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A magnetostructural study of three novel iron(III) complexes of tripodal amine phenolate ligands
Inorganica Chimica Acta 384 (2012) 69–75
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
A magnetostructural study of three novel iron(III) complexes of tripodal amine phenolate ligands Elham Safaei a,⇑, Hamid Sheykhi a, Thomas Weyhermüller b, Eckhard Bill b a b
Institute for Advanced Studies in Basic Sciences (IASBS), 45195 Zanjan, Iran Max-Planck Institute for Bioinorganic Chemistry, Stiftstraße 34-36, D-45470 Mülheim an der Ruhr, Germany
a r t i c l e
i n f o
Article history: Received 27 May 2011 Received in revised form 9 November 2011 Accepted 12 November 2011 Available online 19 November 2011 Keywords: Amine bis(phenolate) Amine tris(phenolate) Non-heme Model complex
a b s t r a c t Three new iron(III) aminophenolate complexes, namely LtriFe(CH3OH), L2OHFe2 and L2OMeFe2(l-OH)2, were synthesized. L represents the deprotonated forms of NR2X, where NR2 stands for the bis(2-hydroxy-3,5-ditert-butylbenzyl)amine, basic motive and X is 2-hydroxy-3,5-di-tert-butylbenzyl in Ltri, 2-hydroxyethyl in LOH, 2-methoxyethyl in LOMe. The complexes were characterized by X-ray, IR- and UV–Vis spectroscopies and magnetic susceptibility studies. LtriFe(CH3OH) has a distorted trigonal bipyramidal geometry in which three phenolate oxygen atoms, the amine nitrogen atom of the ligand and a methanol molecule are coordinated to the iron center. LOH2Fe2 and L2OMeFe2(l-OH)2 show a dimeric structure in which the iron centers are surrounded by the amine nitrogen atom, two phenolate groups and two bridging alkoxo or hydroxo groups, which originate from the alkanole arm of LOH in LOH2Fe2 or water in L2OMeFe2(l-OH)2. Magnetic susceptibility measurements indicate moderate antiferromagnetic coupling between the two iron centers in LOH2Fe2 and L2OMeFe2(l-OH)2 whilst the complex LtriFe(CH3OH) was paramagnetic. Ó 2011 Published by Elsevier B.V.
1. Introduction Metalloenzymes are a broad division of enzymes in which metal ions are coordinated at functional sites in proteins. They regulate a wide range of biological and physiological functions essential for life, including oxidations, reductions and other metal-driven electron transfers in enzymatic reactions [1–3]. The activity of these enzymes depends on the presence of metal ions such as copper and iron. The readily accessibility of oxidation states II–V make iron unique in biological redox chemistry. Iron enzymes are a diverse group of enzymes that catalyze a variety of chemical reactions, including oxidations, hydroxylations, epoxidations, reversible oxygen binding and oxygenations [4–9]. Several research groups have synthesized biomimetic iron model complexes with N-capped ligands providing an O, N, O coordination sphere [10–15]. More recently, iron complexes of chelating amine-phenolate ligands have been studied because of their close relationship with tyrosine ligands found in non-heme iron-containing metalloenzymes [16–24]. Besides the mononuclear FeIII–Ophenolate complexes already described, binuclear iron complexes [25–27] represents an additional interesting model for the active site of binuclear iron enzymes such as purple acid phosphatases (PAPs) [28]. On the other hand, spin interaction between iron centers in binuclear iron model complexes provides a powerful tool for investigation of
⇑ Corresponding author. Tel.: +98 241 4153200; fax: +98 241 4153232. E-mail address: [email protected] (E. Safaei). 0020-1693/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.ica.2011.11.036
bridging groups (oxo, hydroxo, etc.) in iron enzymes. These complexes are classified according to their magnetic properties. In the strongly interacting type, the oxo bridge is responsible for the strong antiferromagnetic coupling (usually 50 > J > 200 cm1) and alkoxo, phenoxo, or hydroxo bridges lead to moderate spin–spin interaction (usually 0 > J > 30 cm1) [29–34]. As continuation of our previous work dealing with synthesis and characterization of metal complexes of amino- or imino-phenolate ligands as models for metalloenzymes [35–38], we report here the coordination chemistry, magnetic and redox properties of three new iron(III) complexes with bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-2-methoxy ethylamine H2LOMe, bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-ethanolamine H2LOH, and tris(3,5-di-tert-butyl-2hydroxy-benzyl)amine H3Ltri (Scheme 1).
2. Experimental 2.1. Materials and physical measurements Reagents and analytical grade materials were obtained from commercial suppliers and used without further purification. Elemental analyses (C, H, N) were performed by the Research Institute of Petroleum Industry (RIPI). Fourier transform infrared spectroscopy on KBr pellets was performed on a FT-IR Bruker Vector 22 instrument. NMR measurements were done on a Bruker 250 instrument. UV–Vis absorbance digitized spectra were collected using a CARY 100 Bio spectrophotometer. Magnetic susceptibility
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Scheme 1. Ligands used in this work.
data were measured from powder samples of solid material in the temperature range of 2–290 K by using a SQUID susceptometer with a field of 1.0 T (MPMS-7, Quantum Design, calibrated with standard palladium reference sample, error <2%). Multiple-field variable-temperature magnetization measurements were done at 1, 4, and 7 T also in the range of 2–290 K with the magnetization equidistantly sampled on a 1/T temperature scale. The experimental data were corrected for underlying diamagnetism by use of tabulated Pascal’s constants [32,39] as well as for temperatureindependent paramagnetism. The susceptibility and magnetization data were simulated with our own package julX for exchange coupled systems [40]. The simulations are based on the usual spin-Hamilton operator for binuclear complexes with two spins S1 = S2 = 5/2. * ^
* ^
* ^
* ^
*
^ ¼ 2J½S 1 S 2 þ gbðS 1 þ S 2 Þ B H
ð1Þ
where J is the spin coupling constant and g is the average of the electronic g matrix components (kept equal to both Cu sites). Diagonalization of the Hamiltonian was performed with the routine ZHEEV from the LAPACK Library [41] and the magnetic moments were obtained from first order numerical derivative dE/dB of the eigenvalues. The powder summations were done by using a 16-point Lebedev grid [42,43]. Intermolecular interactions were considered by using a Weiss temperature, HW, as a perturbation of the temperature scale, kT = k(T HW) for the calculation. Powder summations were done by using a 16-point Lebedev grid. Dark red-brownish single crystals of LtriFe, (LOH)2Fe2 and OMe L2 Fe2 (l-OH)2 were coated with perfluoropolyether, picked up with nylon loops and were mounted in the nitrogen cold stream of the diffractometer. The X-ray data for the reported complexes were collected on a Bruker–Nonius Kappa CCD diffractometer at 100(2) K using graphite-monochromated Mo Ka radiation (k = 0.71073 Å) from a Mo-target rotating-anode X-ray source. Final cell constants were obtained from least squares fits of several thousand strong reflections. Crystal faces were determined and intensity data were corrected for absorption using the GAUSSIAN method in SADABS [44]. Structures were readily solved by the Patterson methods and subsequent difference Fourier techniques. The Siemens SHELXTL [45] software package was used for solution and artwork of the structures, SHELXL97 [46] was used for the refinement. All non-hydrogen atoms were anisotropically refined, and hydrogen atoms bound to carbon were placed at calculated positions and refined as riding atoms with isotropic displacement parameters. Split atom models were refined for disordered parts, where possible. Split positions were refined with restrained bond length, angles and thermal displacement parameters using the SADI, SAME and EADP instructions of SHELXL97.
