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Cite this: Dalton Trans., 2013, 42, 12274

Received 2nd May 2013,Accepted 21st June 2013

DOI: 10.1039/c3dt51153g

www.rsc.org/dalton

Synthesis, crystal structure, magnetic property and DFTcalculations of an unusual dinuclear µ2-alkoxidobridged iron(III) complex†

Rituparna Biswas,a Carmen Diaz,b Antonio Bauzá,c Antonio Frontera*c andAshutosh Ghosh*a

A new dinuclear di(μ-alkoxido) bridged complex [Fe2L2] (1) (H3L = N,N’-bis{1-(2-hydroxyphenyl)-

ethylidene}-2-hydroxy-1,3-propanediamine) has been synthesized and characterized. The structure of

1 consists of a centrosymmetric dimer where two crystallographically equivalent metal ions are asymme-

trically bridged by two alkoxido oxygen atoms. In the structure, each ligand coordinates to one Fe(III)

centre and consequently the imino nitrogen atoms are in cis positions, which is rather unusual for this

ligand. Variable-temperature magnetic susceptibility measurements of the complex indicate that the two

iron(III) centres are antiferromagnetically coupled (J = −17.46 cm−1). The exchange mechanism has been

investigated by means of DFT calculations. In addition, the theoretical study has been also used to ration-

alize the unusual coordination mode of the Schiff base ligand. Moreover, the influence of the weak

forces or solvent molecules present in the solid-state structure for the conformational change is also ana-

lysed theoretically. This theoretical calculation incorporates two more similar structures, [Fe2L’2]·CH2Cl2 (2)

and [Fe2L’2]·2CH3CN (3) (H3L’ = N,N’-bis(salicylidene)-1,3,-diaminopropan-2-ol), for a comparative study.

Complex 2 has been reported previously in the literature with the usual binding mode of the ligands.

We have repeated its synthesis and recrystallized it from another solvent (acetonitrile) to get complex

3 which is isostructural with 2 but has acetonitrile as solvent molecule.

Introduction

The chemistry of iron(III) complexes is dominated by oxygen-bridged species because of the high oxygen affinity of theiron(III) ion.1 The interest in oxygen-bridged dinuclear iron(III)units is largely due to their biological relevance.2 Recently,special attention has been paid to oxygen-bridged diiron(III)complexes, stimulated mainly by the discovery of the Fe–O–Fecore in the active sites of a number of non-heme metallo-enzymes and metalloproteins such as hemerythrin, a dioxygencarrier protein.3 Synthetic efforts are made to prepare severalinteresting oxygen-bridged polyiron compounds, which have

been of considerable interest for several decades, in relation tomagnetic superexchange studies.4 Oxygen-bridged diiron(III)complexes fall into three classes: oxido-bridged,5 hydroxidobridged,5a,6 and alkoxido or phenoxido7 bridged species. It hasbeen noted that the antiferromagnetic interaction mediatedvia oxido-bridge is much stronger than that mediated via otherbridges.8

Among the ligands that produce µ-alkoxido bridged com-plexes, the binucleating Schiff base ligand N,N′-bis(salicyli-dene)-1,3,-diaminopropan-2-ol (H3L′) and its derivatives are ofspecial importance because they afford binuclear complexesrather easily with most of the 1st transition elements. Theseligands bind to the metal centres in two different ways(Scheme 1). Most commonly, it is shared by the two adjacentmetal centres (Scheme 1a).9 However, in a few cases it is alsochelated to one metal centre (Scheme 1b).10 In the case of tri-valent metal ions [M(III) = Fe(III), Mn(III) and Co(III)], usuallybinuclear complexes of MIII

2L′2 type are formed with theseligands. Among them, in all the Mn(III) complexes11 theligands are shared but in the only reported Co(III) complex,each Schiff base ligand binds to one metal center.10a However,in Fe(III) complexes, both types of coordination are found.Among the seven10b,12 reported dinuclear bis-µ-alkoxido-

†Electronic supplementary information (ESI) available: Cyclic voltammogramsof complex 1 and crystal data, structure refinement and dimensions in the metalcoordination sphere of complex 3 are included. CCDC 930142 (for 1) and930143 (for 3). For ESI and crystallographic data in CIF or other electronicformat see DOI: 10.1039/c3dt51153g

aDepartment of Chemistry, University College of Science, University of Calcutta, 92,

A.P.C. Road, Kolkata-700 009, India. E-mail: ghosh_59@yahoo.combDepartament de Química Inorgànica, Universitat de Barcelona, Marti i Franques

1-11, 08028 Barcelona, SpaincDepartament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km

7.5, 07122 Palma de Mallorca (Balears), Spain. E-mail: toni.frontera@uib.es

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bridged Fe(III) complexes, the ligand is shared by two adjacentFe(III) centres in four compounds and in the other three wherethe Schiff base has been reduced, it coordinates to one metalion. It may also be noted that imino nitrogen atoms occupythe trans positions when the ligand is shared by the twometals but are bonded to the cis positions when the ligand ischelated to one metal ion.

