New Highly Charged Iron(III) Metal-Organic Cube Stabilized by a Bulky Amine.

In this work, we report a new octanuclear cluster based on FeIII and the ligand 1H-imidazole-4,5-dicarboxylic acid, [Et3NH]12[Fe8(IDC)12]·10DMF·13H2O (1), with a metal core containing eight FeIII connected by only one type of organic ligand. A peak at 573 m/z in the mass spectra of the compound suggests the adduct species {[Fe8(IDC)12]+8H}4-. By X-ray photoelectron spectroscopy, the oxidation state of the iron cation was confirmed to be 3+, also identifying the presence of a quaternary nitrogen species, which act as a countercation of the anionic metal core [Fe8(IDC)12]12-. Mössbauer spectra recorded at different temperatures show an isomer shift and quadrupole splitting parameters that confirm the existence of only FeIII-HS in the structure of 1. X-ray analysis reveals that compound 1 crystallizes in the orthorhombic system space group Ibam, confirming a molecular cluster structure with an almost regular cube as geometry, with the FeIII atoms located at the corners of the cube and connected by μ-1κ2 N,O:2κ2 N',O‴-IDC3- bridges. Additionally, the magnetic measurements reveal a weak antiferromagnetic coupling in the Fe8 III coordination cluster (J = -3.8 cm-1). To the best of our knowledge, 1 is the first member of the family of cubes assembled with 1H-imidazole-4,5-dicarboxylic acid and FeIII cation, exhibiting high pH stability over a broad pH range, making it an ideal candidate for the design of supramolecular structures and metal-organic frameworks.

Introduction

One of the fascinating fields in coordination chemistry is the research in polynuclear molecular compounds, also called coordination clusters. This type of compound corresponds to molecular entities formed by multiple metal cation arrangements, connected by organic or inorganic ligands.[1] The molecular structure and properties of the coordination cluster are intrinsically related to the nature of the chosen metal center and the interaction between them through the bridging ligands.[2−4] Then, coordination clusters presenting cations with unpaired electrons are attractive in the field of molecular magnetism.[5−8] The slow relaxation of the magnetization reported for the disc-shaped Mn12II/III compound, [Mn12O12(O2CR)16(H2O)4] (R = methyl, phenyl), is a product of the high-spin state and magnetic anisotropy of the paramagnetic centers.[9,10] Furthermore, two Cr10III wheels bridged by acetate and alkoxide ligands, [Cr10(O2CMe)10(OR)20], show a different magnetic behavior depending on the alkoxide ligand present in the wheel. The authors report that the methoxide ligand favors a ferromagnetic behavior, while the ethoxide ligand favors an antiferromagnetic behavior.[11] Additionally, two mixed-valence coordination clusters [Mn12IIIMn7IIO8(HL)12(N3)3(MeO)5.5(MeOH)3.5(H2O)1.5 (OH)0.5](OAc)·10H2O and [Na2Mn11IIIMn4IIO8(HL)10(OAc)2(H2O)2(OCH3)1.5(N3)2.5](OAc)·10H2O·2MeOH (H3L = 2,6-(hydroxymethyl)phenol), reported in the literature exhibit the magnetocaloric effect at low temperatures due to the large spin, small anisotropy, and high density of low-lying excited states.[12] Research in coordination clusters based on FeIII is interesting in molecular magnetism not only by its rich coordination chemistry but also because this spin carrier can present spin values ranging from 5/2 to 1/2, depending on the coordination environment. These features make these types of compounds attractive for applications like spin switches and magnetic refrigerants.[13−16]

From a structural viewpoint, the literature reports several FeIII coordination clusters with a wide variety of nuclearities, topologies, and geometries, such as [Fe6(O2)O2(O2CPh)12(H2O)2] reported by McCusker et al.[17] and {[Fe8(O)2(OH)12(tacn)6]Br7(H2O)}Br·8H2O reported by Wieghardt et al.[18] (O2CPh– = benzoate; tacn = 1,4,7-triazacyclononane). Both compounds present planar moieties as central cores, constructed by oxides/peroxides anions in the case of Fe6 and oxides/hydroxides anions in Fe8. Using phosphonic acids, Konar et al. obtained [Fe9(μ3-O)4(O3PPh(Me)2)3(O2CCMe3)13] and [Fe12(μ2-O)4(μ3-O)4(O2CCHPh2)14(4-buPhPO3H)6] (O3PPh(Me)22– = 3,5-dimethylphenyl phosphonate; O2CCMe3– = pivalate; O2CCHPh2– = diphenylacetate; 4-buPhPO3H– = p-butylphenyl phosphonate), with Fe9 and Fe12 central cores, showing an icosahedral and a double butterfly structure, respectively.[19] Kang et al. reported the mixed-valence Fe18IIIFe24II coordination cluster [{Fe(Tp)(CN)3}24{Fe(H2O)2}6{Fe(dpp)(H2O)}12(CF3SO3)6]·18H2O (Tp = hydrotris(pyrazolyl)borate; dpp = 1,3-di(4-pyridyl)propane).[20] The Fe18IIIFe24II cluster has a number large enough of metal centers to be considered as a nanocage.[20]

From the experimental viewpoint, several polynuclear FeIII compounds found in the literature were synthesized using two or more ligands, giving rise in many cases to complex structures.[21−23] In this article, we report a novel Fe8III octanuclear cube-type coordination cluster [Et3NH]12[Fe8(IDC)12]·10DMF·13H2O (1) obtained using 1H-imidazole-4,5-dicarboxylic acid (H3IDC) as a unique ligand. The structure and composition of the octanuclear cluster were obtained by single-crystal X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and other complementary techniques. Furthermore, the characterization of 1 was complemented with Mössbauer spectroscopy, electronic paramagnetic resonance, and magnetic susceptibility measurements.

