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DOI: 10.1039/B610649H (Paper) New J. Chem., 2007, 31, 93-101

Formation of oxo-bridged tetrairon(III) complexes mediated by oxygen activation. Structure, spectroscopy, magnetism and electrochemistry†

Sujit K. Dutta a, Meenakshi Ghosh a, Papu Biswas a, Ulrich Flörke b, Cristoph Saal c, Wolfgang Haase c and Kamalaksha Nag *a
aDepartment of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India. E-mail: ickn@iacs.res.in
bAnorganische und Analytische Chemie, Universität Paderborn, D-33098, Paderborn, Germany
cInstitut für Physikalische Chemie, Technische Hochschule Darmstadt, D-64287, Darmstadt, Germany

Received (in Montpellier, France) 25th July 2006 , Accepted 22nd September 2006

First published on 17th October 2006

Abstract

The reaction involving 2,6-diformyl-4-methylphenol, 1,3-diaminopropane, iron(II) perchlorate and sodium acetate in the ratio (2 : 2 : 2 : 3) in methanol produces the acetate-bridged macrocyclic diiron complex [FeII2L(μ-O2CCH3)(CH3OH)2](ClO4) (1). The direct reaction between the perchlorate salt of the macrocyclic ligand [LH4](ClO4)2 and iron(II) perchlorate in the presence of a mixture RCO2H and triethylamine also produces in solution the diiron complex [FeII2L(μ-O2CR)]+ (where R = H, CH3, C2H5, (CH3)3C and C6H5), which on exposure to air readily transforms to the corresponding oxo-bridged tetrairon(III) complex [{FeIII2L(μ-O2CR)}2(μ-O)2](ClO4)2 (2–6). The X-ray crystal structures of the acetate (3) and benzoate-bridged (6) complexes have been determined. Variable temperature (4–300 K) magnetic susceptibility measurements carried out for 3 and 6, have been analyzed in terms of molecular field approximation. The dominant antiferromagnetic exchange interaction (J) occurring through the oxo-bridged diiron(III) is −115 cm−1 for 3 and −109 cm−1 for 6, while the magnitude of spin exchange coupling occurring through the phenoxide bridges (J′) is −4 cm−1 for both 3 and 6. Complexes 2–6 exhibit identical proton NMR spectra in CD3CN, indicating that the capped carboxylate ligand gets detached from the metal centres. The Fe⋯H distances estimated from relaxation time (T1 and T2) measurements have been found to agree well with the crystallographic distances. Complexes 2–6 exhibit identical electrochemical behaviour and undergo reduction in three steps involving a first two-electron transfer reaction to [{FeIILFeIII}2(μ-O)2]2+, followed by two one-electron two-proton coupled reactions to [FeIILFeII]2+.

Introduction

Nonheme iron proteins such as hemerythrin,1 ribonucleotide reductase,2 methane monooxygenase3 and Δ–9-desaturase,4 which despite having structurally similar carboxylate-bridged diiron units in their active sites,5–7 activate molecular oxygen to perform biological functions in different ways.8–10 For instance, hemerythrin reversibly binds oxygen to act as oxygen carrier, ribonucleotide reductases catalyze conversion of nucleotides to deoxynucleotides and generate tyrosyl radical, methane monoxygenase catalytically converts methane to methanol, while conversion of alkanes to alkenes is catalyzed by Δ–9-desaturase. These proteins in their native state have a carboxylate–bridged iron(II) pair, although when isolated in oxy form they contain the μ-oxo or hydroxo (or aqua) exogeneous ligands in the carboxylate-bridged diiron(III) core. It is generally considered that manifestation of divergent catalytic reactions by non-heme iron proteins involve binding of oxygen to the diiron(II) unit producing peroxo-diiron(III) species, which, in turn, generate oxo-bridged iron species in higher oxidation states (FeIIIFeIV, FeIVFeIVetc.) that interact with substrates in different ways.

Numerous studies have been made on synthetic analogues modeling the diiron(II), and diiron(III) core structures of these proteins.9,11–14 The activation of dioxygen by diiron(II) complexes, characterization of intermediate peroxo-diiron(III) species and their reactivities have been the focus of interest. These studies have accrued a wealth of information concerning structure, spectroscopy, magnetism and reaction mechanisms of synthetic models that have led to better understanding of biological systems.

In continuation to our biomimetic studies using the macrocyclic ligand [LH4](ClO4)2,15–17 we have recently reported the chemistry involving FeIII, FeIII–O–FeIII, and ZnIIFeIII–O–FeIIIZnII complexes.18 The present study is concerned with reactivity of dioxygen with carboxylate-bridged diiron(II) complexes [FeII2L(μ-O2CR)]+. Cofacial oxo-bridged tetranuclear iron(III) complexes [{FeIII2L(μ-O2CR)}2(μ-O)2](ClO4)2 isolated as the reaction product have been structurally characterized and their magnetic exchange behaviour, proton NMR spectroscopy and redox property have been investigated.

ugraphic, filename = b610649h-u1.gif

Experimental

Materials

Reagent grade chemicals obtained from commercial sources were used as received. Solvents were purified and dried according to standard methods.19 2,6-Diformyl-4-methylphenol was prepared according to the literature method.20 The perchlorate salt of the macrocyclic ligand [LH4](ClO4)2 was prepared as reported earlier.21

Preparation of the complexes

Caution! All the perchlorate salts reported in this study are potentially explosive and therefore should be handled with care.

All the preparations were carried out under oxygen-free dry dinitrogen using standard Schlenk techniques.

