Inorganica Chimica Acta 405 (2013) 309–317

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Inorganica Chimica Acta

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Models for enzyme–substrate adduct of non-heme iron-containingenzymes, synthesis and characterization

0020-1693/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ica.2013.06.023

⇑ Corresponding author. Tel.: +98 241 4153200; fax: +98 241 4153232.E-mail address: safaei@iasbs.ac.ir (E. Safaei).

Touraj Karimpour a, Elham Safaei a,⇑, Andrzej Wojtczak b, Zvonko Jaglicic c

a Institute for Advanced Studies in Basic Sciences (IASBS), 45195 Zanjan, Iranb Nicolaus Copernicus University, Department of Chemistry, 87-100 Torun, Polandc Institute of Mathematics, Physics and Mechanics & Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia

a r t i c l e i n f o

Article history:Received 9 April 2013Received in revised form 11 June 2013Accepted 12 June 2013Available online 22 June 2013

Keywords:Iron model complexCatechol dioxygenasesEnzyme–substrate adduct

a b s t r a c t

Mononuclear iron(III) complexes of (HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)] and (HNEt3)[FeLNEM

(TBC)] as new models for enzyme–substrate adducts of catechol dioxygenases have been prepared andcharacterized, where H2LNEC, H2LNEB and H2LNEM are amine bis(phenolate) ligands with chlorine, bromineand methyl substituted phenol groups and TBC is a tetrabromo catecholate dianion. These complexeswere characterized by IR, UV–vis, elemental analysis and magnetic measurements. X-ray structure anal-ysis of complexes has revealed that the iron(III) core in the model compounds is a distorted octahedralcoordination sphere and surrounded by two phenolate oxygen atoms, two amine nitrogens and twooxygen atoms of TBC. UV–vis spectroscopy exhibits two intense absorption bands in the range of450–650 nm in dichloromethane. These bands are assigned to the catecholate and ligand to Fe(III)charge-transfer bands. The electrochemical features of the adduct complexes were investigated indichloromethane solution by employing cyclic voltammetry (CV).

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

In the aerobic catabolism of catechol to aliphatic acids and otherdihydroxy aromatics, by catechol dioxygenases, oxygen activatingis a key step. These non-heme enzymes which catalyze the biodeg-radation process of environmental pollutants are a class of bacterialiron-containing enzymes found in a diverse range of soil bacteria[1–11]. Based on their cleavage mechanisms, they are categorizedinto two distinctive types: The intradiol-cleaving enzymes containhigh-spin iron(III) center whereas the extradiol-cleaving enzymeswhich are characterized by an iron(II) active site [12–15]. The for-mer cleaves C–C bond between two hydroxyl groups while the latercleaves the adjacent C–C bond (Scheme 1).

Biomimetic models can provide valuable information to betterunderstand the mentioned biodegradation mechanisms. On theother hand, mimicking the process provides scientists with valu-able tools to synthesize biomaterials. In recent years, a numberof studies on the enzyme–substrate analogue complexes with dif-ferent structural features by various ligands (for example, pheno-late, carboxylate, pyridine) have been reported. The structuralcharacterization of model complexes and their dioxygen reactivityhave been described. Electronic and steric effects of the ligands onthe Lewis acidity of the iron(III) center, consequently on the reac-tivity of model complex have been studied [16–26].

Here, we report a series of novel mononuclear iron(III) com-plexes with amine bis(phenolate) ligands (Scheme 2) as modelsfor the enzyme–substrate adduct of the catechol dioxygenase en-zyme. The structures and purity of complexes have been confirmedby X-ray single crystal analysis and other techniques.

2. Experimental

2.1. Materials and physical measurements

Reagents or analytical grade materials were obtained from com-mercial suppliers and used without further purification, exceptthose for electrochemical measurements. Fourier transform infra-red spectroscopy on KBr pellets was performed on a FT-IR BrukerVector 22 instrument. UV–vis absorbance digitized spectra werecollected using a CARY 100 spectrophotometer. Magnetic suscepti-bility was measured for powder samples of solid material over atemperature range of 2–300 K by using a SQUID susceptometer(Quantum Design MPMS-XL-5) at the constant field of 1000 Oe.

Voltammetric measurements were made with a computer con-trolled electrochemical system (ECO Chemie, Utrecht, The Nether-lands) equipped with a PGSTA 30 model and driven by GPES (ECOChemie). A glassy carbon electrode with a surface area of 0.035 cm2

was used as a working electrode and a platinum wire served as thecounter electrode. The reference electrode was an Ag wire as thequasi reference electrode. Ferrocene was added as an internal

Scheme 1. Scheme of cleavage by intradiol and extradiol dioxygenases [15].

N N

OH HO

R2

R1R1

R2

R2=R1 : Cl, Br, Me

Scheme 2. Amine bis(phenolate) ligand used in this study [27].

310 T. Karimpour et al. / Inorganica Chimica Acta 405 (2013) 309–317

standard after completion of the experiment set, and potentials arereferenced vs. the ferrocenium/ferrocene couple (Fc+/Fc).

