Synthesis, structure, magnetic properties and theoretical calculations of methoxy bridged dinuclear...

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Cite this: Dalton Trans., 2013, 42, 2803

Received 1st August 2012,Accepted 2nd October 2012

DOI: 10.1039/c2dt31751f

www.rsc.org/dalton

Synthesis, structure, magnetic properties andtheoretical calculations of methoxy bridged dinucleariron(III) complex with hydrazone based O,N,N-donorligand†

Rahman Bikas,a Hassan Hosseini-Monfared,*a Giorgio Zoppellaro,b Radovan Herchel,*c

Jiri Tucek,b Anita M. Owczarzak,d Maciej Kubickid and Radek Zborilb

Biomimetic complexes are artificially engineered molecules that aim to reduce the structural complexity

of biological systems in order to unveil the key electronic and structural factors relevant to a protein’s

function. In this work, a novel coordination compound (L2Fe2) which mimics non-heme binuclear pro-

teins was synthesized from the Schiff-base ligand HL = (E)-N’-(phenyl(pyridin-2-yl)methylene)isonicotino-

hydrazide. The crystal structure of L2Fe2 showed that the intramolecular Fe–Fe distances (3.1–3.2 Å) were

analogous to those found in non-heme binuclear ferric proteins. However, in L2Fe2, two methoxide

groups act as bridging units for oxidized iron (Fe3+). Such a bridging motif is unprecedented in the bio-

logical realm. Magnetic susceptibility measurements demonstrated that L2Fe2 is characterized by a

singlet (S = 0) ground state and a very small magnetic coupling constant J (≪ −1 cm−1). The J value fea-

tured by L2Fe2 differs considerably from the values observed in non-heme binuclear proteins in the oxi-

dized form (−100 cm−1 < J < −10 cm−1), which encompass oxo/hydroxo and carboxylate bridging

residues. The singlet ground state of L2Fe2 as well as the weak magnetic interaction between the two

ferric cations was successfully predicted by density functional theory (DFT).

Introduction

Binuclear non-heme iron systems comprise a variety ofenzymes known to act as key components in several metabolicprocesses, such as activation, transport or detoxification ofmolecular oxygen.1 The proteins in this class share, in themajority of known cases, similar folding (a four-helix bundle)and contain two iron centers in the active site, separated fromeach other by less than 4 Å. The coordination sphere of themetal ions in the ferric form includes one or more bridgingcarboxylate ligands/terminal carboxylate (Glu, Asp), histidine

(His) ligands, bridging oxo, hydroxo and/or water molecule(s).2

Members of this family comprise hundreds of proteins whichcover the entire spectrum of life-domains from archaea tomammals. Illustrative examples are shown in Fig. 1 and

Fig. 1 Schematic drawings of some binuclear coupled non-heme iron oxo(μ-O)/hydroxo(μ-OH)/water and carboxylate bridged proteins in the oxidized(ferric, +3) state.

†Electronic supplementary information (ESI) available: DFT analyses. CCDCreference numbers 870258. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/c2dt31751f

aDepartment of Chemistry, University of Zanjan 45195-313, Zanjan, I. R. Iran.

E-mail: [email protected] Centre of Advanced Technologies and Materials, Faculty of Science,

Department of Physical Chemistry, Palacky University in Olomouc, 77146 Olomouc,

Czech RepubliccRegional Centre of Advanced Technologies and Materials, Faculty of Science,

Department of Inorganic Chemistry, Palacky University in Olomouc, 77146 Olomouc,

Czech Republic. E-mail: [email protected] of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780

Poznan, Poland

This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 2803–2812 | 2803

www.rsc.org/dalton

include (i) ribonucleotide reductase proteins (Class Ia/Ib, RNRR2),3–5 enzymes that provide de novo production of precursorsfor both DNA synthesis and repair (Fig. 1A), (ii) the stearoyl-acyl carrier protein Δ9-desaturase (Δ9D)6,7 which is activetowards the desaturation of fatty acids, (iii) the hydroxylasecomponent in soluble methane monooxygenase (sMMO)8,9

and related bacterial monooxygenases (alkene and toluenemonooxygenases, phenol hydroxylase) which are competent inhydroxylating reactions against numerous organic substrates(Fig. 1B), (iv) rubrerythrin systems10–12 found in anaerobic bac-teria and archaea, exhibiting superoxide dismutase (SOD)-cata-lase oxidative stress protection (Fig. 1C), (v) ferritins13–15 whichare capable of storing iron as well as featuring ferroxidaseactivities, and (vi) hemerythrin proteins16,17 found in marineinvertebrates and in some methanotrophic bacteria (e.g.Methylococcus capsulatus), which are currently the sole oxygencarrier known in this family (Fig. 1D). The structures of severaldinuclear non-heme iron proteins have been elucidated bycrystallographic studies and their specific function and theelectronic fingerprints associated to the active site have beenfurther dissected by biochemical, site directed mutagenesis,spectroscopic and theoretical studies.1,2,18–29 The synthesis ofcoordination compounds mimicking the binuclear non-hemeiron motif aims to decrease the complex molecular organiz-ation found in proteins in order to dissect the geometric andelectronic factors associated with the active site and whichcontribute to control/gate protein function.21–26