[FeII2L(μ-O2CCH3)(CH3OH)2](ClO4)2(1). Dry methanol (50 cm3) was freed from dissolved oxygen and sequentially treated with 2,6-diformyl-4-methylphenol (0.66 g, 4 mmol), Fe(ClO4)2·6H2O (1.45 g, 4 mmol), 1,3-diaminopropane (0.3 g, 4 mmol), and NaOAc (0.44 g, 6 mmol). The resulting red solution was stirred at room temperature for 2 h after which the bulk of the solvent was removed under vacuum using a liquid nitrogen cold trap. The remaining solution (10 cm3) was filtered and cooled to ca. 5 °C when a red crystalline compound deposited. This was filtered and washed first with chilled dry ethanol and then with dry diethyl ether. Yield 0.45 g (60%). Found: C, 45.28; H, 4.93; N, 7.38. C28H37Fe2N4O10Cl requires: C, 45.64; H, 5.06; N, 7.60%. Selected IR data on KBr (ν/cm−1): 3410(br), 1632(s), 1576(s), 1420(m), 1322(m), 1320(m), 1095(s), 628(m). μeff/Fe at 300 K, 4.85 μB. Spectroscopic characterization of the compound in solution could not be made because of its air sensitivity.

Complex 1 and similar other carboxylate-bridged diiron(II) complexes were obtained in situ as described below for the preparation of bis-μ-oxo tetrairon(III) complexes 2–6.

[{FeIII2L(μ-O2CR)}2(μ-O)2](ClO4)2 (2–6) [R = H (2), CH3 (3), C2H5 (4), (CH3)3C (5), C6H5 (6)]. All complexes were prepared in the same way, as typified by the description given for complex 6.

To an acetonitrile solution (20 cm3) of [LH4](ClO4)2 (0.605 g, 1 mmol) solid Fe(ClO4)2·6H2O (0.725 g, 2 mmol) was added and was then slowly treated with a methanol solution (20 cm3) containing benzoic acid (0.244 g, 2 mmol) and triethylamine (0.606 g, 6 mmol). The solution was stirred at room temperature for 1 h, following which the solvent was removed under vacuum. The residue was stirred with methanol (20 cm3) for 0.5 h and filtered. The filtrate was then exposed to air and the solution was concentrated at 50 °C until crystalline product began to deposit. After cooling to room temperature, the product was collected by filtration and recrystallised from acetonitrile–methanol (1 : 2).

[{Fe2L(μ-O2CH)}2(μ-O)2](ClO4)2 (2). Yield 0.52 g (85%). Found: C, 44.60; H, 4.08; N, 8.34. C50H54Cl2Fe4N8O18 requires: C, 44.51; H, 4.03; N, 8.30%. MS(ESI positive in acetonitrile) m/z = 1250.09 (4%) [{Fe2L(μ-O2CH)}2(μ-O)2(ClO4)]+, 576.08 (100%) [{Fe2L(μ-O2CH)}2(μ-O)2]2+. IR (KBr, ν/cm−1) 1634(s), 1566(s), 1438(m), 1408(m), 1341(m), 1320(m), 1101(s), 843(m), 810(m), 626(m). UV-Vis [in acetonitrile, λmax/nm (ε/dm3 mol−1 cm−1)] 390 (14[thin space (1/6-em)]200), 355 (21[thin space (1/6-em)]000).
[{Fe2L(μ-O2CCH3)}2(μ-O)2](ClO4)2 (3). Yield 0.58 g (90%). Found: C, 45.39; H, 4.30; N, 8.11. C52H58Cl2Fe4N8O18 requires: C, 45.34; H, 4.24; N, 8.14%. MS(ESI positive in acetonitrile) m/z = 1277.12 (5%) [{Fe2L(μ-O2CCH3)}2(μ-O)2(ClO4)]+, 590.09 (100%) [{Fe2L(μ-O2CCH3)}2(μ-O)2]2+. IR (KBr, ν/cm−1) 1636(s), 1562(s), 1434(m), 1408(s), 1322(m), 1102(s), 1082(s), 838(m), 805(m), 625(m). UV-Vis [in acetonitrile, λmax/nm (ε/dm3 mol−1 cm−1)] 390 (13[thin space (1/6-em)]500), 356 (18[thin space (1/6-em)]500).
[{Fe2L(μ-O2C2H5)}2(μ-O)2](ClO4)2 (4). Yield 0.52 g (80%). Found: C, 46.25; H, 4.48; N, 7.89. C54H62Cl2Fe4N8O18 requires: C, 46.15; H, 4.45; N, 7.97%. MS(ESI positive in acetonitrile) m/z = 1305.15 (5%) [{Fe2L(μ-O2C2H5)}2(μ-O)2(ClO4)]+, 604.11 (100%) [{Fe2L(μ-O2C2H5)}2(μ-O)2]2+. IR (KBr, ν/cm−1) 1639(s), 1562(s), 1435(m), 1408(s), 1321(m), 1112(s), 1103(s), 1080(s), 838(m), 805(m), 624(m). UV-Vis [in acetonitrile, λmax/nm (ε/dm3 mol−1 cm−1)] 390 (20[thin space (1/6-em)]400).
[{Fe2L(μ-O2C(CH3)3)}2(μ-O)2](ClO4)2 (5). Yield 0.54 g (80%). Found: C, 47.60; H, 4.73; N, 7.75. C58H70Cl2Fe4N8O18 requires: C, 47.64; H, 4.79; N, 7.66%. MS(ESI positive in acetonitrile) m/z = 1362.22 (5%) [{Fe2L(μ-O2C(CH3)3)}2(μ-O)2(ClO4)]+, 632.14 (100%) [{Fe2L(μ-O2C(CH3)3)}2(μ-O)2]2+. IR (KBr, ν/cm−1) 1642(s), 1564(s), 1481(m), 1443(m), 1412(s), 1364(w), 1319(m), 1097(s), 840(m), 805(m), 625(m). UV-Vis [in acetonitrile, λmax/nm (ε/dm3 mol−1 cm−1)] 390 (14[thin space (1/6-em)]200), 355 (21[thin space (1/6-em)]000).
[{Fe2L(μ-O2C6H5)}2(μ-O)2](ClO4)2 (6). Yield 0.65 g (94%). Found: C, 49.41; H, 4.19; N, 7.44. C62H62Cl2Fe4N8O18 requires: C, 49.56; H, 4.13; N, 7.46%. MS(ESI positive in acetonitrile) m/z = 1402.16 (4%) [{Fe2L(μ-O2C6H5)}2(μ-O)2(ClO4)]+, 652.11 (100%) [{Fe2L(μ-O2C6H5)}2(μ-O)2]2+. IR (KBr, ν/cm−1) 1641(s), 1561(s), 1436(m), 1405(s), 1319(m), 1099(s), 841(m), 805(m), 729(m), 625(m). UV-Vis [in acetonitrile, λmax/nm (ε/dm3 mol−1 cm−1)] 390 (13[thin space (1/6-em)]500), 356 (19[thin space (1/6-em)]500).