Complex LtriFe(CH3OH) was found to be completely disordered. Split positions of two conformers were refined yielding occupation factors of about 0.57 and 0.43. The hydrogen atom at the coordinated methanol molecule was easily located from the difference map and was refined with a restrained bond distance (0.84 Å) and displacement parameter. The t-butyl group containing carbon atoms C(30)–C(33) in the compound LOH2Fe2 was found to be disordered by rotation and a split atom model was refined giving occupations of 0.65 and 0.35. L2OMeFe2(l-OH)2 crystallizes in space group C2/c and resides in a crystallographic twofold axis passing through O(50) and O(60). The unusual thermal displacement parameters of O(50) and O(60) indicate, that the four-membered Fe2O2-ring is not planar but folded, which forces the oxygen atoms to be disordered on two positions each. It was therefore impossible to find the attached hydrogen atom positions. The t-butyl group containing carbon atoms C(10)– C(13) is disorder by rotation and was split (0.55:0.45). The – CH2CH2OCH3 pendent arm of the ligand was found to be disordered on two positions. The main component having an occupation factor of about 0.65 is weakly bound to the iron center. The minor component (0.35) is not coordinated. Details of data collection and refinement of the structures are summarized in Table 1. 2.2. Preparations 2.2.1. Synthesis of ligands Bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-2-methoxy ethylamine H2LOMe, bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-ethanolamine H2LOH and tris(3,5-di-tert-butyl-2-hydroxy-benzyl)-amine, H3Ltri were prepared according to the literature [38,47,48]. 2.2.2. Synthesis of complexes An early attempt to synthesize iron complexes by refluxing methanolic solutions of the ligands with iron chloride and a base (triethylamine) were unsuccessful and all three complexes decomposed after evaporation of the solvent. The addition of sodium phenoxide, pyridine N-oxide and sodium methoxide to the reaction mixture made it possible to isolate the compounds in good yield. The preparation of the iron complexes are given below. 2.2.2.1. Synthesis of LtriFe(CH3OH). H3Ltri (0.653 g, 1 mmol) was added to the deoxygenated (by degassing for 30 min) solution of pyridine-N-oxide (0.190 g, 2 mmol) and triethylamine (0.42 ml, 1 mmol) in methanol (50 ml). The solution was stirred for 10 min at room temperature. Later FeCl24H2O (0.199 g, 1 mmol) was added to the solution that then was stirred for 60 min under reflux conditions. Subsequently, the solution was cooled to room temperature and exposed to air. The green solution turned to a dark-red color. Then the solvent was removed and the dark-red microcrystalline precipitate was resulted, which was filtered and washed with methanol. X-ray quality dark red crystals were grown from a 1:1 solvent mixture of dichloromethane/methanol. Yield = 0.220 g (29%). Anal. Calc. for C46H69FeNO4 (1190.65 g/mol): C, 73.00; H, 9.20; N, 1.90. Found: C, 73.00; H, 9.20; N, 2.30%. IR(KBr, cm1):3602, 2956, 2905, 2864, 1470, 1358, 1299, 1252, 1204, 1167, 1128, 1076, 998, 918, 876, 838, 747, 608, 556, 482. UV–Vis in CH2Cl2: kmax, nm (e, M1 cm1): 335 (8676), 430 (7961). 2.2.2.2. Synthesis of L2OHFe2. H2LOH (0.497 g, 1 mmol) was added to the deoxygenated (by degassing for 30 min) solution of sodium methoxide (0.232 g, 2 mmol) and triethylamine (0.42 ml, 1 mmol) in methanol (50 ml). The solution was stirred for 10 min at room temperature. Later FeCl24H2O (0.199 g, 1 mmol) was added to the solution that then was stirred for 60 min under reflux conditions. Subsequently, the solution was cooled to room temperature and exposed to air.
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E. Safaei et al. / Inorganica Chimica Acta 384 (2012) 69–75 Table 1 Crystal data and structure refinement for LtriFe(CH3OH), LOH2Fe2 and LOMe2Fe2(l-OH)2. Identification code
LOMe2Fe2(l-OH)2
L2OHFe2
LtriFe LtriFe(CH3OH)
Empirical formula Formula weight T (K) k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Calculated density (Mg m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) Theta range for data collection (°) Index ranges
C66H104Fe2N2O8 1165.21 100(2) 0.71073 monoclinic C2/c, No. 15
C64H96Fe2N2O6 1101.13 100(2) 0.71073 monoclinic P2/c, No. 13
C46H70FeNO4 756.88 100(2) 0.71073 monoclinic P2(1)/n, No. 14
23.005(2) 16.4665(12) 19.4826(15) 91.741(4) 7376.8(10) 4 1.049 0.439 2520 0.50 0.30 0.28 3.04–32.50 34 6 h 6 34, 24 6 k 6 24, 29 6 l 6 29 99 486 13 327 (0.0494) 13 327/13/394 1.045 R1 = 0.0603, wR2 = 0.1513 R1 = 0.0817, wR2 = 0.1715 1.112 and 0.624
9.0525(8) 12.6129(12) 29.369(3) 102.571(3) 3272.9(5) 2 1.117 0.489 1188 0.16 0.14 0.10 2.92–32.50 13 6 h 6 13, 19 6 k 6 17, 44 6 l 6 44 58 233 11 808 (0.0777) 11 808/7/356 1.030 R1 = 0.0611, wR2 = 0.1461 R1 = 0.0979, wR2 = 0.1667 0.831 and 0.609
13.7485(8) 11.7296(8) 26.690(2) 93.681(3) 4295.3(5) 4 1.170 0.392 1644 0.12 0.11 0.08 2.97–27.50 17 6 h 6 17, 15 6 k 6 15, 34 6 l 6 34 67 993 9847 (0.0995) 9847/160/562 1.033 R1 = 0.0632, wR2 = 0.1387 R1 = 0.1009, wR2 = 0.1583 0.915 and 0.474
Reflections collected Unique reflections (Rint) Data/restraints/parameters Goodness-of-fit (GOF) on F2 Final R indices [I > 2r(I)] R indices (all data) Largest difference in peak and hole (e A3)
The green solution turned to a dark-red color. Then the solvent and was removed and the dark-red microcrystalline precipitate was resulted, which was filtered and washed with methanol. Xray quality dark red crystals were grown from a 1:1 solvent mixture of dichloromethane/methanol. Yield = 0.477 g (42%). Anal. Calc. for C64H96Fe2N2O6 (1190.65 g/mol): C, 69.8; H, 8.80; N, 2.50. Found: C, 69.0; H, 8.90; N, 2.80%. IR(KBr, cm1): 3442, 2955, 2905, 2866,1605, 1464, 1361, 1302, 1250, 1242, 1171, 1072, 1030, 881, 835, 752, 622, 563, 506. UV–Vis in CH2Cl2: kmax, nm (e, M1 cm1): 325 (16 532), 435 (9724). 2.2.2.3. Synthesis of L2OMeFe2(l-OH)2. H2LOMe (0.511 g, 1 mmol) was added to the deoxygenated (by degassing for 30 min) solution of sodium phenoxide (0.232 g, 2 mmol) and triethylamine (0.42 ml, 1 mmol) in methanol (50 ml). The solution was stirred for 10 min at room temperature. Later FeCl24H2O (0.199 g, 1 mmol) was added to the solution that then was stirred for 60 min under reflux conditions. Subsequently, the solution was cooled to room temperature and exposed to air. The green solution turned to a dark-red color. Then the solvent was removed and the dark-red microcrystalline precipitate was resulted, which was filtered and washed with methanol. X-ray quality dark red crystals were grown from a 1:1 solvent mixture of dichloromethane/methanol. Yield = 0.477 g (42%). Anal. Calc. for C66H102Fe2N2O8 (1190.65 g/mol): C, 68.1; H, 8.80; N, 2.50. Found: C, 66.8; H, 8.90; N, 2.90%. IR(KBr, cm1): 3408, 2955, 1713, 1599, 1465, 1360, 1274, 1203, 1171, 1122, 1026, 920, 872, 842, 750, 610, 554, 478. UV–Vis in CH2Cl2: kmax, nm (e, M1 cm1): 333 (13 797), 545 (8916). 3. Results and discussion Bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-2-methoxy ethylamine H2LOMe, bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-ethanolamine H2LOH and tris(3,5-di-tert-butyl-2-hydroxy-benzyl)-amine, H3Ltri were prepared according to the literatures [38,47,48]. They were synthesized from related amine, formaldehyde and 2,4-di-tert-butyl phenol in a Mannich condensation.