From a structural perspective, the iron coordinationenvironment strongly contributes to dictate how the proteinwill function in oxygen transport or in oxygen activation forcatalysis. For example, in hemerythrin (Fig. 1D), the oxygen-binding site contains more histidine donors than thosepresent in sMMO and RNR R2 as well as in other members ofthis family. When hemerythrin is in the reduced state, it con-tains hydroxo bridging ligands, leaving only a single opencoordination position to one Fe2+ site available for terminaloxygen binding. Therefore, in hemerythrin, the active site isstructurally constrained in a way that can act as an oxygencarrier. On the contrary, other proteins in this class containvacant and/or labile coordination positions on both irons.Thus, they can activate molecular oxygen for catalysis. In theoxidized form these proteins contain two ferric centres (Fe–XmYn–Fe) which are antiferromagnetically coupled. The mag-netic coupling (J) between Fe(III) cations is usually large andmediated by super-exchange paths (XmYn) provided by brid-ging ligands (∼−100 cm−1).1,2 Several synthetic biomimeticcomplexes containing oxo, hydroxo, alkoxo and carboxylatobridged binuclear and polynuclear iron systems have beenengineered and structurally characterized in order to repro-duce/mimic the ligand-to-metal arrangements witnessed inthese metalloproteins.1,2,22 Magneto-structural correlationstudies have shown that the chemical nature of the bridgingunit/s (XY) connecting the metal centres, the Fe–X/Y bond dis-tances, the Fe–Fe distance and the Fe–X–Fe/Fe–Y–Fe bondangles are variables that can significantly alter the magneticcoupling through a diamagnetic bridging path. The

mechanism of spin-coupling mediated by μ-oxo/μ-hydroxomoieties has been experimentally and theoretically studied indetail by several groups, as reported in the literature.27–29 Inparticular, it has been observed that upon protonation of theμ-oxo-bridging ligand/s, the strength of the antiferromagneticinteraction is substantially weakened and may even drop by afactor of ten from ∼−100 cm−1 to ∼−10 cm−1.2,27–29 The mag-netic interaction in binuclear non-heme iron systems contain-ing μ-methoxo bridging units (MeO−) has not been studiedpreviously, both experimentally and theoretically, since such abridging motif has not been observed in any of the knownstructures solved for proteins belonging to this class. However,it is interesting to note that the hydroxylase component ofsMMO (Fig. 1B), where CH4 is converted into CH3OH via acomplex oxygen activation and substrate hydroxylation mech-anism, is not yet fully unveiled.30 Biomimetic complexesencompassing the methanolate bridging arrangements maythen provide an intriguing and perhaps alternative structuralmodel for such enzymes in the final stage of their turnoveractivity, namely, before reaching the MeOH release andprotein resting state. In this work, we experimentally andtheoretically analysed such a scenario and engineered a coordi-nation compound (L2Fe2) that contained two methoxo (MeO−,X) bridging residues (Scheme 1) and an intramolecular Fe–Fedistance of 3.1–3.2 Å, consistent with those found in binuclearexchange coupled non-heme iron metalloproteins (see Fig. 1).The subtle balance between the structural parameters andmagnetic coupling has been addressed in L2Fe2 by theoreticalanalyses (DFT) in order to gain further knowledge of the effec-tiveness of the magnetic interaction in spin-coupled systemswhen such unconventional bridging moieties are present.

Results and discussion

We employed an aroylhydrazone scaffold (HL) as a Schiff-baseligand for the assembly of the coordination compound L2Fe2.The organic backbone can formally provide a limited set ofdonors for metal coordination i.e. two nitrogens and oneoxygen (Scheme 1), leaving the metal coordination sphere

Scheme 1 Schematic drawings of the keto–enol tautomerism of the aroyl-hydrazone ligand (HL) and the binuclear ferric complex (L2Fe2).

Paper Dalton Transactions

2804 | Dalton Trans., 2013, 42, 2803–2812 This journal is © The Royal Society of Chemistry 2013

open to solvent and/or counter anion molecules. The ligandHL was synthesized by reacting equimolar amounts of 4-pyridine-carboxylic acid hydrazide and 2-benzoylpyridine in methanolunder reflux, which provided, upon recrystallization, thedesired asymmetric tridentate Schiff base ligand in excellentyield and purity (yield 80%). In solution, two tautomeric struc-tures were expected to exist in equilibrium due to the enoliza-tion of the amide functionality (Scheme 1).

The formation of L2Fe2 occurred through a self-assemblyprocess by working in a methanolic solution (MeOH) and fol-lowing the simple mixing of HL and ferrous iron (FeCl2) in anequimolar ratio under air (4 h, 60 °C). The synthetic reactioncan be written in the form of eqn (1)

4FeIICl2 þ 4HLþ 4MeOHþ O2 !2½FeIII2L2ðOMeÞ2Cl2� þ 4HClþ 2H2O

ð1Þ

The metal coordination to ligand HL and the oxidation ofFe(II) to Fe(III) by O2 from the air appeared to occur with theconcomitant formation of MeO− anions.

