Synthesis and Properties of Dinuclear μ-Oxodiiron(III) Complexes of Amide-Based Macrocyclic Ligands

Job/Unit: I20683 /KAP1 Date: 26-09-12 10:26:11 Pages: 10

FULL PAPER

DOI: 10.1002/ejic.201200683

Synthesis and Properties of Dinuclear μ-Oxodiiron(III) Complexes ofAmide-Based Macrocyclic Ligands

Sushil Kumar,[a] Shefali Vaidya,[a] Michael Pissas,[b] Yiannis Sanakis,*[b] andRajeev Gupta*[a]

Keywords: Macrocyclic ligands / Iron / Mössbauer spectra / Magnetic properties

We report the syntheses, structural, Mössbauer, magnetic,and redox characterization of a series of μ-oxido-bridged di-iron(III) complexes in a set of 13-membered amide-basedmacrocyclic ligands that contain electron-donating and-withdrawing substituents (–H, –Cl, and –CH3) on the ligandperiphery. For all three complexes, the Mössbauer spectraindicate that two iron sites are indistinguishable, a fact thatis well supported by X-ray crystallographic results. The threecomplexes exhibit almost identical isomer shifts but distinc-

Introduction

The μ-oxido-bridged diiron(III) complexes are importantentities due to their presence in several metalloenzymes,[1]

their role in metal-mediated oxidation chemistry,[2] andtheir importance as molecule-based magnetic materials.[3]

Although the diiron moieties in the biological systems areoften bridged by other groups such as carboxylate,[1d–1f] un-supported oxido-bridged dimers provide a unique opportu-nity to study the effect of ligand architecture on their physi-cal and chemical properties. Whereas the majority of thestructurally characterized μ-oxido-bridged diiron(III) com-plexes are supported by carboxylato and other bridging li-gands,[4,5] Fe–porphyrins,[6] Fe–salen,[7] and Fe–macrocyclicsystems,[8] on the other hand, have mostly produced unsup-ported single oxido-bridged systems. A general feature ofall these examples is the presence of an anionic square-planar basal plane that houses the FeIII ion.[6–8] Certainstructural features of the μ-oxido-bridged diiron(III) com-

[a] Department of Chemistry, University of Delhi,Delhi 110007, IndiaFax: +91-11-27666605E-mail: [email protected]: http://people.du.ac.in/~rgupta/

[b] Department of Materials Science, IAMPPNM, NCSRDemokritos,15310 Ag. Paraskevi, Attiki, GreeceE-mail: [email protected] information for this article is available on theWWW under http://dx.doi.org/10.1002/ejic.201200683.

Eur. J. Inorg. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1

tively different quadruple splitting values. The variable-tem-perature magnetic susceptibility measurements show thatthe unsupported single μ-oxido group mediates a strong anti-ferromagnetic coupling between two FeIII ions. The three di-iron complexes show quite different magnetic coupling (J =191, 194, and 230 cm–1, respectively; Hiso = JS1·S2) that hasbeen related to the structural differences. We also show thatthe electron-donating and -withdrawing substituents presenton the ligands influence the redox properties.

plexes, such as the Fe–O–Fe bond angle and Fe–O bondlengths and the relative disposition of Fe-containing planes,have been suggested to dictate the fate of magnetic interac-tions between two iron centers.[3] Thus, a considerable inter-est lies in the versatility of magnetic behavior of μ-oxido-bridged diiron(III) complexes and its relationship with thestructural parameters.

Our group has been working on amide-based macro-cyclic ligands[9] as well as their open-chain analogues[10] andstudying their coordination chemistry. In all such studies,the emphasis has been given to understand the effect ofligand architectural features on the structural, electronic,and redox properties of metal ions. We have been successfulin evaluating the role of electron-donating and -with-drawing substituents on the ligand framework that influ-ence the relative stability of metal ions in the +2 and +3oxidation states. Such studies also showcased that the li-gand architecture plays a crucial role in the redox regulationof a metal center and subsequent stabilization and/or isola-tion of the product.[9,10] The present study is in continuationof our earlier work on the coordination chemistry and re-dox investigation of transition-metal ions with amide-basedmacrocyclic ligands,[9a–9d] and herein we extend the work toiron. In the present work, the dianionic form of macrocyclicligands H2LH, H2LCl, and H2LMe have been used to synthe-size μ-oxido-bridged diiron(III) complexes (Scheme 1). Weshow that the electron-donating and -withdrawing substitu-ents present on the ligand (–H, –Cl, and –CH3) influencethe electronic, magnetic, and electrochemical properties ofμ-oxido-bridged diiron(III) complexes.


S. Kumar, S. Vaidya, M. Pissas, Y. Sanakis, R. GuptaFULL PAPER

Scheme 1. μ-Oxido-bridged diiron(III) complexes used in the pres-ent study.

