Single-Molecule-Magnet FeII4FeIII2 and Antiferromagnetic FeIII4

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Single-Molecule-Magnet FeII4FeIII2 and Antiferromagnetic FeIII4 Coordination Clusters Suman K. Barman,† Joan Cano,‡ Francesc Lloret,‡ and Rabindranath Mukherjee*,† †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India Departament de Química, Inorgànica/Instituto de Ciencia Molecular (ICMOL), Universitat de València, Polígono de la Coma, s/n, 46980 Paterna (València), Spain

‡

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S Supporting Information *

ABSTRACT: Supported by endogenous (part of the ligand, in-built) phenoxo bridges provided by the ligand 2,6-bis[{{(5bromo-2-hydroxybenzyl)}{(2-(pyridylethyl)}amino}methyl]4-methylphenol) (H3L), in its deprotonated form, exogenous (not part of the ligand, externally added or generated) oxo-/ hydroxoand acetato-bridged [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1) and [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2) coordination clusters have been synthesized and structurally characterized. Complexes 1 and 2 have μ4-O and μ3-OH bridges, respectively. Magnetic studies on 1 reveal slow magnetic relaxation below 2 K. Both in-phase (χ′M) and outof-phase (χ″M) magnetic susceptibility were found to be frequency dependent. This is typical of a single-molecule magnet (SMM) with τ0 = 1.9(2) × 10−7 s−1 and Ea = 5.1(3) cm−1. Assuming that Ea corresponds to the energy splitting of the ground spin state (S = 2) by the zero-filed-splitting (zfs), Ea = 4|D| (D is the axial zfs parameter), D ≈ − 1.3 cm−1 could be estimated. For 2, three types of magnetic interactions are observed: JA = −56.5(3), JB = −71.6(4), and JC = +4.5(2) cm−1. Considering the observed structural parameters, the magnetic behavior for both of the coordination clusters 1 and 2 has been rationalized.

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INTRODUCTION Synthesis of high-nuclearity transition-metal clusters through “coordination-driven self-assembly” continues to be of special interest, owing to their aesthetically pleasing structures, interesting magnetic properties,1,2 and the possibility of observing slow magnetic relaxation effects at the molecular level.3 Application potentials of such coordination clusters include high-density information storage, quantum computing, and spintronic technology.4,5 In order to utilize these properties, it is important to examine the topology of the synthesized clusters for a better understanding of the origin of the observed properties, which are associated with structural features of the polynuclear complexes. The increasing interest in this field is justified by the intellectual challenge in controlling and manipulating the self-assembly process. The choice of the bridging ligand(s) is crucial for the formation of these clusters. To successfully synthesize such coordination clusters, carboxylates act as ideal linkers between metal ions for propagation of metal−ligand coordination units due to their divergent bridging capabilities.6 In fact, we have investigated magnetostructural properties of acetato-bridged coordination polymers7a,b and discrete coordination clusters of various nuclearity.7c−f It is appropriate to mention here that understanding the electronic and magnetic properties of μ-oxo-, μhydroxo-, and μ-carboxylato-bridged high-nuclearity iron © XXXX American Chemical Society

complexes is of significant importance because of their existence in the active sites of several nonheme diironcontaining proteins and enzymes.8 From the viewpoint of magnetic property driven applications, single-molecule magnets (SMMs)3−5,9−11 are very useful, as these discrete molecules are capable of retaining a net magnetization even after removal of the applied magnetic field. In other words, SMMs refer to slow relaxation of magnetization of purely molecular origin.3 Two fundamental criteria for obtaining efficient 3d-SMM behavior are (i) a large ground-state spin (S) and (ii) a high negative magnetic anisotropy, quantified by the axial zero-field splitting parameter (D). In order to maximize molecular ground-state spin, synthesis of polynuclear coordination clusters has been a good strategy.9e Here an ensemble of paramagnetic ions couple their spins via superexchange pathways to achieve large-spin ground states.9d,10a−c,11b,g However, in most cases the maximum high-spin regime is observed only at low temperatures (T ≲ 30 K).11g Stronger electronic alignment to observe large ground-state spin is constrained by superexchange interaction. The extent of superexchange is dependent on structural features: e.g., arrangement of metal ions, nature of Received: March 22, 2019