Although the mechanism of formation of the alkoxide isunclear due to the absence of any strong base in the startingmixture of the reactants, no mononuclear LFe compound wasobtained from the reaction reported in eqn (1) and only thedimeric coordination compound L2Fe2 was isolated. The UV-vis spectrum (MeOH) of the bare ligand HL showed two strongbands at 217 nm (ε = 27.3 × 103 M−1 cm−1) and 268 nm (ε =26.5 × 103 M−1 cm−1), followed by a weaker absorption(325 nm, ε = 12.7 × 103 M−1 cm−1) at longer wavenumbers. Thesignals arise from transitions involving the aromatic backboneand are assigned as π → π* and the weaker n → π*. The ferricbinuclear complex L2Fe2 exhibits similar strong bands at218 nm (145.9 × 103 M−1 cm−1) and 264 nm (147.3 × 103 M−1

cm−1) together with a strong LMCT band at 395 nm (ε = 30.1 ×103 M−1 cm−1) for the coordinated ferric metals. Structuralanalyses of L2Fe2 when obtained as a crystalline material(Table 1) revealed that the unit cell contained two symmetry-independent molecules (thereafter termed L2Fe2A and L2Fe2B),both having Ci symmetry and lying across two different centresof inversion (1/2,1/2,1/2 for molecule L2Fe2A and 0,1/2,0 formolecule L2Fe2B). Fig. 2 shows their structural arrangementwith atom numbering while a close up view of the iron coordi-nation sphere is given in Fig. 3 with selected bond lengths.The detailed geometric parameters are collected together inTable 2. In both L2Fe2A and L2Fe2B, the iron coordinationenvironment is characterized by a fac-FeO3N2Cl motif, withone oxygen and two nitrogen atoms provided by the Schiffbase ligand and a coordinated chlorine atom and two oxygenatoms from the methoxy-bridging ligands. The two Fe3+

cations form nearly planar four-membered Fe2O2 cyclic units.The coordination polyhedron of the Fe(III) cations can bedescribed as a distorted octahedron with minor axialelongation and evident angular distortion (Table 2).

The Fe⋯Fe distance within the four-membered Fe2O2 cyclicunit is 3.1465(10) Å in L2Fe2A and 3.1863(10) Å in L2Fe2B. Theintra-ring O–Fe–O and Fe–O–Fe angles are 75.48(11)° and

104.52(11)° in molecule L2Fe2A and 74.73(11)° and 105.27(11)°in molecule L2Fe2B. The Schiff base ligand forms twofive-membered chelate rings encompassing bite angles ofabout 74°.

Table 1 Crystallographic data of L2Fe2

Formula C38H32Cl2Fe2N8O4·CH4OFormula weight 879.36Crystal system TriclinicSpace group P1a (Å) 9.9949(3)b (Å) 10.7974(3)c (Å) 19.3657(5)α (°) 91.921(2)β (°) 104.190(2)γ (°) 97.509(2)V (Å3) 2004.10(10)Z 2Dcalc (g cm−3) 1.46F (000) 904μ (mm−1) 0.91Crystal size (mm) 0.2 × 0.2 × 0.08Θ range (°) 2.99–26.08hkl range −12 ≤ h ≤ 12

−12 ≤ k ≤ 13−23 ≤ l ≤ 23

Reflections collected 19 639Unique (Rint) 7767 (0.029)With I > 2σ(I) 5745Number of parameters 509Weighting scheme:A 0.0865B 1.1973R(F) [I > 2σ(I)] 0.053wR(F2) [I > 2σ(I)] 0.149R(F) [all data] 0.077wR(F2) [all data] 0.162Goodness of fit 1.05max/min Δρ (e Å−3) 1.51/−0.64

Fig. 2 X-ray structure of the two symmetry-independent molecules (L2Fe2Aand L2Fe2B) of the complex L2Fe2. The thermal ellipsoids were drawn at 30%probability. The hydrogen atoms and solvent (MeOH) molecules are not shownfor clarity.

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The bulk magnetic properties (polycrystalline powder) ofL2Fe2A/B show a strong decrease in the magnetic couplingbetween Fe3+ ions. The recorded magnetic trend is reported inFig. 4. The effective magnetic moment at room temperaturefalls to 8.49 μB, a value that is in good agreement with thetheoretical spin only value of 8.37 μB, calculated for two non-interacting (S = 5/2)–(S = 5/2) spin centers having a gavg of 2.0.The μeff/μB value decreased slightly upon cooling, reaching7.54 μB at T = 1.9 K. The observed behaviour confirms theweakness of the through-bond antiferromagnetic interaction

and the negligible through-space magnetic interaction amongthe dimers.

In order to address the value of the isotropic exchangemediated by the methoxy-bridges in more detail, the followingspin Hamiltonian for the dimer was postulated (eqn (2))

H ¼ �Jð~S1 �~S2Þ þ μBBgðS1;z þ S2;zÞ ð2Þ

where a simple analytical formula of molar susceptibility isavailable.31 The best numerical fitting of the experimental datagave an exchange term of J = −0.08 cm−1 and g = 2.02. Takingthe Fe(III)–(μO)2–Fe(III) core motif as the arrangement thatfavors the largest exchange energy, protonation of the oxo-brid-ging units (Fe(III)–(OH)2–Fe(III)) or replacement with methoxidegroups (Fe(III)–(OCH3)2–Fe(III)) leads to the sequentialdecrease (in magnitude) of the antiferromagnetic exchangeterm J (–100 cm−1, ∼ −10 cm−1, ≪ −1 cm−1).8b,27–29 The calcu-lated small and negative value of J indicates that a pooroverlap between the Fe(III) d-orbitals and the p-orbitals of thediamagnetic bridges (–OCH3) exists in L2Fe2 and isaccompanied by a strong decrease in their π interactions. Ithas been shown by previous theoretical studies on binuclearnon-heme iron model complexes that the dominant exchangepathway in oxo-bridged ferric systems, Fe1(dxz):μ-O(px):Fe2(dxz), is dominated by π interactions whereas in hydroxo-bridged systems, Fe1(dz