In the present work we report the synthesis, crystal struc-ture, magnetic property, electrochemistry and DFT calculationsof a new dinuclear double µ2-alkoxido bridged Fe(III) com-pound [Fe2L2] (1) (H3L = N,N′-bis{1-(2-hydroxyphenyl)-ethyli-dene}-2-hydroxy-1,3-propanediamine). The complex is a rareexample in which each Schiff base ligand coordinates to oneFe(III) centre. The two different coordination modes of theSchiff base ligand as described above have not been rationali-zed theoretically to date. We, therefore, perform a DFT calcu-lation on complex 1 along with two other Fe(III) compounds(2 and 3) in which the ligands are shared by the two adjacentmetal centres. Complex 2, a bis-µ-alkoxido bridged dinuclearFe(III) compound, [Fe2L′2]·CH2Cl2 (H3L′ = N,N′-bis(salicylidene)-1,3-diaminopropan-2-ol), has been reported previously.12a Werepeated the synthesis of 2 and crystallized it from a differentsolvent (acetonitrile instead of dichloromethane) and obtained[Fe2L′2]·2CH3CN (3). Single crystal X-ray analysis confirms thatit has the same molecular structure as 2. We have computedthe energies of dimers 1–3 and the results established thatweak forces, such as hydrogen bonds and the presence ofdifferent solvent molecules, are capable of directing the for-mation of a particular geometrical isomer either by compen-sating for the energy differences or by providing stabilizationthrough weak noncovalent interactions.

Experimental sectionStarting materials

1,3-Diaminopropan-2-ol, 2-hydroxyacetophenone and anhy-drous iron(III) chloride were purchased from Merck and usedas received. All other solvents were of reagent grade and wereused without further purification.

Physical measurements

Elemental analyses (carbon, hydrogen and nitrogen) were per-formed using a 2400 series II CHN analyzer. IR spectra in KBr

(4500–500 cm−1) were recorded using a Perkin-Elmer RXIFT-IR spectrophotometer. Electronic spectra (1200–200 nm) inCH3CN (for 1 and 3) were recorded in a Hitachi U-3501 spec-trophotometer. The magnetic measurements were carried outin the “Servei de Magnetoquimica (Universitat de Barcelona)”on polycrystalline samples (20 mg) with a Quantum DesignSQUID MPMSXL susceptometer in an applied field of 5000 Gand 300 G in the temperature ranges of 2–300 K and 2–30 K,respectively. Field dependence of magnetization measure-ments were carried out in the same instrument at 2 K. Theexperimental magnetic susceptibility data were corrected forthe diamagnetism estimated from Pascal’s tables and sampleholder calibration.13

Theoretical methods

The energies of all compounds described in the present studywere computed at the BP86-D3/def2-TZVPD level of theorywithin the program TURBOMOLE version 6.4.14 This DFTmethod (BP86-D3)15 has been successfully used before tostudy energetic features of coordination compounds.16 Tocompare the relative stabilities of the compounds, the geome-tries have been fully optimized in the gas phase without sym-metry constraints starting from the crystallographiccoordinates. Frequency calculations have been performed atthe same level of theory to confirm that the geometries corre-spond to true minima. The experimental and theoretical dis-tances and angles obtained for compounds 1 and 3 are verysimilar (see Table S2†), giving reliability to the level of theory.To evaluate the noncovalent interactions in the solid state, wehave used the crystallographic coordinates. The interactionenergies were calculated with correction for the basis setsuperposition error (BSSE) by using the Boys–Bernardi coun-terpoise technique.17

For the magnetic properties, we have used the crystallo-graphic coordinates. The magnetic coupling constants aredescribed using the Heisenberg model (see the DFT calcu-lations of magnetic properties subsection). For DFT-based wavefunctions, a reasonable estimate of the exchange coupling con-stants can be obtained from the energy difference between thestate with the highest spin, EHS, and the low spin wave function,EBS (namely the broken-symmetry18 solution) obtained by justflipping one of the spins through eqn (1), where J is theexchange coupling constant and S1 and S2 the local spins oncentres 1 and 2 (for dinuclear FeIII complexes, S1 = S2 = 5/2):

J ¼ ðEBS � EHSÞ=ð2S1S2 þ S2Þ ð1ÞThe hybrid B3LYP functional19 has been used in all calcu-

lations as implemented in Gaussian-0920 using the 6-31+G*basis set for all atoms. Due to the small magnitude of theexchange coupling constant, all energy calculations must beperformed including the SCF = tight option of Gaussian toensure sufficiently well converged values for the calculatedenergies. The approach used for determining the exchangecoupling constants for dinuclear complexes has beendescribed before in the literature.21

Scheme 1 (a) Sharing of ligands by two metal ions; (b) chelation of ligands toone metal ion.