The crystals of the (HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)]and (HNEt3)[FeLNEM(TBC)] suitable for the X-ray diffraction exper-iment were grown from the EtOH–CH2Cl2 solutions. The X-ray datawere collected with an Oxford Sapphire CCD diffractometer usingMo Ka radiation k = 0.71073 Å, at 293(2) K, by x-2h method. Allstructures have been solved by direct methods and refined withthe full-matrix least-squares method on F2 with the use of SHELX97[28] program package. The analytical absorption corrections wereapplied (RED171 package of programs [29] Oxford Diffraction,2000). The absolute structure for the complexes was determinedwith the Flack method [30]. No extinction correction was applied.For all structures, positions of hydrogen atom were found from theelectron density maps, and hydrogen atoms were constrained inthe refinement.

2.2. Preparations

2.2.1. Synthesis of (HNEt3)[FeLNEX(TBC)] complexesTo a stirred mixture of H2LNEX (1.00 mmol) and FeCl3 (0.16 g,

1.00 mmol) in ethanol (50 ml), triethylamine (0.20 g, 2.00 mmol)was added under continuous stirring. After 30 min, an ethanolsolution of tetrabromocatechol (0.20 g, 2.00 mmol) and triethyl-amine (0.20 g, 2.00 mmol) was added to the reaction mixture,and the initial blue color changed to red brown. reaction mixturewas then stirred for 3 h at room temperature. Crystals suitablefor X-ray diffraction were obtained by the slow evaporation ofdichloromethane and ethanol mixture.

2.2.1.1. Synthesis of (HNEt3)[FeLNEC(TBC)]. Yield: 0.46 g (78%). Anal.Calc. for C30H34Br4Cl4FeN3O4 (1017.88 g/mol): C, 35.40; H, 3.37;N, 4.13. Found: C, 34.53; H, 3.18; N, 3.92%. IR (KBr, cm�1): 3443,2925, 2342, 1634, 1444, 1314, 1246, 1164, 1028, 930, 862, 811,744, 450. UV–vis in CH2Cl2: kmax, nm (e, M�1 cm�1): 300 (18220),473 (4830) and 604 (3040).

2.2.1.2. Synthesis of (HNEt3)[FeLNEB(TBC)]. Yield: 0.46 g (70%). Anal.Calc. for C30H34Br8FeN3O4 (1195.69 g/mol): C, 30.14; H, 2.87; N,3.51. Found: C, 30.07; H, 2.52; N, 3.36%. IR (KBr, cm�1): 3443,2975, 1630, 1440, 1315, 1249, 1154, 1031, 931, 809, 722, 527.UV–vis in CH2Cl2: kmax, nm (e, M�1 cm�1): 301 (15400), 475(4010) and 601 (2590).

2.2.1.3. Synthesis of (HNEt3)[FeLNEM(TBC)]. Yield: 0.46 g (83%). Anal.Calc. for C34H46Br4FeN3O4 (936.21 g/mol): C, 43.62; H, 4.95; N,

4.49. Found: C, 42.75; H, 4.61; N, 4.20%. IR (KBr, cm�1): 3443,2976, 2709, 1624, 1445, 1312, 1256, 1026, 927, 809, 736, 512.UV–vis in CH2Cl2: kmax, nm (e, M�1 cm�1): 300 (13410), 525 (4620).

2.2.2. Spectrophotometric investigation of FeCl3–H2LNEX complexeswith TBC

In an experiment, 2 ml of FeCl3 (1.0 � 10�4 M) and H2LNEX

(1.0 � 10�4 M) solution in dichloromethane was transferred intoa cuvette. UV–vis spectra were recorded in the range of 200–800 nm about 3 min after each addition of 30 lL of TBC(1.0 � 10�4 M) solution. Changes in the absorbance of complexupon addition of TBC solution were monitored at the LMCT maxi-mum wavelength of complexes.

3. Results and discussion

The reported (HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)] and(HNEt3)[FeLNEM(TBC)] complexes were prepared by the followingprocedure (Scheme 3).

The structures and purity of complexes have been confirmed byX-ray single crystal analysis and IR spectroscopy, as well as the ele-mental analysis. The IR spectrum of the ligand showed the strongand sharp OH stretching band of the phenols around 3400–3500 cm�1 [27]; however, the IR spectrum of the complexes dis-played a declined band centered at 3400–3500 cm�1 relative tothat of ligand, proving the coordination of phenol groups to themetal. It is worth noting that the weak band at 3400–3500 cm�1

of the IR spectra of complexes can be assigned to the trace wateradsorbed on the complexes.

The electronic absorption spectra of complexes have been mea-sured in dichloromethane in the range of 200–800 nm (Fig. 1,Table 1). The visible spectra show that there are three importantabsorption bands for complexes. The absorption band in the higherenergy region (300 and 301 nm) is caused by p ? p⁄ transitionsinvolving the phenolate and catecholate units. In these modelcomplexes there are two electronic absorption bands for ligand-to-metal charge-transfer (LMCT) that are assigned to the catecho-lato-to-metal and phenolate-to-metal LMCT transition. Theabsorption features at 473, 475 and 525 nm for (HNEt3)[FeLNEC