2):μ-OH(p||):Fe2(dz2), σ interactions

prevail.29 The weak exchange J value calculated in L2Fe2 is sur-prisingly similar in magnitude and sign to the magnetic inter-action found in non-heme binuclear iron proteins andmimicking synthetic systems when the metal centers arereduced (ferrous).1,2,32 For example, from MCD studies, theexchange term J in the reduced RNR R2 protein from E. colihas been estimated as −0.4 cm−1,33 similar to hp53R2(human, J ∼ −0.6 cm−1)34 and mouse R2 (|J| < 0.5 cm−1),35

while in RNR R2 from B. cereus, J has been estimated to be

Fig. 3 The close-up view of the iron coordination environment in the two sym-metry-independent molecules (L2Fe2A and L2Fe2B). The grey planes are gener-ated from the Fe1–O23–O23i atoms and the depicted bond distances (X⋯Y) areexpressed in Å.

Table 2 Selected geometrical parameters (distances in Å, angles in deg, °)with s.u.’s reported in parentheses

X–Y Bond L2Fe2A (Å) L2Fe2B (Å)

Fe1–Cl1 2.314(2) 2.296(1)Fe1–O7 2.015(3) 2.005(3)Fe1–N9 2.106(3) 2.115(3)Fe1–N18 2.165(3) 2.153(3)Fe1–O23 1.957(2) 1.944(2)Fe1–O23i 2.021(2) 2.064(3)C7–O7 1.290(4) 1.288(4)N8–N9 1.396(4) 1.385(4)

X–Y–Z angle L2Fe2A (°) L2Fe2B (°)

N9–Fe1–O23 167.08(11) 165.83(12)O7–Fe1–N18 147.71(11) 148.12(11)Cl1–Fe1–O23i 169.71(7) 169.49(8)C7–N8–N9 107.9(3) 107.3(3)

i = 1 − x, 1 − y, 1 − z for L2Fe2A, 2 − x, 1 − y, 2 − z for L2Fe2B.

Fig. 4 The magnetic data for complex L2Fe2 in powder form. The circles (○)represent the experimental data for the effective magnetic moment (μeff ). Thesquares (□) represent the experimental data for the molar susceptibility (χmol).The solid line (----) represents the fit to eqn (2) with J = −0.08 cm−1 and g = 2.02.



slightly stronger (−2.2 cm−1).33 Similar results hold for otherproteins belonging to this family, such as reduced sMMO anddeoxy-Hr, where the magnetic interactions remain weak andantiferromagnetic in nature.1,2 In these systems, the carboxy-late groups acting as μ-1,3 bridging units between the twoFe(II) cations give only weak super-exchange pathwaysavailable for the electron transfer process. It is importantto note that the O2 reactivity of the carboxylate-bridgedbinuclear iron proteins is strongly linked to their abilitiesin favoring the formation of a peroxo-biferric structure, Fe(III)–(μO)2–Fe(III), during enzymatic turnover.1,2 This structuralcharacteristic of the active site is intimately linked tothe specific coordination environment of the metals, namelytheir effective local charges, and the availability ofopen coordination positions for oxygen binding on bothmetal sites. Solomon and coworkers used a de novo protein(DF2t) that reproduces the active site of non-heme biferrousenzymes to demonstrate that neutral structures which do notcontain an additional OH/O bridge can efficiently act in O2-activation processes.36 RNR R2 and sMMO, for example,contain four negatively charged donors (four Glu/Asp) and twoneutral His ligands that provide, in the ferrous form, anuncharged active site for oxygen binding in a bridged confor-mation. In this way, fast electron transfer from the metals tothe oxygen occurs, leading to formation of a μ-1,2 peroxo-difer-ric complex. On the contrary, in the reduced Hr protein, thereis a large overall positive charge in the active site because onlytwo negatively charged carboxylate ligands and five neutral Hisligands are available for metal coordination. In this case, thepresence of a hydroxide bridge precludes the formation ofoxygen bridging in a 1,2 fashion and hence a terminal bindingof O2 and its interaction with only one metal center isfavored. However, the presence of a super-exchange pathwayallows the second electron to be efficiently transferredfrom the distant Fe(II) to O2 and the so-formed peroxideis further stabilized by the nearby hydroxyl proton(see Fig. 1D). The self-assembly of the L2Fe2 complexfrom FeCl2 and HL in MeOH under air suggests thatthe oxygen consumption and metal oxidation (Fe(II) → Fe(III))processes should follow a different path with respect to theprocess of O2-activation and metal oxidation witnessed in thenon-heme binuclear iron proteins discussed previously. Nomononuclear ferric complex was in fact isolated at the end ofthe reaction and the formation of a dimeric ferric complexcontaining bridging oxo/hydroxo groups was not witnessed.Even though we did not follow the kinetics of this process,and hence we do not have direct knowledge of any reactionintermediate/s, we suggest that the presence of two ferriccentres weakly coupled to each other in L2Fe2 might preventdestabilization of the –OCH3 bridging units towards, forexample, the decomposition of the complex into the mono-meric LFe species. We therefore analysed the electronicand magnetic properties of L2Fe2 in more detail withdensity functional theory (DFT), in order to probe whetherthe two bridging methoxide groups represent the keychemical variable responsible for such destabilization of the

singlet ground state and the decreased effectiveness of themagnetic interaction between the Fe(III) cations in this coordi-nation complex.