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It should be mentioned that the widely and successfullyused22 broken symmetry DFT approach is not a uniquemethodology to compute and interpret magnetic properties inquantum chemistry. For instance ab initio methods based onDifference Dedicated Configuration Interaction23 (e.g. CASSCF/DDCI) give excellent results and offer the possibility to finelyanalyse the mechanisms and origin of magnetic propertiestaking advantage of the access to the wavefunction of all spinstates of interest. This approach has been recently used for thestudy of cis/trans isomeric effects on the magnetic propertiesof copper complexes.24

Crystallographic data collection and refinement

Suitable single crystals of each of the complexes were mountedon a Bruker SMART diffractometer equipped with a graphitemonochromator and Mo-Kα (λ = 0.71073 Å) radiation. Thestructures were solved by the Patterson method using SHELXS97. Subsequent difference Fourier synthesis and least-squarerefinement revealed the positions of the remaining non-hydrogen atoms. Non-hydrogen atoms were refined with in-dependent anisotropic displacement parameters. All thehydrogen atoms were placed in idealized positions and theirdisplacement parameters were fixed at 1.2 times larger thanthose of the attached non-hydrogen atom except the hydrogenatoms on C(6) in 3 which were located in the differenceFourier map. Successful convergence was indicated by themaximum shift/error of 0.001 for the last cycle of the leastsquares refinement. All calculations were carried out usingSHELXS 97,25 SHELXL 97,26 PLATON 99,27 ORTEP-3228 andWinGXsystemVer-1.64.29 Data collection and structure refine-ment parameters and crystallographic data for the two com-plexes are given in Table 1.

Synthesis of the complexes

[Fe2L2] (1). A 5 ml methanolic solution of FeCl3 (0.324 g,2 mmol) was allowed to react with the Schiff-base ligand H3L,formed by the in situ condensation of 1,3-diaminopropan-2-ol(0.118 g, 2 mmol) and 2-hydroxy acetophenone (0.48 ml,4 mmol) in methanol (10 ml) with constant stirring. Triethyl-amine (0.84 ml, 6 mmol) was added dropwise to the solutionand the stirring was continued for about 15 minutes. Thecolour of the solution turned to dark red and a crystalline pre-cipitate was separated out within a few minutes. The precipi-tate was collected by filtration, washed, dried andrecrystallized from acetonitrile. X-ray quality dark red singlecrystals of complex 1 were obtained on slow evaporation of thesolvent.

Complex 1. Yield 0.493 g (65%). Anal. Calcd forC38H38Fe2N4O6 (758.42): C, 60.18; H, 5.05; N, 7.39. Found: C,60.07; H, 5.01; N, 7.28. IR (KBr pellet, cm−1): 1595.6 ν(CvN).λmax/nm (CH3CN), 459, 312, 252.

[Fe2L′2]·2CH3CN (3). A methanolic solution of FeCl3 (0.324 g,2 mmol) was reacted with the Schiff base ligand H3L′ (0.596 g,2 mmol), synthesized by the previously reported procedure30

in the presence of triethylamine (0.84 ml, 6 mmol). The colourof the reaction mixture turned to dark brown on stirring and aprecipitate was separated out which was filtered, washed withcold methanol and dried under vacuum. It was then dissolvedin acetonitrile solvent and dark-red X-ray quality single crystalsof 3 appeared at the bottom of the vessel on slow evaporationof the solvent.

Complex 3. Yield 0.588 g (75%). Anal. Calcd forC38H36Fe2N6O6 (784.43): C, 58.18; H, 4.63; N, 10.71. Found: C,59.07; H, 4.59; N, 10.67. IR (KBr pellet, cm−1): 1633.3 ν(CvN).λmax/nm (CH3CN), 421, 322, 260.

Result and discussionSynthesis of complexes 1 and 3

The Schiff base was synthesized by the condensation of 1,3-diaminopropan-2-ol and 2-hydroxyacetophenone in a1 : 2 molar ratio. FeCl3 reacted with the Schiff base ligand in a1 : 1 molar ratio in the presence of triethylamine to result inthe formation of the dinuclear di-μ-alkoxido bridged iron(III)complex 1 (Scheme 2). Following a similar reaction procedureto complex 1, we synthesized another complex 3 using aslightly different Schiff base ligand (N,N′-bis(salicylidene)-1,3,-diaminopropan-2-ol).30 Complex 3 is also a dinuclear di-μ-alkoxido bridged complex but with a different coordinationmode of the ligand (Scheme 2). The same molecule has beenreported earlier12a with a different solvent of crystallization(dichloromethane instead of acetonitrile that is present in 3).