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DOI: 10.1021/acs.inorgchem.9b00828 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry bridging groups, bridge angle, dihedral angle between the planes containing metal ions, etc. Thus, systematic investigation on magnetostructural studies is very important and relevant to obtain coordination clusters that can exhibit thermally well isolated ground states, even at room temperature and beyond.12a The second requirement for SMM behavior, large magnetic anisotropy, is very difficult to achieve in a controllable manner, and that is why clusters with large spin values do not always show SMM behavior. Magnetic anisotropy (D) is related to Jahn−Teller distortion. The ligand-field approach is handy in this regard to judiciously choose the metal ion, with potentially high magnetic anisotropy.12b In this context, FeII ion is a suitable candidate, as it can provide a high-spin ground state and, by manipulating the coordination geometry of the FeII ion, magnetic anisotropy can be controlled. Thus, the synthesis of high-nuclearity iron(II)13 and iron(III)14 coordination clusters has been of continued interest. It is appropriate to mention here that recently there has been a totally new synthetic procedure established for the guaranteed preparation of high-spin coordination clusters and polymers with exciting magnetic properties, including SMM behavior.15 Moreover, synthesis of mixed-valent coordination clusters has attracted much attention owing to their electrochemical properties, small-molecule activation, catalysis, and magnetic applications.14d,f,16 In mixed-valent clusters, the existence of several redox sites with different spins makes it challenging to understand their ground-state electronic properties due to the presence of delocalization. Thus, the development of mixed-valent coordination clusters is quite relevant from the viewpoint of understanding their magnetostructural as well as electronic structural properties. From this perspective, herein we describe the synthesis and magnetic properties of phenolato-, oxo-, and acetato-bridged hexanuclear mixedvalent [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1) and tetranuclear [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2) coordination clusters. The coordination clusters are endogenously supported by two phenol-based 2,6-bis[{{(5-bromo-2hydroxybenzyl)}{(2-(pyridylethyl)}amino}methyl]-4-methylphenol) (H3L) (Figure 1), in their deprotonated form (L3−). To the best of our knowledge, 1 is the first example of a mixedvalent FeII4FeIII2 coordination cluster to exhibit SMM behavior. A temperature-dependent magnetic study reveals that for 2 three types of magnetic interactions are observed. On the basis of structural parameters, the observed magnetic behavior of coordination clusters 1 and 2 has been rationalized. For a better understanding and estimation of the magnetic-exchange parameters DFT calculations have been performed on 1.