Iron complexes were formed in good yields by refluxing methanolic solution of the ligands with iron(III) chloride hexa-aqua and base in suitable ratio (Eqs. (2) and (3)). Reflux
H2 L þ FeCl2 4H2 O þ Base ! L2 Fe2 MeOH=1 h
Reflux
H3 L þ FeCl2 4H2 O þ Base ! LFeðCH3 OHÞ MeOH=1 h
ð2Þ ð3Þ
In IR spectra of complexes, the strong and sharp OH stretching band of the phenols around 3300–3500 cm1 for the mOH stretch of ligands were replaced by a broad band, proving the coordination of phenol groups to the metal. The electronic absorption spectra of all the complexes have been measured in dichloromethane in the 300–800 nm range and they all exhibit a band in the near UV and one more in the visible region with the high absorption coefficient (8000–16 000 M1 cm1). For all the present, the lower energy band (400–500 nm) is assigned to a charge-transfer transition from the phenolate (pp) to the half-filled dp orbital of the iron atom, and thus its band position falls in the range of other phenolato compounds(Section 2.2.2). The higher energy band (300 nm) is associated with p/p⁄ interaligand charge transfer, ILCT [49,50]. 3.1. Description of crystal structures Red brown crystals of LtriFe(CH3OH), L2OHFe2 and L2OMeFe2 (l-OH)2 for X-ray analysis were obtained from methanol/dichloromethane (1:2) solution. A complete listing of bond lengths and angles for all these complexes can be found in Supplementary material. The structure of LtriFe(CH3OH) is composed of a ferric iron center ligated by an amine tris(phenolate) ligand and a methanol molecule coordinated to the metal as shown in Fig. 1. The iron center is surrounded by oxygen atoms O(9), O(29) and O(49) of the phenolates in an equatorial position. An amine nitrogen N(1) and one oxygen atom O(60) of the coordinated methanol molecule as the apices; forming a distorted trigonal bipyramidal coordination geometry. Selected bond parameters are shown in Table 2. The
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N(1)–Fe(1)–O(60) angle is almost linear at 173.6(3)°. The observed Fe–N (2.137(4) Å) and equatorial Fe–O bond distances; Fe–O(49), 1.859(4) Å; Fe–O(29), 1.871(4) Å and Fe–O(9), 1.871(4) Å are in the range of those reported for other trigonal bipyramidal iron(III) complexes containing related ligands [41–55,14,56]. The Fe–O(60) distance at 2.120(4) Å is in the range of the average bond lengths previously determined for terminally bound methanol molecules in trigonal bipyramidal iron complexes [57–59]. The geometrical parameters s ([(U1 U2)/60], in which U1 and U2 are the two largest L–M–L angles of the coordination sphere [58]) for this complex is 0.75 for Fe(1) (Table 2). Comparing these s values with corresponding values for ideal trigonal bipyramid (s = 1) and square pyramidal (s = 0) suggest that this complex has a distorted trigonal bipyramid geometry. The distortion from ideal TBP geometry is observed in equatorial bond angles of 113°, 118.1°, and 128.9°, showing deviation from ideal 120° angles. On the other hand, deviation of axial bond angle of 173.6 from ideal 180° angle confirms the idea of distortion in the structure of complex. The FeNO4 coordination sphere is not very common for trivalent iron with O, N-based ligation, octahedral coordination is by far the most common for Fe(III) [25,60]. Selected bond lengths and angles for L2OHFe2 are presented in Table 3 and the labeling scheme is shown on Fig. 2. The structure of this complex consists of C2-symmetrical dinuclear unit L2FeIII2(l-OH)2. Each LOH ligand forms three coordination bonds to one Fe ion via O and N atoms. Each Fe(III) has a geometry of the distorted trigonal bipyramid. The coordination sphere consists of the phenolate oxygen atoms O(29), O(9) and the bridging alkoxo oxygen O(44) which form the equatorial plane for the Fe(1) center and the phenolate O(44A) and amine N(1) lie at the axial position of the trigonal bipyramid. The iron atoms of the dimer are bridged by the alkoxide oxygen atoms (O(44) and O(44A)) of the two ligands to form a four-membered ring showing iron–oxygen bond distances of 1.9846(14), Fe(1)–O(44); 1.9681(15) Å, Fe(1)–O(44A). The central four-membered Fe2O2 ring is not planar but folded by about 6.8° along the Fe–Fe vector. The Fe–N(1) bond distance of 2.1771(16) and the Fe Fe separation at 3.12 Å are consistent with Fe(III) centers with amine nitrogen and phenolate oxygen-donor ligands [51–55,14,56]. The Fe–O bond formed by the phenolate oxygen atoms 1.8507(15) Å, Fe–O(29) and 1.8549(15) Å, Fe–O(9) are shorter than those formed by the alkoxo oxygen by
Fig. 1. ORTEP diagram and atom labeling scheme for complex LtriFe(CH3OH).
Table 2 Selected bond lengths [Å] and angles [°] for LOMe2Fe2(lOH)2. Identification code
LOMe2Fe2(l-OH)2
Fe(1)–O(29) Fe(1)–O(9) Fe(1)–O(60) Fe(1)–O(50) Fe(1)–N(1) O(50)–Fe(1)#1 O(60)–Fe(1)#1 O(29)–Fe(1)–O(9) O(29)–Fe(1)–O(60) O(9)–Fe(1)–O(60) O(29)–Fe(1)–O(50) O(9)–Fe(1)–O(50) O(60)–Fe(1)–O(50) O(29)–Fe(1)–N(1) O(9)–Fe(1)–N(1) O(60)–Fe(1)–N(1) O(50)–Fe(1)–N(1) Fe(1)#1–O(50)–Fe(1) Fe(1)#1–O(60)–Fe(1)
1.8801(14) 1.8829(13) 1.9778(13) 1.9784(14) 2.2153(14) 1.9783(13) 1.9778(13) 104.77(6) 99.29(4) 98.79(6) 104.73(4) 150.50(4) 75.86(7) 90.97(5) 89.55(6) 164.60(6) 90.49(6) 104.12(10) 104.16(10)
Table 3 Selected bond lengths [Å] and angles [°] for L2OHFe2. Identification code
L2OHFe2
Fe(1)–O(29) Fe(1)–O(9) Fe(1)–O(44) Fe(1)–O(44)#1 Fe(1)–N(1) O(44)–Fe(1)#1 O(29)–Fe(1)–O(9) O(29)–Fe(1)–O(44) O(9)–Fe(1)–O(44) O(29)–Fe(1)–O(44)#1 O(9)–Fe(1)–O(44)#1 O(44)–Fe(1)–O(44)#1 O(29)–Fe(1)–N(1) O(9)–Fe(1)–N(1) O(44)–Fe(1)–N(1) O(44)#1–Fe(1)–N(1) C(43)–O(44)–Fe(1) C(43)–O(44)–Fe(1)#1 Fe(1)–O(44)–Fe(1)#1
1.8507(15) 1.8549(15) 1.9681(15) 1.9845(14) 2.1771(16) 1.9845(14) 115.69(7) 126.59(7) 116.98(7) 100.08(6) 103.85(6) 75.50(6) 90.50(6) 92.38(6) 79.25(6) 154.16(6) 120.18(12) 134.92(12) 104.34(6)
0.11–0.13 Å, an effect similar to that found in hydroxyl bridged binuclear amine–phenolate iron complexes [58–60]. The angle Fe(1)–O(44)–Fe(1A) and Fe(1)–O(44A)–Fe(1A) at the bridging ligands are 104.34(6)° and 104.34(6)°, respectively and comparable with the reported values for similarly bridged iron(III) complexes [61–63]. The Fe–Ohydroxo bond distances are equivalent. This similarity is not usual in other similar bis(l-hydroxo) complexes [64–66]. Both L2OMeFe2(l-OH)2 and L2OHFe2 represent very similar distances and angles at the bridging units in these complexes. The structure of complex L2OMeFe2(l-OH)2 consists of a symmetrical dinuclear unit residing on a crystallographic twofold axis, C2 which passes through the bridging hydroxo oxygen atoms, O(50) and O(60). Selected bond distances and angles are listed in Table 4. An ORTEP view of the dimeric unit is shown in Fig. 3. The iron ion, Fe(1) is in deformed square pyramidal coordination sphere with weakly coordinated ligand occupying a sixth site. The phenolate O(9), O(29) and the bridging O(50) occupy the equatorial positions whilst the amine N(1) and O(60) lie at the apical position of the trigonal bipyramid. The iron ion is weakly coordinated to O(44) of the pendent methoxy-moiety, at 2.47 Å. The pendent arm is statistically disordered and only about 65% of the
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Fig. 2. ORTEP diagram and atom labeling scheme for complex L2OHFe2.