Experimental Section

Synthesis of [Et3NH]12[Fe8(IDC)12]·10DMF·13H2O (1)

Caution: The reaction involves the mixture of a moderate amount of triethylamine (Et3N) and glacial acetic acid (CH3COOH). Fuming vapors are released, so care must be taken in this procedure.

To a suspension of Fe(acac)3 (0.5 mmol) and H3IDC (1 mmol) in a mixture of 10 mL of water and N,N′-dimethylformamide (DMF) (1:1 H2O/DMF), 5 mL of Et3N was added with constant stirring. After a few minutes, the suspension disappears, and the resulting dissolution changes the initial orange color to a deep red color. After 10 min of stirring, 5 mL of glacial CH3COOH was added, giving a deep reddish-brown solution, which was left undisturbed for a few weeks, giving yellow prismatic crystals of 1 (Figure S1). MW = 4475.86 g/mol. Yield: 50% based on iron(III) salt. Anal. Calc. for: C162H300N46O71Fe8 (1): C, 43.47%; H, 6.77%; N, 14.40%; Fe, 9.98%. Found: C, 43.35%; H, 6.71%; N, 14.36%; Fe, 9.98%.

Physical Measurements

Elemental Analysis

The quantitative content of C, H, and N was obtained on a Thermos CHNS flash 2000 elemental analyzer using solid crystalline samples of 1. The quantitative content of Fe was determined using atomic absorption spectroscopy. Three samples of 1 from different batches were measured in triplicate. Solid crystalline samples of 1 were digested in a 0.5% HNO3 solution, and the iron concentration was measured with a Perkin Elmer PinAAcle 900F atomic absorption spectrophotometer equipped with an acetylene burner using a flux of acetylene:air of 2.51:10 L min–1 as an oxidant mixture. An Fe lamp of 248.33 nm wavelength was used with standard solutions in a linear range of 0.1–3,0 mgFe L–1.

Mass Spectrometry

Mass spectra were measured in a Linear Ion Trap LTQ-XL Thermo Scientific spectrometer and were analyzed with the software MMass 5.5.0.[24−26] The analysis was carried out in a DMF:H2O solution of 1 (pH ≈ 6.0) in negative ion mode, scanning in a range of m/z of 150–1500 (Table S2 shows the peak list of the mass spectrum).

Thermogravimetric Analysis

Thermogravimetric (TG) analysis was performed on a Mettler Toledo TGA/DSC STAR system. The samples were introduced in an alumina holder and heated under a nitrogen atmosphere from room temperature to 900 °C at a heating rate of 5 °C/min.

Single-Crystal X-ray Diffraction

A single crystal of 1 was directly selected from the reaction beaker and glued on the tip of a capillary glass using epoxy resin. Scans on the Bruker APEXII diffractometer confirmed enough crystal quality to perform full recording using X-ray radiation of Mo Kα (λ = 0.71073 Å). The data were reduced by SAINT,[27] and empirical absorption corrections were applied using SADABS.[28] The structure was solved with the ShelXT[29] structure solution program using direct methods and refined with the ShelXL[30] refinement package with least-squares minimization. Additional data concerning the crystallographic and the refinement parameters are detailed in the Supporting Information. ToposPro 5.0 software[31] was used for structure drawing. During the structure completion process by difference Fourier synthesis, it was clear that there was a high amount of space left by the Fe8 cubes of the complex within the cell, having an ill-defined electron density. Although the triethylammonium and water molecules were identified preliminarily at this point, refinement by any model we tried was unsuccessful. Efforts to measure at low temperatures failed since crystals crack. Synthetic modifications were also tried to change triethylammonium by an alkaline metal or other ammonium such as tetramethylammonium or tetraethylammonium without any good results. Since efforts to model the disorder of the solvating molecules by using a meaningful scheme failed, a well-documented method for the modeling of unresolved electron density, PLATON-SQUEEZE, was used.[32,33] Alert B is present in the check cif file. CHEMS01 Type_1 is due to the use of the “[Fe8C60H12 N24O48 + solvent]” expression to remark that the reported formula Fe8C60H12N24O48 lacks a specific amount of solvent and countercations molecules because of the use of SQUEEZE. PLAT084 and PLAT230 can be assigned to the unresolved effects such as the disorder in chemical species. Despite this, most of the structural determination alerts are related to cation/solvent disorder, thus allowing one to consider that the geometry for the Fe8 cluster is reliable. Therefore, based on the complementary analyses, 12 triethylammonium (Et3NH+) as counterions, 10 N,N′-dimethylformamide, and 13 water crystallization molecules per Fe8 cube unit were included in the final formula (C162H300N46O71Fe8).

X-ray Photoelectron Spectroscopy

XPS data were recorded using a PHOIBOS-150 hemispherical electron analyzer (Specs) under a pressure lower than 2 × 10–9 mbar using Al Kα radiation. The X-ray gun was operated at a power of 100 W to minimize radiation-induced damage to the sample. All the spectra were recorded at a constant pass energy of 30 eV. The binding energy scale was referenced to the main C 1s contribution due to aliphatic carbon, which was set at 284.6 eV. All spectra were computer-fitted to a sum of pseudo-Voigt profiles using Shirley background subtraction.