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Figure 1. organic layers were dried over anhydrous Na 2 SO4. Solvent evaporation led to isolation of the amine. Yield: 1.6 g, 76%. 1H NMR (CDCl3, 500 MHz): δ 3.02 (2H, t, −CH2−), 3.108 (2H, t, −CH2−), 3.96 (2H, s, −CH2−), 6.69 (2H, s, aromatic H), 7.12 (1H, t, pyridine 5-H), 7.20 (1H, d, pyridine 3-H), 7.60 (1H, t, pyridine 4H), 8.52 (1H, d, pyridine 6-H). Step 2. To a solution of 2,6-bis(chloromethyl)-4-methylphenol (0.414 g, 0.002 mol) dissolved in dry THF (10 mL) was added dropwise a solution in dry THF (7 mL) of the amine (1.228 g, 0.004 mol), obtained in step 1, followed by addition of Et3N (0.408 g, 0.004 mol) at 273 K. The mixture was stirred for 24 h at 298 K and filtered to remove a white solid. The filtrate was evaporated to dryness, and CH2Cl2 (20 mL) was added. The solution was washed with brine solution and water. The organic layer was dried over anhydrous Na2SO4. Solvent was evaporated to obtain the desired ligand H3L, as a yellowish white solid. Yield: 1.1 g, 73%. 1H NMR (CDCl3, 500 MHz): δ 2.16 (3H, s, −CH3), 2.82 (4H, t, −CH2−), 3.06 (4H, t, −CH2−), 3.68 (4H, s, −CH2−), 3.72 (4H, s, −CH2−), 6.60 (2H, d, aromatic H), 6.74 (2H, s, aromatic H), 7.00 (2H, d, aromatic H), 7.09 (2H, s, aromatic H),7.15 (2H, t, pyridine 5-H), 7.18 (2H, d, pyridine 3-H), 7.54 (2H, t, pyridine 4-H), 8.50 (2H, d, pyridine 6-H). Synthesis of Complexes. [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1). Method A. To a mixture of H3L (0.100 g, 0.13 mmol) and NaO2CMe·3H2O (0.088 g, 0.65 mmol) in MeOH (4 mL) was added solid [FeII(H2O)6](CF3SO3)2 (0.120 g, 0.26 mmol), under anaerobic conditions (mBraun glovebox). After 5 min of stirring, solid FeIII(ClO4)3·6H2O (0.050 g, 0.13 mmol) was added portionwise to it and the mixture was stirred. After 6 h, the reaction mixture was filtered through a G-4 frit and the filtrate was kept for Et2O diffusion under anaerobic conditions, affording single crystals suitable for X-ray diffraction studies. Yield: 0.090 g, 58%. Anal. Calcd for C98H122Br4Fe6N8O20 (fw 2386.78): C, 49.31; H, 5.12; N, 4.70. Found: C, 49.10; H, 4.85; N, 4.82. UV−vis (MeCN): λmax, nm (ε, M−1 cm−1) 415 (1900). Method B. To a stirred mixture of H3L (0.100 g, 0.13 mmol) and NaO2CMe·3H2O (0.088 g, 0.65 mmol) in MeOH (5 mL) was added solid [Fe(H2O)6](CF3SO3)2 (0.120 g, 0.26 mmol), under anaerobic conditions (mBraun glovebox). After 10 min, solid FeIII(CF3SO3)3 (0.065 g, 0.13 mmol) was added portionwise and the mixture was stirred for 5 h. The reaction mixture was then filtered through a G-4 frit, and the solution was kept for Et2O diffusion, under anaerobic

EXPERIMENTAL SECTION

General Considerations. All reagents were obtained from commercial sources and used as received. Solvents were dried/ purified as reported previously.7 2,6-Bis(chloromethyl)-4-methylphenol17 and [Fe(H2O)6](CF3SO3)218 were prepared by following the reported procedures. Synthesis of H3L. The ligand H3L was synthesized in two steps. Step 1. To a solution of 2-pyridylethylamine (0.850 g, 0.007 mol) in MeOH (20 mL) was added solid 5-bromo-2-hydroxybenzaldehyde (1.40 g, 0.007 mol) portionwise. This mixture was refluxed for 3 h and then cooled to 298 K. Solid NaBH4 (0.304 g, 0.008 mol) was then added portionwise with continuous stirring. After 12 h of stirring, the volatiles of the mixture were removed under reduced pressure, and then water (20 mL) was added. The mixture was neutralized with MeCO2H and extracted with CH2Cl2 (3 × 15 mL). The combined B