IR and electronic spectra

The IR spectra of complex 1 showed characteristic CvNstretching at 1595 cm−1, slightly lower in comparison tocomplex 3 which shows CvN stretching at 1633 cm−1. Twoother Fe(III) complexes in which the ligand is the same as in

Table 1 Crystal data and structure refinement of complex 1

1

Formula C38H38Fe2N4O6M 758.42Crystal system TriclinicSpace group P1̄ (No. 2)a/Å 8.823(5)b/Å 9.225(5)c/Å 10.639(5)α/° 83.287(5)β/° 83.219(5)γ/° 84.359(5)V/Å3 850.8(8)Z 1Dc/g cm−3 1.480µ/mm−1 0.907F (000) 394R(int) 0.024Total reflections 6347Unique reflections 3208I > 2σ(I) 2756R1, wR2 0.0364, 0.1087Temp./K 293

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complex 3 exhibit CvN stretching at 1625 and 1629 cm−1.12a,c

Another noticeable difference is the appearance of a weakband at 2248 cm−1 in the IR spectra of 3 due to the presence ofa solvent acetonitrile molecule.

Since the high-spin Fe(III) is under an octahedral field, thed–d transitions should be very weak because they are not spin-allowed. The electronic spectra of complexes 1 and 3 show ahigh intensity band at 459 and 421 nm assignable to a chargetransfer transition from the pπ orbital of phenoxido oxygen tothe half filled dπ* orbitals of the iron(III).31 The higher energytransition bands for 1 and 3 at 312, 322 nm and 252, 260 nmrespectively arise from the intraligand transitions.12a

Structure of complex [Fe2L2] (1)

The asymmetric unit of 1 is shown in Fig. 1 together with theatomic numbering scheme. Selected bond lengths and anglesare summarized in Table 2. The crystal structure of 1 consistsof a centrosymmetric dimer where the two six-coordinatedcrystallographically equivalent metal ions are bridged by twodeprotonated alkoxido oxygen atoms of the pentadentate

Schiff base ligand. The four donor atoms of the ligand (twoimino, N(1) and N(2), and two phenoxido, O(2) and O(3)) co-ordinate to only one Fe(III) center, while the two remaining sites ofFe(III) are coordinated by the two bridging µ2-alkoxido atoms (O(1)and O(1)′). The central Fe2O2 core is significantly asymmetricwith Fe(1)–O(1) = 2.058(2) Å and Fe(1)–O(1)′ = 1.981(2) Å whichis a common feature of such dimers. The longest Fe–O(alko-xido) bond is trans to the phenoxido oxygen atom O(2), and theshortest one is trans to the imino nitrogen atom N(1). The brid-ging Fe(1)–O(1)–Fe(1)′ angle is 103.49(7)° leading to a Fe⋯Fedistance of 3.172 Å, comparable to that in the other dialkoxidobridged iron(III) complexes.12 Each Fe(III) ion possesses a dis-torted octahedral geometry coordinated by the ligand in sucha way that one phenoxido oxygen atom O(3) and one iminonitrogen atom N(2) are in trans axial positions, while the otherphenoxido oxygen O(2) atom and imino nitrogen atom N(1)are cis. These two cis atoms (O(2) and N(1)) along with the twobridging alkoxido oxygen atoms O(1) and O(1)′ define theequatorial plane. The four donor atoms in the basal planeshow a tetrahedral distortion with deviations of −0.257(2),−0.161(2), 0.217(2) and 0.201(2) Å for O(1), O(2), O(1)′, andN(1), respectively from the mean plane passing through them.The iron atom Fe(1) is 0.126(4) Å from this plane in the direc-tion of the axial O(3) atom. The basal bond distances are inthe range of 1.902(2)–2.127(2) Å. The axial bond distances areFe(1)–N(2) 2.188(2) Å and Fe(1)–O(3) 1.927(2) Å. In general, allthe cis and trans angles deviate from 90 or 180°; the range ofcis bond angles is 75.59(7)–109.76(7)° and of the trans anglesis 150.30(8)–163.36(7)°. The CvN bond distance in thecomplex is 1.291 Å indicating its double bond character. Asshown in Fig. 1, the phenoxido oxygen atom O(3) forms anintramolecular C–H⋯O hydrogen bond with the hydrogenatom H(9B) of the Schiff base ligand having C⋯O = 3.204(4) Å,O⋯H = 2.40(3) Å and ∠C–H⋯O = 131(2)°.

It is worth mentioning that in complex 1 each ligand isbonded to one metal center. As a consequence, the two iminonitrogen atoms (N(1) and N(2)) coordinate to the Fe(III) centresat the cis positions unlike the cases where the ligands areshared between the two adjacent metal centres with the iminonitrogen atoms at the trans positions. It should also be men-tioned that compound 1 is the first report of a Fe(III) complex

Scheme 2 Synthetic route to 1 and 3.

Fig. 1 ORTEP-3 view of the asymmetric unit of 1 with ellipsoids at the 30%probability level. The intramolecular hydrogen bonds are shown as dashed lines.