Results and Discussion

Synthesis and Characterization

The 13-membered macrocyclic ligands H2LH, H2LCl, andH2LMe have been used in this work to prepare their ironcomplexes (Scheme 1). All three ligands are identical exceptfor the types of substituents (H, Cl, or CH3) that are pres-ent on the 3- and 4-positions of the phenylene ring. Thus,whereas ligand H2LCl contains electron-withdrawing –Clgroups, ligand H2LMe offers electron-donating –CH3 sub-stituents.[9c,9d] All three diiron complexes were synthesizedby treating the deprotonated form of the respective macro-cyclic ligand with ferrous tosylate salt. The final ferric statewas achieved through dioxygen oxidation, which also pro-vided the source of the oxido bridge. Interestingly, otheriron salts, such as [Fe(H2O)6](ClO4)2, [Fe(CH3CN)4]-(ClO4)2, FeCl2, or FeCl3 did not result in a clean reaction.[11]

We believe that the noninterfering tosylate ion when using[Fe(H2O)6](OTs)2 as the metal salt has helped in the isola-tion of a single product. All three diiron complexes wereisolated as highly crystalline products in decent to goodyields. The microanalytical data suggest that all three com-plexes had crystallized with additional solvent molecule(s)in the lattice, subsequently confirmed by the diffractionstudies, which showed the presence of solvent molecules inall three cases (vide infra). The nonelectrolytic nature forall three complexes was confirmed by the solution molarconductivity.[12] In the FTIR spectra, the disappearance ofN–H stretches and the bathochromic shift in the C=Oamide

stretch (relative to the free ligand) clearly suggest the in-volvement of deprotonated Namide in the bonding.[13] A sim-ilar observation was noted for our earlier metal complexesof macrocyclic ligands.[9] In addition, the asymmetrical Fe–O–Fe stretch was observed at 856, 836, and 848 cm–1 forcomplexes 1, 2, and 3, respectively. In the literature, theasymmetrical Fe–O–Fe stretch for a single oxido bridge be-tween two iron(III) centers has been typically observed be-tween 790 and 870 cm–1.[4] These stretches are in the follow-ing order: 1 � 3 � 2, which suggests the influence of elec-tron-donating and -withdrawing substituents present on thephenylene backbone of the ligand. The absorption spectrafor μ-oxido-bridged dimers were recorded in DMF to ob-tain the information about various transitions in these com-plexes (Figure 1). For complexes 1–3, the λmax was found at

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430, 410, and 455 nm, respectively. In addition, high-energyfeatures between 310 and 330 nm were also observed for allthree complexes (Figure S1 in the Supporting Information).Such features at approximately 440 nm and approximately330 nm are typically observed for the single oxido-bridgeddiiron(III) complexes and have been assigned as the oxido-to-iron(III) charge-transfer transitions.[4] As noted for theasymmetrical Fe–O–Fe stretch, here too a trend is observedthat could be related to the electronic groups present on theligand.

Figure 1. UV/Vis spectra of complexes 1 (solid line), 2 (dashedline), and 3 (dotted line) in DMF.

To investigate the solution-state structure, paramagnetic1H NMR spectral studies were performed for all three di-iron complexes. Importantly, the proton spectra were ob-served between δ = –5 and 70 ppm, thus indicating strongmagnetic interactions between two equivalent high-spinFeIII centers, which was confirmed by the variable-tempera-ture magnetic susceptibility studies and Mössbauer spec-troscopy (vide infra). All three spectra were of a similarnature, and an equivalent number of resonances were ob-served for all protons (Figure 2). For all three μ-oxido-bridged dimers, the methylene protons (–CH2– groups)were found to appear at two different positions (axial andequatorial positions). Such an observation suggests the in-herent strain within the macrocycle due to the frozen con-formation of the chelate rings that places all protons in dif-ferent chemical environments. A similar observation wasnoted for our earlier NiII complexes of similar ligands.[9a–9c]

For complex 1, the set of methylene protons a/a�, b/b�, andc/c� were observed as broad singlets at δ = –1.6/1.6, 8.6/12.8, and 36.0/63.7 ppm, respectively. The intensity ratio fora/a�, b/b�, and c/c� was found to be 1/1:2/2:2/2. The aromaticprotons d and e were found to resonate at δ = 9.5 and7.8 ppm, respectively, in the intensity ratio of 2 each. Simi-larly, the signal of the N–CH3 groups was noted at δ =24.6 ppm with the intensity of 6 protons. Similar patternsand intensity ratios were observed for the remaining twocomplexes (Figure 2). It should be noted, however, that thesignals for protons labeled as e were not observed for com-plexes 2 and 3 as these positions were occupied by Cl andCH3 groups, respectively. For complex 3, the CH3 groupspresent on the phenylene ring were found to merge with theadventitious water present in the solution.


Dinuclear μ-Oxodiiron(III) Complexes

Figure 2. Paramagnetic proton NMR spectra of complexes 1 (bot-tom), 2 (middle), and 3 (top) recorded in [D6]DMSO. The in-terpretation is provided only for complex 1 (bottom trace). See thetext for details. The peaks at δ = 2.5 and 3.3 ppm are assigned toresidual solvent and adventitious water, respectively. The peak at δ≈ 8.0 ppm adjacent to Ar–H is assigned to the formamide protonof DMF.