methoxy unit bind weakly to iron, about 35% is not coordinated. The Fe(1) Fe(2) separation of 3.122 Å fall in the range reported earlier and the angle Fe(1)–O(50)–Fe(1A), Fe(1)–O(60)–Fe(1A) at the bridging ligands are 104.12(10)° and 104.16(10)°, respectively and comparable with the reported values for similarly bridged iron(III) complexes [51–55,14,56]. The Fe–N and Fe–O bond distances of Fe–N(1), 2.2153(14); Fe–O(9), 1.8829(13); Fe–O(50), 1.9784 (14); Fe–O(60), 1.9778(13) and Fe–O(29) 1.8801(14) are consistent with Fe(III) centers with amine nitrogen and phenolate oxygen-donor ligands [51–55,14,56]. The Fe–O bond at Fe2O2-‘‘plane’’ formed by the phenolate oxygen atoms are shorter than those formed by the hydroxo oxygen by 0.11 Å, an effect similar to that found in hydroxyl bridged binuclear amine–phenolate iron complexes [61– 63]. The Fe–Ohydroxo bond distances are equivalent. This similarity is not usual in other similar bis(l-hydroxo) complexes [64–66]. The two nitrogen donors are cis-configured at Fe2O2-‘‘plane’’. The central Fe2O2-ring is not really planar, as seen from the unusual thermal displacement parameters of the hydroxo oxygen atoms. In fact, this disorder is stemming from a folding of the two planes defined by Fe1–O(50)–Fe(1A) and Fe1–O(60)–Fe(1A). Both L2OMeFe2(l-OH)2 and L2OHFe2 represent very similar distances and angles at the bridging units in these complexes.
Table 4 Selected bond lengths [Å] and angles [°] for LtriFe(CH3OH). Identification code
LtriFe(CH3OH)
Fe(1)–O(49) Fe(1)–O(29) Fe(1)–O(9) Fe(1)–O(60) Fe(1)–N(1) O(49)–Fe(1)–O(29) O(49)–Fe(1)–O(9) O(9)–Fe(1)–O(29) O(9)–Fe(1)–O(60) O(29)–Fe(1)–O(60) O(49)–Fe(1)–O(60) O(49)–Fe(1)–N(1) O(29)–Fe(1)–N(1) O(9)–Fe(1)–N(1) O(60)–Fe(1)–N(1) C(42)–N(1)–Fe(1) C(22)–N(1)–Fe(1)
1.859(4) 1.871(4) 1.871(4) 2.120(4) 2.137(4) 118.1(2) 113.0(2) 128.9(2) 92.0(3) 84.0(3) 93.6(3) 90.7(2) 89.9(2) 90.6(2) 173.6(3) 110.4(4) 106.18(10)
Fig. 3. ORTEP diagram and atom labeling scheme for complex LOMe2Fe2(l-OH)2.
3.2. Magnetic susceptibility measurements Magnetic susceptibility data for polycrystalline samples of complexes L2OHFe2 and L2OMeFe2(l-OH)2 and LtriFe(CH3OH) were collected at the temperature range 2–290 K with a magnetic field of 1 T applied. For L2OHFe2 and L2OMeFe2(l-OH)2 complexes, the value of the magnetic moment is 7.2–7.4 lB, in the temperature range of 230–290 K (room temperature), much less than the expected P spin-only value (8.37 lB) [l = g[ ZS(S + 1)]1/2] of two independent high spin iron(III) ions (S = 5/2). This value decreases dramatically with decreasing temperature until it reaches a value of 0.02 lB at 1.92 K. This temperature dependence of leff is a clear indication of an antiferromagnetic exchange coupling between two iron centers (Fig. 4). The experimental data were simulated with the parameter set J = 8.49 cm1, at g values of 2.00 (g1 = g2) for L2OHFe2 and J = 6.00 cm1, at g values of 1.9 (g1 = g2) for L2OMeFe2(l-OH)2. The antiferromagnetic coupling constant J for the Fe2(l-OH) and Fe2(l-OMe) cores lie in the range but at the lower end found for di-l-alkoxo or hydroxo bridged Fe2III complexes (usually 0 > J c> 30 cm1) [67–70]. The extent of antiferromagnetic interaction in such complexes is affected by the structural parameters,
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8
OH)2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2011.11.036.
7
µ (B.M.)
6 5
References
4 3 2 1 0
0
50
100
150
200
250
300
T (K) Fig. 4. Magnetic measurements for complexes LtriFe(CH3OH) (d), L2OHFe2 () and LOMe2Fe2(l-OH)2 (j).
Fe–Oalkoxo bond distances, the coordination environment around the Fe centers, Fe–O(H)–Fe bridge angle and specially the planarity or nonplanarity of the Fe2(l-OH) and Fe2(l-OMe) cores. In fact, the nonplanarity of the bridging cores and large distances (over 3 Å) between the Fe ions in these two complexes compared to other bis(l-alkoxo) complexes, may be responsible for the lower degree of antiferromagnetic coupling [25,71]. Above T 15 K, the effective magnetic moment (leff) of complex LtriFe(CH3OH) is essentially temperature-independent and has leff value of 5.9 lB, what is in excellent agreement with the expected value (5.92 lB) for the isolated S = 5/2 Fe(III) ion. Thus, the magnetic measurement of this complex shows unambiguously that LtriFe contains the magnetically diluted high-spin d5 iron(III) ion. 4. Conclusions Three iron(III) complexes of the type L2OHFe2 and L2OMeFe2(lOH)2 and LtriFe(CH3OH) where L is [N–O]-donor tripodal amine phenol ligand, have been synthesized and characterized. X-ray crystal structures of L2OHFe2 and L2OMeFe2(l-OH)2 complexes reveal that both complexes have dimeric structures while LtriFe(CH3OH) has a monomer structure with distorted trigonal bipyramid geometry. In all complexes, Fe(III) centers are surrounded by amine nitrogen’s, phenolate and hydroxo oxygen atoms of ligands or solvent hydroxyl groups. Variable temperature magnetic susceptibility measurements displayed moderate antiferromagnetic coupling in the order of about 10 cm1 between the two iron centers through bridging alkoxo or hydroxo groups in LOH2Fe2 and LOMe2Fe2(l-OH)2 to yield an S = 0 ground state whilst paramagnetic property was found for iron(III) in monomer complex LtriFe(CH3OH). Acknowledgments Authors are grateful to the Institute for Advanced Studies in Basic Sciences (IASBS) and Max-Planck-Institute for Bioinorganic Chemistry. E. Safaei gratefully acknowledges the support by the Institute for Advanced Studies in Basic Sciences (IASBS) Research Council under Grant No. G2011IASBS127. Thanks are due to Mr. Andreas Göbels and Mrs. Heike Sukht (MPI-BAC) for skillful technical assistance. Appendix A. Supplementary material CCDC 802028, 802029 and 802030 contain the supplementary crystallographic data for LtriFe(CH3OH), L2OHFe2 and L2OMeFe2(l-
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A magnetostructural study of three novel iron(III) complexes of tripodal amine phenolate ligands Elham Safaei a,⇑, Hamid Sheykhi a, Thomas Weyhermüller b, Eckhard Bill b a b
Institute for Advanced Studies in Basic Sciences (IASBS), 45195 Zanjan, Iran Max-Planck Institute for Bioinorganic Chemistry, Stiftstraße 34-36, D-45470 Mülheim an der Ruhr, Germany
a r t i c l e
i n f o
Article history: Received 27 May 2011 Received in revised form 9 November 2011 Accepted 12 November 2011 Available online 19 November 2011 Keywords: Amine bis(phenolate) Amine tris(phenolate) Non-heme Model complex
a b s t r a c t Three new iron(III) aminophenolate complexes, namely LtriFe(CH3OH), L2OHFe2 and L2OMeFe2(l-OH)2, were synthesized. L represents the deprotonated forms of NR2X, where NR2 stands for the bis(2-hydroxy-3,5-ditert-butylbenzyl)amine, basic motive and X is 2-hydroxy-3,5-di-tert-butylbenzyl in Ltri, 2-hydroxyethyl in LOH, 2-methoxyethyl in LOMe. The complexes were characterized by X-ray, IR- and UV–Vis spectroscopies and magnetic susceptibility studies. LtriFe(CH3OH) has a distorted trigonal bipyramidal geometry in which three phenolate oxygen atoms, the amine nitrogen atom of the ligand and a methanol molecule are coordinated to the iron center. LOH2Fe2 and L2OMeFe2(l-OH)2 show a dimeric structure in which the iron centers are surrounded by the amine nitrogen atom, two phenolate groups and two bridging alkoxo or hydroxo groups, which originate from the alkanole arm of LOH in LOH2Fe2 or water in L2OMeFe2(l-OH)2. Magnetic susceptibility measurements indicate moderate antiferromagnetic coupling between the two iron centers in LOH2Fe2 and L2OMeFe2(l-OH)2 whilst the complex LtriFe(CH3OH) was paramagnetic. Ó 2011 Published by Elsevier B.V.