The effectiveness of the super-exchange pathways stronglydepends on the Fe–ligand bonding interaction, core geometryand chemical nature of the bridging ligands. The four-mem-bered Fe2(OMe)2 cyclic motif seen in the X-ray structure isnearly planar in both the L2Fe2A and L2Fe2B units and theOMe moieties represent the sole exchange paths. The other rel-evant structural factors which affect J, such as the Fe–Fe, Fe–Nand Fe–O distances, are also very similar to each other in thetwo L2Fe2 subunits and therefore, it was expected that theL2Fe2A and L2Fe2B molecules should exhibit nearly identicallow-spin/high-spin energy gaps. For the calculation of the twolimiting spin-configurations (ST = 5 and ST = 0), the broken-symmetry approach was employed by performing single point(energy) calculations using the experimental X-ray structuresas input parameters and employing the B3LYP/def2-TZVP(-f )functional.

The calculation results (atomic spin density, ρ; expectationvalue, <S2>) are summarized in Table 3. The minor structuraldifferences between L2Fe2A and L2Fe2B are in harmony withthe calculated similar J values, −41.6 cm−1 for L2Fe2A and−44.7 cm−1 for L2Fe2B. The details of the spin density distri-butions obtained for the high-spin (HS) and broken-symmetry(BS) states are visually illustrated in Fig. 5 for the case ofL2Fe2A. Even though the singlet (S = 0) has been correctly pre-dicted by theoretical calculations to be a ground state spin-configuration, there is considerable discrepancy between theexperimentally derived and calculated J values (Jsusc ≈−0.1 cm−1 vs. JDFT ≈ −43 cm−1).

However, as found in other computational works and forcases encompassing weakly exchange coupled systems, it hasbeen previously highlighted that the major limit in the accu-rate prediction of the exchange constant arises from the extre-mely small antiferromagnetic energy splitting compared to the

Table 3 The B3LYP/def2-TZVP(-f ) calculated (from single point) net Mullikenspin densities (ρ), the expectation <S2> values and derived J values from the HSand BS states in L2Fe2A and L2Fe2B. The atom coding scheme used here is thesame as that employed in the X-ray structures depicted in Fig. 2 and 3

L2Fe2A L2Fe2B

HS (MS = 5) BS (MS = 0) HS (MS = 5) BS (MS = 0)

ρ(Fe) 4.18 4.17 4.18 4.16ρ(Cl) 0.24 0.23 0.24 0.23ρ(N18) 0.08 0.08 0.09 0.09ρ(N9) 0.07 0.07 0.06 0.06ρ(O7) 0.12 0.11 0.12 0.11ρ(O23) 0.24 0.02 0.24 0.05ρ(O23i) 0.24 −0.02 0.24 −0.05ρ(O7i) 0.12 −0.11 0.12 −0.11ρ(N9i) 0.07 −0.07 0.06 −0.06ρ(N18i) 0.08 −0.08 0.09 −0.09ρ(Cli) 0.24 −0.23 0.24 −0.23ρ(Fei) 4.18 −4.17 4.18 −4.16<S2> 30.02 4.96 30.02 4.96J −41.6 (cm−1) −44.7 (cm−1)



total molecular energies and the difficulties encountered bytheoretical methods on obtaining an appropriate electronicdescription of the real systems in such limiting scenarios.27–29

We theoretically employed two different methods, the B3LYPand B2PLYP methods, by calculating the impact on the mag-netic exchange J when minor structural alterations of the Feligand sphere are enforced, namely Fe–Fe through-space dis-tances and Fe–O(Me)–Fe angles. For such analyses, we used asimplified structural model for L2Fe2 (Fig. 6A, L2Fe2opt). InL2Fe2opt, the phenyl residue and one of the pyridyl groups ofthe Schiff-base ligand, the moieties that do not directly takepart in the metal coordination environment, were replaced bymethyl groups in order to simplify the computational work.The double hybrid functional B2PLYP was chosen as analternative method to the B3LYP because it recently demon-strated a better performance for the calculation of J in a Mn(II)-dimer (5/2–5/2 coupled system) when only weak AF exchangeswere found to be active (J ∼ −0.2 cm−1).37

The geometrically optimised structure (L2Fe2opt, C1 sym-metry), which mimics and simplifies the L2Fe2 system (Fig. 6),was obtained using the UBP86 functional and def2-TZVP(-f )basis set on all atoms. In L2Fe2opt, the Fe–Fe through space dis-tance of 3.242 Å and Fe–O(Me)–Fe angle of 104.64° closelyreproduce the corresponding values found by X-ray analyses inL2Fe2A/B. The four-membered Fe2OMe2 cyclic motif seen inthe X-ray structure is also planar in the optimized structure.The other Fe–N and Fe–O distances from atoms belongingto the ligand backbone are also consistent with thoseobserved in the X-ray structure and the same holds for theFe–Cl bond lengths. However, one of the bond lengthsbetween the Fe and bridging oxygen from the MeO groupsbecame larger in the optimized structure (Fe–O(i) with d =2.230 Å). The full details for L2Fe2opt (XYZ-data) are given

in the ESI.† Following the single point calculations atthe B3LYP/def2-TZVP(-f ) level on the UBP86 optimized geo-metry gave a calculated J value (from the HS and BS approach)falling to −35.2 cm−1, a value close to those estimated pre-viously for L2Fe2A and L2Fe2B using the same level of theory(see Table 4).