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Inorganic Chemistry conditions. Single crystals suitable for X-ray diffraction studies were obtained. Yield: 0.100 g, 64%. It should be noted that both methods provided identical products, as verified by X-ray crystallography. [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2). To a solution of H3L (0.051 g, 0.068 mmol) in MeOH (3 mL), first solid NaO2CMe· 3H2O (0.051 g, 0.375 mmol) and then solid FeIII(ClO4)3·6H2O (0.048 g, 0.136 mmol) were added. The reddish brown solution thus obtained was stirred for 6 h and filtered. The filtrate was kept for Et2O vapor diffusion. A dark brown crystalline solid was obtained. Slow evaporation of a MeCN solution of this solid led to single crystals, suitable for X-ray diffraction studies. Yield: 0.045 g, ∼65%. Anal. Calcd for C86H94Br4ClFe4N11O20 (fw 2178.9): C, 47.36; H, 4.31; N, 7.07. Found: C, 47.51; H, 4.21; N, 6.95. UV−vis (MeCN): λmax, nm (ε, M−1 cm−1): 465 (9180), 345 (sh) (11 100). Caution! Perchlorate salts of compounds containing organic ligands are potentially explosive! Physical Measurements. Elemental analyses were obtained using a Thermo Quest EA1110 CHNS-O instrument (Italy). UV−vis spectra were recorded on an Agilent 8453 diode-array spectrophotometer. 1H NMR spectra (CDCl3) were obtained on a JEOL LA 500 (500 MHz) spectrometer. Chemical shifts are reported in ppm referenced to TMS. Variable-temperature magnetic susceptibility measurements in the solid state were performed with a Quantum Design (València, Spain) SQUID magnetometer. Variable-temperature (2−300 K) direct current (dc) magnetic susceptibility measurements on crushed crystals of 1 and 2 under applied fields of 0.25 (T < 30 K) and 5.0 kG (T ≥ 30 K) and variable-field (0−5 T) magnetization measurements in the temperature range 2.0−10.0 K were carried out with a Quantum Design SQUID magnetometer. Variable-temperature (2.0−10.0 K) alternating current (ac) magnetic susceptibility measurements at frequencies in the range 0.6−10.0 kHz under ±4.0 G oscillating field and different applied static dc fields in the range 0−1.0 kG were performed for 1 with a Quantum Design Physical Property Measurement System (PPMS). Data were corrected for the diamagnetism of the constituent atoms and the sample holder. The crushed samples of 1 and 2 were restrained with n-eicosane to prevent any displacement due to their significant magnetic anisotropy. Crystal Structure Determination. Single crystals of suitable dimension were used for data collection. Diffraction intensities were collected on a Bruker SMART APEX CCD diffractometer, with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at 100(2) K. For data reduction the “Bruker Saint Plus” program was used. Data were corrected for Lorentz and polarization effects; empirical absorption correction (SADABS) was applied. For 1, the structure was solved by SHELXT and refined with SHELXL-2016,19a,b incorporated into the Olex2 1.2-alpha crystallographic package.20a For 2, the structure was solved by SIR-97 and refined by full-matrix least-squares methods, based on F2 using SHELXL-97, expanded by difference Fourier syntheses, and refined with SHELXL-2014,19a as incorporated in the WinGX 2014.1 crystallographic collective package.19c The positions of the hydrogen atoms were calculated by assuming ideal geometries but not refined. All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix leastsquares procedures on F2. The structural analysis of 1 revealed distorted Et2O molecules. The atoms O11, O112, and O13 of the Et2O molecules are situated on a crystallographically imposed 2-fold axis. C44, C45, C46, C47, C52, and C53 are generated by the symmetry operator −3/2 − x, 3/2 − y, z. Fe1, Fe2, and Fe3 of 1 are situated on a crystallographically imposed glide plane. The other half of the molecules is generated by the symmetry operator −1/2 − x, 3/2 − y, z. The anisotropic displacement parameters of the highly distorted Et2O molecules were constrained, using distance restraints. To make the ADPs of the atoms reasonable, the rigid-bond restraints DELU, SADI, SIMU, and ISOR were used on the solvent molecules. For 1 the convergence of the Flack parameter was taken into account, during the final cycles of least-squares refinement. At this stage, all the x, y, z coordinates for 1 were inverted (using both TWIN and BASF entry) to test the absolute configuration. In fact, before refinement the R1 (wR2) values