Mössbauer Spectroscopy

The Mössbauer spectra of finely ground crystals of 1 were recorded between 295 and 4.2 K. The measurements were recorded using two 57Co(Rh) γ-ray sources mounted on both ends of an electromagnetic transducer operated in the triangular mode. One of the sources was used for energy calibration with α-Fe (6 μm) foil. The spectra were analyzed by a nonlinear fit using the NORMOS program,[34] and the energy calibration was made using α-Fe (6 μm) foil. Variable temperatures were obtained with an Oxford Spectromag 4000 M cryostat connected to an ITC-503 Oxford Instruments temperature controller. To avoid saturation effects and optimize the signal-to-noise ratio, the sample thickness was 10 mg of natural Fe cm–2.

Electronic Paramagnetic Resonance

EPR spectra were collected on a Bruker EMX-1572 spectrometer working at 9.39 GHz (X-band) using polycrystalline samples of 1 at 298 K.

Magnetic Measurements

Magnetic measurements were carried out using a Quantum Design Dynacool Physical Properties Measurement System (PPMS) equipped with a vibrating sample magnetometer (VSM). The dc data were collected under externally applied fields of 1 kOe in the 1.8–300 K temperature range. Also, magnetization measurements at the variable field were performed at 1.8, 3, 5, and 8 K from 0 to 90 kOe. The magnetic signal from the sample holder was negligible to affect our data accuracy and was not considered. Diamagnetic corrections (estimated from Pascal constants) were considered.[35] The simulation of the temperature-dependent magnetic susceptibility data was performed by exact diagonalization of the spin Hamiltonian employing the PHI software.[36]

Results and Discussion

Synthesis

Concerning the synthesis of 1, the organic ligand H3IDC can be considered as a polyprotic acid. Depending on the pH of the media, this ligand can be present in the final product with different protonation degrees (Scheme ).

Scheme 1

pKa Values of the Organic Ligand H3IDC

The initial suspension of H3IDC in an H2O/DMF mixture gave a pH near 3, indicating that the full protonated H3IDC should be the predominant species in this media. When triethylamine (Et3N; 36 mmol) was added, the pH increased to 11, causing a displacement of the equilibrium to the more reactive ionic species HIDC2–/IDC3–, which are now able to coordinate the FeIII centers. Then, at the end of the reaction, the acidification with glacial acetic acid (87 mmol) decreased the pH of the solution to 4. Regarding the pH fluctuation in the reaction and the different protonation degrees of the H3IDC ligand (IDC3–/HIDC2–/H2IDC–), it is possible to infer that the assembly of iron(III) with the organic species should generate multiple possibilities for the final formula of Fe8III cluster. Thus, an anionic [Fe8(IDC)12]12–, neutral [Fe8(HIDC)12], or cationic [Fe8(H2IDC)12]12+ species could be considered as a product. Considering that the species of the ligand are dependent on the pH, it is reasonable to infer that IDC3– anion should be the predominant species at pH = 11 (pKa3 = 11.8), being also able to coordinate the FeIII cations to give the Fe8 cluster [Fe8(IDC)12]12–. Later, the pH decreases to 4, causing the quantitative protonation of Et3N (pKa = 10.8) needed to induce the crystallization of the anionic cluster, [Fe8(IDC)12]12–. The fact that the Fe8 cluster was assembled under a basic medium and crystallized under a acidic medium can be taken as evidence about the chemical stability of [Fe8(IDC)12]12– over a broad pH range.

Figure S2 shows the mass spectrum of 1 obtained by the linear ion trap in the range of 150–1500 m/z. The base peak at 573 m/z correlates very well with the adduct species {[Fe8(IDC)12]+8H}4–. Other adducts species with lower intensities were also identified at 1147, 764, and 458 m/z, which correlate with {[Fe8(IDC)12]+10H}2–, {[Fe8(IDC)12]+9H}3–, and {[Fe8(IDC)12]+7H}5–, respectively. This result corroborates that the anionic species [Fe8(IDC)12]12– is the correct representation for the Fe8 cluster.[37,38]