Following the calculations with B2PLYP/def2-TZVP(-f )method, the derived J value became considerably smaller (J =−22.4 cm−1) but remained far from the experimentally derivedvalue. A comparison of the calculated atomic spin densitiesresulting from the analyses following these two approaches isgiven in Table 4. Fig. 7 illustrates the variation of J upon relax-ing or compressing the Fe1–Fe1(i) distance in L2Fe2opt aroundthe value obtained from the optimized geometry (see Fig. 6Afor notation), in the range of 2.900 to 3.400 Å. It is importantto underline that the Fe2OMe2 cyclic motif remained coplanarin each optimized structure obtained under frozen Fe–Fe1(i)distances. The exchange energy |J| strongly decreased upondecreasing the Fe1–Fe1(i) distance. However, calculations per-formed with B2PLYP/def2-TZVP(-f ) showed that J turned posi-tive (ferromagnetic) for distances less than 3 Å. A similaroverall trend was observed when the B3LYP/def2-TZVP(-f )method was employed (a decrease in the magnitude of J upondecreasing the metals through-space distances) but, in con-trast to the results witnessed with B2PLYP/def2-TZVP(-f ),B3LYP consistently predicted a singlet ground state in the

Fig. 5 The calculated spin density distribution in the (A) high-spin system and(B) the broken symmetry (BS) low-spin system, from single point energy calcu-lations using the B3LYP/def2-TZVP(-f ) functional from the X-ray structural co-ordinates of L2Fe2A. Positive and negative spin densities are shown with dark-blue and dark-green surfaces (cut-off set to 0.007 e bohr−3). (C) and (D) showthe relevant frontier orbitals (α/β-HOMO, 0.02 IsoVal) for the low-spin (BS)configuration, where the negligible overlap of the magnetic orbitals betweenthe two ferric spin-centers (Fe1 and Fe1i Sαβ = 0.039) becomes evident.

Fig. 6 (A) The optimized structure of L2Fe2opt as a simplified model of L2Fe2obtained by UBP86/def2-TZVP(-f ). (B) The magnified view of the iron coordi-nation environments in the optimized geometry (no structure imposed) withbond distances (in Å). In (A), the plane in grey (xy) was generated using theFe1–O–Fe1(i) atoms and the broken arrow line lying on this plane and passingthrough Fe1 and Fe1(i) highlights the direction of compression or relaxation ofthe Fe–Fe distance from where the further optimization of L2Fe2opt underfrozen distances were performed.



examined range. We further probed the impact on J whendifferent halogen groups (fluoride and bromide) replace thechloride anion. The computational results are shown schema-tically in Table 5. Each modified structure was subjected togeometry optimization (UBP86/def2-TZVP(-f )) followed bysingle point calculations using both the B3LYP/def2-TZVP(-f )and B2PLYP/def2-TZVP(-f ) methods in order to obtain a mean-ingful trend for comparison. The computational resultsshowed that changes in the Fe–X chemical identity (with X =F, Cl, Br) only slightly modify some of the key structural para-meters in the binuclear complex (Fe–Fe distances and Fe–O

(Me)–Fe angles). However, besides more substantial andexpected changes in the Fe–F and Fe–Br bond lengths withrespect to Fe–Cl, the calculated effects on J were not as large asthose witnessed upon the alteration of the Fe1–Fe1(i) distance.Thus, this later factor represents the dominant structural vari-able which is able to alter strongly the efficiency of theexchange interaction. In conclusion, the present work demon-strates the ability of methoxide bridges to considerablydecrease the magnetic communication in the Fe–Fe spindimer.

Furthermore, the strong dependence of J to minor altera-tions to the Fe–Fe distance, when methoxide groups act as thesole exchange paths (e.g., in B2PLYP/def2-TZVP, ∼0 < J <−20 cm−1 when the Fe–Fe distance varies between 3.1 ± 0.1 Å)is illustrated by the theoretical calculations. This work furtherdemonstrates that unfortunately, there is no “preferred” com-putational method that can be applied a priori with great con-fidence of success when the aim is a highly accurateprediction of J. This is especially true if very weakly coupled(|J| ≪ 1 cm−1) spin systems become the target of the theoreti-cal analyses.