Physical measurements

Elemental C, H and N analyses were performed on a Perkin-Elmer 2400II elemental analyzer. IR spetra were recorded using KBr disks on a Shimadzu FTIR 8400S spectrometer. The electronic spectra were recorded using a Perkin–Elmer 950 UV/VIS/NIR spectrophotometer. The electrospray ionization mass spectra (ESI-MS) were measured on a Micromass Qtof YA 263 mass spectrometer. The 1H NMR (300 MHz) spectra were recorded on a Bruker Avance DPX-300 spectrometer. Longitudinal relaxation times (T1) were measured by the inversion-recovery method. The T1 values were obtained by fitting of magnetization recovery curves, which were exponential in nature. The transverse relaxation times (T2) were estimated from the peak half-widths. Magnetic susceptibility of powdered samples were measured in the same way, as reported earlier,22 on a Faraday-type magnetometer using a Cahn RG electrobalance in the temperature range 4–300 K; the magnetic field applied was ≈1.2 T. Experimental susceptibility data were corrected for diamagnetism using Pascal’s constants.23 The cyclic voltammetric (CV) and square wave voltammetric (SWV) measurements were carried out in acetonitrile solution at room temperature under a nitrogen atmosphere using a BAS 100B electrochemical analyzer. The solutions were 10−3 mol dm−1 in complexes and 0.1 mol dm−1 in tetrabutylammonium perchlorate (TBAP) used as the supporting electrolyte. A three-electrode assembly comprising a glassy carbon working electrode, a platinum auxiliary electrode and a Ag/AgCl reference electrode was used. IR compensation was made automatically during each run. Under the experimental condition the ferrocene/ferrocenium couple was observed at 410 mV.

X-ray crystallography

Crystals suitable for structure determinations of [{Fe2L(μ-OAc)}2(μ-O)2](ClO4)2 (3) and [{Fe2L(μ-OBz)}2(μ-O)2](ClO4)2 (6) were obtained by slow evaporation of their acetonitrile–ethanol solutions. The crystals were mounted on glass fibres using perfluoropolyether oil. Intensity data were collected at 120 K on a Bruker–AXS SMART CCD diffractometer for 3, while for 6 at 293 K on a Siemens R3 m/v diffractometer. Graphite-monochromated Mo-Kα radiation (λ = 0.710 73 Å) was used in both cases. In the case of 3, for data processing and absorption correction the packages SAINT24 and SADABS24 were used. The structure was solved by direct and Fourier methods and refined by full-matrix least-squares based on F2 using SHELXTL24 and SHELXL-9725 software packages. In the case of 6, the intensity data were corrected for Lorentz and polarization effects and semiempirical absorption correction was made from psi-scans. The structure solution and refinement were made in the same way as 3 using the programs SHELXTL24 and SHELXL-97.25 The non-hydrogen atoms were refined anisotropically, while the hydrogen atoms were placed at the geometrically calculated positions with fixed isotropic thermal parameters. The perchlorate anion in 6 is disordered over two sites with half occupation each and the oxygens were treated with a split model. Crystal data and selected details of structure determinations are given in Table 1.‡
Table 1 Crystallographic data for [Fe2L(μ-OAc)2(μ-O)2](ClO4)2 (3) and [Fe2L(μ-OBz)2(μ-O)2](ClO4)2 (6)
  3 6
a R 1(F) = ∑‖Fo| − |Fc‖/∑|Fo|. b wR 2(F2) = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2. c S = [∑w(Fo2−Fc2)2/(N−P)]1/2. Where N is the number of data and P the total number of parameters refined.
Formula C52H58Cl2Fe4N8O18 C62H62Cl2Fe4N8O18
M 1377.36 1501.5
Crystal size/mm3 0.22 × 0.12 × 0.10 0.58 × 0.35 × 0.20
Crystal system Orthorhombic Monoclinic
Space group Pbcn C2/c
a/Å 16.936(3) 21.958(4)
b/Å 15.203(3) 15.182(3)
c/Å 21.457(3) 19.591(4)
α/° 90 90
β/° 90 90.61(2)
γ/° 90 90
U/Å3 5524.7(18) 6528(2)
Z 4 4
D/g cm−3 1.656 1.528
T/K 120(2) 293(2)
μ/mm−1 1.209 1.030
No. measured/observed reflections 43989/6578 7732/7547
Parameters refined 381 482
Final R1a, wR2b [I > 2σ (I)] 0.0583, 0.0876 0.0597, 0.1212
R 1 a, wR2b (all data) 0.2755, 0.1355 0.1307, 0.1511
S c 0.733 1.012