X-ray Single-Crystal Structural Characterization

X-ray single-crystal analysis reveals that 1 corresponds to a discreet octanuclear cluster forming a cubic polyhedron, which crystallizes in the highly symmetric Ibam orthorhombic system (Table ). Each corner of the cube is occupied by one FeIII center (Figure a), being the edges organic ligands. Continuous shape measurement (CShM) was used to evaluate the distortion of the cube. The iron moiety was compared with an ideal cube using the SHAPE software,[39] obtaining a CShM value of 0.004 and confirming an almost perfect cubic arrangement of the coordination cluster (Table S3). This cube is obtained by the asymmetric unit containing two crystallographically independent iron centers and three full deprotonated IDC3– ligands. Fe(1) and Fe(2) present the same coordination environment, FeN3O3, produced by the coordination of three κN,O-IDC3– molecules, which also form three independent FeNCCO chelate rings (Figure b). The geometry of each FeN3O3 moiety was also compared with an ideal octahedron using the SHAPE. CShM values of 1.13 and 1.00 were obtained for Fe(1) and Fe(2), respectively, confirming that the geometry of both cations is well described as octahedron (Tables S4 and S5). On the other hand, as depicted in Figure c, the IDC3– anion is acting as the μ-1κ2N,O:2κ2N′,O‴-IDC3– bis-chelating bridge, leading to an intercation distance of Fe(1)···Fe(2) = 6.3754(3) Å (Figure c). Additionally, each [Fe(IDC)3] moiety is chiral, being the handedness of Fe(1) Λ and Fe(2) Δ. Because of the Λ–Δ alternating arrangement, a centrosymmetric compound is obtained. In compound 1, the Fe–N and Fe–O bond distances are in the range of 2.028(7)–2.093(7) Å and 1.985(6)–2.003(7) Å for Fe(1) and in the range of 2.069(7)–2.098 (7) Å and 1.979(7)–2.005(7) for Fe(2). Usually, in coordination compounds, the bond distance can be associated with the oxidation and spin state of the cation. For example, Angaridis et al. reported two molecular systems [Fe(3,5-Bu2salpn)(MeDCBI)Fe(3,5-Bu2-salpn)] and {[Fe(3,5-Bu2salpn)]2(HDCBI)} (3,5-Bu2salpn = dianion of 1,3-bis-[(di-tert-butylsalicylidene)amino]propane); H2MeDCBI = 4,5-dicarboxy-1-methyl-1H-imidazole; H3DCBI = 4,5-dicarboxyimidazole).[40] Both compounds are based on FeIII in a high-spin state (HS), having an FeN3O3 environment formed by κ2N,O′-HDCB2–/κ2N,O′-MeDCBI2– and κ4O,N,N′,O′-3,5-Bu2salpn, being the cations bridged by μ-1κ2N,O:2κ2N′,O‴-imidazole ligand as in 1 and presenting Fe–N = 2.126(3)–2.107(1) and Fe–O = 1.883(2)Å–2.223 Å bond distances.[40] Also, in other imidazoles FeIII-HS compounds, the average Fe–N is 2.091 Å .[41] All these data permitted us to infer that the Fe(1) and Fe(2) cations present in 1 can be assigned as FeIII-HS. Therefore, the iron cluster previously determined from structural X-ray measurements can be represented as [Fe8(IDC)12]12–.

Table 1

Crystallographic Refinement Data for 1

empirical formula	C162H300N46O71Fe8a
formula weight (g mol–1)	4475.86a
temperature (K)	293(2)
crystal system	orthorhombic
space group	Ibam
a (Å)	18.051(8)
b (Å)	29.079(13)
c (Å)	33.766(15)
α (°)	90
β (°)	90
γ (°)	90
volume (Å3)	17,724(13)
Z	4
ρcalc (g/cm3)	0.856
μ (mm–1)	0.692
F(000)	4528.0
crystal size (mm3)	0.986 × 0.363 × 0.274
radiation	Mo Kα (λ = 0.71073)
2θ range for data collection (°)	5.954–52
index ranges	–22 ≤ h ≤ 22, −35 ≤ k ≤ 35, −41 ≤ l ≤ 41
reflections collected	67,448
independent reflections	8864 [Rint = 0.0848, Rsigma = 0.0601]
data/restraints/parameters	8864/0/320
goodness-of-fit on F2	1.297
final R indices [I ≥ 2σ (I)]	R1 = 0.1537, wR2 = 0.3714
final R indices [all data]	R1 = 0.2003, wR2 = 0.4078
largest diff. peak/hole (e Å–3)	2.08/–1.05

The empirical formula and molecular weight given in the table consider the counterions and solvate molecules determined by all the analysis techniques used in the experimental section.

Figure 1

(a) Molecular structure of 1 and view of the packing through the c axis. (b) Coordination environment of FeIII centers in 1 and (c) μ-1κ2N,O:2κ2N′,O‴ bis-chelating bridge mode of IDC3–.

Literature reports different analogous M8-HIDC compounds based on other transition metal cations (2–18), which are summarized in Table . For example, Liu et al. obtained under solvothermal conditions an anionic NiII cluster, [Ni8(HIDC)12]8– (2). This Ni8II cube is surrounded by a large number of solvent molecules and protonated 4,4′-trimethylenedipiperidine as countercations.[42] Likewise, by a solvothermal reaction, Xu et al. reported the synthesis of another 0D NiII-neutral cubic cluster [(Ni8(H2IDC)8(HIDC)4)]·8(C2H5OH)·18(H2O) (3), in which the H3IDC ligand is found both as di- and monoprotonated species and also surrounded by solvate molecules.[43] Meanwhile, Cheng et al. reported the synthesis at room temperature of another anionic cluster without solvate molecules [Me4N]20[Co8(IDC)12] (4),[44] but unfortunately, the crystalline structure was not reported in the publication. Moreover, extended networks obtained from the M8 anionic cluster and stabilized by the incorporation of a second metal cation have also been reported (5–10). In these cases, the imidazole ligand is present in the full deprotonated form (IDC3–), giving a cluster with a highly negative charge. For compounds 6–10, the anionic cluster is stabilized by the linkage of alkaline cations to the carboxylate groups belonging to the imidazole ligand.[45] Besides, Cheng et al. reported a mixed-valence CoII/CoIII cubic cluster forming a 1D heterometallic coordination polymer [Ni(cyclam)]4[Ni(cyclam)(H2O)2]2{[Ni(cyclam)][Co8(IDC)12]}·41H2O (5). This extended compound is assembled by the [Co6IICo2III(IDC)12]14– cluster, which is coordinated by the carboxylate group to the [Ni(cyclam)]2+ complexes.[44]