Table 2 Dimensions in the metal coordination sphere, distances (Å) and angles (°)of complex 1

Atom labels Dist./angle Atom labels Angle

Fe(1)–O(1) 2.058(2) O(2)–Fe(1)–O(3) 89.53(8)Fe(1)–O(2) 1.902(2) O(2)–Fe(1)–N(1) 105.97(8)Fe(1)–O(3) 1.927(2) O(2)–Fe(1)–N(2) 83.12(8)Fe(1)–N(1) 2.127(2) O(1)′–Fe(1)–O(2) 103.73(8)Fe(1)–N(2) 2.188(2) O(3)–Fe(1)–N(1) 83.23(7)Fe(1)–O(1)′ 1.981(2) O(3)–Fe(1)–N(2) 163.36(7)O(1)–Fe(1)–O(2) 160.62(8) O(1)′–Fe(1)–O(3) 96.86(7)O(1)–Fe(1)–O(3) 109.76(7) N(1)–Fe(1)–N(2) 84.47(7)O(1)–Fe(1)–N(1) 75.59(7) O(1)′–Fe(1)–N(1) 150.30(8)O(1)–Fe(1)–N(2) 77.78(7) O(1)′–Fe(1)–N(2) 99.38(7)O(1)–Fe(1)–O(1)′ 76.51(7) Fe(1)–O(1)–Fe(1)′ 103.49(7)

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in which each Schiff base ligand (H3L or its derivatives) is co-ordinated to one metal ion. In the other three Fe(III) complexeswhere this coordination of the ligand was found, the Schiffbase had been reduced (either partially or fully). The higherflexibility of CH2NHCH2 linkage in the reduced Schiff baseligand was thought to be responsible for coordination of oneligand to one metal centre as such a coordination requiresconsiderable puckering of the ligand.12a However, in complex1, the Schiff base is not reduced suggesting that other factors,e.g. various weak interactions, may play an important role instabilizing each structure. Therefore, we undertake DFT calcu-lations to gain insight into the stability of these two differentstructures.

Electrochemistry

Cyclic voltammetric studies of complex 1 were carried out inacetonitrile solvent using a platinum electrode at scan rates of20 to 100 mV s−1. It shows two irreversible waves at E1/2 =−0.92 V and −1.7 V (vs. Ag–Ag+) due to the reduction of FeIII toFeII species at a scan rate of 100 mV s−1. On decreasing thescan rate, the peaks shifted towards less negative potentials,corroborating the irreversible nature of the redox behaviour ofthe complex (see Fig. S1, ESI†). In a previously reported dia-lkoxido bridged Fe(III) complex,12a [Fe2L′2]·CH2Cl2, (H3L′ = N,N′-bis(salicylidene)-1,3,-diaminopropan-2-ol) where the ligandshave been shared by two metal centers, a reversible wave atE1/2 = −0.75 V (ΔE = 80 mV) followed by a quasi-reversible waveat −1.35 V (ΔE = 200 mV) was observed. Thus, the change ofthe coordination modes of the ligands seems to influence theredox property. However, the electrochemical behaviour ofsome other Fe(III) complexes with these two types of cores isrequired to be studied to observe any general trend (Fig. 2).

Magnetic study of complex 1

The variable temperature magnetic susceptibility of complex 1has been recorded in the region of 2–300 K, under an applied

field of 5000 G in all ranges of temperature and 300 G at lowtemperatures. A plot of χMT vs. T, for complex 1, is shown inFig. 3. The χMT value at 300 K is 7.78 emu mol−1 K, which iswell below the spin-only value of 8.75 emu mol−1 K (assumingg = 2) expected for two non-interacting high-spin Fe3+ ions.Upon cooling, the χMT product decreases continuously andreaches 0 emu mol−1 K at 2 K, indicating the occurrence ofantiferromagnetic intramolecular interaction between the twoFe3+ ions that leads to a diamagnetic (S = 0) spin ground state.Indeed the susceptibility curve reaches a maximum value of0.044 emu mol−1 at 80 K and decreases upon further cooling.

The experimental susceptibility data were analysed bymeans of the theoretical equation32 derived for a Heisenbergexchange model (H = −JS1S2) with S1 = S2 = 5/2. The best fitgave J = −17.46 cm−1, g = 2.15 and R = 42.6 × 10−6 (Fig. 3). Theagreement factor R is defined as R = Σi[(χMT)obsd − (χMT)calcd]2/Σi[(χMT)obsd]. The value of the exchange coupling constant ofcomplex 1 is perfectly in line with the values reported for otherdimethoxido-bridged and dialkoxido-bridged iron(III) com-plexes with similar structural features: Fe⋯Fe distances, Fe–O–Fe angles and Fe⋯O distances.7b,8a,12b,c,33

In Table 3 we list the examples of dimethoxido-bridged iron(III)complexes for which both structures and magnetic studieshave been performed, along with J values and the three struc-tural parameters. Although it is clear that the above geometri-cal parameters play a role in their contribution to the couplingconstant J, the trend of the J parameter in systems with analo-gous geometrical parameters and similar J values is still underinvestigation. In this regard, several works have been devotedto correlate the magnetic behaviour of dialkoxido-bridgediron(III) systems with either the Fe–O–Fe bond angle or theFe⋯Fe distance.34 Recently, a combined experimental andtheoretical study in dialkoxido bridged diiron(III) complexeswas conducted35 where the authors were able to correlate themagnetic exchange interaction in these systems. They havefound that there exists a correlation between the Fe–O distanceand the Fe–O–Fe angle (the smaller the angle, the longer the

Fig. 2 Cyclic voltammograms at 298 K in CH3CN solvent (0.1 mol dm−3, Ag–AgCl reference electrode) at a platinum working electrode of complex 1 at ascan rate of 100 mV s−1.