1. Introduction Metalloenzymes are a broad division of enzymes in which metal ions are coordinated at functional sites in proteins. They regulate a wide range of biological and physiological functions essential for life, including oxidations, reductions and other metal-driven electron transfers in enzymatic reactions [1–3]. The activity of these enzymes depends on the presence of metal ions such as copper and iron. The readily accessibility of oxidation states II–V make iron unique in biological redox chemistry. Iron enzymes are a diverse group of enzymes that catalyze a variety of chemical reactions, including oxidations, hydroxylations, epoxidations, reversible oxygen binding and oxygenations [4–9]. Several research groups have synthesized biomimetic iron model complexes with N-capped ligands providing an O, N, O coordination sphere [10–15]. More recently, iron complexes of chelating amine-phenolate ligands have been studied because of their close relationship with tyrosine ligands found in non-heme iron-containing metalloenzymes [16–24]. Besides the mononuclear FeIII–Ophenolate complexes already described, binuclear iron complexes [25–27] represents an additional interesting model for the active site of binuclear iron enzymes such as purple acid phosphatases (PAPs) [28]. On the other hand, spin interaction between iron centers in binuclear iron model complexes provides a powerful tool for investigation of
⇑ Corresponding author. Tel.: +98 241 4153200; fax: +98 241 4153232. E-mail address: [email protected] (E. Safaei). 0020-1693/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.ica.2011.11.036
bridging groups (oxo, hydroxo, etc.) in iron enzymes. These complexes are classified according to their magnetic properties. In the strongly interacting type, the oxo bridge is responsible for the strong antiferromagnetic coupling (usually 50 > J > 200 cm1) and alkoxo, phenoxo, or hydroxo bridges lead to moderate spin–spin interaction (usually 0 > J > 30 cm1) [29–34]. As continuation of our previous work dealing with synthesis and characterization of metal complexes of amino- or imino-phenolate ligands as models for metalloenzymes [35–38], we report here the coordination chemistry, magnetic and redox properties of three new iron(III) complexes with bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-2-methoxy ethylamine H2LOMe, bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-ethanolamine H2LOH, and tris(3,5-di-tert-butyl-2hydroxy-benzyl)amine H3Ltri (Scheme 1).
2. Experimental 2.1. Materials and physical measurements Reagents and analytical grade materials were obtained from commercial suppliers and used without further purification. Elemental analyses (C, H, N) were performed by the Research Institute of Petroleum Industry (RIPI). Fourier transform infrared spectroscopy on KBr pellets was performed on a FT-IR Bruker Vector 22 instrument. NMR measurements were done on a Bruker 250 instrument. UV–Vis absorbance digitized spectra were collected using a CARY 100 Bio spectrophotometer. Magnetic susceptibility
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E. Safaei et al. / Inorganica Chimica Acta 384 (2012) 69–75
Scheme 1. Ligands used in this work.
data were measured from powder samples of solid material in the temperature range of 2–290 K by using a SQUID susceptometer with a field of 1.0 T (MPMS-7, Quantum Design, calibrated with standard palladium reference sample, error <2%). Multiple-field variable-temperature magnetization measurements were done at 1, 4, and 7 T also in the range of 2–290 K with the magnetization equidistantly sampled on a 1/T temperature scale. The experimental data were corrected for underlying diamagnetism by use of tabulated Pascal’s constants [32,39] as well as for temperatureindependent paramagnetism. The susceptibility and magnetization data were simulated with our own package julX for exchange coupled systems [40]. The simulations are based on the usual spin-Hamilton operator for binuclear complexes with two spins S1 = S2 = 5/2. * ^
* ^
* ^
* ^
*
^ ¼ 2J½S 1 S 2 þ gbðS 1 þ S 2 Þ B H
ð1Þ
where J is the spin coupling constant and g is the average of the electronic g matrix components (kept equal to both Cu sites). Diagonalization of the Hamiltonian was performed with the routine ZHEEV from the LAPACK Library [41] and the magnetic moments were obtained from first order numerical derivative dE/dB of the eigenvalues. The powder summations were done by using a 16-point Lebedev grid [42,43]. Intermolecular interactions were considered by using a Weiss temperature, HW, as a perturbation of the temperature scale, kT = k(T HW) for the calculation. Powder summations were done by using a 16-point Lebedev grid. Dark red-brownish single crystals of LtriFe, (LOH)2Fe2 and OMe L2 Fe2 (l-OH)2 were coated with perfluoropolyether, picked up with nylon loops and were mounted in the nitrogen cold stream of the diffractometer. The X-ray data for the reported complexes were collected on a Bruker–Nonius Kappa CCD diffractometer at 100(2) K using graphite-monochromated Mo Ka radiation (k = 0.71073 Å) from a Mo-target rotating-anode X-ray source. Final cell constants were obtained from least squares fits of several thousand strong reflections. Crystal faces were determined and intensity data were corrected for absorption using the GAUSSIAN method in SADABS [44]. Structures were readily solved by the Patterson methods and subsequent difference Fourier techniques. The Siemens SHELXTL [45] software package was used for solution and artwork of the structures, SHELXL97 [46] was used for the refinement. All non-hydrogen atoms were anisotropically refined, and hydrogen atoms bound to carbon were placed at calculated positions and refined as riding atoms with isotropic displacement parameters. Split atom models were refined for disordered parts, where possible. Split positions were refined with restrained bond length, angles and thermal displacement parameters using the SADI, SAME and EADP instructions of SHELXL97.