Result and discussion

Synthesis and characterization

The dinuclear iron(II) complex [FeII2L(μ-O2CCH3)(CH3OH)2](ClO4) (1) is readily obtained by the template condensation reaction involving 2,6-diformyl-4-methylphenol, 1,3-diaminopropane, iron(II) perchlorate and sodium acetate in the ratio (1 : 1 : 1 : 1.5) in methanol under strictly anaerobic conditions. Alternatively, the carboxylate-bridged complex species [FeII2L(μ-O2CR)]+ (R = H, CH3, C2H5, (CH3)3C, C6H5) can be generated in solution by reacting the preformed macrocyclic ligand [LH4](ClO4)2 with iron(II) perchlorate and a mixture of the carboxylic acid (RCO2H) and triethylamine. The diiron(II) complexes [FeII2L(pyridine)4](BF4)2 and [FeII2L(N-methylimidazole)4](BF4)2 have been studied previously,26 including their magnetic exchange behaviour and structure determination of the later compound. Although these compounds have been reported to be air-sensitive, their reaction with oxygen in solution has not been investigated. [FeII2L(μ-O2CCH3)(CH3OH)2](ClO4) (1) has been isolated in the solid state and characterized by its elemental analysis, IR spectrum (νC=N = 1632 cm−1, νasCO2− = 1576 cm−1, νsCO2− = 1420 cm−1, νClO4− = 1095 and 628 cm−1) and room temperature magnetic moment (4.85 μB/per Fe), which indicates that the iron(II) centres are high-spin (S = 2).

Methanol solution of [FeII2L(μ-O2CR)]+ (R = H, CH3, C2H5, (CH3)3C, C6H5) on exposure to air immediately transforms to corresponding oxo-bridged tetranuclear iron(III) complex [{FeIII2L(μ-O2CR)}2(μ-O)2](ClO4)2 (2–6). The formation of 2–6 according to eqn (1) takes place quantitatively.

 
2 [FeII2L(μ-O2CR)(CH3OH)2]+ + O2 → [{FeIII2L(μ-O2CR)}2(μ-O)2]2+ + 4 CH3OH(1)
The mechanism of formation of (μ-oxo)-diiron(III) compound from dinuclear iron(II) compound by oxygen uptake has been suggested27 to take place via a bimolecular pathway involving a tetranuclear diiron(II) diiron(III) intermediate.

In the macrocyclic systems under consideration, presumably a four-centre μ, μ-η2:η2 type binding of oxygen to the diiron(II) centres of two macrocyclic units takes place in a concerted way (Scheme 1). Four electrons transfer simultaneously from the iron(II) centres to the dioxygen and consequent to its homolytic cleavage (O2 + 4e− → 2O2−), leads to the formation of the dioxo-tetrairon(III) [FeIII4O2] core.


scheme, filename = b610649h-s1.gif
Scheme 1

ESI (positive) mass spectra of the tetranuclear iron(III) complexes [{FeIII2L(μ-O2CR)}2(μ-O)2](ClO4)2 (2–6) in acetonitrile exhibit in all cases two peaks due to the cations [{FeIII2L(μ-O2CR)}2((μ-O)2(ClO4)]+ and [{FeIII2L(μ-O2CR)}2((μ-O)2]2+. The intensity of the monopositive peak is much weaker (4–5%) relative to the dipositive cation, which forms the base peak. As a typical case, the observed and the simulated spectra of the pivalate-bridged compound (5), taking into consideration the relative isotopic abundances of the constituent elements, are shown in Fig. S1.†

The IR spectra of complexes 2–6 exhibit characteristic νC=N vibration between 1634 cm−1 (for 2) and 1641 cm−1 (for 6). The νasCO2− and νsCO2− bands are observed in the ranges 1565 to 1560 cm−1 and 1410 to 1405 cm−1, respectively. The difference in energy between these two bands by about 155 cm−1 is consistent with the bridging mode of carboxylate binding.28 A medium intensity band observed between 843 and 838 cm−1 in 2–6 is due to the νas Fe–O–Fe vibration.14 Finally, the two characteristic bands due to the ionic perchlorate are observed at ca. 1100 and 625 cm−1.

In the UV-Vis region (300–900 nm), complexes 2–6 in acetonitrile exhibit an absorption peak at 355 nm (ε = 18[thin space (1/6-em)]500–21[thin space (1/6-em)]500 dm3 mol−1 cm−1) and a shoulder at about 390 nm (ε ≈ 14 000 dm3 mol−1 cm−1). The oxo-bridged diiron(III) complexes are known to exhibit several symmetry related oxo → iron(III) charge-transfer transitions whose energies depend on the Fe–O–Fe bridged angle.29–31 When the Fe–O–Fe angle is close to 180°, a single band is observed between 300 and 500 nm due to an oxo → iron(III) dxz and dyz transition of π-symmetry. However, as the Fe–O–Fe angle deviates from linearity, this band gets split. As will be seen, the Fe–O–Fe angle in 3 and 6 are 155.8 (3)° and 161.7 (2)°, respectively. Thus, the bands observed at 355 and 390 nm can be attributed to oxo → iron(III) charge-transfer transition, albeit the band at 355 nm may have a contribution from internal transition of the macrocyclic ligand.