Table 2

Summary of M8 Cubes Reported in the Literature

formula of the compound	synthesis type	dimension	charge of the cluster	H3IDC species	ref
[Et3NH]12[Fe8(IDC)12]·10DMF·13H2O (1)	R.T.e	0D	–12	IDC3–	this work
Ni8(HIDC)12(H2TMDP)4(DMF)4(EtOH)4 (H2O)6a (2)	solvothermal	0D	–8	HIDC2–	(42)
[(Ni8(H2IDC)8(HIDC)4)]·8(C2H5OH)·18(H2O) (3)	solvothermal	0D	0	H2IDC–/HIDC2–	(43)
[Me4N]20[Co8(IDC)12]b (4)	R.T.	0D	–20	IDC3–	(44)
[Ni(cyclam)]4[Ni(cyclam)(H2O)2]2 (5)	R.T.	1D	–14	IDC3–	(44)
{[Ni(cyclam)][Co8(IDC)12]}·41H2Oc
K20[Ni8IDC12]·74(H2O) (6)	R.T.	3D	–20	IDC3–	(43)
K20[Ni8IDC12]·50(H2O) (7)	R.T.	3D	–20	IDC3–	(43)
K20[Ni8IDC12]·29(H2O) (8)	R.T.	3D	–20	IDC3–	(43)
{[Li11(Ni8IDC12)(H2O)12]Li9(H2O)20} (9)	solvothermal	3D	–20	IDC3–	(45)
{[Na20(Ni8IDC12)(H2O)28](H2O)13 (CH3OH)2} (10)	solvothermal	3D	–20	IDC3–	(45)
Zn12(guanidinium)8(IDC)8(HIDC)4·(DMF)8(H2O)3 (11)	solvothermal	3D	–16	HIDC2–/IDC3–	(46)
Cd8Na8(HIDC)8(IDC)4(H2Pip)2·(EtOH)5(H2O)37d (12)	solvothermal	3D	–12	HIDC2–/IDC3–	(46)
Zn8K8(HIDC)12(DMF)5(H2O)16 (13)	solvothermal	3D	–8	HIDC2–	(46)
Cd8K8(HIDC)12(DMF)5(H2O)16 (14)	solvothermal	3D	–8	HIDC2–	(46)
Co8K8(HIDC)12(DMF)5(H2O)16 (15)	solvothermal	3D	–8	HIDC2–	(46)
Mn8K8(HIDC)12(DMF)5(H2O)16 (16)	solvothermal	3D	–8	HIDC2–	(46)
[Cr4In4(HIDC)12]·H2O (17)	solvothermal	0D	0	HIDC2–	(47)
[Cr7.28In0.72(HIDC)12]·H2O (18)	solvothermal	0D	0	HIDC2–	(47)

H2TMDP2+ = 4,4′-trimethylenedipiperidinium.

Me4N+ = tetramethylammonium, crystalline structure not reported.

cyclam = 1,4,8,11-tetraazacyclotetradecane.

H2Pip2+ = piperazinium.

R.T. = room temperature.

On the other hand, Alkordi et al. reported six 3D networks built from M8II anionic cubes (11–16).[46] Zn12(guanidinium)8(IDC)8(HIDC)4·(DMF)8(H2O)311 and Cd8Na8(HIDC)8(IDC)4(H2Pip)2(EtOH)5(H2O)3712 contain HIDC2– and IDC3– species as ligands. In the case of 11, the negative charge is balanced by both guanidinium and zinc cations, being the zinc cations also coordinating the carboxylates groups belonging to the anionic cubes originating in this form the 3D structure. For 12, sodium and a protonated amine (piperazinium) are also balancing the charge of the cubic cluster. The authors also report a series of compounds M8K8(HIDC)12(DMF)5(H2O)16 (M = ZnII, CdII, CoII, and MnII) (13–16), containing only the HIDC3– species as a ligand, but in these cases, only alkaline cations are stabilizing the [M8(HIDC)12]8– cluster in the 3D networks. The two last examples (17 and 18) were reported by Zhai et al.[47] and correspond to InIII-CrIII heterometallic 0D cubes [Cr4In4(HIDC)12]·H2O and [Cr7.28In0.72(HIDC)12]·H2O, being neutral species and only including the HIDC2– in the structure.

In our case, although the molecular structure of the Fe8III core was well identified by X-ray single-crystal analysis due to the size of the cluster and the high symmetry of the cell, part of the electronic density surrounding the cluster remained invisible for this technique, as has also been reported in an analogous cobalt octanuclear cluster.[44] However, according to the examples mentioned above, the full deprotonated IDC3– species generates M8 cubes with the highest negative charge, which is stabilized by other metal cations or bulky protonated amines. Then, 12 counterions must be present in the compound to neutralize the negative charge of the resulting [Fe8(IDC)12]12– cluster. Since no additional iron centers were found in the intercluster space, only the triethylammonium cation should be acting as a counterion. This counterion also provides enough steric hindrance to avoid the assembly between the iron clusters, thus producing a molecular crystal packing for 1, as was also reported for the Ni8II analogous system with 4,4′-trimethylenedipiperidinium.[42] Accordingly, XPS, TG, and FTIR were performed to complement the structural characterization.