Crystal Structure Studies

All three μ-oxidodiiron(III) complexes were charac-terized by X-ray crystallography to understand their molec-ular structure, bonding, and disposition of iron-containingplanes with respect to the bridging oxido group. The dif-fraction studies show that all three complexes have a singleoxido bridge between two iron(III) centers (Figures 3, 4,and 5; Table 1). In each case, the iron center is five-coordi-nate in which the basal plane is comprised of an N4 macro-cyclic cavity and the bridging oxido group occupies the ax-ial position. Thus, the iron ion in each complex adopts adistorted square-pyramidal geometry. Addison’s distortionparameter τ5, which is 0 and 1 for a perfect square-pyrami-dal and trigonal-pyramidal geometry, is 0.11, 0.009, and 0for complexes 1–3, respectively.[14] A small value of τ5 sug-gests that in all three cases the geometry is predominantlysquare-pyramidal. Every FeIII center is coordinated throughtwo anionic Namide and two neutral Namine atoms of themacrocyclic ligand. The average Fe–Namide bond is shorterby 0.114–0.134 Å than the average Fe–Namine bond(Table 1). This fact is attributed to the strong σ-donationability of the deprotonated amidate group relative to theneutral amine group. For dimer 1, the two Fe–O bondlengths are not completely symmetrical and differ margin-ally by 0.015 Å. Of course, a highly symmetrical cell systemhas resulted in identical Fe–O bond lengths in dimers 2 and3. Further, the Fe–O bond lengths observed in the presentcomplexes are within the range observed for other structur-ally characterized examples with a single oxido bridge be-tween two FeIII ions (see Table S1 in the Supporting Infor-mation).[15–28] Around the iron center, the Namide–Fe–Namide bond angle is quite a bit smaller than the Namine–Fe–Namine angle, which reflects a tight chelation due to the

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deprotonated amidate donors. The lateral Namide–Fe–Namine

bond angle is close to 80° in all three cases. The iron atomis significantly deviated from the N4 basal plane of themacrocycle towards the oxido group by approximately0.7 Å in complexes 1 and 2 and by approximately 0.63 Å incomplex 3. Although the two iron ions are linked througha single oxido bridge, two macrocyclic-based basal planesare not parallel to each other and in fact make an angle ofapproximately 140° and approximately 148° for dimers 1and 2, respectively. This suggests that both the halves arenot symmetrically placed, possibly due to electronic repul-sion between the arene rings and/or attached groups. No-tably, a linear Fe–O–Fe angle and a perfect trans dispositionof Fe-based basal planes was noted for complex 3 owing tothe symmetry reasons. Interestingly, in all three cases, theNMethyl groups are in the same direction; however, they aredirected away from the Fe–O–Fe linkage. The six-memberedtrimethylenediamine chelate ring in all three complexes pos-sesses the typical chair conformation as generally observedfor six-membered chelate rings.[9c,9d] The lateral five-mem-

Figure 3. Molecular structure of complex 1 (thermal ellipsoids aredrawn at 30% probability level whereas the hydrogen atoms areomitted for clarity).



S. Kumar, S. Vaidya, M. Pissas, Y. Sanakis, R. GuptaFULL PAPERbered chelate rings [–C(O)–CH2– fragment] in all complexesadopt the envelope conformation as noted before.[9a–9d]


Table 1. Bond lengths [Å] and bond angles [°] for complexes 1–3.

Bond/Angle 1 2[a] 3[b]

Fe1–N1 2.013(3) 2.019(5) 2.005(12)Fe1–N2 2.013(3) 2.023(6) 2.139(13)Fe1–N3 2.142(3) 2.137(6) –Fe1–N4 2.150(3) 2.130(5) –Fe2–N5 2.003(3) – –Fe2–N6 2.012(3) – –Fe2–N7 2.152(3) – –Fe2–N8 2.140(2) – –Fe1–OX[c] 1.759(2) 1.767(2) 1.760(9)Fe2–OX[c] 1.774(2) – –N1–Fe1–N2 77.55(12) 78.3(2) 143.23(5)N1–Fe1–N3 138.21(11) 140.4(2) –N1–Fe1–N4 79.59(11) 80.0(2) –N2–Fe1–N3 80.71(13) 80.1(3) –N2–Fe1–N4 140.01(11) 138.2(2) –N3–Fe1–N4 95.53(11) 95.0(2) –N5–Fe2–N6 78.33(11) – –N5–Fe2–N7 139.03(11) – –N5–Fe2–N8 80.73(10) – –N6–Fe2–N7 80.39(11) – –N6–Fe2–N8 141.79(11) – –N7–Fe2–N8 95.49(10) – –Fe1–O3–Fe2 148.75(13) 140.6(4) –N1–Fe1–N1#1 – – 78.95(7)N1–Fe1–N2#1 – – 80.63(5)N2–Fe1–N2#1 – – 98.59(7)Fe1–O2–Fe1#2 – – 180

[a] Symmetry operations used to generate equivalent atoms forcomplex 2: x, y, –z + 1. [b] Symmetry operations used to generateequivalent atoms for complex 3: #1: x, –y + 1, z; #2: –x + 1, –y +1, –z. [c] X = 3 for complexes 1 and 2; X = 2 for complex 3.