Crystal structure

The X-ray crystal structures of the tetrairon(III) complexes [{FeIII2L(μ-O2CCH3)}2(μ-O)2](ClO4)2 (3) and [{FeIII2L(μ-O2CC6H5)}2(μ-O)2](ClO4)2 (6) have been determined. Compound 3 crystallized in the orthorhombic form with the space group Pbcn, whereas 6 crystallized in the monoclinic form with the space group C2/c. The Fe4-cation in compound 3 has crystallographically imposed twofold symmetry with atoms C13, C14, C27 and C28 on the twofold axis. In the case of compound 6, the geometric centre of the Fe4-cation, however, lies on a crystallographic inversion centre. The structural projections of the cations in 3 and 6 along with their numbering schemes are shown in Figs. 1 and 2(a), respectively. Selected bond distances and angles of the two compounds are compared in Table 2.
The ORTEP representation of the cation [{Fe2L(μ-O2CCH3)}2(μ-O)2](ClO4)2 in 3 showing 50% probability displacement ellipsoids. The letter ‘A’ in atom labels indicates atoms at (1 − x, y, 1/2 − z).
Fig. 1 The ORTEP representation of the cation [{Fe2L(μ-O2CCH3)}2(μ-O)2](ClO4)2 in 3 showing 50% probability displacement ellipsoids. The letter ‘A’ in atom labels indicates atoms at (1 − x, y, 1/2 − z).

(a) A structural projection for the cation [{Fe2L(μ-O2CC6H5)}2(μ-O)2](ClO4)2 in 6. The letter ‘A’ in atom labels indicates atoms at (1/2 − x, ½ − y, −z). (b) The ORTEP representation of the major conformation of the cation in the asymmetric unit of complex 6 showing 50% probability displacement ellipsoids.
Fig. 2 (a) A structural projection for the cation [{Fe2L(μ-O2CC6H5)}2(μ-O)2](ClO4)2 in 6. The letter ‘A’ in atom labels indicates atoms at (1/2 − x, ½ − y, −z). (b) The ORTEP representation of the major conformation of the cation in the asymmetric unit of complex 6 showing 50% probability displacement ellipsoids.
Table 2 Selected bond lengths (Å) and angles (°) for [Fe2L(μ-OAc)2(μ-O)2](ClO4)2 (3)a and [{Fe2L(μ-OBz)}2(μ-O)2](ClO4)2 (6)b
3 6
a For 3: ‘A’ indicates atoms at (1 − x, y, 1/2 − z). b For 6: ‘A’ indicates atoms at (1/2 − x, 1/2 − y, −z).
Fe(1)–O(3) 1.795(6) Fe(1)–O(5) 1.786(3)
Fe(2)–O(3) 1.787(6) Fe(2)–O(5A) 1.787(3)
Fe(1)–O(2) 2.036(7) Fe(1)–O(1) 2.064(3)
Fe(2)–O(4) 2.016(7) Fe(2)–O(1) 2.070(3)
Fe(1)–O(2A) 2.051(7) Fe(1)– O(2) 2.024(3)
Fe(2)–O(4A) 2.038(6) Fe(2)– O(2) 2.027(3)
Fe(1)–O(1) 2.118(6) Fe(1)–O(3) 2.181(3)
Fe(2)–O(5) 2.147(6) Fe(2)–O(4) 2.150(3)
Fe(1)–N(1) 2.093(8) Fe(1)–N(1) 2.064(4)
Fe(2)–N(3) 2.054(9) Fe(2)–N(4) 2.056(4)
Fe(1)–N(2) 2.075(8) Fe(1)–N(2) 2.097(4)
Fe(2)–N(4) 2.047(9) Fe(2)–N(3) 2.096(4)
Fe(1)⋯Fe(1A) 3.071(3) Fe(1)⋯Fe(2) 3.0347(9)
Fe(1)⋯Fe(2) 3.5027(16) Fe(1)⋯Fe(2A) 3.5281(11)
O(2)–Fe(1)–O(2A) 82.5(3) O(1)–Fe(1)–O(2) 84.32(12)
O(4)–Fe(2)–O(4A) 81.9(3) O(1)–Fe(2)–O(2) 84.11(12)
N(1)–Fe(1)–N(2) 97.5(3) N(1)–Fe(1)–N(2) 98.0(2)
N(3)–Fe(2)–N(4) 99.4(3) N(3)–Fe(2)–N(4) 96.5(2)
O(2)–Fe(1)–N(2) 88.4(3) O(1)–Fe(1)–N(1) 88.80(14)
O(4)–Fe(2)–N(4) 88.5(3) O(2)–Fe(2)–N(3) 87.2(2)
O(2A)–Fe(1)–N(1) 90.2(3) O(2)–Fe(1)–N(2) 86.95(14)
O(4A)–Fe(2)–N(3) 88.5(3) O(1)–Fe(2)–N(4) 89.72(14)
O(2)–Fe(1)–N(1) 167.9(3) O(1)–Fe(1)–N(2) 169.71(14)
O(4)–Fe(2)–N(3) 165.5(3) O(2)–Fe(2)–N(4) 165.58(14)
O(2A)–Fe(1)–N(2) 168.3(3) O(2)–Fe(1)–N(1) 162.61(14)
O(4A)–Fe(2)–N(4) 167.7(3) O(1)–Fe(2)–N(3) 167.3(2)
O(1)–Fe(1)–O(3) 178.2(3) O(3)–Fe(1)–O(5) 179.40(14)
O(3)–Fe(2)–O(5) 179.1(3) O(4)–Fe(2)–O(5A) 176.30(14)
Fe(1)–O(3)–Fe(2) 155.8(3) Fe(1)–O(5)–Fe(2A) 161.7(2)
Fe(1)–O(2)–Fe(1A) 97.4(3) Fe(1)–O(1)–Fe(2) 94.46(12)
Fe(2)–O(4)–Fe(2A) 98.1(3) Fe(1)–O(2)–Fe(2) 97.05(13)

The structures of 3 and 6 consist of two {Fe2L(μ-O2CR)} units, which are cofacially linked to each other by the two Fe–O–Fe bridges and the two ClO4− anions are well-separated from the cationic species. In each {Fe2L(μ-O2CR)} unit, the two iron centres are triply bridged by the two phenoxides of the macrocyclic ligand and the carboxylate ligand. Each of the metal centres is additionally coordinated by two imine nitrogens of the macrocycles ligand. Thus, each of the iron centres obtains an axially distorted octahedral geometry where the equatorial plane is provided by the N2O2 donors, and a carboxylate oxygen and an oxo oxygen act as the axial donors.