were 0.0654 (0.01513) and the Flack parameter (x) value was 0.041(2). After twin refinement, the values became 0.0618 (0.01513) with x = 0.027(6), suggesting that the absolute configuration is best described by the latter coordinate assignment. The figures were made with the Mercury program.20b Pertinent crystallographic parameters are summarized in Tables S1 and S2. CCDC 1849015 (1) and 1849001 (2) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Computational Details. DFT calculations were carried out using the Gaussian 09 package21 to estimate the magnitude of the magnetic coupling between the magnetic centers in 1. These calculations were performed with the B3LYP hybrid functional; the quadratic convergence approach and a guess function was generated with the fragment tool using the same program.22−24 Two ways to build the atomic orbitals were used: (i) triple-ζ and double-ζ all-electron basis sets proposed by Ahlrichs et al. were employed for the metal ion and the rest of the atoms, respectively; (ii) the same triple-ζ basis set was employed for all atoms.25,26 The magnetic coupling states were obtained from the relative energies of the broken-symmetry (BS) singlet spin state from the high-spin state with parallel local spin moments. More details about the use of the broken-symmetry approach to evaluate magnetic coupling constants can be found in the literature.27−29 A polarizable continuum model (PCM) was introduced in the calculations with the parameters corresponding to acetonitrile.30 High-spin iron(II) complexes show an incomplete t2g shell, and therefore searching for the most stable electronic configuration is a difficult task, even more so when a large number of FeII ions are present. This situation can be made simpler by reducing the number of FeII ions. Only two metal ions are involved in each magnetic coupling in our molecular models (see below). Therefore, they were kept and the remaining FeII and FeIII ions were replaced by diamagnetic ZnII and GaIII ions, preserving the overall charge of the complex and avoiding a cutting of the molecule that could introduce unwanted electronic effects.

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RESULTS AND DISCUSSION Synthesis of the Ligand H3L and the Coordination Complexes Thereof. The ligand H3L was synthesized in two steps. The first step involved Schiff base condensation between 2-pyridylethylamine and 5-bromo-2-hydroxybenzaldehyde, followed by reduction with NaBH4 to produce the secondary amine. In the second step, the amine was treated with 2,6bis(chloromethyl)-4-methylphenol and Et3N in THF. This afforded H3L (Figure 1) as a yellowish white solid, which was characterized by its 1H NMR spectrum (Figure S1). The choice of H3L in this work derives impetus from our recent results7e on the hydrolysis of carboxy esters under ambient conditions by dinickel(II) complexes; recently we reported the series of dinuclear complexes [NiII2(L*)(H2O)2(MeOH)](ClO4)·MeOH, [NiII2(HL*)(μ-O2CMe)(H2O)2](ClO4)·4H2O, [NiII2(H2L*)(μ-O2CMe)2](ClO4)·CH2Cl2· 2H2O, [NiII2(HL*)(μ-O2CEt)(H2O)2](ClO4), and particularly the pentanuclear complex [NiII5(H2L*)2(μ3-OH)2(μO2CMe)4](ClO4)2·MeCO2Et, with use of H3L* (=2,6-bis[{{(5-bromo-2-hydroxybenzyl)(N′,N′′-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol). The difference between the two ligands is only replacing the N′,N′′-dimethyl arms of H3L* by 2-pyridyl arms, present in H3L. Notably, in none of these NiII complexes did bromophenolate units act as bridging ligands. Barring only one case, in all other complexes these units coordinated as phenol(s), not in the common phenolate form. Moreover, using asymmetrical ligands providing both a bridging and a terminal phenolate to a pair of iron ions, mixed-valent binuclear FeIIFeIII complexes have C