The linearity versus nonlinearity of the Fe–O–Fe anglein μ-oxidodiiron(III) complexes is crucial due to its impor-tance in magnetic interaction and thus requires special men-


tion. Fe–O–Fe angles of approximately 149° and 141° wereobserved for complexes 1 and 2, respectively; however, ow-ing to symmetry reasons, a linear angle of 180° was notedfor 3. As can be seen from Table S1 in the Supporting Infor-mation, the assorted values of the Fe–O–Fe angle have beenobserved for the single oxido bridge between two FeIII cen-ters.[15–28] Several factors including type of ligand, symmet-rical arrangement of ligand, and coordination number af-fect the Fe–O–Fe angle and might have a significant impli-cation on the fate of exchange coupling between two ferricions.[3,4] Recently, Ghosh and co-workers have investigateda few μ-oxido-bridged diiron(III) systems with varying Fe–O–Fe bond angles.[7k] Their combined experimental andtheoretical results indicate that, although 180° is the mostabundant situation for five-coordinate cases, the energy costof bending the Fe–O–Fe angle is not high, and conse-quently, is easily compensated by other structural factors(including weak hydrogen-bonding interactions). The crys-tallographically observed Fe–O–Fe bond angles for thepresent complexes suggest that, while dimers 1 and 2 arelikely to display similar magnetic properties, dimer 3 is ex-pected to show a much larger superexchange between twoFeIII ions (vide infra).

Weak Interactions

All three diiron complexes show various kinds of weakinteractions that result in interesting solid-state packing(Figures S2–S4 and Table S2 in the Supporting Infor-mation). For 1, linear chains are formed through hydrogenbonding between O5amide and a methylene proton (C25–H25B) and between O1amide and a methylene proton (C10–H10B). Additionaly, Oamide atoms further interact withmethylene (C29–H29B) as well as methyl protons (C13–H13A) to generate a two-dimensional (2D) layer. Such lay-ers are interlinked to each other through DMF moleculesto create a 2D network. For 2, every dimer is linked to threeneighboring dimers and a water molecule. As a result, a 2Dnetwork is formed. Further, a weak hydrogen bond con-nects O2amide and methylene proton H11A, and Cl2 to thatof methylene proton H11B, thus leading to a chain.Furthermore, a weak hydrogen bond between O1amide andmethyl proton (C13–H13C) joins two individual chains to-gether. In the case of 3, the dimers are connected to eachother through DMSO molecules present in the lattice. TheO atom of DMSO acts as the hydrogen-bond acceptor,whereas the CH3 groups act as hydrogen-bond donors toOamide atoms. As a consequence, every dimer is in interac-tion with eight DMSO molecules (four on each side), anda combination of such interactions generates an interesting2D network (Figure S4 in the Supporting Information).

Mössbauer Spectroscopy

Mössbauer spectra were recorded for the three complexesto understand the oxidation state as well as the geometricalsymmetry around the iron ions. Mössbauer spectra at 78 K



and at zero magnetic field for complexes 1–3 are shown inFigure 6. The parameters used to simulate the spectra arelisted in Table 2. The spectrum from each sample is com-prised of one quadruple doublet of Voigt shape with fairlynarrow lines, thus indicating that the two iron sites of thedimers have indistinguishable parameters. In fact, the crys-tal structures for dimers 2 and 3 display identical Fe–O dis-tances due to the symmetrical cell system. However, in thecase of dimer 1, two iron sites have marginally different Fe–O distances (1.759 and 1.774 Å). Nevertheless, such a smalldifference is not reflected in the Mössbauer spectrum, anda quadruple doublet with a narrow line was obtained. Thevalue of the isomer shift (δ) is almost identical for all threecomplexes. In contrast, the quadruple splitting, ΔEQ, de-pends characteristically on each complex. ΔEQ also exhibitslittle temperature dependence. This parameter relates to theasymmetry of the charge distribution around the Fe ion.The observations suggest that the asymmetry is smaller forcomplex 1 and larger for complex 3. For complex 2, Möss-bauer spectra were also recorded at liquid helium tempera-tures in the absence or in the presence of magnetic fields upto 7 kOe applied perpendicularly to the γ-rays. Representa-tive spectra recorded at 1.5 K are shown in Figure S5 of theSupporting Information. The spectrum consists of a narrowquadruple doublet that undergoes a slight broadening uponapplication of the external magnetic field. This broadeningis entirely due to the external magnetic field, which indi-

Figure 6. Mössbauer spectra of complexes 1–3 recorded at zeromagnetic field at 78 K. The gray lines are fits assuming quadrupledoublets with the parameters listed in Table 2.

Table 2. Mössbauer hyperfine parameters, exchange coupling con-stants, and molar fractions of paramagnetic impurities for com-plexes 1–3.

Complex δ [mms–1] ΔEQ [mms–1] Γ [mms–1] J [cm–1] ρ [%](�0.01) (�0.01)

1 0.31 1.14 0.35 194(�2) 1.12 0.31 1.33 0.34 191(�1) 3.03 0.31 1.47 0.35 230(�1) 0.3


cates that each iron atom exhibits no internal magnetic fielddue to the hyperfine interaction of the nucleus with un-paired electrons. This observation is consistent with an anti-ferromagnetically coupled dimer that yields an S = 0ground state in agreement with the variable-temperaturemagnetic susceptibility measurements (vide infra). In gene-ral, the isomer shifts in the range of 0.30–0.60 mms–1 arecharacteristic of five-coordinate single μ-oxido-bridgedhigh-spin diiron complexes.[4] Similarly, the majority of theμ-oxido-bridged diiron complexes have quadruple splittingof �1 mms–1.[4] As an example, complex [Fe2O(LISQ)4][HLISQ: 4,6-di-tert-butyl-2-(p-tert-butylanilino)phenol] ex-hibits δ and ΔEQ values of 0.38 and 1.11 mm s–1, respec-tively.[22] For this five-coordinate complex, the Fe–O dis-tance and Fe–O–Fe angle were found to be approximately1.78 Å and approximately 175°, respectively (see Table S1in the Supporting Information).