In the case of 6, the perchlorate anion in the asymmetric unit is disordered over two sites with half occupation each. Such disordering of perchlorate, however, is absent in 3. Moreover, in 6, the positions of C(2) and C(28) are split over two sites C(2′) and C(28′) with site occupancies 0.8/0.2 for C(2)/C(2′) and 0.85/0.15 for C(28)/C(28′). Fig. 2(b) shows the major conformation. Further, the anisotropic displacement parameters of the benzoate phenyl ring atoms C(23), C(24) and C(25) indicate thermal disorder. It appears that this phenyl ring vibrates to a small extent parallel to its plane.

The average in-plane Fe–O(phenoxo) distances in 3 and 6 are 2.035(18) Å and 2.046(18) Å, respectively, while corresponding Fe–N distances are 2.067(22) Å for 3 and 2.078(21) Å for 6. The average axial Fe–O(acetate) distance in 3, 2.132(14) Å, is considerably longer relative to its trans Fe–O(oxo) distance, 1.791(4) Å, due to the trans influence of the oxo group. Similar distances in the benzoate compound 6 are 2.166(16) Å and 1.786(1) Å, respectively. The Fe–O–Fe phenoxo bridge angle in 3 and 6 are 97.8 ± 0.3° and 95.8 ± 1.3°, respectively, while the Fe–O–Fe bridge angles in the two compounds are 155.8(3)° (for 3) and 161.7(2)° (for 6). The distances between the iron pairs bridged by the phenoxides and the oxo group respectively in the two compounds are 3.071(6) Å and 3.503(8) Å in 3 and 3.035(4) Å and 3.528(4) Å in 6. In the {Fe2L(μ-O2CR)} unit, each of the iron centres are displaced from their N2O2 equatorial mean planes towards the oxo group by about 0.17 Å (in 3) and 0.21 Å (in 6). Because the axial Fe–O(oxo) bond length is considerably shorter relative to those of the in-plane Fe–O(phenoxo) and Fe–N bond lengths, the macrocyclic ligand is folded to acquire a somewhat bowel-like shape. Indeed, in the tetranuclear complex cation, the two macrocyclic units have concave–convex relationship. The extent of deviation of the macrocycles from planarity can be gauged from the dihedral angle formed by the mean planes of the two aromatic rings, which is 15.2° for 3 and 9.8° for 6.

Magnetic properties

Variable-temperature magnetic susceptibility measurements have been carried out for compounds 3 and 6 between 4 and 300 K. The plots of χM and μeffvs. T for 3 and 6 are shown Fig. 3 and Fig. S2, respectively. For both the compounds, the magnetic moments decrease from about 3.8 μB at 300 to ca. 0.6 μB at 4 K, indicating strong antiferromagnetic coupling between S = 5/2 centres. As shown in Scheme 2, ignoring any effective exchange coupling through the carboxylate bridge, the magnetic properties of the tetrairon(III) complexes are controlled by two exchange coupling constants, one involving the oxo bridges (J) and the other involving the phenoxo bridges (J′). Accordingly, the spin Hamiltonian is given by eqn (2).
 
Ĥ = −2J (Ŝ1Ŝ2 + Ŝ3Ŝ4) −2J′ (Ŝ1Ŝ4 + Ŝ2Ŝ3)(2)
where Ŝ1 = Ŝ2 = Ŝ3 = Ŝ4 = 5/2
scheme, filename = b610649h-s2.gif
Scheme 2

Molar magnetic susceptibility (□) and effective magnetic moment (○) vs. temperature for [{Fe2L(μ-O2CCH3)}2(μ-O)2](ClO4)2 (3). The solid lines result from a least-squares fit to the theoretical expression given in eqn (3).
Fig. 3 Molar magnetic susceptibility (□) and effective magnetic moment (○) vs. temperature for [{Fe2L(μ-O2CCH3)}2(μ-O)2](ClO4)2 (3). The solid lines result from a least-squares fit to the theoretical expression given in eqn (3).

Taking into consideration the dimer of dimer symmetry of the complexes and the fact that exchange coupling via the oxo bridges is much stronger relative to that via the phenoxide bridges14 (|J| > |J′|), the intramolecular spin exchange in 3 and 6 can be treated by the molecular field approximation.23,32–34

Accordingly, the molar susceptibility data have been fitted to the following expression

 
χM = [(1 − p) 2Ng2μB2 (A/B)/k{T − 2J′(A/B)}] + (4.375p/T) + TIP(3)
In this expression, A = 2 exp (2x) + 10 exp (6x) + 28 exp (12x) + 60 exp (20x) + 110 exp (30x), B = 1 + 3 exp (2x) + 5 exp (6x) + 7 exp (12x) + 9 exp (20x) + 11 exp (30x), x = J/kT, p = mole percentage of paramagnetic impurity and TIP is the temperature independent paramagnetism. The contribution of TIP has been considered to be zero because for high-spin iron(III) the ground state is much lower in energy than the first excited state.35

As shown in Fig. 3, the least-squares fit of the susceptibility data of 3 to eqn (3) with g = 2.0 provides J = −115 cm−1, J′ = −4 cm−1 and p = 0.25. Similar good fit has also been obtained for the susceptibility data of 6 (Fig. S2) with J = −109 cm−1, J′ = −4 cm−1 and p = 0.20. Since the magnitude of J is many-fold larger than J′, it has been necessary to construct an error surface plot to determine whether a global minimum is reached. A view of the error surface plot as a function of both J and J′ for 3, as shown in Fig. S3, reveals that indeed global minimization has been obtained with J = −115 cm−1, J′ = −4 cm−1.