Fig. 3 Plot of χMT and χM vs. T of compound 1. The solid line is the best fit tothe experimental data.

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distance). Curiously, the reported magneto-structural corre-lation based on the average Fe–O distance is not good (r2 =0.72).35 However, a good linear relationship between the calcu-lated J values and the Fe–O–Fe angles was found (r2 = 0.97).Unfortunately, if this last correlation is applied to complex 1,the value obtained for J is too high in absolute value (J ≈−34 cm−1), twice the experimental value. In addition, the Fe–Odistance cannot be used to predict the J since it is out of therange used in the representation reported by Hänninen et al.35

In any case, the extrapolated value is too small (J ≈ −2 cm−1)compared to the experimental value. This is due to the factthat complex 1 is characterized by having a long Fe–O bondand a large Fe–O–Fe angle compared to similar structures (seeTable 3), behaving in the opposite way to the trend reported byHänninen et al.35

DFT calculations of magnetic properties

A theoretical study of the electronic structure and magneticproperties of the dinuclear complex 1 was performed at theB3LYP/6-31+G* level of theory (see the theoretical methodssection). The calculated J value is −10.1 cm−1, which is inreasonable agreement with the experimental value(−17.4 cm−1, see above), and confirms the presence of anti-ferromagnetic coupling. Such behaviour is quite common forFe(III) complexes, due to the presence of unpaired electrons inthe five d orbitals of the metal, favouring the overlap betweenthe magnetic orbitals of both paramagnetic centres, regardlessof the s or p nature of the bridging ligands.

In order to further study the magnetic coupling mechan-ism, the spin density distribution is analysed. According to themolecular orbital theory, spin delocalization is the result ofelectron transfer from the magnetic centers to the ligand

atoms. The spin density of compound 1 in the broken-sym-metry state is represented in Fig. 4 and the spin density valuesare summarized in Table 4, where positive and negative signsdenote α and β spin states, respectively. In Table 4 it is shownthat the spin densities on the two Fe(III) ions have identicalabsolute values but opposite signs. The spin densities of +4.22on one Fe(III) and −4.22 on the other reveals that they areindeed the magnetic centres; however, some of the spindensity delocalizes onto the ligands. The spin densities on theligand atoms have the same signs as that of the Fe(III) atoms towhich they are bonded (see Table 4). The spin delocalization isstrong enough that ∼16% of the spin for the unpaired elec-trons on the Fe(III) centres is delocalized to the ligand atoms.In spite of the fact that the actual S = 0 ground state of the

Table 3 Relative bond angle and bond distances of the reported methoxido-bridged and alkoxido-bridged diiron(III) complexes

Fe⋯Fe (Å) Fe–O–Fe (°) Fe⋯O (Å) J (cm−1)

[Fe2L2(µ-OMe)2(NCS)2]a 3.178 105.8 1.993 −29.4

[Fe2(H3L3)2(OMe)2]8MeOHb 3.145 104.2 1.992 −24.2

[Fe2(mxba)2(OMe)2]CH2Cl2c 3.164 104.97 1.994 −27

[Fe2(HL)4(OMe)2]d 3.085 102.73 1.967 −27.4

[Fe2Cl4(amp)4(OMe)2]e 3.18 104.44 1.955 −29.4

[Fe2(Cl)2(salEen)2(OMe)2]f 3.11 104.35 2.00 −27.3

[Fe2(dbm)4(OMe)2]g 3.087 102.0 1.987 −15.4

[Fe2(dpm)4(OMe)2]g 3.105 103.7 1.974 −19.0

[Fe2(chp)4(dmbipy)2(OMe)2]2MeOHh 3.194 104.7 1.987 −26.8[Fe2(chp)4(phen)2(OMe)2]2MeOHh 3.153 104.3 1.948 −28.6[Fe2Cl2L(OMe)2]

i 3.106 103 1.920 −16.3[Fe2(L

Br-A)2(µ-OMe)2(OMe)2]j 3.068 100.18 2.030 −12.0

[Fe2(LI-A)2(µ-OMe)2(OMe)2]

j 3.073 99.95 2.006 −12.4[Fe2(SON(CNEt2)2)2(µ-OMe)2]