Magnetic Susceptibility Measurements

The temperature dependence of the magnetic suscep-tibility as a function of temperature for the three μ-oxido-bridged diiron(III) complexes is shown in Figure 7. At300 K, the χT product is 0.944, 0.989, and0.766 cm3 mol–1 K for dimers 1, 2, and 3, respectively. Thesevalues are much smaller than those expected for two non-interacting FeIII(S = 5/2) ions (8.75 cm3 mol–1 K), whichsuggests strong antiferromagnetic interactions. As the tem-perature decreases, χT also decreases reaching a plateauaround approximately 70 K. The plateau varies from sam-ple to sample and is attributed to a small amount of para-magnetic impurities (Table 2).

Figure 7. Temperature dependence of the product χT obtained at5 kOe for complexes 1–3. The gray lines are fits to the experimentaldata as described in the text.

The temperature dependence of the magnetic suscep-tibility was fitted on the basis of the isotropic Heisenberg–Dirac–van Vleck Hamiltonian [Equation (1)], pertinent to abinuclear cluster that comprises two high-spin FeIII(S =5/2) centers.[3]

Hiso = JS1·S2 (1)

To account for the presence of paramagnetic impurities, weassumed a high-spin monomeric FeIII(S = 5/2) species with


S. Kumar, S. Vaidya, M. Pissas, Y. Sanakis, R. GuptaFULL PAPERa mole percentage, ρ. The obtained parameters are listed inTable 2. It should be noted that the amount of paramag-netic impurity as derived from the analysis of the magneticsusceptibility data is too small to be resolved in the Möss-bauer spectra of the complexes. The J values for the threeμ-oxido-bridged diiron(III) complexes 1–3 indicate that thesingle oxido group mediates a strong antiferromagneticcoupling between two iron(III) ions (Table 2).

A comparison of the exchange coupling constants (Jvalue) suggests that the extent of antiferromagnetic cou-pling is similar for dimers 1 and 2 (J = 194 and 191 cm–1,respectively) but is larger in the case of dimer 3 (J =230 cm–1). In the literature, such a significant difference hasbeen attributed to a few structural parameters, such as thelinearity of the Fe–O–Fe angle, Fe–O bond length, and thesymmetrical versus asymmetrical nature of two Fe–O bondlengths.[3–8,15–28] In the case of complex 3, a comparison ofbonding parameters does display a linear Fe–O–Fe bondangle, short Fe–O bond lengths, and symmetrical Fe–O dis-tances, thus justifying the largest J value of 230 cm–1. Ad-ditionally, the average Fe–N4 bond lengths for dimers 1 and2 were found to be 2.08 Å, whereas an average value of2.07 Å was observed for 3, which is clearly shorter by0.01 Å. We believe that the strong Fe–N4 bond increases theelectron density at the FeIII center and thus increases theextent of exchange coupling. Importantly, dimer 3 has elec-tron-donating CH3 groups on the ligand framework andthus supports this hypothesis. Min and co-workers[17] haveobserved a similar correlation between J and bond lengthsfor their μ-oxido-bridged diiron(III) dimers. Moreover, theobserved J values for the present diiron complexes arewithin the range for other single μ-oxido-bridged diiron(III)dimers supported with various multidentate capping ligands(see Table S1 in the Supporting Information).[15–28] In ad-dition, the Fe···Fe separation for unsupported single μ-ox-ido-bridged diiron(III) complexes falls in the range of ap-proximately 3.30–3.60 Å for the examples contained inTable S1, and the present complexes 1–3 are within this li-mit. Furthermore, the observed Fe–O distances for the pres-ent diiron complexes again fall within the range observedfor similar systems as mentioned in Table S1 (1.75–1.81 Å).It may be noted, however, that, whereas in the majority ofμ-oxido-bridged diiron(III) complexes the J value has beenfound as a function of structural parameters, a precise cor-relation between the mentioned parameters could not berationalized.[3,29] In a landmark paper, Weihe and Güdel[29]

expressed J in terms of overlap between iron d orbitals andthe bridging oxido p and s orbitals by using an angular andradial overlap model. These authors were able to provide arationale for a large number of μ-oxido-bridged diiron(III)dimers.

Electrochemical Studies

The redox behavior for all three complexes was studiedby cyclic voltammetry to understand the accessibility andstability of different oxidation states of iron. In addition,


these studies were also targeted to understand the influenceof electron-donating and -withdrawing substituents on theredox properties. For complexes 1–3, the FeIII/FeII redoxcouple was reversible in nature and appeared at the negativepotential region (E1/2 = –0.48 to –0.65 V versus SCE) (Fig-ure 8, Table 3). The observed potentials clearly indicate thehigher stability of the FeIII state over the FeII state.[11] Thereversibility of the FeIII/FeII redox couple is quite interest-ing and suggests that the present macrocyclic ligand sys-tems are nicely tuned to support both FeIII and FeII statesunder the electrochemical conditions.[30] A probable reasoncould be the balance of two anionic amidate and two neu-tral amine donors from the macrocyclic ligand and the 13-membered cavity size. For complex 2 (with –Cl substitu-ents), an E1 value of –0.48 V was obtained, which is 130 mVhigher than that of complex 1 (with –H substituents).Clearly, the electron-withdrawing –Cl substituents have re-duced the electron density at the iron center and thus desta-bilize the FeIII state. An opposite situation was observed forcomplex 3 with electron-donating –CH3 substituents on theligand in which the FeIII state is better stabilized by 40 mVrelative to complex 1. In essence, a potential difference of170 mV separates the two extremes of cases 2 and 3 andjustifies the electronic influence.