In oxo-bridged iron(III) complexes, the dependence of J on the Fe–O–Fe angle φ and the average Fe–O distance r has been analyzed by an angular and radial overlap model36 and is given by eqn (4).

 
−2Jmodel = 1.337 × 108(3.536 + 2.488 cosφ + cos2φ) exp (−7.909 r)(4)
Using the values φ = 155.8 (for 3) and 161.7 (for 6) and r = 1.791 Å (for 3) and 1.786 Å (for 6), the calculated J values turn out to be −99 cm−1 for 3 and −102 cm−1 for 6. As compared to the experimentally determined J values −115 cm−1 (3) and −109 cm−1 (6), the calculated J values are somewhat less. One possibility for this small deviation could be that the iron(III) centres involved in exchange coupling through the oxo bridge are also engaged in exchange coupling to the phenoxide bridges.

Proton NMR spectra

The proton NMR spectra of complexes 3–6 have been measured in CD3CN and all of them exhibit identical spectra, as could be seen in Fig. 4 for complexes 3 and 6. It may be noted that the two compounds aside from having identical hyperfine-shifted resonances also exhibit in the case of 3 an additional signal at 4.42 ppm due to the free acetate, while in the case of 6 two such signals due to the free benzoate are observed at 7.68 and 7.12 ppm (intensity ratio 4 : 1). Clearly, in solution, the carboxylate caps get detached from the metal centres. Fig. 4 shows that a total number of five hyperfine-shifted signals are observed at 84, 41, 19.5, 10.83 and 5.63 ppm and their intensity ratios being 2 : 2 : 2 : 2 : 3. No additional peak could be detected when the scan range was extended from −50 to +200 ppm. The spectra recorded in (CD3)2SO also exhibit similar features with the peaks appearing at 85, 44.5, 16.7, 10.97 and 5.90 ppm.
Proton NMR spectra of [{Fe2L(μ-O2CCH3)}2(μ-O)2](ClO4)2 (3) and in [{Fe2L(μ-O2CC6H5)}2(μ-O)2](ClO4)2 (6) in CD3CN.
Fig. 4 Proton NMR spectra of [{Fe2L(μ-O2CCH3)}2(μ-O)2](ClO4)2 (3) and in [{Fe2L(μ-O2CC6H5)}2(μ-O)2](ClO4)2 (6) in CD3CN.

Spectral assignment has been made by measuring proton longitudinal relaxation time T1 and transverse relaxation time T2 and also taking into consideration the number of protons associated with each of the signals. Both T1 and T2 correlate the proximity of the proton site from the paramagnetic centre. While T1 values are obtained by the inversion recovery method, T2 values are obtained from the relation 1/T2 = πΔ1/2, where Δ1/2 is the full width of a signal at half-height. Thus, for closure proximity of a proton to the paramagnetic centre, a shorter T1 and a broader line width are to be expected.37 Since a dipolar mechanism plays a dominant role to induce paramagnetic chemical shifts, T1 has a r6 dependence on the metal–proton distance (rFe⋯H). Therefore, using relative T1 values corresponding to rFe⋯H distances can be obtained by using the relation rFe⋯H = rref (T1/Tref)1/6, where rref and Tref are the reference values. The proton site which is farthest from the metal centre and hence, experiencing minimum contact/pseudocontact shift(s) and also can be recognized unambiguously, is taken as the reference point.

In Table 3, T1, Δ1/2 and relative areas of the signals of [{FeIII2L}(μ-O)2}]4+ species are listed. Based on these values assignments of the proton NMR signals have been made. The symmetry of [{FeIII2L}(μ-O)2}]4+ requires that if all the CH2 protons of the macrocyclic ligands are diastereotopic in that case the total number of signals expected to be observed are seven and their intensity ratios would be 3 : 2 : 2 : 2 : 2 : 1 : 1. However, since only five signals have been observed and with the intensity ratio 3 : 2 : 2 : 2 : 2, it can be concluded that the two signals with the ratio 1 : 1 have collapsed to a single one and another signal has not been seen. This means that although CH2CH2CH2 protons are anisochronous (CHax and CHeq), the CH2CH2CH2 protons are isochronous. The missing signal is probably due to the CH[double bond, length as m-dash]N proton which might be too broad to be observed. Good agreements between the Fe⋯H distances obtained from the X-ray structure and from T1 measurements provide strong support to the correctness of the signal assignments.