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Inorganic Chemistry been reported.31 Anticipating that the bromophenolate units of H3L may act as bridging ligands, we attempted the synthesis of FeII/FeIII coordination cluters with H3L and the present work is the outcome of such a synthetic exploration. The reaction of H3L, NaO2CMe·3H2O, [FeII(H2O)6](CF3SO3)2, and FeIII(ClO4)3·6H2O (1:5:2:1 mol ratio, respectively) in MeOH, under anaerobic conditions, followed by vapor diffusion of Et2O, afforded [FeII4FeIII2 (O)2(O2CMe)4(L)2]·4Et2O (1). The identical complex in better yield was obtained when FeIII(CF3SO3)3 was used instead of FeIII(ClO4)3·6H2O. The reaction of H3L, NaO2CMe·3H2O, and Fe(ClO4)3·6H2O, (1:5.5:2 mol ratio, respectively) in MeOH, followed by vapor diffusion of Et2O, afforded a microcrystalline solid. The slow evaporation of a MeCN solution of this solid yielded [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2). We should admit that, after initial serendipity in the synthesis, we were successful in the designed synthesis of 1 and 2. The formation of the coordination clusters 1 and 2 is summarized in eqs 1 and 2, respectively. 2H3L + 10NaO2 CMe·3H 2O + 4[Fe II(H 2O)6 ](CF3SO3)2 + 2Fe III(CF3SO3)3 (MeOH, Et 2O) → [Fe II 4Fe III 2(O)2 (O2 CMe)4 (L)2 ]·4Et 2O + 6MeCO2 H + 10NaO3SCF3 + 4CF3SO3H + 52H 2O

(1)

2H3L + 11NaO2 CMe·3H 2O + 4Fe III(ClO4 )3 · 6H 2O(MeOH, MeCN, Et 2O) → [Fe III 4(OH)2 (O2 CMe)3 (L)2 ](ClO4 )·3MeCN· 2H 2O + 8MeCO2 H + 11NaClO4 + 53H 2O

(2)

Complexes 1 and 2 display UV−vis spectra assignable to metal-to-ligand (FeII → pyridine)13a and ligand-to-metal (PhO−/OH− to FeIII) charge-transfer (MLCT/LMCT) transitions at 415 nm for 1 and at 465 and 345 nm (sh) for 2. X-ray structure determinations of 1 and 2 (see below) authenticated the composition of the coordination clusters. It should be mentioned here that a sizable number of iron(II)13 and a large number of iron(III)14 coordination clusters are known. To the best of our knowledge, only a handful of mixedvalent FeIIFeIII (≥Fe3) coordination clusters are known,14d,f,32 with oxo, alkoxo, and acetato bridges. Hence, the successful synthesis of coordination cluster 1, supported by phenoxo, oxo, and acetato bridges, deserves special attention. From a magnetostructural perspective, therefore, synthesizing FeII and mixed-valent FeIIFeIII (≥Fe3) coordination clusters and understanding their structural as well as topological features are of paramount importance. Description of the Structure of [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1). An X-ray structural analysis of 1 reveals a hexameric core (Figure 2), bridged by two endogenous phenoxides and two exogenous oxo and four exogenous acetates. The molecule sits on a crystallographically imposed mirror plane. Selected metric parameters are given in Table 1 and Table S3. From the bond distances (Table 1) and bond valence sum (BVS) analysis (Table 2),33 it is evident that the Fe1 and Fe3 centers are FeII and the Fe2 center is FeIII (Table 2). Thus, if there is any ambiguity, it is due to delocalization. Two dimeric FeII2 units are connected by two FeIII ions: Fe2 and symmetry-related Fe2*. In addition to terminal phenolato bridges from L3− connecting a FeII2 unit,

Figure 2. (a) Perspective view of metal coordination environment and (b) schematic representation and (c) butterfly-like FeII4FeIII2 core of [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1).

oxo (μ4-bridging mode) and acetate bridges connect Fe2 and Fe2* with other symmetry-related FeII2 units. A schematic presentation of the FeII4FeIII2 core is displayed in Figure 2. The FeII4FeIII2 core assumes a butterfly-like topology. It should be mentioned here that syntheses of mixed-valent coordination clusters14d,f,16,32 not only are challenging from synthetic aspects but are also of significant interest because of their application in magnetic material, redox catalysis, etc. The Fe1 (and symmetry-related Fe1*) centers are held by the endogenous phenoxo O1 with Fe1−O1−Fe1* = 103.3(5)° and exogenous oxo O3 with Fe1−O3−Fe1* = 101.1(5)°. Other four-coordinations to Fe1 (and Fe1*) centers consist of pyridyl N1, tertiary amine N2, a phenolate O2 from one L3−, and O4 from an acetate. Thus, Fe1/Fe1* centers have a distorted FeIIN2O4 coordination environment. The Fe1 (and Fe1*) centers are 3.208 Å apart. The Fe1 and Fe2 centers are held by the oxo bridge O3 with Fe1−O3−Fe2 = 134.21(7)°, at a distance of 3.75 Å. Fe1 and Fe2* centers are bridged by phenolate O2 with Fe1−O2−Fe2* = 94.7(3)° and oxo O3 with Fe1−O3−Fe2* = 98.86(6)°. In addition, an acetate bridges Fe1 and Fe2* in a μ2-1,3 syn-syn bridging mode (O4 D