k 3.148 103.96 1.998 −26.6[Fe2(L

5)2]2MeOHl 3.262 106.80 2.032 −19.32[Fe2(L

A)2(OMe)(Cl2) MeOH]m 3.149 102.76 2.009 −21.2Complex 1 3.247 106.45 2.035 −17.46

aHL [2-[(2-dimethylamino-ethylimino)-methyl]-phenol], ref. 8a; bH5L3 = 2-hydroxy-N-{3-hydroxy-5-[(2-hydroxybenzoyl)amino]pentyl}benzamide,ref. 7b; cmxba = m-xylylenebis(acetylacetone); ref. 33c; dH2L = 2-salicyloylhydrazono-1,3-dithiolate; ref. 33d; e amp = 2-aminomethylpyridin; ref.33e; fHsalEen = N-ethyl-N-(2-aminoethyl)salicylaldimine; ref. 33a; g dbm = dibenzoylmethane, dpm = dipivaloylmethanate; ref. 33f; hHchp =6-chloro-2-pyridone, dmbipy = 4,4′-dimethyl-2,2′-bipyridine, phen = 1,10-phenantroline; ref. 33g; i L = 1,4-piperazinebis(N-ethylenesalicylaldimine),ref. 33b; j LBr-A = 2,4-dibromo-6(2-pyridylmethylaminomethyl)phenolato and LI-A = 2,4-diiodo-6(2-pyridylmethylaminomethyl)phenolato, ref. 33h;k (SON(CNEt2)2)2 = ethyl derivative of 1,1,5,5-tetraalkyl-2-thiobiuret, ref. 33i; lH3L5 = N,N′-bis-(3-methoxy-salicyldiene)-1,3-diamino-2-propano,ref. 12c; mH3L

A = N,N′-bissalicylidene-1,3-diaminopropan-2-ol, ref. 12b.

Fig. 4 Representation of the spin density corresponding to the “broken-sym-metry” configuration computed for complex 1. The blue surface corresponds topositive spin density values and the green surface to negative values. Hydrogenatoms have been omitted for clarity.

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complex (resulting from the combination of the two “broken-symmetry” wavefunctions) has no spin density, we haveplotted in Fig. 4 the spin density of one of the “broken-sym-metry” wavefunctions. This representation shows that the spindensity on the iron atoms (±4.2) in Mulliken population ana-lysis is distributed spherically due to the presence of oneunpaired electron in each d orbital. The analysis of distri-bution of the spin density likely indicates the predominance ofthe delocalization mechanism over that of polarization, asdeduced by the contribution from the atoms coordinated tothe metal centres.36 We have also computed the singly occu-pied molecular orbitals (SOMO) of compound 1 and we haveselected two of them to illustrate the magnetic coupling inter-action (see Fig. 5), where the p orbitals of the bridging ligandspresent the major contribution. The SOMO diagrams showthat the dx2−y2 orbitals of the Fe(III) centres have a large inter-action with the py orbitals of the bridging ligands that contrib-ute to the magnetic coupling (Fig. 5, left). In addition, the pyorbitals of the bridging oxygen atoms also interact with the dz2orbitals of the Fe(III) centers that further contribute to the mag-netic coupling interaction (Fig. 5, right).

Theoretical study on the stability of the complexes

Experimentally, two different binding modes have beenobserved for the coordination of Schiff base ligands in di-nuclear Fe complexes, as explained above. In one case the iminonitrogen atoms are bonded at the trans positions (3), which iscommon in these types of complexes and in the other one thenitrogen atoms are bonded at the cis position (1). Another

significant difference is that in complex 1 both nitrogen atomsof the same ligand are coordinated to the same Fe metalcenter, while in complex 3 each ligand is coordinated to bothFe ions using the two imino nitrogen atoms. The unique struc-tural difference between the ligands is the presence of amethyl group attached to the imino group (see Fig. 6). We alsoshow in Fig. 6 the optimized structures at the BP86-D3/def2-TZVPD level of theory and the geometries are almost identicalcompared to the experimental ones, giving reliability to thelevel of theory. It can be observed that the X-ray structure of 1has two intramolecular hydrogen bonds that are not formed instructure 3. In the optimized structure these hydrogen bondsare maintained and the C–H⋯O distance is almost equivalent.

We have computed the energetic difference between bothconformations. Since the structure that adopts a cis con-formation has a methyl group and the trans does not, we haveadded a methyl group to structure 3 (3-Me) in order to be ableto compare the energies. We have optimized the geometry and,as a result, the energetic difference between both con-formations (3-Me vs. 1) is only 0.4 kcal mol−1 favourable to thecis, which is the one observed experimentally. Therefore, alikely explanation is that the double H-bonding interactionobserved in 1 stabilizes the unfavourable cis conformationwith respect to the more common trans conformation. In anycase, the difference is very small and any other effect related tothe crystal packing may control the formation of one or theother.