Complex LtriFe(CH3OH) was found to be completely disordered. Split positions of two conformers were refined yielding occupation factors of about 0.57 and 0.43. The hydrogen atom at the coordinated methanol molecule was easily located from the difference map and was refined with a restrained bond distance (0.84 Å) and displacement parameter. The t-butyl group containing carbon atoms C(30)–C(33) in the compound LOH2Fe2 was found to be disordered by rotation and a split atom model was refined giving occupations of 0.65 and 0.35. L2OMeFe2(l-OH)2 crystallizes in space group C2/c and resides in a crystallographic twofold axis passing through O(50) and O(60). The unusual thermal displacement parameters of O(50) and O(60) indicate, that the four-membered Fe2O2-ring is not planar but folded, which forces the oxygen atoms to be disordered on two positions each. It was therefore impossible to find the attached hydrogen atom positions. The t-butyl group containing carbon atoms C(10)– C(13) is disorder by rotation and was split (0.55:0.45). The – CH2CH2OCH3 pendent arm of the ligand was found to be disordered on two positions. The main component having an occupation factor of about 0.65 is weakly bound to the iron center. The minor component (0.35) is not coordinated. Details of data collection and refinement of the structures are summarized in Table 1. 2.2. Preparations 2.2.1. Synthesis of ligands Bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-2-methoxy ethylamine H2LOMe, bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-ethanolamine H2LOH and tris(3,5-di-tert-butyl-2-hydroxy-benzyl)-amine, H3Ltri were prepared according to the literature [38,47,48]. 2.2.2. Synthesis of complexes An early attempt to synthesize iron complexes by refluxing methanolic solutions of the ligands with iron chloride and a base (triethylamine) were unsuccessful and all three complexes decomposed after evaporation of the solvent. The addition of sodium phenoxide, pyridine N-oxide and sodium methoxide to the reaction mixture made it possible to isolate the compounds in good yield. The preparation of the iron complexes are given below. 2.2.2.1. Synthesis of LtriFe(CH3OH). H3Ltri (0.653 g, 1 mmol) was added to the deoxygenated (by degassing for 30 min) solution of pyridine-N-oxide (0.190 g, 2 mmol) and triethylamine (0.42 ml, 1 mmol) in methanol (50 ml). The solution was stirred for 10 min at room temperature. Later FeCl24H2O (0.199 g, 1 mmol) was added to the solution that then was stirred for 60 min under reflux conditions. Subsequently, the solution was cooled to room temperature and exposed to air. The green solution turned to a dark-red color. Then the solvent was removed and the dark-red microcrystalline precipitate was resulted, which was filtered and washed with methanol. X-ray quality dark red crystals were grown from a 1:1 solvent mixture of dichloromethane/methanol. Yield = 0.220 g (29%). Anal. Calc. for C46H69FeNO4 (1190.65 g/mol): C, 73.00; H, 9.20; N, 1.90. Found: C, 73.00; H, 9.20; N, 2.30%. IR(KBr, cm1):3602, 2956, 2905, 2864, 1470, 1358, 1299, 1252, 1204, 1167, 1128, 1076, 998, 918, 876, 838, 747, 608, 556, 482. UV–Vis in CH2Cl2: kmax, nm (e, M1 cm1): 335 (8676), 430 (7961). 2.2.2.2. Synthesis of L2OHFe2. H2LOH (0.497 g, 1 mmol) was added to the deoxygenated (by degassing for 30 min) solution of sodium methoxide (0.232 g, 2 mmol) and triethylamine (0.42 ml, 1 mmol) in methanol (50 ml). The solution was stirred for 10 min at room temperature. Later FeCl24H2O (0.199 g, 1 mmol) was added to the solution that then was stirred for 60 min under reflux conditions. Subsequently, the solution was cooled to room temperature and exposed to air.
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E. Safaei et al. / Inorganica Chimica Acta 384 (2012) 69–75 Table 1 Crystal data and structure refinement for LtriFe(CH3OH), LOH2Fe2 and LOMe2Fe2(l-OH)2. Identification code
LOMe2Fe2(l-OH)2
L2OHFe2
LtriFe LtriFe(CH3OH)
Empirical formula Formula weight T (K) k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Calculated density (Mg m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) Theta range for data collection (°) Index ranges
C66H104Fe2N2O8 1165.21 100(2) 0.71073 monoclinic C2/c, No. 15
C64H96Fe2N2O6 1101.13 100(2) 0.71073 monoclinic P2/c, No. 13
C46H70FeNO4 756.88 100(2) 0.71073 monoclinic P2(1)/n, No. 14
23.005(2) 16.4665(12) 19.4826(15) 91.741(4) 7376.8(10) 4 1.049 0.439 2520 0.50 0.30 0.28 3.04–32.50 34 6 h 6 34, 24 6 k 6 24, 29 6 l 6 29 99 486 13 327 (0.0494) 13 327/13/394 1.045 R1 = 0.0603, wR2 = 0.1513 R1 = 0.0817, wR2 = 0.1715 1.112 and 0.624
9.0525(8) 12.6129(12) 29.369(3) 102.571(3) 3272.9(5) 2 1.117 0.489 1188 0.16 0.14 0.10 2.92–32.50 13 6 h 6 13, 19 6 k 6 17, 44 6 l 6 44 58 233 11 808 (0.0777) 11 808/7/356 1.030 R1 = 0.0611, wR2 = 0.1461 R1 = 0.0979, wR2 = 0.1667 0.831 and 0.609
13.7485(8) 11.7296(8) 26.690(2) 93.681(3) 4295.3(5) 4 1.170 0.392 1644 0.12 0.11 0.08 2.97–27.50 17 6 h 6 17, 15 6 k 6 15, 34 6 l 6 34 67 993 9847 (0.0995) 9847/160/562 1.033 R1 = 0.0632, wR2 = 0.1387 R1 = 0.1009, wR2 = 0.1583 0.915 and 0.474
Reflections collected Unique reflections (Rint) Data/restraints/parameters Goodness-of-fit (GOF) on F2 Final R indices [I > 2r(I)] R indices (all data) Largest difference in peak and hole (e A3)
The green solution turned to a dark-red color. Then the solvent and was removed and the dark-red microcrystalline precipitate was resulted, which was filtered and washed with methanol. Xray quality dark red crystals were grown from a 1:1 solvent mixture of dichloromethane/methanol. Yield = 0.477 g (42%). Anal. Calc. for C64H96Fe2N2O6 (1190.65 g/mol): C, 69.8; H, 8.80; N, 2.50. Found: C, 69.0; H, 8.90; N, 2.80%. IR(KBr, cm1): 3442, 2955, 2905, 2866,1605, 1464, 1361, 1302, 1250, 1242, 1171, 1072, 1030, 881, 835, 752, 622, 563, 506. UV–Vis in CH2Cl2: kmax, nm (e, M1 cm1): 325 (16 532), 435 (9724). 2.2.2.3. Synthesis of L2OMeFe2(l-OH)2. H2LOMe (0.511 g, 1 mmol) was added to the deoxygenated (by degassing for 30 min) solution of sodium phenoxide (0.232 g, 2 mmol) and triethylamine (0.42 ml, 1 mmol) in methanol (50 ml). The solution was stirred for 10 min at room temperature. Later FeCl24H2O (0.199 g, 1 mmol) was added to the solution that then was stirred for 60 min under reflux conditions. Subsequently, the solution was cooled to room temperature and exposed to air. The green solution turned to a dark-red color. Then the solvent was removed and the dark-red microcrystalline precipitate was resulted, which was filtered and washed with methanol. X-ray quality dark red crystals were grown from a 1:1 solvent mixture of dichloromethane/methanol. Yield = 0.477 g (42%). Anal. Calc. for C66H102Fe2N2O8 (1190.65 g/mol): C, 68.1; H, 8.80; N, 2.50. Found: C, 66.8; H, 8.90; N, 2.90%. IR(KBr, cm1): 3408, 2955, 1713, 1599, 1465, 1360, 1274, 1203, 1171, 1122, 1026, 920, 872, 842, 750, 610, 554, 478. UV–Vis in CH2Cl2: kmax, nm (e, M1 cm1): 333 (13 797), 545 (8916). 3. Results and discussion Bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-2-methoxy ethylamine H2LOMe, bis(3,5-di-tert-butyl-2-hydroxy-benzyl)-ethanolamine H2LOH and tris(3,5-di-tert-butyl-2-hydroxy-benzyl)-amine, H3Ltri were prepared according to the literatures [38,47,48]. They were synthesized from related amine, formaldehyde and 2,4-di-tert-butyl phenol in a Mannich condensation.