(TBC)], (HNEt3)[FeLNEB(TBC)] and (HNEt3)[FeLNEM(TBC)] respec-tively that are strongly dependent on the Lewis acidity of ferricmetal center, are assigned to phenolate-to-iron charge-transfer(LMCT) transitions. The energies of phenolate ? Fe(III) charge-transfer band show a negative shift (blue shift) when substituentson the phenolate groups varied from electron-donating to electronwithdrawing groups. The catecholate(p) ? Fe(III) (dp⁄) charge-transfer transitions appear as a shoulder close to the pheno-late(p) ? Fe(III) (dp⁄) charge-transfer bands in regions around400–600 nm for (HNEt3)[FeLNEC(TBC)] and (HNEt3)[FeLNEB(TBC)]while in (HNEt3)[FeLNEM(TBC)] these band overlap with the pheno-late(p) ? Fe(III) (dp⁄) charge-transfer bands. The Position of thecatecholate-to-iron charge-transfers bands shifted to higher wave-lengths (lower energy) when electron-accepting substituents arebound to the phenolate groups. The energy of charge-transfer tran-sitions are correlated to the Lewis acidity of the Fe(III) metal center(Fig. 1, Table 1) [24,25].

2 NEt3

N N

OH HO

R2

R1

R2

R1

N

NO

O

R2

R1

R2

R1

Fe

OH

OH

Br

Br

BrBr

O

O

Br

Br

BrBr

+ FeCl3EtOH N

N

O

O

R2

R1

R2

R1

FeCl

2N

Et3

(HNEt3)[FeLNEX(TBC)]

NH

Scheme 3. The reaction pathway for the synthesis of (HNEt3)[FeLNEX(TBC)] complexes.

Fig. 1. Electronic absorption spectra of (HNEt3)[FeLNEX(TBC)] in (1.0 � 10�4 M)CH2Cl2 solution.

Table 1kmax/nm (e/M�1cm�1) LMCT band of (HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)] and(HNEt3)[FeLNEM(TBC)].

(HNEt3)[FeLNEX(TBC)] kmax/nm (e /M�1cm�1)

kmax/nm (e /M�1cm�1)

kmax/nm (e /M�1cm�1)

(HNEt3)[FeLNEC(TBC)] 473 (4830) 300 (18220) 604 (3040)(HNEt3)[FeLNEB(TBC)] 475 (4010) 301 (15400) 601 (2590)(HNEt3)[FeLNEM(TBC)] 525 (4620) 300 (13410) –

T. Karimpour et al. / Inorganica Chimica Acta 405 (2013) 309–317 311

The overlap of LMCT bands in (HNEt3)[FeLNEX(TBC)] complexescan be nicely illustrated by monitoring the formation of the com-plex in time by UV–vis spectroscopy. The titrations of FeCl3 andligand (H2LNEX) solutions have been conducted at fixed concentra-tion of FeCl3 and H2LNEX solutions and varying concentration ofTBC. When TBC is added to FeCl3 and H2LNEC or H2LNEB solution atroom temperature, two original LMCT bands are decreased andtwo bands are grown around 400–600 nm. These bands can beassigned to catecholato-to-iron(III) and ligand-to-iron(III) charge-transfer transitions (Figs. 2–4). The appearance of isosbestic points

in the spectra clearly indicates the existence of simple equilibriumbetween (HNEt3)[FeLNEX(TBC)] with other substrates (Eq. 1).

FeCl3 þH2LNEX ! FeLNEXðClÞ þ TBC� ðHNEt3Þ½FeLNEXðTBCÞ� ð1Þ

3.1. Description of crystal structure

The diffraction experiment and the structure refinement for the(HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)] and (HNEt3)[FeLNEM

(TBC)] complexes reported here, have been summarized in Table 2.The selected bond lengths and angles have been given in Table 3.The molecular structures of complexes are shown in Figs. 7–9.

All three complexes reported here crystallize in the non-centro-symmetric space group Pna21, as confirmed by the systematicabsences. The absolute structures were determined with the Flackmethod [30], the absolute structure parameter x being 0.043(17);�0.010(5); 0.049(10) for (HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB

(TBC)] and (HNEt3)[FeLNEM(TBC)], respectively. The conformationand absolute configuration observed for (HNEt3)[FeLNEB(TBC)] and(HNEt3)[FeLNEM(TBC)] is almost identical (Fig. 5), with the (S,S) con-figuration on N1 and N2 atoms. Since for all complexes reportedhere the space group has the symmetry of glide planes, complex

Fig. 2. UV–vis spectral changes upon mixing FeCl3, H2LNEB and TBC (1.0 � 10�4 M)in CH2Cl2 solution. Arrows indicate the decrease or increase in absorption over time.

Fig. 3. UV–vis spectral changes upon mixing FeCl3, H2LNEC and TBC (1.0 � 10�4 M)in CH2Cl2 solution. Arrows indicate the decrease or increase in absorption over time.

Fig. 4. UV–vis spectral changes upon mixing FeCl3, H2LNEM and TBC (1.0 � 10�4 M)in CH2Cl2 solution. Arrows indicate the decrease or increase in absorption over time.

312 T. Karimpour et al. / Inorganica Chimica Acta 405 (2013) 309–317

molecules with opposite (R,R) configuration on N1,N2 are also pres-ent in the crystal lattice, as was found for molecule constituting theasymmetric unit in (HNEt3)[FeLNEC(TBC)] complex (Fig. 6).