Another experimental difference between the solid statestructure of 1 and 3 is the presence of solvent molecules in 3.We have observed that the compound that incorporates aceto-nitrile molecules in the solid state (3) does not have themethyl group bonded to the imino group. Interestingly, thelocation of the methyl group in the solid state of compound 1is close to the Fe2O2 rhombus core of another molecule andvice versa. We have analysed the energetic contribution of thismethyl group that mainly interacts with oxygen atoms of the

Fig. 5 Two selected SOMOs of complex 1. Hydrogen atoms have been omittedfor clarity.

Fig. 6 Experimental (sticks) and theoretical (ball and sticks) geometries ofcompounds 1 and 3. Distances are in Å.

Table 4 The spin densities on the selected atoms for compound 1 at theUB3LYP/6-31G* level of theory. See Fig. 1 for labelling

Atom Spin density

Fe1 −4.22O1 −0.17O2 −0.02O3 −0.17N1 −0.11N2 −0.08Fe1′ +4.22O1′ +0.17O2′ +0.02O3′ +0.17N1′ +0.11N2′ +0.08

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core by computing the dimers shown in Fig. 7 (left). The inter-action energy of the dimer is reduced by 12 kcal mol−1 whenthe methyl groups are replaced by hydrogen atoms (see thestructure and energy in the middle of Fig. 7). Therefore themethyl groups clearly contribute, among many other inter-actions, to the final architecture in 1. Curiously, compound 3that does not have the methyl groups incorporates solventmolecules into the structure. The location of the solvent mole-cules resembles the position of the methyl groups observed incompound 1, since the methyl group of the solvent is alsolocated near the Fe2O2 rhombus core. The interaction energyis −9.8 kcal mol−1, which is similar to the interaction energydifference between the both dimers computed for compound1. Therefore, the presence/absence of the methyl group likelyinfluences the different coordination modes observed in thesolid state for the Schiff base ligands in the dinuclear Fecomplexes 1 and 3.

To further analyse this aspect, we found in the literature acrystal structure (2) similar to compound 3 (the same ligandand the same Fe2O2 core) that incorporates a different solventmolecule into the structure. In this case a dichloromethaneinteracts with the rhombus core in the same position as theacetonitrile does in 3 (see Fig. 8). A fragment of the X-ray struc-ture (CSD code LOQDIE) is shown in Fig. 8. The computedinteraction energy is comparable (−10.9 kcal mol−1) to 3, con-firming the ability of the core to interact with hydrogen bonddonors, either coming from the solvent molecule as in 3 orfrom the self-assembly of the coordination complex as in 1.

Concluding remarks

A new dinuclear di(μ-alkoxido)-bridged complex with anunusual coordination mode (imino nitrogen atoms in cis posi-tion) has been synthesized and characterized. The antiferro-magnetic coupling of the complex has been measured byvariable-temperature magnetic susceptibility measurements.DFT calculations also confirm that the magnetic exchangebetween these high spin (S = 5/2) centres is antiferromagnetic.The calculated J value is in agreement with the value obtainedexperimentally. The analysis of the SOMO orbitals and spindensity surface gives hints about the main contributions to themagnetic coupling. The theoretical study also demonstratedthat the cis arrangement of the imino nitrogen atoms in thecomplex is stabilized by the presence of two intramolecularC–H⋯O hydrogen bonding interactions. In addition, themethyl group plays a prominent role in stabilizing the solidstate structure of 1 by means of relevant CH3⋯Fe2O2 inter-actions, which are possible thanks to the cis arrangement ofthe N atoms. In compounds 2 and 3, which adopt the usualtrans conformation, the ligand (L′) lacks the methyl group and,interestingly, both compounds incorporate a solvent moleculeinto the structure that precisely occupies the position of themethyl group establishing similar Cl2CH2⋯Fe2O2 (2) orCNCH3⋯Fe2O2 (3) interactions, which are energetically verysimilar and comparable to the strength of the L–CH3⋯Fe2O2

interaction in 1.

Acknowledgements

We thank CSIR, Government of India [Senior Research Fellow-ship to R.B.; sanction no. 09/028(0746)/2009-EMR-I]. Crystallo-graphy was performed at the DST-FIST, India-funded SingleCrystal Diffractometer Facility at the Department of Chemistry,University of Calcutta. A.B. and A.F. acknowledge the SpanishMinisterio de Economía y Competitividad (MINECO) forfunding (CTQ2011-27512/BQU, and CONSOLIDER INGENIO2010, CSD2010-00065), FEDER funds and the Direcció Generalde RecercaiInnovació del Govern Balear (project 23/2011,FEDER funds). C.D. acknowledges the Spanish Government(grant CTQ2012-30662, FEDER funds).

Notes and references

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Fig. 8 X-ray fragment of LOQDIE (2) and the interaction energy of the solventmolecule with the Fe2O2 core.

Fig. 7 X-ray fragments of compounds 3 (right) and 1 (left) and a model ofcompound 1 (middle) without methyl groups.

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