII(HL)][BPh4] were obtained by diffusing a dichloro-methane solution of the complex into hexane and that of [FeIII

2 -

L(OAc)2][BPh4] � H2O � CH3OH was obtained from a methanolicsolution by slow evaporation. The X-ray intensity data were col-lected on a Bruker AXS SMART APEX CCD diffractometer (Mo Ka,k = 0.71073 Å) at 293 K. The detector was placed at a distance of6.03 cm from the crystal. A total of 606 frames were collected witha scan width of 0.3� in different settings of u. The data were reducedin SAINTPLUS [8] and an empirical absorption correction was appliedusing the SADABS package [8]. The metal atoms were located by thePatterson Method and the rest of the non-hydrogen atoms emergedfrom successive Fourier synthesis. The structures were refined by afull-matrix least-square procedure on F2. All non-hydrogen atomswere refined anisotropically. All hydrogen atoms were directly lo-cated in different Fourier maps. All calculations were performedusing the SHELXTL V 6.14 program package [9]. Molecular structureplots were drawn using ORTEP [10]. Relevant crystal data are givenin Table 1.

2.4. Computational details

All the calculations were carried out with the Density Func-tional Theory (DFT) [11] method and the calculations have been

performed using the B3LYP exchange correlation function, [12] asimplemented in the GAUSSIAN03 (G03) program package [13]. Thegeometry of complexes 1 and 2 were fully optimized in the gasphase using the 6-31G basis set [14] for the H, C, N, O atoms andthe 6-31+g(d) basis set for iron without any symmetry constrain.

3. Results and discussion

The trianionic N4O3 coordinating ligand [7] has been used forthe present study. The ligand possesses two sets of mono-acidicONN chelating donor sites along with a central phenolate group.The reaction of FeCl3 � 6H2O with H3L in a ratio 2:1 in the presenceof sodium acetate offered a l-phenoxo l-acetato bridged diiron(III)complex of the formula [Fe2

IIIL(OAc)2]+, (2+) in good yields. Inter-estingly, when the same reaction was carried out with a 1:1 me-tal-to-ligand ratio in the presence of excess triethylamine insteadof sodium acetate it yielded mononuclear [FeIII(HL)]+, 1+. The Fe(III)complexes are isolated as salts of the BPh4

� anion.

H3L

N

NMe2

N

NMe2 HO

CH3

OH

OH

The m(C–Ophenolate) vibrations in both cases are observed around1270 cm�1. Complex 2 displays carboxylate stretches near 1564and 1452 cm�1. The splitting between the two peaks is 112 cm�1,which indicates that the carboxylate group bridges the two ironions in a syn–syn fashion [15a].

The UV–vis spectra of the complexes were recorded in acetoni-trile (CH3CN) solution and display two well resolved peaks alongwith a shoulder. The absorption near 500–545 nm is probably dueto a phenolate(axial) ? Fe(III) or phenolate(equatorial) ? Fe(III)LMCT transition [15b,c]. The band near 300 nm is associated withligand p–p* transition.

R. Singh et al. / Polyhedron 27 (2008) 2751–2756 2753

All the complexes are electroactive at a platinum electrodeversus SCE in acetonitrile solution. Complexes 1 and 2 exhibitan anodic response at �0.191 and �0.131 V, respectively. Thevoltammogram response is probably due to Fe(III) ? Fe(II) andFe(III)–Fe(III) ? Fe(III)–Fe(II) reduction couples [16], respectively.

4. Crystal structures

The molecular structures of 1 and 2 � H2O � CH3OH have beendetermined by the X-ray diffraction method. The molecular viewsof the cationic part of the complexes are shown in Figs. 1 and 2,respectively. Selected bond parameters are listed respectively inTables 2 and 3.

Fig. 1. Perspective view and atom-labelling scheme for [FeIIILH+]. All non-hydrogenatoms are represented by their 30% thermal probability ellipsoids.

Fig. 2. Perspective view and atom-labelling scheme for [FeIII2 L(OAC)2]+. All non-

hydrogen atoms are represented by their 30% thermal probability ellipsoids.

[FeIII(HL)][BPh4] (1). The coordination environment around theiron atom in [FeIIILH+] is a distorted trigonal bipyramid (s = 0.84)[17]. The ONN donor atoms along with the central phenolato groupoccupy four coordinating sites in which the terminal phenolate andtripodal nitrogen occupy equatorial positions whereas the centralphenolate and terminal phenolate groups are trans to each otherin the axial sites. The fifth coordination position is occupied bythe terminal phenolato group of another ONN site of H3L and theother two nitrogen atoms remain uncoordinated. It is interestingto note that the pentadentate nitrogen becomes protonated inthe complex. In the lattice, the protonated N–H atom is hydrogenbonded with the coordinated O(1), the O(1)� � �N(4) distance being2.833(6) Å. The involvement in hydrogen bonding of the O(1) atomexplains the large Fe(1)–O(1) bond distance. The Fe–O(phenoxide)bond distances reveal that all three phenol groups are coordinatedto iron(III) as phenoxides. The observed Fe–O bond distances(Fe(1)–O(1), 1.900(3), Fe(2)–O(2), 1.849(3) and Fe(1)–O(3),1.872(3) Å) are shorter than the average octahedral Fe–O(phenox-ide) bond distance of 1.92 Å. Fe–N bond distances are usual andspan the range 2.15–2.31 Å [4b].