For all structures reported here, the asymmetric unit consists ofthe complex anion [FeLNEX(TBC)]� and the triethylammonium cat-ion. In all the complex anions, the Fe(III) has an octahedral coordi-nation sphere (Table 3). The Fe–O bonds formed by theaminophenolate ligands are shortest, while the Fe–N bonds arelongest within the coordination sphere. In all the complexes, thetetrabromo catecholate (TBC) is bidentately coordinated to Fe(III)via the phenolate O atoms. These atoms are positioned trans rela-tive to one N atom and the phenolic O atom of the aminophenolateligand. In (HNEt3)[FeLNEC(TBC)] and (HNEt3)[FeLNEB(TBC)] com-plexes, the Fe–O bond formed by the TBC oxygen positioned transrelative to the N atom is significantly longer than the other Fe–Obond formed by TBC. However, in (HNEt3)[FeLNEM(TBC)], the differ-ence in the Fe–O bond lengths is not statistically significant.

Crystal structure information is available for enzyme–substratecomplex of the enzyme protocatechuate 3,4-dioxygenase fromPseudonomas putida (3,4-PCD) [31]. The active-site geometry ofthe enzyme–substrates complex has a distorted octahedral coordi-nation sphere. The Fe–Ocat bond lengths differ significantly(�0.3 Å) in the enzyme. The asymmetric chelation in the en-zyme–substrate complex is attributed to the different trans influ-ences exerted by the histidine and tyrosine residues and theinvolvement of one of the catecholato oxygens in a hydrogen bond.Both factors are effective in (HNEt3)[FeLNEC(TBC)] and (HNEt3)-[FeLNEB(TBC)] complexes similar to the enzyme but for (HNEt3)-[FeLNEM(TBC)], the hydrogen bonding interactions counter-balancethe effect of the different trans ligands. From this comparison,(HNEt3)[FeLNEX(TBC)] can be regarded as a close structural mimicof the enzyme–substrate (E–S) complex of the catechol dioxygen-ase enzymes.

3.1.1. X-ray crystal structure of (HNEt3)[FeLNEC(TBC)]Structures of Fe(III) complexes with acetylacetonate and the

same LNEX ligands have been reported previously [27]. Structuralanalysis of protocatechuate 3,4-dioxygenase (3,4-PCD) and its vari-ants with Tyr408 substitutions suggested a role of Y408 in the pro-posed catalytic mechanism [32]. One of the elements is the rapidexchange of anionic ligands during the catalytic cycle, with theligand positioned trans to Y408 exchanged. Structure of (HNEt3)-[FeLNEC(TBC)] reveals that the Fe1–O4 bond formed by the TBCoxygen positioned trans relative to LNEC phenolic O1 is shorter thanFe–O3 (Table 3), what seems to be consistent with the mechanismproposed for 3,4-PCD. However, structural analysis of FeLNEC re-veals the opposite effect for acac dianion binding.[27]. The statisti-cally significant difference is found between two Fe–O bondsformed by LNEC with the longer bond formed by O1 positionedtrans to TBC.

The largest angular deformation in the coordination sphere isO1–Fe1–N2 of 165.57(11)�. Conformation of the ethylenediaminemoiety is synclinal, with N1–C8–C9–N2 torsion angle being�57.1(5)�, similar to that found for the FeLNEC complex. The dihe-dral angle between two phenolic rings of LNEC is 87.5�, almost10� larger than that for FeLNEC. The TBC ligand is positioned almostperpendicular to the best planes of the phenolic rings of LNEC

(Fig. 7) with the dihedral angles between the ring best planes being72.1 and 83.2� for C1–C6 and C11–C16, respectively. In such posi-tion, the deviation of Fe1, O2 and N1 atoms, constituting the samecoordination plane with TBC, from the best plane of TBC are�0.235, �0.562 and �0.748 Å, respectively.

The Fe1–N1–C8–C9–N2 chelate ring is an envelope on C8. TheFe1–O1–C1–C6–C7–N1 has an envelope conformation, whileFe1–O2–C16–C11–C10–N2 is a twist-boat. The Fe1–O3–C19–C24–O4 chelate ring is planar.

Table 2Crystallographic data and structure refinement for compounds (HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)] and (HNEt3)[FeLNEM(TBC)].

(HNEt3)[FeLNEB(TBC)] (HNEt3)[FeLNEC(TBC)] (HNEt3)[FeLNEM(TBC)]

Empirical formula C30H34Br8FeN3O4 C30H34Br4Cl4FeN3O4 C34H46Br4FeN3O4

Formula weight 1195.73 1017.89 936.23T (K) 293(2) 293(2) 293(2)k (Å) 0.71073 0.71073 0.71073Crystal system orthorhombic orthorhombic orthorhombicSpace group Pna2(1) Pna2(1) Pna2(1)Unit cell dimensionsa (Å) 16.7615(17) 16.6385(4) 16.684(2)b (Å) 21.276(4) 21.1817(5) 21.456(2)c (Å) 10.3631(11) 10.3011(3) 10.4396(9)V (Å3) 3695.7(8) 3630.43(15) 3737.2(7)Z, Calculated density (Mg/m3) 4, 2.149 4, 1.862 4, 1.664Absorption coefficient (mm�1) 9.096 5.152 4.720F(000) 2292 2004 1876Crystal size (mm) 0.39 � 0.22 � 0.05 0.36 � 0.21 � 0.12 0.61 � 0.20 � 0.11Theta range for data collection (�) 2.19–28.25 2.20–28.13 2.17–27.96Limiting indices �21 6 h 6 21,