XPS

To gather more information about the chemical composition of 1, Fe 2p, N 1s, C 1s, and O 1s high-resolution XPS spectra were also collected (Figure a). The Fe 2p spectrum consists of an asymmetric spin-orbit doublet with binding energies of the Fe 2p3/2 and Fe 2p1/2 core levels of 711.5 and 725.2 eV, respectively, and a small shake-up satellite at 717.8 eV. The shape of the spectrum, the binding energies of its different components, and the presence of the small satellite are all characteristic of iron in 3+ oxidation state and high-spin configuration,[48,49] confirming the observation in the structural characterization. The N 1s spectrum was best-fitted to two contributions. The major one, located at a binding energy of 398.7 eV, can be ascribed to an amine-type or aromatic-type N–H bond belonging to the imidazole ring of the organic ligand.[50,51] The higher binding energy component at 401.1 eV is characteristic of a quaternary nitrogen species,[50,51] which must belong to the triethylammonium (Et3NH+) present in the intercluster space of 1 and acting as a countercation. A fit considering also the contribution to the N 1s spectrum of a ternary amine was not successful, discarding the presence of trimethylamine in the final product. The C 1s spectrum contains three different carbon species at 284.6, 285.8, and 288.4 eV, which can be associated with C–C or C–H bonds; C–N, C–O, C=N, or C ≡ N bonds; and O—C=O bonds, respectively.[50,51] The O 1s spectrum contains three contributions. The main one at 531.4 eV can be associated with C=O bonds in O=C—O groups, while the second one at 533.0 eV may correspond to C–O bonds in O=C—O groups.[50,51] The minor component at 535.0 eV might be ascribed to a small oxygen satellite.

Figure 2

(a) High-resolution Fe 2p, N 1s, C 1s, and O 1s XPS spectra. (b) Fe 2p spectra recorded after different exposure times to the X-ray beam of the XPS spectrometer: (a) 7 min, (b) 42 min, (c) 107 min, (d) 172 min, (e) 237 min, (f) 302 min, (g) 367 min, (h) 432 min, (i) 497 min, and (j) 562 min. Note the progressive transformation of the initial FeIII-HS species in an FeII-HS species. (c) N 1s spectra recorded after different exposure times to the X-ray beam of the XPS spectrometer: (a) 12 min, (b) 77 min, (c) 142 min, (d) 205 min, (e) 270 min, (f) 367 min, (g) 432 min, (h) 497 min, (i) 562 min, and (j) 592 min. Note how the intensity of the initial triethylammonium (Et3NH+) species decreases with increasing X-ray irradiation times.

All the identified bonds are part of the IDC3– anionic ligand present in the cluster [Fe8(IDC)12]12–. Nevertheless, from a chemical viewpoint, the presence of a quaternary nitrogen species corroborates that triethylammonium (Et3NH+) is the countercation of the anionic iron cluster, which is also consistent with the reaction condition used to obtain 1. Therefore, the molecular structure of the iron cluster is deduced to be [Et3NH]12[Fe8(IDC)12].

An important fact that was found during the development of this work is that the sample is very sensitive to the irradiation by the X-rays of the XPS spectrometer and that it undergoes important chemical changes after relatively short irradiation times. To minimize the occurrence of these changes, we used a low power in the X-ray gun (100 W against the more usual 300 W) and slightly higher pass energy (30 eV vs the usual 20 eV) to record the data. Using a constant pass energy of 30 eV instead of 20 eV allows recording the spectra with a larger number of counts in a given time without compromising too much the energy resolution. So, for the Fe 2p spectrum shown in Figure b, we used only one scan (less than 5 min of irradiation) since increasing the number of scans can lead to chemical changes.Figure b collects the Fe 2p spectra recorded after several increasing irradiation times. It is evident that, even after short irradiation times (see spectra (b) and (c) in Figure b), the lines of the Fe 2p spectrum shift to lower binding energies and a strong shake-up satellite starts developing at around 714 eV. These results clearly indicate that the initial FeIII-HS is progressively reduced to an FeII-HS species upon X-ray irradiation. In fact, the spectra recorded at intermediate irradiation times contain both FeII-HS/FeIII-HS contributions. After long irradiation times (Figure b), the Fe 2p spectrum only shows the presence of FeII-HS species.[49] Important changes are also observed in the N 1s spectra 2c. In this case, a decrease in the intensity of the protonated Et3NH+ species is observed upon X-ray irradiation. However, and contrary to the iron case, the quaternary nitrogen species appears to be more resilient, as it is still present after long irradiation times. The mechanisms giving place to chemical changes under X-ray irradiation in XPS are complex and have been commented in detail elsewhere.[51] In particular, in the case of the reduction of metal cations in oxides, interionic Auger decay processes appear to play an important role.[50] Among all the chemical changes reported, the degradation of quaternary nitrogen species (deprotonation) has also been reported for some organic compounds.[50] The results are relevant because they stress the need of being extremely careful when recording XPS data for this type of compound. Otherwise, erroneous conclusions can be drawn from the recorded data.