Table 3 Chemical shifts, T1, T2, line widths, Fe⋯H distances and spectral assignments for 3–6 in CD3CN
        Fe⋯H/Å  
δ/ppm T 1/ms Δ1/2/Hza T 2/ms Calc. X-ray Assignment
a Full width at half-height. b Reference values.
84 1.5 220 1.4 3.69 3.39 CH2CH2CH2(Heq)
41 1.6 165 1.9 3.72 3.87 CH2CH2CH2(Hax)
19.5 2.6 160 2.0 4.04 3.75 CH2CH2CH2
10.83 13.4 40 8.0 5.32 5.21 Ar
5.63 98b 12 25.5   7.41b CH3

Electrochemistry

The electrochemical behaviour of complexes 2–6 have been studied in acetonitrile. In view of the dissociation of the bridging carboxylate moiety in solution, all the complexes exhibited identical electrochemical behaviour. Typical cyclic voltammograms observed for 6 at two different scan rates are shown in Fig. 5. In the potential range 0 to 1000 mV, at a scan rate of 50 mV s−1, three cathodic waves with their peaks at −350, −535 and −785 mV are observed, while in the return sweep broad anodic responses at ca. −650, −460 and ca. −250 mV could be observed (Fig. 5(a)). On increasing the scan rate to 1000 mV s−1, the cathodic waves are shifted to less negative potentials, viz. −320, −500 and −710 mV (Fig. 5(b)), whereas in the return sweep a broad envelop at −650 mV and a peak at −410 mV are observed. The square wave voltammogram of the compound (Fig. 6) exhibits three peaks at −325, −500 and −750 mV with their current heights in the ratio 2 : 1 : 1.
Cyclic voltammograms of [{Fe2L(μ-O2CC6H5)}2(μ-O)2](ClO4)2 (6) in acetonitrile at a scan rate of (a) 50 mV s−1 and (b) 1000 mV s−1.
Fig. 5 Cyclic voltammograms of [{Fe2L(μ-O2CC6H5)}2(μ-O)2](ClO4)2 (6) in acetonitrile at a scan rate of (a) 50 mV s−1 and (b) 1000 mV s−1.

Square-wave voltammogram of [{Fe2L(μ-O2CC6H5)}2(μ-O)2](ClO4)2 in acetonitrile.
Fig. 6 Square-wave voltammogram of [{Fe2L(μ-O2CC6H5)}2(μ-O)2](ClO4)2 in acetonitrile.

Redox properties of oxo-bridged iron(III) complexes derived from polydentate ligands have been the subject of several studies.38–40 We have recently reported18 that the oxo-bridged heteronuclear complex [{ZnIILFeIII(μ-OAc)}2(μ-O)]2+ undergoes one electron quasi-reversible reduction with E1/2 at −300 mV to oxo-bridged FeIII–O–FeII species which then gets irreversibly reduced at −700 mV. Other reported studies on FeIII–O–FeIII systems have shown38–40 that the addition of the first electron occurs either reversibly or quasi-reversibly to produce oxo-bridged mixed-valence FeIII–O–FeII species. The second electron addition, however, occurs irreversibly indicating that along with the electron transfer cleavage of the oxo-bridge takes place. So far, there is only one reported study41 where a mixed-valent FeIII–O–FeII compound has been isolated in the solid state and has been characterized both structurally and spectroscopically.

The observed redox behaviour of the oxo-bridged tetrairon(III) complexes are, clearly, quite complex in nature. Nevertheless, a tentative interpretation of the voltammetric responses is given in Scheme 3. As indicated in Fig. 6, the first reduction step involves two electron transfers occurring simultaneously at ca. −320 mV generating the mixed-valence [{FeIII⋯FeII}2(μ-O)2] species II. The second reduction step involves one electron and two proton transfer reactions in which along with the reduction of one iron(III) centre, the associated oxo-bridge also gets removed as H2O. In the third step, III is reduced in the same way at ca. −700 mV producing the diiron(II) species IV. Thus, the overall reaction (5) involves four electron and four proton transfers

 
[{FeIIILFeII}2(μ-O)2]4+ + 4e− + 4H+ → 2[FeIIILFeII]2+ + 2H2O(5)

Conclusion

A series of oxo-bridged tetranuclear iron(III) complexes [{FeIII2L(μ-O2CR)}2(μ-O)2](ClO4)2 [R = H, CH3, C2H5, (CH3)3C, C6H5] derived from the tetraimidiphenolate macrocyclic ligand L2− have been obtained in high yield from the corresponding diiron(II) complexes [{FeII2L(μ-O2CR)]+ through activation of molecular oxygen. The binding of the oxygen molecule to two macrocyclic diiron(II) complex units presumably occurs in μ, μ-η2:η2 mode. Simultaneous release of one electron from each of the four iron(II) centres to the oxygen molecule leads to its reduction and hemolytic cleavage of the O–O bond. The two O2− ions thus produced bind the two diiron(III) macrocyclic units in a face-to-face manner. The binding of oxygen and intramolecular redox reaction seems to be a concerted process.
scheme, filename = b610649h-s3.gif
Scheme 3

In solution, the carboxylate caps of the iron(III) compounds become dissociated and the paramagnetic proton spectroscopic behaviour of the resulting [{FeIII2L}2(μ-O)2]4+ cation has been studied. Variable-temperature magnetic properties of the acetate and benzoate-bridged compounds have been modeled by considering two antiferromagnetic exchange pathways through the phenoxo bridges and the oxo bridge. The exchange coupling constant involving the oxo bridge (J ≈ −110 cm−1) is much stronger as compared to that of the phenoxo bridges (J′ ≈ −4 cm−1). The electrochemical study has indicated that [{FeIII2L}2(μ-O)2]4+ undergoes two electron reduction to generate chemically unstable [{FeIILFeIII}2(μ-O)2]2+ species, which then gets reduced to [FeIILFeII]2+ in two steps by involving coupled one-electron two-proton transfers.

Acknowledgements

MG and PB are thankful to CSIR, India for the award of a research fellowship.

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Footnotes

† Electronic supplementary information (ESI) available: observed and simulated ESI-MS of compound 5 (Fig. S1), variable-temperature (4–300 K) magnetic susceptibility (χM) and magnetic moment (μeff) plots for compound 6 (Fig. S2), error-surface plot for compound 3 (Fig. S3).
‡ CCDC reference numbers CCDC 615601 (3) and 615602 (6). For crystallographic data in CIF or other electronic format see DOI: 10.1039/b610649h.
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