�27 6 k 6 27,�11 6 l 6 13

�21 6 h 6 20,�26 6 k 6 27,�12 6 l 6 12

�20 6 h 6 22,�26 6 k 6 28,�9 6 l 6 13

Reflections collected/unique 23825/7430 [R(int) = 0.1284] 24297/7958 [R(int) = 0.0386] 24766/6277 [R(int) = 0.0526]Completeness to theta 26.00 deg; 99.9% 26.00 deg, 99.9% 26.00 deg, 99.9%Absorption correction analytical analytical analyticalMax. and min. transmission 0.6546 and 0.1269 0.5857 and 0.2616 0.6299 and 0.1605Refinement method full-matrix least-squares on F2 full-matrix least-squares on F2 full-matrix least-squares on F2

Data/restraints/parameters 7430/1/415 7958/1/415 6277/1/415Goodness-of-fit on F2 0.868 0.805 0.899Final R indices [I > 2r(I)] R1 = 0.0586,

wR2 = 0.1184R1 = 0.0336,wR2 = 0.0559

R1 = 0.0391,wR2 = 0.0902

R indices (all data) R1 = 0.1476,wR2 = 0.1360

R1 = 0.0686,wR2 = 0.0598

R1 = 0.0733,wR2 = 0.1000

Absolute structure parameter 0.043(17) �0.010(5) 0.049(10)Largest difference peak and hole (e �3) 0.888 and �0.662 0.492 and �0.489 0.855 and �0.672

Table 3Selected bond lengths (Å) and angles (deg) for compounds (HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)] and (HNEt3)[FeLNEM(TBC)].

(HNEt3)[FeLNEC(TBC)] (HNEt3)[FeLNEB(TBC)] (HNEt3)[FeLNEM(TBC)]

Bond lengths/Å

Fe1–O1 1.943(3) Fe1–O1 1.937(9) Fe1–O1 1.926(4)Fe1–O2 1.927(3) Fe1–O2 1.955(8) Fe1–O2 1.903(4)Fe1–O3 2.007(2) Fe1–O3 2.011(8) Fe1–O3 2.029(4)Fe1–O4 1.992(3) Fe1–O4 1.981(8) Fe1–O4 2.035(4)Fe1–N1 2.312(3) Fe1–N1 2.335(9) Fe1–N1 2.209(5)Fe1–N2 2.201(3) Fe1–N2 2.209(11) Fe1–N2 2.340(5)

Bond angles/�

O1–Fe1–O3 100.05(10) O1–Fe1–O3 93.9(3) O1–Fe1–O4 102.27(16)O2–Fe1–O3 94.45(10) O2–Fe1–O3 101.5(3) O2–Fe1–O4 93.14(16)O4–Fe1–O3 81.08(11) O4–Fe1–O3 81.2(3) O3–Fe1–O4 79.59(16)O1–Fe1–N2 165.57(11) O1–Fe1–N2 97.4(4) O1–Fe1–N2 164.90(16)O2–Fe1–N2 84.64(10) O2–Fe1–N2 85.9(4) O2–Fe1–N2 84.27(16)O4–Fe1–N2 88.95(11) O4–Fe1–N2 86.5(4) O4–Fe1–N2 92.81(16)O3–Fe1–N2 94.38(10) O3–Fe1–N2 165.9(3) O3–Fe1–N2 88.29(16)N2–Fe1–N1 78.28(12) N2–Fe1–N1 78.6(4) N2–Fe1–N1 77.83(17)

T. Karimpour et al. / Inorganica Chimica Acta 405 (2013) 309–317 313

The valence geometry of LNEC, TBC and triethylammonium ionare typical. The C–Cl distances range from C15–Cl4 1.730(4) toC13–Cl3 1.758(5) Å. The C–Br distances vary from C20–Br11.861(5) to C22–Br3 1.913(4) Å.

Analysis of the crystal packing reveals the H-bond N3–H3 N� � �O3[�1/2 + x ,1/2 � y, z] with N3� � �O3 distance of 2.972 Å(Table 4). Also the intramolecular interaction C7–H7B� � �O4 isfound with C� � �O distance 2.929 Å.

3.1.2. X-ray crystal structure of (HNEt3)[FeLNEB(TBC)]Structure of (HNEt3)[FeLNEB(TBC)] reveals that the Fe1–O4 bond

formed by the TBC oxygen positioned trans relative to LNEB pheno-lic O1 is shorter than Fe–O3 positioned trans to N2, distances being1.981(8) and 2.011(8) Å, respectively (Table 3), what seems to be

consistent with the mechanism proposed for 3,4-PCD. In FeLNEB

similar effect was found for bonds involving acac dianion [27]and the corresponding Fe–O distances were very similar. There isa statistically significant difference between two Fe–O bondsformed by LNEB with the shorter bond formed by O1 positionedtrans to TBC. That is opposite to the difference found for (HNEt3)[FeLNEC(TBC)] reported here. In FeLNEB complex, there was nostatistically significant difference between the Fe–O bonds for ledby LNEB. These distances seem to be affected by the steric effectwithin the coordination sphere, but also might be related to thenature of acac vs. TBC. The largest angular deformation withinthe coordination sphere is O2–Fe1–N1 164.0(3)�.