Iron complexes were formed in good yields by refluxing methanolic solution of the ligands with iron(III) chloride hexa-aqua and base in suitable ratio (Eqs. (2) and (3)). Reflux
H2 L þ FeCl2 4H2 O þ Base ! L2 Fe2 MeOH=1 h
Reflux
H3 L þ FeCl2 4H2 O þ Base ! LFeðCH3 OHÞ MeOH=1 h
ð2Þ ð3Þ
In IR spectra of complexes, the strong and sharp OH stretching band of the phenols around 3300–3500 cm1 for the mOH stretch of ligands were replaced by a broad band, proving the coordination of phenol groups to the metal. The electronic absorption spectra of all the complexes have been measured in dichloromethane in the 300–800 nm range and they all exhibit a band in the near UV and one more in the visible region with the high absorption coefficient (8000–16 000 M1 cm1). For all the present, the lower energy band (400–500 nm) is assigned to a charge-transfer transition from the phenolate (pp) to the half-filled dp orbital of the iron atom, and thus its band position falls in the range of other phenolato compounds(Section 2.2.2). The higher energy band (300 nm) is associated with p/p⁄ interaligand charge transfer, ILCT [49,50]. 3.1. Description of crystal structures Red brown crystals of LtriFe(CH3OH), L2OHFe2 and L2OMeFe2 (l-OH)2 for X-ray analysis were obtained from methanol/dichloromethane (1:2) solution. A complete listing of bond lengths and angles for all these complexes can be found in Supplementary material. The structure of LtriFe(CH3OH) is composed of a ferric iron center ligated by an amine tris(phenolate) ligand and a methanol molecule coordinated to the metal as shown in Fig. 1. The iron center is surrounded by oxygen atoms O(9), O(29) and O(49) of the phenolates in an equatorial position. An amine nitrogen N(1) and one oxygen atom O(60) of the coordinated methanol molecule as the apices; forming a distorted trigonal bipyramidal coordination geometry. Selected bond parameters are shown in Table 2. The
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N(1)–Fe(1)–O(60) angle is almost linear at 173.6(3)°. The observed Fe–N (2.137(4) Å) and equatorial Fe–O bond distances; Fe–O(49), 1.859(4) Å; Fe–O(29), 1.871(4) Å and Fe–O(9), 1.871(4) Å are in the range of those reported for other trigonal bipyramidal iron(III) complexes containing related ligands [41–55,14,56]. The Fe–O(60) distance at 2.120(4) Å is in the range of the average bond lengths previously determined for terminally bound methanol molecules in trigonal bipyramidal iron complexes [57–59]. The geometrical parameters s ([(U1 U2)/60], in which U1 and U2 are the two largest L–M–L angles of the coordination sphere [58]) for this complex is 0.75 for Fe(1) (Table 2). Comparing these s values with corresponding values for ideal trigonal bipyramid (s = 1) and square pyramidal (s = 0) suggest that this complex has a distorted trigonal bipyramid geometry. The distortion from ideal TBP geometry is observed in equatorial bond angles of 113°, 118.1°, and 128.9°, showing deviation from ideal 120° angles. On the other hand, deviation of axial bond angle of 173.6 from ideal 180° angle confirms the idea of distortion in the structure of complex. The FeNO4 coordination sphere is not very common for trivalent iron with O, N-based ligation, octahedral coordination is by far the most common for Fe(III) [25,60]. Selected bond lengths and angles for L2OHFe2 are presented in Table 3 and the labeling scheme is shown on Fig. 2. The structure of this complex consists of C2-symmetrical dinuclear unit L2FeIII2(l-OH)2. Each LOH ligand forms three coordination bonds to one Fe ion via O and N atoms. Each Fe(III) has a geometry of the distorted trigonal bipyramid. The coordination sphere consists of the phenolate oxygen atoms O(29), O(9) and the bridging alkoxo oxygen O(44) which form the equatorial plane for the Fe(1) center and the phenolate O(44A) and amine N(1) lie at the axial position of the trigonal bipyramid. The iron atoms of the dimer are bridged by the alkoxide oxygen atoms (O(44) and O(44A)) of the two ligands to form a four-membered ring showing iron–oxygen bond distances of 1.9846(14), Fe(1)–O(44); 1.9681(15) Å, Fe(1)–O(44A). The central four-membered Fe2O2 ring is not planar but folded by about 6.8° along the Fe–Fe vector. The Fe–N(1) bond distance of 2.1771(16) and the Fe Fe separation at 3.12 Å are consistent with Fe(III) centers with amine nitrogen and phenolate oxygen-donor ligands [51–55,14,56]. The Fe–O bond formed by the phenolate oxygen atoms 1.8507(15) Å, Fe–O(29) and 1.8549(15) Å, Fe–O(9) are shorter than those formed by the alkoxo oxygen by
Fig. 1. ORTEP diagram and atom labeling scheme for complex LtriFe(CH3OH).
Table 2 Selected bond lengths [Å] and angles [°] for LOMe2Fe2(lOH)2. Identification code
LOMe2Fe2(l-OH)2
Fe(1)–O(29) Fe(1)–O(9) Fe(1)–O(60) Fe(1)–O(50) Fe(1)–N(1) O(50)–Fe(1)#1 O(60)–Fe(1)#1 O(29)–Fe(1)–O(9) O(29)–Fe(1)–O(60) O(9)–Fe(1)–O(60) O(29)–Fe(1)–O(50) O(9)–Fe(1)–O(50) O(60)–Fe(1)–O(50) O(29)–Fe(1)–N(1) O(9)–Fe(1)–N(1) O(60)–Fe(1)–N(1) O(50)–Fe(1)–N(1) Fe(1)#1–O(50)–Fe(1) Fe(1)#1–O(60)–Fe(1)
1.8801(14) 1.8829(13) 1.9778(13) 1.9784(14) 2.2153(14) 1.9783(13) 1.9778(13) 104.77(6) 99.29(4) 98.79(6) 104.73(4) 150.50(4) 75.86(7) 90.97(5) 89.55(6) 164.60(6) 90.49(6) 104.12(10) 104.16(10)
Table 3 Selected bond lengths [Å] and angles [°] for L2OHFe2. Identification code
L2OHFe2
Fe(1)–O(29) Fe(1)–O(9) Fe(1)–O(44) Fe(1)–O(44)#1 Fe(1)–N(1) O(44)–Fe(1)#1 O(29)–Fe(1)–O(9) O(29)–Fe(1)–O(44) O(9)–Fe(1)–O(44) O(29)–Fe(1)–O(44)#1 O(9)–Fe(1)–O(44)#1 O(44)–Fe(1)–O(44)#1 O(29)–Fe(1)–N(1) O(9)–Fe(1)–N(1) O(44)–Fe(1)–N(1) O(44)#1–Fe(1)–N(1) C(43)–O(44)–Fe(1) C(43)–O(44)–Fe(1)#1 Fe(1)–O(44)–Fe(1)#1
1.8507(15) 1.8549(15) 1.9681(15) 1.9845(14) 2.1771(16) 1.9845(14) 115.69(7) 126.59(7) 116.98(7) 100.08(6) 103.85(6) 75.50(6) 90.50(6) 92.38(6) 79.25(6) 154.16(6) 120.18(12) 134.92(12) 104.34(6)
0.11–0.13 Å, an effect similar to that found in hydroxyl bridged binuclear amine–phenolate iron complexes [58–60]. The angle Fe(1)–O(44)–Fe(1A) and Fe(1)–O(44A)–Fe(1A) at the bridging ligands are 104.34(6)° and 104.34(6)°, respectively and comparable with the reported values for similarly bridged iron(III) complexes [61–63]. The Fe–Ohydroxo bond distances are equivalent. This similarity is not usual in other similar bis(l-hydroxo) complexes [64–66]. Both L2OMeFe2(l-OH)2 and L2OHFe2 represent very similar distances and angles at the bridging units in these complexes. The structure of complex L2OMeFe2(l-OH)2 consists of a symmetrical dinuclear unit residing on a crystallographic twofold axis, C2 which passes through the bridging hydroxo oxygen atoms, O(50) and O(60). Selected bond distances and angles are listed in Table 4. An ORTEP view of the dimeric unit is shown in Fig. 3. The iron ion, Fe(1) is in deformed square pyramidal coordination sphere with weakly coordinated ligand occupying a sixth site. The phenolate O(9), O(29) and the bridging O(50) occupy the equatorial positions whilst the amine N(1) and O(60) lie at the apical position of the trigonal bipyramid. The iron ion is weakly coordinated to O(44) of the pendent methoxy-moiety, at 2.47 Å. The pendent arm is statistically disordered and only about 65% of the
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Fig. 2. ORTEP diagram and atom labeling scheme for complex L2OHFe2.