Figure 8. Cyclic voltammograms for complexes 1 (solid line), 2(dotted line), and 3 (dashed line) in DMF. See the text for condi-tions.

Table 3. Electrochemical data for complexes 1–3. The potentials arereported versus SCE.

Complex[a] E1 [V] E2 [V](ΔEp [mV]) [ΔEp [mV])

1 –0.61 (80) 0.84 (65)2 –0.48 (90) 1.0[b]

3 –0.65 (80) 0.77 (100)

[a] Conditions: solvent DMF; complex ca. 1 mm; TBAP ca. 0.1 m;scan rate 100 mVs–1; working electrode: glassy carbon; counterelectrode: Pt wire; reference electrode: Ag/AgCl. [b] Epa value.

The E2 potentials (the second redox process) for com-plexes 1–3 appeared in the positive potential range of 0.77–1.0 V. Notably, the E2 redox response was reversible in na-ture for complexes 1 and 3, whereas an irreversible responsewas observed for complex 2. Figure S6 in the SupportingInformation displays a representative voltammogram forcomplex 3. The E2 oxidative responses are most likely li-



gand-centered oxidations located at the o-benzenediamidatefragment. This assignment has been made on the basis ofour earlier nickel and copper complexes with similar macro-cyclic ligands as well as a few literature examples.[9a–9d,10e,31]

Conclusion

This work has reported the syntheses, structural charac-terization, Mössbauer, magnetic, and redox studies of aseries of single μ-oxido-bridged diiron(III) complexes in aset of 13-membered amide-based macrocyclic ligands. Themacrocyclic ligands contain electron-donating and -with-drawing substituents (–H, –Cl, and –CH3) on the ligandperiphery that influence the structural, electronic, and re-dox properties. The Mössbauer spectra are comprised ofone quadruple doublet of symmetrical nature, which indi-cates that two iron sites are indistinguishable, a fact that iswell supported by the crystallography results. The unsup-ported single μ-oxido group mediates a strong antiferro-magnetic coupling between two FeIII ions as revealed by thevariable-temperature magnetic susceptibility studies. Ourresults show that minor structural alterations cause signifi-cant variation in the magnetic coupling between two ferricions.

Experimental SectionMethods and Materials: All reagents were obtained from commer-cial sources and were used as received. DMF was dried and distilledfrom 4 Å molecular sieves and was stored over moleular sieves.Acetonitrile (CH3CN) was dried by distillation from anhydrousCaH2. Diethyl ether was dried by heating it to reflux in the presenceof sodium metal pieces under an inert gas. [Fe(H2O)6](OTs)2 wasprepared according to a reported method.[32] The ligands H2LH,H2LCl, and H2LMe were synthesized according to our recent litera-ture report.[9c]

Physical Measurements: Conductivity measurements were per-formed in organic solvents with a digital conductivity bridge fromPopular Traders, India (model number: PT-825). Microanalyticaldata were obtained with a Vario EL-III instrument from ElementarAnalysen System GmbH. Infrared spectra (either as KBr pellet oras a mull in mineral oil) were recorded with a Perkin–Elmer FTIR2000 spectrometer. Absorption spectra were recorded with a Per-kin–Elmer Lambda 25 spectrophotometer. Cyclic voltammetric ex-periments were performed with a CH Instruments electrochemicalanalyzer (Model 1120A). The cell contained a glassy-carbon or aPt working electrode, a Pt wire auxiliary electrode, and an Ag/AgClreference electrode. Although an Ag/AgCl reference electrode wasused, the potentials were converted with respect to the saturatedcalomel electrode (SCE) reference throughout the text. For coulo-metric experiments, a Pt mesh was used as the working electrode.The solutions were approximately 1 mm in the complex and approx-imately 0.1 m in the supporting electrolyte, tetrabutylammoniumperchlorate (TBAP). Under our experimental conditions, theE1/2 value (in V) for the couple ferrocenium/ferrocene (Fc+/Fc) wasobserved at 0.47 V in DMF versus SCE.

Magnetic and Mössbauer Measurements: Variable-temperaturemagnetic susceptibility measurements were carried out on pow-dered samples of complexes 1–3 in the 2–300 K temperature range


with a Quantum Design PPMS SQUID susceptometer. Diamag-netic corrections for the complexes were estimated from Pascal’sconstants.[33] 57Fe Mössbauer spectra were recorded in the constantacceleration mode at temperatures controlled with a Janis or anOxford cryostat. External magnetic fields were provided with anelectromagnet and were applied perpendicularly to the γ-rays. Iso-mer shifts are reported relative to iron metal at room temperature.Simulations of the Mössbauer spectra were obtained with theWMOSS software (See Co., Edina, Minnesota) or with locally writ-ten routines.