Conformation of the ethylenediamine moiety is synclinal, withN1–C8–C9–N2 torsion angle of 56.8(15)�, the absolute value

Fig. 7. ORTEP diagram and atom labeling scheme for (HNEt3)[FeLNEC(TBC)] Ellip-soids are plotted at 30% probability level.

Fig. 8. ORTEP diagram and atom labeling scheme for (HNEt3)[FeLNEB(TBC)] Ellip-soids are plotted at 30% probability level.

Fig. 5. Superposition of (HNEt3)[FeLNEM(TBC)] (solid lines) and (HNEt3)[FeLNEB(TBC)](thin lines) revealing similar conformation and identical (S,S) configuration on N1,N2of the ligands. Superposition was based on the TBC O atoms, LNEX atoms positionedtrans to TBC and Fe.

Fig. 6. Opposite configuration on N1, N2 atoms, as revealed by the superposition of(HNEt3)[FeLNEM(TBC)] (solid lines) and (HNEt3)[FeLNEC(TBC)] (thin lines). Superpo-sition was based on the TBC O atoms, LNEX atoms positioned trans to TBC and Fe.Both S,S and R,R enantiomers are present in the crystal lattices of the reportedcomplexes.

314 T. Karimpour et al. / Inorganica Chimica Acta 405 (2013) 309–317

similar to that found for the (HNEt3)[FeLNEC(TBC)] complex re-ported here, and almost identical to that of 56.5(6)� found forFeLNEB [27]. The dihedral angle between two phenolic rings of LNEB

is 85.9�, the value similar to that found for (HNEt3)[FeLNEC(TBC)].The corresponding angle between the rings for FeLNEB is 73.2�.The difference between TBC and acac complexes with the sameLNEX ligand are consistent for all complexes reported here and theirequivalents with acac [27]. The dihedral angles between TBC ringand the LNEB phenolic rings (Fig. 8) are 83.5 and 69.0� for C1–C6and C11–C16, respectively, similar to those reported for (HNEt3)[FeLNEC(TBC)]. The similar tilt of TBC is found in the (HNEt3)[FeLNEB(TBC)] as in the (HNEt3)[FeLNEC(TBC)] mentioned above.The deviation of Fe1, O1 and N2 from the best plane of TBC are0.308, 0.740 and 0.847 Å, respectively.

The Fe1–O3–C19–C24–O4 chelate ring has an envelope confor-mation on Fe1, the Fe1–N1–C8–C9–N2 ring is an envelope on C9.

Conformation of Fe1–O1–C1–C6–C7–N1 is a twist-boat, whileFe1–O2–C16–C11–C10–N2 is an envelope.

The valence geometry of LNEB, TBC and triethylammonium ionare typical. In LNEB the C–Br distances range from C4–Br21.882(14) to C13–Br3 1.916(12) Å. The C–Br distances in TBC varyfrom C23–Br8 1.845(13) to C22–Br7 1.889(12) Å.

The N–H group of triethylammonium cation participates in thepair of the intramolecular H-bonds N3–H3 N� � �Br5 and N3–H3 N� � �O3 with N3� � �Br5 and N3� � �O3 distances of 3.552 and2.976 Å (Table 4). There are also the C–H� � �O intramolecular inter-action C10–H10B� � �O4 and C17–H17C� � �O4, with the C� � �O dis-tances being 2.978 and 2.978 Å, respectively.

3.1.3. X-ray crystal structure of (HNEt3)[FeLNEM(TBC)]In (HNEt3)[FeLNEM(TBC)] no statistically significant difference

between two Fe1–O bonds formed by the TBC is found. Contrary,

Fig. 9. ORTEP diagram and atom labeling scheme for (HNEt3)[FeLNEM(TBC)]Ellipsoids are plotted at 30% probability level.

Fig. 10. Temperature dependence of susceptibility v(T) and leff (lB) of (HNEt3)[-FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)] and (HNEt3)[FeLNEM(TBC)] measured in mag-netic field of H = 1000 Oe.

T. Karimpour et al. / Inorganica Chimica Acta 405 (2013) 309–317 315

in the FeLNEM structure, the acac Fe–O bond trans to the LNEM Natom was significantly shorter than that trans to the phenolic Oatom of LNEM [27]. There is a statistically significant difference be-tween two Fe–O bonds formed by LNEB with the shorter bondformed by O2 positioned trans to TBC, the effect similar to thatfound in (HNEt3)[FeLNEB(TBC)]. The largest angular deformationwithin the coordination sphere is O1–Fe1–N2 164.90(16)�.