methoxy unit bind weakly to iron, about 35% is not coordinated. The Fe(1) Fe(2) separation of 3.122 Å fall in the range reported earlier and the angle Fe(1)–O(50)–Fe(1A), Fe(1)–O(60)–Fe(1A) at the bridging ligands are 104.12(10)° and 104.16(10)°, respectively and comparable with the reported values for similarly bridged iron(III) complexes [51–55,14,56]. The Fe–N and Fe–O bond distances of Fe–N(1), 2.2153(14); Fe–O(9), 1.8829(13); Fe–O(50), 1.9784 (14); Fe–O(60), 1.9778(13) and Fe–O(29) 1.8801(14) are consistent with Fe(III) centers with amine nitrogen and phenolate oxygen-donor ligands [51–55,14,56]. The Fe–O bond at Fe2O2-‘‘plane’’ formed by the phenolate oxygen atoms are shorter than those formed by the hydroxo oxygen by 0.11 Å, an effect similar to that found in hydroxyl bridged binuclear amine–phenolate iron complexes [61– 63]. The Fe–Ohydroxo bond distances are equivalent. This similarity is not usual in other similar bis(l-hydroxo) complexes [64–66]. The two nitrogen donors are cis-configured at Fe2O2-‘‘plane’’. The central Fe2O2-ring is not really planar, as seen from the unusual thermal displacement parameters of the hydroxo oxygen atoms. In fact, this disorder is stemming from a folding of the two planes defined by Fe1–O(50)–Fe(1A) and Fe1–O(60)–Fe(1A). Both L2OMeFe2(l-OH)2 and L2OHFe2 represent very similar distances and angles at the bridging units in these complexes.
Table 4 Selected bond lengths [Å] and angles [°] for LtriFe(CH3OH). Identification code
LtriFe(CH3OH)
Fe(1)–O(49) Fe(1)–O(29) Fe(1)–O(9) Fe(1)–O(60) Fe(1)–N(1) O(49)–Fe(1)–O(29) O(49)–Fe(1)–O(9) O(9)–Fe(1)–O(29) O(9)–Fe(1)–O(60) O(29)–Fe(1)–O(60) O(49)–Fe(1)–O(60) O(49)–Fe(1)–N(1) O(29)–Fe(1)–N(1) O(9)–Fe(1)–N(1) O(60)–Fe(1)–N(1) C(42)–N(1)–Fe(1) C(22)–N(1)–Fe(1)
1.859(4) 1.871(4) 1.871(4) 2.120(4) 2.137(4) 118.1(2) 113.0(2) 128.9(2) 92.0(3) 84.0(3) 93.6(3) 90.7(2) 89.9(2) 90.6(2) 173.6(3) 110.4(4) 106.18(10)
Fig. 3. ORTEP diagram and atom labeling scheme for complex LOMe2Fe2(l-OH)2.
3.2. Magnetic susceptibility measurements Magnetic susceptibility data for polycrystalline samples of complexes L2OHFe2 and L2OMeFe2(l-OH)2 and LtriFe(CH3OH) were collected at the temperature range 2–290 K with a magnetic field of 1 T applied. For L2OHFe2 and L2OMeFe2(l-OH)2 complexes, the value of the magnetic moment is 7.2–7.4 lB, in the temperature range of 230–290 K (room temperature), much less than the expected P spin-only value (8.37 lB) [l = g[ ZS(S + 1)]1/2] of two independent high spin iron(III) ions (S = 5/2). This value decreases dramatically with decreasing temperature until it reaches a value of 0.02 lB at 1.92 K. This temperature dependence of leff is a clear indication of an antiferromagnetic exchange coupling between two iron centers (Fig. 4). The experimental data were simulated with the parameter set J = 8.49 cm1, at g values of 2.00 (g1 = g2) for L2OHFe2 and J = 6.00 cm1, at g values of 1.9 (g1 = g2) for L2OMeFe2(l-OH)2. The antiferromagnetic coupling constant J for the Fe2(l-OH) and Fe2(l-OMe) cores lie in the range but at the lower end found for di-l-alkoxo or hydroxo bridged Fe2III complexes (usually 0 > J c> 30 cm1) [67–70]. The extent of antiferromagnetic interaction in such complexes is affected by the structural parameters,
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8
OH)2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2011.11.036.
7
µ (B.M.)
6 5
References
4 3 2 1 0
0
50
100
150
200
250
300
T (K) Fig. 4. Magnetic measurements for complexes LtriFe(CH3OH) (d), L2OHFe2 () and LOMe2Fe2(l-OH)2 (j).
Fe–Oalkoxo bond distances, the coordination environment around the Fe centers, Fe–O(H)–Fe bridge angle and specially the planarity or nonplanarity of the Fe2(l-OH) and Fe2(l-OMe) cores. In fact, the nonplanarity of the bridging cores and large distances (over 3 Å) between the Fe ions in these two complexes compared to other bis(l-alkoxo) complexes, may be responsible for the lower degree of antiferromagnetic coupling [25,71]. Above T 15 K, the effective magnetic moment (leff) of complex LtriFe(CH3OH) is essentially temperature-independent and has leff value of 5.9 lB, what is in excellent agreement with the expected value (5.92 lB) for the isolated S = 5/2 Fe(III) ion. Thus, the magnetic measurement of this complex shows unambiguously that LtriFe contains the magnetically diluted high-spin d5 iron(III) ion. 4. Conclusions Three iron(III) complexes of the type L2OHFe2 and L2OMeFe2(lOH)2 and LtriFe(CH3OH) where L is [N–O]-donor tripodal amine phenol ligand, have been synthesized and characterized. X-ray crystal structures of L2OHFe2 and L2OMeFe2(l-OH)2 complexes reveal that both complexes have dimeric structures while LtriFe(CH3OH) has a monomer structure with distorted trigonal bipyramid geometry. In all complexes, Fe(III) centers are surrounded by amine nitrogen’s, phenolate and hydroxo oxygen atoms of ligands or solvent hydroxyl groups. Variable temperature magnetic susceptibility measurements displayed moderate antiferromagnetic coupling in the order of about 10 cm1 between the two iron centers through bridging alkoxo or hydroxo groups in LOH2Fe2 and LOMe2Fe2(l-OH)2 to yield an S = 0 ground state whilst paramagnetic property was found for iron(III) in monomer complex LtriFe(CH3OH). Acknowledgments Authors are grateful to the Institute for Advanced Studies in Basic Sciences (IASBS) and Max-Planck-Institute for Bioinorganic Chemistry. E. Safaei gratefully acknowledges the support by the Institute for Advanced Studies in Basic Sciences (IASBS) Research Council under Grant No. G2011IASBS127. Thanks are due to Mr. Andreas Göbels and Mrs. Heike Sukht (MPI-BAC) for skillful technical assistance. Appendix A. Supplementary material CCDC 802028, 802029 and 802030 contain the supplementary crystallographic data for LtriFe(CH3OH), L2OHFe2 and L2OMeFe2(l-
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