Crystallography: Single crystals suitable for the diffraction studieswere obtained by the vapor diffusion of diethyl ether into a solutionof the compound in DMF or DMSO at room temperature. TheX-ray diffraction intensities were collected with an Oxford CCDdiffractometer with an Xcalibur sapphire diffraction measurementdevice using graphite-monochromated Mo-Kα radiation (λ =0.71073 Å)[34] for complexes 1 and 3 and Cu-Kα radiation (λ =1.54184 Å)[34] for complex 2. Multiscan absorption correction wasapplied for all three complexes.[34] The structure was solved by di-rect methods using SIR-92[35] and refined by full-matrix least-squares refinement techniques on F2 using SHELXL97.[36] Thenon-hydrogen atoms were anisotropically refined. The hydrogenatoms were placed into calculated positions and included in thelast cycles of the refinement. All calculations were carried out withthe Wingx crystallographic package.[37] For complex 1, the refine-ment showed two disordered DMF molecules. For one of the DMFmolecules that was lying on a general position, the disorder couldbe resolved by splitting each atom into two positions with their siteoccupancy and Uiso values refined as free variables, keeping thebond lengths to be the same in both the fragments. A flat commandwas also used to keep them as planar moieties. This DMF moleculehas been refined anisotropically as well. The second molecule ofDMF (C31, C32, C33, N9, and O6) was much diffused and highlydisordered with its N atom lying on the center of symmetry withan SOF of 0.5, thus making its contribution half of the asymmetricunit. This DMF could be modeled by keeping an SOF of 0.5 forall three C and the O atoms as well, and restraints on their bondlengths. As expected, the thermal parameters were high. Therefore,this DMF molecule was removed by using the SQUEEZE routineof PLATON.[38] The remaining structure was then refined againwith the new solvent-free reflection data to give a converged im-proved model. The squeeze shows that 23 electrons were removedfrom two centers; the number of electrons recovered is almost equalto half a molecule of DMF, thus creating a solvent-accessible vol-ume of 206 A3. The results of the SQUEEZE have been appendedin the relevant CIF file. However, the molecular formula includesthis half of a molecule of DMF. For complex 2, there was onedisordered uncoordinated water molecule (O4) in the lattice, whichcould only be refined isotropically as an isolated oxygen atom. TheCIF file thus shows errors due to the isolated nature of the oxygenatom. For complex 3, along with the dimer, there were four uncoor-dinated DMSO molecules present in the unit cell. The complex islocated on a 2/m special position; thus, the asymmetric unit con-tains only a quarter. The whole molecule is generated by symmetryoperations (twofold rotation and mirror). As a consequence ofsymmetry, a total of four DMSO molecules resulted. Details of thecrystallographic data collection and structure solution parametersare presented in Table 4. CCDC-886615 (1), -886616 (2), and-886617 (3) contain the crystallographic data for this paper. Thesedata can be obtained free of charge from The Cambridge Crystallo-graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Synthesis of Diiron(III) Complex [(FeLH)2O] (1): The ligand H2LH

(0.10 g, 0.34 mmol) was dissolved in DMF (5 mL) under magnetic


S. Kumar, S. Vaidya, M. Pissas, Y. Sanakis, R. GuptaFULL PAPERTable 4. Crystallographic data collection and structure solution parameters for complexes 1–3.

1·DMF 2·H2O 3·4DMSO

Empirical formula C33H47Fe2N9O6 C30H36Cl4Fe2N8O6 C42H72Fe2N8O9S4

Mr 777.50 858.17 1073.02T [K] 293(2) 293(2) 150(2)Crystal system monoclinic tetragonal monoclinicSpace group P21/c P41212 C2/ma [Å] 15.8001(7) 10.7053(4) 13.686(5)b [Å] 9.3107(3) 10.7053(4) 22.427(5)c [Å] 26.8117(9) 33.370(2) 9.653(5)α [°] 90 90 90β [°] 96.472(3) 90 118.827(5)γ [°] 90 90 90V [Å3] 3919.1(3) 3824.3(3) 2595.7(17)Z 4 4 2Dcalcd. [Mgm–3] 1.318 1.490 1.373µ [mm–1] 0.792 9.088 0.777F(000) 1632 1760 1136Crystal size [mm] 0.30 �0.20�0.15 0.24�0.23�0.22 0.23�0.22�0.20θ range 3.04 to 26.50° 4.34 to 67.49° 3.02 to 26.98°Index ranges –19 �h�19 –12�h�12 –16�h�17

–11� k�11 –10�k�12 –28�k�28–33� l�33 –39� l�34 –12� l�12

Reflections collected 21229 9815 10789Independent reflections 8090 [R(int) = 0.0467] 3403 [R(int) = 0.0826] 2886 [R(int) = 0.0234]Data completeness [%] 99.7 100.0 99.8Absorption correction multiscan multiscan multiscanMax./min. transmission 0.8904/0.7971 0.2397/0.2190 0.8601/0.8415Data/restraints/parameters 8090/12/472 3403/0/226 2886/0/202Goodness-of-fit on F2 0.861 0.982 1.057Final R indices [I�2σ(I)][a,b] R1 = 0.0472, wR2 = 0.0976 R1 = 0.0623, wR2 = 0.1580 R1 = 0.0270, wR2 = 0.0726R indices (all data) R1 = 0.1012, wR2 = 0.1059 R1 = 0.0813, wR2 = 0.1690 R1 = 0.0324, wR2 = 0.0739Largest diff. peak/hole [e Å–3] 0.319/–0.222 0.932/–0.517 0.374/–0.180