Conformation of the ethylenediamine moiety is synclinal, withN1–C8–C9–N2 torsion angle of 58.7(6)�, similar to that found forthe (HNEt3)[FeLNEB(TBC)] complex reported here, and of identicalabsolute value to that reported for FeLNEM [27]. The dihedral anglebetween two phenolic rings of LNEM is 81.2�, the value similar tothat found in other (HNEt3)[FeLNEX(TBC)] reported here, while thecorresponding angle between the rings for FeLNEM is 74.0�. Thedihedral angles between TBC ligand and the best planes of the phe-nolic rings of LNEB (Fig. 9) are 63.9 and 75.9� for C1–C6 and C11–C16, respectively, similar to other (HNEt3)[FeLNEX(TBC)]. The tiltof TBC relative to the plane defined by Fe and donor atoms posi-tioned trans to TBC, is also found in (HNEt3)[FeLNEM(TBC)], withthe deviation of Fe1, O2 and N1 from the best plane of TBC being0.377, 0.814 and 1.081 Å, respectively.

Conformation of the Fe1–O3–C19–C24–O4 chelate ring istwisted on O4–Fe1, the Fe1–N1–C8–C9–N2 ring is twisted onC8–C9. Conformation of Fe1–O1–C1–C6–C7–N1 is an envelope,while Fe1–O2–C16–C11–C10–N2 is a twist-boat.

The valence geometry of LNEM, TBC and triethylammonium ionis typical. In TBC the C–Br distances range from C24–Br1 1.886(7)to C27–Br4 1.902(7) Å.

The triethylammonium cation participates in the intramolecu-lar H-bond N3–H3 N� � �O4 with N3� � �O4 distance 2.904 Å (Table 4).There are also intramolecular C–H� � �O interaction C7–H7A� � �O3,C22–H22A� � �O2 and C33–H33A� � �O2, with the C� � �O distancesbeing 2.998 and 2.826 and 3.211 Å, respectively.

Table 4Hydrogen bonds lengths (Å) and angles (deg) for compounds (HNEt3)[FeLNEC(TBC)], (HNEt

D–H� � �A

(HNEt3)[FeLNEC(TBC)] N3–H3N� � �O3N3–H3N� � �Br1

(HNEt3)[FeLNEB(TBC)] N3–H3N� � �O3[�x + 1,�y + 1,z � 1/2]N3–H3N� � �Br5[�x + 1,�y + 1,z � 1/2]

(HNEt3)[FeLNEM(TBC)] N3–H3N� � �O4[x,y,z � 1]N3–H3N� � �Br4[x,y,z � 1]

3.2. Magnetic susceptibility measurement

The magnetic susceptibility data for powder samples of(HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)] and (HNEt3)[FeLNEM(-TBC)] were collected in a magnetic field of 1000 Oe as a functionof temperature in the range 2–300 K with a Quantum DesignMPMS-XL-5 magnetometer. The measured data shown in Fig. 10were corrected for the sample holder contribution, for the temper-ature-independent Larmor diamagnetic susceptibility obtainedfrom the Pascal’s tables [33], and the temperature independentparamagnetism. The susceptibilities of all three samples increasewith decreasing temperature. The analysis of the susceptibilityv(T) was performed by applying the Curie–Weiss law,v ¼ C

ðT�hÞ,for T > 50 K, which gives us information on the magnitude of mag-netic moments leff ¼

ffiffiffiffiffiffi8Cp

through the Curie–Weiss constant C.The obtained fitting parameters C and h are listed in Table 5. TheCurie–Weiss constants C are 3.8, 4.1 and 3.9 emu/mol while thecalculated effective magnetic moments leff are 5.5, 5.7 and 5.6 lB

for (HNEt3)[FeLNEC(TBC)], (HNEt3)[FeLNEB(TBC)] and (HNEt3)-[FeLNEM(TBC)], respectively. A temperature variation of effectivemagnetic moments of all three samples (leff ¼

ffiffiffiffiffiffiffiffiffi8vT

p) are shown

also in inset of Fig. 10. The temperature independence of leff downto the lowest measured temperature of 2 K confirms a perfect para-magnetic behavior of the samples. The magnitudes of effectivemagnetic moments of 5.5, 5.6 and 5.7 lB correspond to theexpected values for Fe(III) ions [34]. The obtained Curie–Weisstemperatures in Table 5, can be considered only be considered asan additional parameters which slightly improve the fits of v(T).No other experimental results suggest any magnet interaction

3)[FeLNEB(TBC)] and (HNEt3)[FeLNEM(TBC)].

d(D–H) d(H� � �A) d(D� � �A) <(DHA)

0.910 2.096 2.972 161.190.910 3.009 3.361 127.04

0.910 2.116 2.976 157.070.910 2.905 3.552 129.31

0.910 2.031 2.904 160.270.910 2.948 3.559 125.91

Table 5Calculated parameters C and h from the fit of the measured data. The effectivemagnetic moment leff per iron ion was calculated using a relation leff ¼

ffiffiffiffiffiffi8Cp

.

C (emu K/mol) h (K) leff (BM)

(HNEt3)[FeLNEC(TBC)] 3.7 �0.18 5.4(HNEt3)[FeLNEB(TBC)] 3.8 0.63 5.5(HNEt3)[FeLNEM(TBC)] 3.6 0.21 5.4

Table 6Electrode peak potentials for oxidation and reduction of complexes(HNEt3)[FeLNEX(TBC)].