Conclusions

A novel complex (L2Fe2) mimicking non-heme binuclear ferricproteins was synthesized from an aroylhydrazone ligand andFe(II) chloride tetrahydrate. L2Fe2 featured short intramolecularFe–Fe distances (3.1–3.2 Å) but contained two methoxidebridges between the Fe(III) cations. This type of bridging motifhas never been observed in non-heme binuclear ferric proteinswhere oxo/hydroxo and carboxylate groups function as brid-ging moieties. The two methoxide molecules in L2Fe2 provideda poor exchange pathway for magnetic communication (J ≪−1 cm−1). This behaviour differs significantly from thatobserved in binuclear ferric systems such as RNR R2 andsMMO, where the exchange interaction is known to be strongerin magnitude (about −10 cm−1 in the hydroxylase componentof sMMO and from −60 cm−1 to −92 cm−1 in RNR R2 fromvarious organisms). Finally, the relevance of the methoxidegroups in hampering the effectiveness of the Fe(III)–(OMe)2–Fe(III) magnetic interaction was studied by density functionaltheory, which accurately predicted the singlet ground state butcould only provide an upper bound value for J.

Fig. 7 The calculated isotropic exchange J as a function of the Fe1–Fe1i dis-tances calculated using either the B3LYP/def2-TZVP(-f ) (squares) or B2PLYP/def2-TZVP(-f ) (circles) methods.

Table 5 The relevant DFT calculated structural parameters and correspondingexchange term J (from an HS-BS approach) obtained from the L2Fe2opt sim-plified model system when the Fe–halogen identity was changed further

X–F X–Cl X–Br

Fe–Fe 3.222 Å 3.242 Å 3.221 ÅFe–O–Fe 104.60° 104.64° 103.92°Fe–halogen 1.846 Å 2.280 Å 2.451J (B3LYP) −32.3 cm−1 −35.2 cm−1 −32.2 cm−1

J (B2PLYP) −19.2 cm−1 −22.4 cm−1 −20.0 cm−1

Table 4 The DFT calculated net Mulliken spin densities (ρ), the expectationvalues <S2> and exchange energy (J) from the HS and BS states of the modelcompound L2Fe2opt based on the UBP86/def2-TZVP(-f ) optimized geometry

B3LYP/def2-TZVP(-f) B2PLYPa/def2-TZVP(-f)

HS (MS = 5) BS (MS = 0) HS (MS = 5) BS (MS = 0)

ρ(Fe1) 4.17 4.16 4.26 4.25ρCl) 0.25 0.24 0.22 0.22ρ(Nax) 0.08 0.08 0.05 0.05ρ(Neq) 0.07 0.07 0.10 0.10ρ(Oax) 0.11 0.11 0.11 0.11ρ(O) 0.23 0.07 0.21 0.06ρ(O(i)) 0.23 −0.07 0.21 −0.06ρ(O(i)ax) 0.11 −0.11 0.11 −0.11ρ(N(i)eq) 0.07 −0.07 0.10 −0.09ρ(N(i)ax) 0.08 −0.08 0.05 −0.05ρ(Cl(i)) 0.25 −0.24 0.22 −0.22ρ(Fe1(i)) 4.17 −4.16 4.26 −4.25<S2> 30.02 4.97 30.02 5.01J −35.2 (cm−1) −22.4 (cm−1)

a The net atomic spin densities are based on relaxed MP2 densitypopulation analysis.



Materials and methods

Iron(II) chloride tetrahydrate, 4-pyridinecarboxylic acid hydra-zide and 2-benzoylpyridine were purchased from Merck andused as received. Solvents of the highest grade commerciallyavailable (Merck) were used without further purification. IRspectra were recorded on KBr disks with a Bruker FT-IRspectrophotometer. UV-Vis spectra in solution were recordedon a thermo spectronic Helios Alpha spectrometer. 1H and 13CNMR spectra of the ligand HL in DMSO-d6 and mixed DMSO-d6 + D2O solutions were recorded on a Bruker Avance 250 MHzspectrometer and the chemical shifts are indicated in ppmrelative to tetramethylsilane (TMS). Atomic absorption spectro-scopy was carried out on a ContrAA 300 (AnalytikJena AG,Germany). Magnetic susceptibility measurements on polycrys-talline samples were performed on a Quantum Design SQUIDMPMS XL7 magnetometer. The DC measurement was per-formed in the temperature range of 1.9–300 K at applied mag-netic fields of 1000 Oe. Diamagnetic corrections of theconstituent atoms were estimated from Pascal’s constants (χdia =−4.2 × 10−4 cm3 mol−1).31 The DFT theoretical calculationswere carried out with the ORCA 2.9.1 computationalpackage.38 The hybrid B3LYP39 and double hybrid B2PLYPmethods40 were used for the calculation of the isotropicexchange constant J using the Noodleman formula with thebroken-symmetry approach.41 The geometry optimization forthe model compound with constrained Fe–Fe distances weredone using the BP86 density functional.42 The polarized triple-z quality basis set (def2-TZVP(-f )) proposed by Ahlrichs and co-workers has been used for all atoms.43 The calculations madeuse of the RI approximation with the decontracted auxiliarydef2-TZVP/J Coulomb fitting basis sets44 and the chain-of-spheres (RIJCOSX) approximation to exact exchange,45 asimplemented in ORCA. Increased integration grids (Grid4 inORCA convention) and tight SCF convergence criteria wereused throughout. The corresponding orbital transformationswere performed using the ORCA program.