[a] R = Σ(||Fo| – |Fc||)/Σ|Fo|. [b] Rw = {Σ[w(Fo2 – Fc

2)2]/Σ[w(Fo2)2]}1/2.

stirring and treated with solid NaH (0.018 g, 0.75 mmol) under N2.The reaction mixture was stirred until the effervescence had ceased.[Fe(H2O)6](OTs)2 (0.16 g, 0.34 mmol) was added as a solid, and thesolution was stirred for 10 min. Dry oxygen was bubbled throughthe reaction mixture for 2 min, and the resultant mixture wasstirred at room temperature for further 3 h. The initial pale greencolor of the reaction mixture changed to dark brown after O2 purg-ing. The solvent was removed under reduced pressure, and thecrude product was isolated after washing with diethyl ether. Thecrude compound thus obtained was dissolved in DMF and passedthrough a pad of Celite on a medium-porosity frit. The resultingfiltrate was concentrated to one-third of its original volume andsubjected to vapor diffusion of diethyl ether. This afforded a highlycrystalline product within 4–5 d. Yield: 0.17 g (70%).C33H49Fe2N9O7 (795.49·H2O·DMF): calcd. C 49.83, H 6.21, N15.85; found C 49.60, H 6.45, N 15.65. FTIR (KBr disk): ν̃ = 3490,3399, 2912, 1617, 1577, 1476, 1388, 970, 856, 774 cm–1. Conducti-vity (DMF, ca. 1 mm solution, 298 K): ΛM = 5 Ω–1 cm2mol–1. UV/Vis (DMF): λmax (ε) = 327 (sh, 12320), 430 nm (4690 m–1 cm–1).

Synthesis of Diiron(III) Complex [(FeLCl)2O] (2): A similar pro-cedure was adopted for the synthesis of complex 2 by using ligandH2LCl (0.10 g, 0.27 mmol), NaH (0.014 g, 0.61 mmol), and[Fe(H2O)6](OTs)2 (0.14 g, 0.27 mmol). In this case, recrystallizationwas achieved by vapor diffusion of diethyl ether into a solution ofcomplex 2 in DMF. Yield: 0.14 g (58%). C33H45Cl4Fe2N9O7

(933.27·H2O·DMF): calcd. C 42.47, H 4.86, N 13.51; found C42.48, H 5.11, N 13.35. FTIR (KBr disk): ν̃ = 3430, 2915, 2858,1636, 1568, 1465, 1385, 974, 836, 760 cm–1. Conductivity (DMF,ca. 1 mm solution, 298 K): ΛM = 6 Ω–1 cm2mol–1. UV/Vis (DMF):λmax (ε) = 320 (sh, 15185), 410 nm (6080 m–1 cm–1).


Synthesis of Diiron(III) Complex [(FeLMe)2O] (3): A similar pro-cedure was used for complex 3 with the following reagents: H2LMe

(0.10 g, 0.31 mmol), NaH (0.016 g, 0.69 mmol), and [Fe(H2O)6]-(OTs)2 (0.16 g, 0.31 mmol). Recrystallization was achieved by vapordiffusion of diethyl ether into a solution of the crude compound inDMF. Yield: 0.13 g (55%). C37H60Fe2N9O8.5 (878.62·DMF·2.5H2O): calcd. C 50.58, H 6.88, N 14.35; found C 50.47, H 6.74,N 14.78. FTIR (KBr disk): 3432, 1610, 1480, 1388, 974, 848,763 cm–1. Conductivity (DMF, ca. 1 mm solution, 298 K): ΛM =9 Ω–1 cm 2mol–1. UV/Vis (DMF): λmax (ε) = 310 (sh, 18330), 455(5350 m–1 cm–1).

Supporting Information (see footnote on the first page of this arti-cle): Figures for UV/Vis and Mössbauer spectra; crystallographicpacking diagrams; tables for magneto-structural correlation andhydrogen-bonding parameters.

Acknowledgments

R. G. sincerely thanks the Council of Scientific & Industrial Re-search (CSIR), New Delhi and the University of Delhi for financialsupport, the CIF-USIC of this university for use of its instrumentalfacilities, and Professor G. Hundal, GNDU-Amritsar, for the crys-tallographic help with complex 1. S. K. thanks the UGC for a fel-lowship.

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Received: June 22, 2012Published Online: �


S. Kumar, S. Vaidya, M. Pissas, Y. Sanakis, R. GuptaFULL PAPER

μ-Oxodiiron Complexes

We report here the syntheses, structures, S. Kumar, S. Vaidya, M. Pissas,Mössbauer, magnetic, and redox charac- Y. Sanakis,* R. Gupta* .................. 1–11terization of a series of μ-oxido-bridged di-iron(III) complexes supported with a set of Synthesis and Properties of Dinuclear μ-amide-based macrocyclic ligands. The un- Oxodiiron(III) Complexes of Amide-Basedsupported single μ-oxido group mediates a Macrocyclic Ligandsstrong antiferromagnetic coupling betweentwo FeIII ions. Keywords: Macrocyclic ligands / Iron /

Mössbauer spectra / Magnetic properties


Synthesis and Properties of Dinuclear μ-Oxodiiron(III) Complexes of Amide-Based Macrocyclic Ligands

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Transcript of Synthesis and Properties of Dinuclear μ-Oxodiiron(III) Complexes of Amide-Based Macrocyclic Ligands