(HNEt3)[FeLNEX(TBC] E1ox/V E2

ox/V E3ox/V E4

ox/V E5ox/V

(HNEt3)[FeLNEC(TBC] �0.74 0.56 0.85 1.12 1.37(HNEt3)[FeLNEB(TBC] – 0.60 0.78 1.11 1.38(HNEt3)[FeLNEM(TBC)] �0.80 0.64 0.80 1.02 1.16

Redox behavior ill-defined.

Fig. 11. Cyclic voltammogram of (HNEt3)[FeLNEC(TBC)] in CH2Cl2 at �70 �C (sc20 mV s�1).

Fig. 12. Cyclic voltammogram of (HNEt3)[FeLNEC(TBC)] in CH2Cl2 at �70 �C (sc20 mV s�1).

Fig. 13. Cyclic voltammogram of (HNEt3)[FeLNEC(TBC)] in CH2Cl2 at �70 �C (sc 20–100 mV s�1).

316 T. Karimpour et al. / Inorganica Chimica Acta 405 (2013) 309–317

between the magnetic moments of Fe(III). To conclude, accordingto the susceptibility measurements all three investigated samplesbelong to a class of non-interacting mononuclear high spin iron(III)S = 5/2 complexes what is in agreement with their structure. Othersimilar iron(III) complexes studied by EPR [21,25] or NMR [19] alsoshow a non-interacting paramagnetic behavior.

3.3. Electrochemistry

The electrochemical behavior of the iron(III) complexes wasinvestigated by employing cyclic voltammetry in CH2Cl2 solutionscontaining 0.1 M [(nBu)4N]ClO4 as a supporting electrolyte. This re-search reveals that there are four oxidation peaks in positive regionfor all complexes with a difference in their redox potentials of(HNEt3)[FeLNEX(TBC)] complexes with FeLNEX complexes [27] thatreflects the interaction of the ferric center with the coordinatedTBC (Table 6 and Fig. 11). The similarity of E1

ox � E5ox values for

all complexes suggests the same oxidation mechanism for all com-plexes. Typical cyclic voltammogram (CV) of (HNEt3)[FeLNEC(TBC)]is presented in Fig. 11.

All the cyclic voltammograms show redox processes associatedwith the TBC/TBSQ (E2

ox and E3ox) and phenolate/phenoxyl (E4

ox

and E5ox) couple in the complexes (Fig. 11). The redox processes

of TBC/TBSQ has quasi-reversible oxidation peak based on devia-tion of DEp from 1 (DEp is the separation of the oxidation andreduction peaks; the value of DEp for reversible peaks is 1) (Figs. 12and 13). In all complexes the TBC/TBSQ couple has positive shift incomparison of free TBC/TBSQ couple. This shift indicates that thecoordination of the TBC to the ferric center significantly stabilizesthe DBC oxidation state.

The metal-centered voltammograms have been observed in thenegative potential range (E1

ox) that irreversible oxidation peak ismost likely arising from oxidation of FeIII/FeIV whereas reductionpeak corresponds to the FeIII/FeII reduction of iron complexes. Com-parison of the peaks position suggested that redox potentials arevery dependent on the Lewis acidity of the ferric metal center.These obtained results have been confirmed by the UV–vis spectro-scopic experiments, too.

4. Conclusions

We synthesized a series of iron complexes and characterizedthem by IR and elemental analysis techniques. X-ray structureanalysis has revealed that complexes (HNEt3)[FeLNEC(TBC)],

T. Karimpour et al. / Inorganica Chimica Acta 405 (2013) 309–317 317

(HNEt3)[FeLNEB(TBC)] and (HNEt3)[FeLNEM(TBC)] are six coordinateand Fe(III) centres were surrounded by two phenolate oxygenatoms, two amine nitrogens and two oxygen atoms of catecolate li-gand similar to the proposed catechol-bound intermediate for cat-echol dioxygenase, structurally characterized by single crystalX-ray diffraction. Magnetic moment measurements confirmparamagnetic iron(III) monomer complexes. Study of modelcomplexes showed that novel complexes have good similarity tothe intermediate of catechol dioxygenase enzyme.

The effect of substitutions on phenolate of the aminophenolligands in these structures has been illustrated by UV–vis andcyclic voltammetry. The oxidation potentials and LMCT ofcomplexes are depend on the substitutions of phenolate ligandsand consequently on the Lewis acidity of the ferric metal centre.

Acknowledgments

Authors are grateful to the Institute for Advanced Studies in Ba-sic Sciences (IASBS), Nicolaus Copernicus University and Universityof Ljubljana. E. Safaei gratefully acknowledges the support by theInstitute for Advanced Studies in Basic Sciences (IASBS) ResearchCouncil under Grant No. G2013IASBS127.

Appendix A. Supplementary material

CCDC 920663–920665 contain the supplementary crystallo-graphic data for compounds (HNEt3)[FeLNEB(TBC)], (HNEt3)[FeLNEC

(TBC)] and (HNEt3)[FeLNEM (TBC)]. These data can be obtained freeof charge from The Cambridge Crystallographic Data Centre viahttp://www.ccdc.cam.ac.uk/data_request/cif. Supplementary dataassociated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/j.ica.2013.06.023.

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