Synthesis of (E)-N′-(phenyl(pyridin-2-yl)methylene)isonicotino-hydrazide (HL)

A methanol (20 mL) solution of 2-benzoylpyridine (3.26 mmol)was added drop-wise to a methanol solution (10 mL) of 4-pyri-dinecarboxylic acid hydrazide (3.26 mmol) and the mixturewas refluxed for 6 h. The solution was then evaporated on asteam bath to 10 mL and cooled to room temperature. Theresulting light yellow precipitates were separated and filteredoff, washed with 10 mL of cooled methanol and recrystallized(788 mg, yield 80%). Anal. Calc. for C18H14N4O (MW = 302.12 gmol−1). Elemental Analysis: C, 71.51; H, 4.67; N, 18.53. Found:C 71.62; H 4.73; N 18.60. FT-IR (KBr pellet): ν (cm−1) = 3120 (w,br), 1692 (vs), 1577 (m), 1549 (s), 1490 (s), 1465 (s), 1444 (s),114 (s), 1320 (vs), 1279 (vs), 1216 (m), 1142 (s), 1071 (s), 992(m), 927 (m), 895 (m), 847 (s), 807 (s), 744 (vs), 702 (vs), 678(vs), 652 (vs), 579 (m), 518 (m), 485 (m), 450 (m). 1H NMR(250.13 MHz, DMSO-d6, 25 °C, TMS): δ (ppm) = 14.81 (s, 1H,N–H), 8.93 (m, 3H), 7.94 (m, 2H), 7.73 (d, 2H), 7.61 (m, 4H),

7.49 (d, 2H). 1H NMR (250.13 MHz, DMSO-d6 + D2O): δ (ppm)= 7.41–8.80 (aromatic) ppm. 13C NMR (62.90 MHz, DMSO-d6):δ (ppm) = 121.46, 124.61, 127.15, 128.61, 131.06, 133.39,136.41, 138.06, 140.69, 148.97, 150.64, 154.87, 164.37, 193.85.UV-vis spectrum in CH3OH [λmax (nm), (ε, dm3 mol−1 cm−1)]:217 (27 340), 268 (26 460), 325 (12 660).

Synthesis of the complex L2Fe2

The ligand HL (453 mg, 1.5 mmol) was dissolved in a solutionof methanol (30 mL) and placed in a round bottomed flask.Solid FeCl2·4H2O (320 mg, 1.6 mmol) was added and the solu-tion was gently refluxed for 4 h with stirring. Upon cooling,the resulting reddish solid was filtered off, washed with cooledmethanol and dried in an oven at 60 °C (yield 75%). Anal.Calc. for 2(C38H32Cl2Fe2N8O4)·2(CH4O) (MW = 1758.72 gmol−1) C, 53.27; H, 4.13; N, 12.74. Found: C, 53.34; H, 4.16; N,12.67%. Atomic Absorption (AAS, λex = 248.3 nm, 0.2 nm slitwidth) 12.79%; calculated Fe 12.70%. FT-IR (KBr, cm−1), ν =3445 (m, broad), 1599 (m), 1565 (s), 1496 (vs), 1450 (vs), 1380(vs), 1331 (s), 1260 (m), 1200 (w), 1148 (s), 1082 (vs), 1021 (s),844 (m), 797 (s), 752 (s), 701 (vs), 698 (vs), 662 (m), 610 (m),508 (m). UV-vis spectrum in CH3OH [λmax (nm), (ε, M−1 cm−1)]:218 (145 900), 264 (147 300), 395 (30 100).

X-ray crystallography

Suitable crystals of L2Fe2 for X-ray analysis were grown from aconcentrated methanol solution following slow evaporation inthe dark at room temperature. X-ray diffraction data were col-lected at room temperature by the ω-scan technique on anAgilent Technologies four-circle diffractometer equipped withan Eos CCD-detector46 using graphite-monochromatized Mo–Kα radiation (λ = 0.71073 Å). The data were corrected forLorentz-polarization effects as well as for absorption.46Accu-rate unit-cell parameters were determined by a least-squares fitof 5930 reflections of the highest intensity, chosen from thewhole experiment. The calculations were mainly performedwithin the WinGX program system.47 The structures weresolved with SIR9248 and refined with the full-matrix least-squares procedure on F2 by SHELXL97.49 Scattering factorsincorporated in SHELXL97 were used. The function Σw(|Fo|2 −|Fc|

2)2 was minimized, with w−1 = [σ2(Fo)2 + (0.0865·P)2 +

1.1973·P], where P = [Max (Fo2,0) + 2Fc

2]/3. All non-hydrogenatoms were refined anisotropically and all hydrogen atomswere placed in calculated positions and were refined as ‘riding’on their parent atoms. The Uiso values for the hydrogen atomswere set as 1.2 (1.5 for methyl groups) times the Ueq value ofthe appropriate carrier atom. The relevant crystal data hasbeen listed in the main text as Table 1 together with refine-ment details.

Acknowledgements

We thank the University of Zanjan, the Adam Mickiewicz Uni-versity, the Academy of Sciences of the Czech Republic(KAN115600801), the Operational Program Research and



Development for Innovations – European Regional Develop-ment Fund (CZ.1.05/2.1.00/03.0058), the Operational ProgramEducation for Competitiveness – European Social Fund(CZ.1.07/2.3.00/20.0017) of the Ministry of Education, Youthand Sports of the Czech Republic for financial support.

Notes and references

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