[FeIII2L(OAc)2][BPh4] � H2O � CH3OH, 2 � H2O � CH3OH. For thecationic part of the complex, two iron centers are bridged by the

Table 2Selected bond distances (Å) and angles (�) for 1, calculated parameters are given inparentheses

DistancesFe(1)–O(1) 1.900(3) [1.945]Fe(1)–O(2) 1.849(3) [1.890]Fe(1)–O(3) 1.872(3) [1.875]Fe(1)–N(1) 2.316(3) [2.398]Fe(1)–N(2) 2.157(3) [2.173]

AnglesO(1)–Fe(1)–O(2) 116.48(12) [119.9]O(1)–Fe(1)–O(3) 95.66(11)O(2)–Fe(1)–O(3) 102.89(12)O(1)–Fe(1)–N(1) 84.88(11)O(2)–Fe(1)–N(1) 88.46(11)O(3)–Fe(1)–N(1) 166.88(13) [164.07]O(1)–Fe(1)–N(2) 129.32(13)O(2)–Fe(1)–N(2) 110.77(13)O(3)–Fe(1)–N(2) 90.72(12)N(1)–Fe(1)–N(2) 78.97(12)

Table 3Selected bond distances (Å) and angles (�) for 2, calculated parameters are given inparentheses

DistancesFe(1)–O(1) 1.874(8) [1.860] Fe(2)–O(2) 2.061(6) [2.052]Fe(1)–O(2) 2.063(7) [2.098] Fe(2)–O(3) 1.864(7) [1.864]Fe(1)–O(4) 2.041(7) [2.082] Fe(2)–O(5) 1.975(7) [1.990]Fe(1)–O(6) 1.981(8) [1.997] Fe(2)–O(7) 2.047(8) [2.118]Fe(1)–N(1) 2.177(8) [2.208] Fe(2)–N(3) 2.175(8) [2.218]Fe(1)–N(2) 2.258(10) [2.272] Fe(2)–N(4) 2.215(8) [2.233]

AnglesO(1)–Fe(1)–O(2) 174.1(3) O(2)–Fe(2)–O(3) 97.0(3)O(1)–Fe(1)–O(4) 88.8(3) O(2)–Fe(2)–O(5) 96.8(3)O(1)–Fe(1)–O(6) 92.1(3) O(2)–Fe(2)–O(7) 83.1(3)O(2)–Fe(1)–O(4) 85.9(3) O(3)–Fe(2)–O(5) 91.5(3)O(4)–Fe(1)–O(6) 98.1(3) O(3)–Fe(2)–O(7) 179.1(3)O(2)–Fe(1)–O(6) 91.2(3) O(5)–Fe(2)–O(7) 89.3(3)O(1)–Fe(1)–N(1) 88.5(3) O(2)–Fe(2)–N(3) 88.9(3)O(2)–Fe(1)–N(1) 89.2(3) O(3)–Fe(2)–N(3) 90.0(3)O(4)–Fe(1)–N(1) 93.3(3) O(5)–Fe(2)–N(3) 173.8(3)O(6)–Fe(1)–N(1) 168.6(3) O(7)–Fe(2)–N(3) 89.2(3)O(1)–Fe(1)–N(2) 91.7(3) O(2)–Fe(2)–N(4) 164.6(3)O(2)–Fe(1)–N(2) 93.5(3) O(3)–Fe(2)–N(4) 95.5(3)O(4)–Fe(1)–N(2) 174.9(3) O(5)–Fe(2)–N(4) 91.9(3)O(6)–Fe(1)–N(2) 87.0(4) O(7)–Fe(2)–N(4) 84.3(3)N(1)–Fe(1)–N(2) 81.7(3) N(3)–Fe(2)–N(4) 82.0(3)Fe(1)– O(2)– Fe(2) 115.81(16)

2754 R. Singh et al. / Polyhedron 27 (2008) 2751–2756

central phenolate (O(2)) of the heptadentate ligand and two biden-tate syn–syn acetato groups, leading to a triply bridged dinuclearspecies with Fe(1)–Fe(2) = 3.496(9) Å. The remaining coordinatingsites of each iron are occupied by the ONN donor atoms of the li-gand. The positions of the donor atoms in both iron centers arenot identical. The bridging phenolato (O(2)) group in the coordina-tion sphere of Fe(1) is trans to a terminal phenoxo group while thatin Fe(2) is trans to the pentadentate nitrogen atom (N(4)). TheFe(1)–O(2)–Fe(2) bridging angle is 115.81(16)�. The Fe–O(terminalphenolato) span the range 1.864–1.874 Å. The Fe–O(bridging), Fe–N and Fe–O(acetate) distances are normal [5].