FTIR and Thermogravimetric Measurements

Figure S3a,b shows the infrared spectra of 1, compared with those of trimethylamine in acidic media and N,N-dimethylformamide. A clear match of the first two spectra between 3000 and 2400 cm–1 can be observed, in agreement with the presence of the cationic Et3NH+ species in 1 (Figure S3a). Also, the strong absorption at 1680 cm–1 (observed in the spectrum of 1) is exactly in the same position of the C=O vibration associated with DMF molecules (Figure S3b), indicating that this molecule must also be present in the final formula. Additionally, thermogravimetric analysis was performed under a nitrogen atmosphere (Figure S4) to characterize the presence of solvent molecules and thermal stability of 1. The cluster is stable until 250 °C, showing two weight losses of 5.2% (30–90 °C) and 16.1% (190–250 °C) associated with solvate molecules. Considering that both H2O and DMF were used as a solvent in the synthesis of 1 and as it has been reported for other compounds,[52] it is reasonable to assign the first loss to the release of 13 water molecules (5.2%) and the second one to 10 DMF molecules (16.3%). At higher temperatures, a third weight loss of 30.2% (250–360 °C) is observed. Literature data show that triethylammonium salts (triethylammonium bis-7,7,8,8-tetracyanoquinodimethane) have a thermal decomposition process between 195 and 220 °C.[53] Additionally, the inorganic cluster ([(HTEA)(TEA)0.75]2[PSb2IIIMo5VMo7VIO40]) shows a continuous weight loss (14.6%) between 44 and 573 °C, which is attributed by the authors to the release of tryethylamine.[54] Therefore, the loss of 30.2% can be associated to the release of 12 trimethylamine (27.4%) belonging to 12 Et3NH+ acting as counterions.[55] Considering the solvate molecules, the final formula of 1 is [Et3NH]12[Fe8(IDC)12]·10DMF·13H2O, which correlates very well with the elemental analysis given in the experimental section (section 2.1.1). With these results in mind, Mössbauer spectroscopy, electronic paramagnetic resonance, and magnetic susceptibility measurements were also performed to characterize the properties of [Et3NH]12[Fe8(IDC)12]·10DMF·13H2O.

EPR and Magnetic Measurements

The X-band EPR spectrum at room temperature of a polycrystalline sample of 1 shows a broad signal characteristic of an FeIII-HS ion with S = 5/2,[56,57] with a line width of 376 G and a resonant field of 3330 G giving a value of g = 2.016 (Figure S5a). Magnetic measurements of 1 as a function of temperature were performed using a polycrystalline sample in the range of 1.8–300 K at 1 kOe. The temperature dependence of the magnetization is shown in Figure a as χm vs T and χm–1 vs T plots. The χm vs T plot shows an increase in the χm value as the temperature is lowered, leading to a maximum of 0.3 emu mol–1 at 50 K. From this temperature, a constant decrease is observed, leading to a non-zero value of 0.2 emu mol–1 at 1.8 K. On the other hand, the χm–1 vs T plot gives C and θ values of 44.6 emu mol–1 and −73.2 K, respectively. Furthermore, a 35.5 emu K mol–1 value is obtained at room temperature (4.44 emu K mol–1 per FeIII center) from the χmT vs T plot (Figure S5b), close to the expected for eight non-interacting FeIII-HS centers, 35.56 emu K mol–1 considering SFe = 5/2 and g = 2.016. Lowering the temperature, the χmT product constantly decreases as the temperature reaches 100 K. Below this temperature, a more pronounced decrease is observed, reaching a minimum value of 0.3 emu K mol–1 at 1.8 K. Clearly, the susceptibility data shows that an antiferromagnetic behavior is the predominant phenomenon of the Fe8III coordination cluster. Field-dependent magnetization measurements were performed at 1.8, 3, 5, and 8 K and represented as Nβ vs H and Nβ vs HT–1 plots (Figure S6). The curves Nβ vs H show a lack of saturation at the studied conditions, leading to a maximum near to 4 μB at 90 kOe, very far than the value expected for eight FeIII cations in high spin (40 μB). For an antiferromagnetic system with a spin state of S = 0 at low temperatures, the observed magnetization only can belong from non-zero spin states that are close to the ground state. Then, the absence of saturation and the small values obtained for the magnetization corroborate that antiferromagnetic interactions predominate in the Fe8III coordination cluster at low temperatures.

Figure 3

(a) Plots of χm vs T and χm–1 vs T for 1. (b) Images of magnetic pathways for the Fe8 moiety in 1 and the binuclear Fe2 simplified model.

As Figure shows, 12 equivalent μ-1κ2N,O:2κ2N′,O‴ magnetic pathways can be established (the edges of the cube) between the eight FeIII-HS in the octanuclear molecular structure of 1, leading to the following spin Hamiltonian:

No analytical model has been developed for such a complex system. For this reason, the use of a simplified model was necessary. Considering the symmetry of the cluster, the cubic system can be simplified to a binuclear system that consists in two FeIII-HS cations, connected by a single μ-1κ2N,O:2κ2N′,O‴ bridge (Figure b), leading to the following spin Hamiltonian:

A fit of the experimental susceptibility data by matrix diagonalization using the Ĥ2 spin Hamiltonian using the PHI[36] software was performed between 300 and 1.8 K, considering SFe = 5/2 and maintaining the g factor fixed to the experimental value obtained by EPR, g = 2.016 (Figure ). A magnetic coupling constant of J = −3.8 cm–1 was obtained, with an agreement factor of R = 8.5 × 10–3 (R = Σ(χmTobs) – (χmTexp)2/Σ(χmTobs)2). The use of a free g value gave a better fit (Figure S5c), obtaining J and g values of −5.28 cm–1 and 2.178, respectively. In both cases, the negative sign and a similar magnitude of the magnetic exchange coupling confirm the antiferromagnetic nature of the superexchange interaction of FeIII-HS cations through μ-1κ2N,O:2κ2N′,O‴ bridges. However, the use of a g value of 2.016, obtained from EPR measurements, seems to be more adequate and in agreement with the structural features of the FeIII-HS cations. The magnitude and sign of the coupling constants also are in agreement with the antiferromagnetic interactions observed by Angaridis et al.[40] between FeIII-HS/FeIII-HS mediated by μ-1κ2N,O:2κ2N′,O‴ bridges with J = −4.8(2) cm–1. The interaction between FeIII-HS/FeIII-HS can be rationalized considering the five magnetic orbitals associated with FeIII-HS in an octahedral environment (two eg and three t2g). Thus, the interaction through the IDC3– ligand generates a greater possibility of overlapping between the five orbitals of each FeIII-HS center.[58] The fact that the binuclear model presents a rather moderate fit of the magnetic behavior can be explained on the base of the crude simplification of the octanuclear structure of 1. An analogous Ni8II cluster was reported by Liu et al.,[42] (H2TMDP)4(DMF)4(EtOH)4(H2O)6[Ni8(HImDC)12] (DMF = N,N′-dimethylformamide, EtOH = ethanol, and H2TMDP = 4,4′-trimethylenedipiperidinium), having the same cubic structure of 1 and the same μ-1κ2N,O:2κ2N′,O‴ connectivity between the metal cations. This compound also presents an antiferromagnetic behavior, with an identical χm vs T tendency compared with 1. Although no exchange coupling values were given for this Ni8 cluster, this example suggests that the bulk magnetic properties of such cubic systems are related not only with the particular magnetic interactions of the metal cations through μ-1κ2N,O:2κ2N′,O‴ but also with the spin topology arrangement within the whole cluster structure. In any case, more experimental and theoretical analyses should be performed to clarify the magnetic behavior of compound 1, perhaps studying the orbitals involved in each magnetic exchange pathway and the role of the topology of the cluster in the bulk magnetic properties.

Figure 4

Fitting of χmT vs T at 1 kOe and using a simplified binuclear model.

Mössbauer Spectroscopy

The temperature evolution of the Mössbauer spectra of 1 at 295, 77, and 4.2 K is shown in Figure . In the studied temperature range, each spectrum consists of a doublet, corroborating that Fe(1) and Fe(2) present the same coordination environment. Furthermore, the Mössbauer parameters at room temperature, isomer shift (δ = 0.336(1) mm s–1) and quadrupole splitting (ΔEQ = 0.369(4) mm s–1), are characteristics of FeIII-HS. As the temperature decreases, the isomer shift increases, δ = 0.447(1) mm s–1 at 4.2 K, according to the second-order Doppler shift; nevertheless, the quadrupole splitting is nearly independent of the temperature (ΔEQ = 0.352(2) mm s–1 at 4.2 K), as expected for FeIII-HS ions for which the quadrupolar interaction is due only to the lattice contribution. In the same way, the presence of only a doublet even at 4.2 K would be indicative of the absence of magnetic order or that the relaxation time is shorter than the lifetime of the nuclear excited state.[59] For the antiferromagnetic FeIII-HS coupled system, a sextet should be observed in the Mössbauer spectrum, below the critical temperature (50 K). Nevertheless, the spectra recorded at room temperature, 77 K, and 4 K are almost the same, consisting of a sharp doublet instead of a sextet. This behavior indicates that the relaxation time of compound 1 is shorter than the lifetime of the nuclear excited state (experimental measurement time, 10–7 s).[51] Such kind of paramagnetic fast spin relaxation has also been reported for other polynuclear FeIII-HS coupled systems.[60,61]

Figure 5

Mössbauer spectra of 1 at different temperatures.

Conclusions

A new iron octanuclear coordination cluster 1 was successfully synthesized under an acid–base reaction using a single ligand, 1H-imidazole-4,5-dicarboxylic acid (H3IDC). The cluster [Fe8(IDC)12]12– is formed in basic media and crystallized in acid media, proving the chemical stability of the anionic Fe8 cluster in a wide pH range. On the other hand, XPS is a very powerful technique that permitted to identify the existence of triethylammonium cations as a charge compensator that balance the charge of the anionic cluster, and to corroborate the chemical identity of the cluster [Fe8(IDC)12]12– determined by single-crystal X-ray diffraction. Moreover, it was possible to identify that a reduction process centered on the metal cation is produced under experimental conditions of the XPS technique. In fact, the recorded data have to be carefully analyzed to obtain the correct information on the oxidation state of the metal cation in this type of compound. Additionally, Mössbauer spectroscopy corroborated the spin and oxidation state of the iron center within the cluster, leaving no doubt that only FeIII-HS ions are present in the structure of 1. The performed magnetic measurements revealed that weak antiferromagnetic coupling is dominant in the whole temperature range. Although the literature reports the existence of MnII, NiII, CoII/III, ZnII, and CrIII-InIII negative or neutral cubic analogous systems forming 0D, 1D, or 3D compounds with either H2IDC–/HIDC2–/IDC3– anionic species, compound 1 is the first member of this family based on FeIII. Despite the challenges presented for the characterization of 1, the present work collects a complete chemical and physical description of a new FeIII-HS coordination cluster with a novel cubic structure.