5. Optimized geometry

The geometry optimization for the cationic part of complexes 1and 2 is performed in the gas phase with the complexes in their

Fig. 3. Optimized molecular structure of (a) [FeIIILH+] and (b) [FeIII2 L(OAC)2]+ (Fe:

Cyan, N: Blue, O: Red, C: Grey). Hydrogen atoms are excluded for clarity. (Forinterpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)

highest spin state. The geometries are optimized using the crystalcoordinates as the initial coordinates. The gas phase optimizedstructures in both cases are shown in Figs. 3a and b, respectively.The crystal structural parameters of 1+ and 2+ (Tables 1 and 2)are in excellent agreement with the optimized parameters.

6. EPR spectra

Complex 2 is EPR silent, both in the solid state and in solution.However, in frozen solution complex 1 exhibits an EPR signal nearg = 4.2 (Fig. 4a). The spectral feature is associated with a rhombic-ally distorted high-spin iron(III) complex [15c,18].

7. Magnetic studies

Variable temperature magnetic susceptibility measurementswere performed in the range 2–300 K. The plot of vMT versus Tand an inset plot of vM versus T are shown in Fig. 4b. The effectivemagnetic moment at 300 K is 7.49 emu cm�1 K and this decreasesrapidly to 0.62 emu cm�1 K at 2 K, which is a characteristic featureof weak antiferromagnetic interactions. The spin Hamiltonian usedto simulate the vMT versus T curve is

H ¼ �JS1S2

where S1 = S2 = 5/2. The best fit of the vMT versus T curve yieldedJ = �14.4(4) cm�1, g = 2.04(2) and q = 0.06(1).

The weak antiferromagnetic interaction is a characteristic fea-ture of diiron(III) complexes interacting via (l-O) bis(l-acetato)/(l-OH) bis(l-acetato). It has been documented in the literaturethat the exchange coupling between two iron(III) centers in sucha class of complexes take place through Fe(III)–(l-O)–Fe(III)/Fe(III)–(l-OH)–Fe(III) bonds and the strength of the exchange pri-marily depends on the Fe–O(bridge) bond distance; whereas theFe(III)–O(bridge)–Fe(III) angle has a secondary role [6b]. In thepresent case, the weak antiferromagnetic coupling is a directconsequence of the large Fe–O(bridging) distance, 2.061 Å,and the exchange coupling is comparable with other reported(l-Ophenoxo)bis(l-acetato) complexes [5].

CASSCF and CASPT2 [19] calculations have been performed for atheoretical investigation of the exchange coupling of complex 2using 6-31G and ANO-DZP [20] basis sets. The calculated J valuesare listed in Table 4. For the CASPT2 calculation the model struc-ture 3 has been used, keeping the coordination environment

Fig. 4a. X-band EPR spectra for 2 in frozen solution (CH2Cl2/Toluene) at 77 K.Instrument settings: microwave frequency, 9.1 GHz; microwave power, 1 mW;modulation frequency,100 kHz; modulation amplitude, 0.5 G; sweep center,2100 G.

Table 4Calculated J values (in cm�1) for complex 2 and model 3

Basis Molecule CASSCF CASPT2

6-31G 2 �3.33 �3.0 �21.7

ANO-DZP 3 �2.8 �20.9

Fig. 4b. vMT vs. T for 2. Solid line shows the best fit obtained. Inset plot of vM vs. T.

R. Singh et al. / Polyhedron 27 (2008) 2751–2756 2755

around the iron centers unchanged. It has been documented thatthis has little or no influence on the magnetic coupling as long asthe geometry of the core of the molecule and the nature of theatoms directly coordinated to the metal ions remain the same[21]. In the present case the CASSCF results for the real and themodel complex confirm the small effect of the modelling and theresults are almost identical (Table 4). The calculated J value is inagreement with the experimental result.

8. Conclusion

In summary, we have synthesized and characterized mononu-clear and dinuclear iron(III) complexes incorporating a N4O3 coor-dinating heptadentate ligand. Interestingly in the mononuclearcomplex the ligand binds in the N2O3 form. Variable temperaturemagnetic moment measurements have been performed in the caseof the dinuclear species. The optimized geometries, in both cases,are in good agreement with the X-ray structures. The study of dif-ferent Fe(III) species using various O,N-coordinating ligands, alongwith theoretical investigations, are in progress.

Acknowledgements

Financial support from the Council of Scientific and IndustrialResearch, New Delhi, India, the Department of Science and Tech-nology, New Delhi, India and the University Grant Commission,New Delhi is greatly acknowledged. We are also thankful to DSTfor the data collection on the CCD facility setup at the Indian Insti-tute of Science, Bangalore, India under the IRHPA-DST program andthe department of Chemistry, IIT Guwahati. The author alsoacknowledges SAIF, IIT Bombay, India for EPR measurements andlastly the financial support from the Spanish Government (GrantNo. BQU2000/0791).

Appendix A. Supplementary material

CCDC 682156 and 682155 contain the supplementary crystallo-graphic data for 1 and 2. These data can be obtained free of chargevia http://www.ccdc.ac.uk/conts/retrieving.html, or from the Cam-bridge Crystallographic Data Centre, 12 Union Road, CambridgeCB2 1EZ, UK; email: deposit@ccdc.cam.ac.uk.

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