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Inorganic Chemistry Table 1. Selected Bond Lengths (Å) in [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1) Fe1−O1 Fe1−O2 Fe1−O3 Fe1−O4 Fe1−N1 Fe1−N2

2.045(7) 2.104(8) 2.077(7) 2.045(8) 2.246(10) 2.189(10)

Fe2−O2 Fe2−O3 Fe2−O5 Fe2−O6 Fe2−O8 Fe2−O9

2.107(8) 1.999(8) 2.070(8) 2.051(8) 2.024(8) 1.936(7)

Fe3−O7 Fe3−O8 Fe3−O9 Fe3−O10 Fe3−N3 Fe3−N4

2.097(8) 2.204(8) 2.146(7) 2.071(7) 2.231(10) 2.243(11)

Table 2. Bond Valence Sums (BVS) for the Fe Centers in 1 and 2 Fe center in 1

FeII a

FeIII b

Fe center in 2

FeII a

FeIII b

Fe1 Fe2 Fe3

2.22 2.72 1.95

2.42 2.91 2.12

Fe1 Fe2 Fe3 Fe4

2.73 2.89 2.89 2.75

2.95 3.09 3.09 2.98

a

Considering all as FeII. bConsidering all as FeIII.

to Fe1 and O5 to Fe2*).6 Fe2 (and Fe2*) centers are further coordinated by O6 of an acetate, an oxo O9, and a phenolate O8, from another L3−. The distance between Fe1 and Fe2* is 3.09 Å. O3 and O9 oxo groups bridge Fe2 and Fe2*, at a distance of 2.96 Å, with Fe2−O3−Fe2* = 95.9(5)° and Fe2− O9−Fe2* = 100.1(5)°. Thus, Fe2/Fe2* centers assume a distorted FeIIIO6 coordination sphere. It should be noted here that oxo groups O3 and O9 exist in μ4-bridging mode: O3 bridges Fe1, Fe1*, Fe2, and Fe2*, and O9 bridges Fe2, Fe2*, Fe3, and Fe3*. The Fe2 and Fe3 centers are held at a distance of 3.783 Å by the oxo bridge O9 with Fe2−O9−Fe3 = 135.88(6)°. Fe2 and Fe3* (and Fe2* and Fe3) centers are held at 3.04 Å by three bridging moieties: an oxo O9 (Fe2− O9−Fe3* = 96.18(7)°), a phenolate O8 (Fe2−O8−Fe3* = 91.9(3)°), and a μ2-1,3 syn-syn bridging acetate O6 and O7 providing coordination to Fe2 and Fe3*, respectively. Fe3 (and Fe3*) are bridged by a phenolate O10 with Fe3−O10− Fe3* = 105.0(5)° and oxo O9 with Fe3−O9−Fe3* = 100.0(5)°. The resultant distance between Fe3 and Fe3* is 3.29 Å. Fe3 (and Fe3*) centers are further coordinated by pyridyl N3, tertiary amine N4, and phenolate O8 from L3− and O7 from an acetate. Thus, Fe3/Fe3* centers attain a distorted FeIIN2O4 coordination sphere. An analysis of the packing diagram of 1 reveals the presence of an intermolecular C−H···Br hydrogen-bonding interaction.34 The Br5 on the phenolate ring is involved in H-bonding with H10 of the pyridine ring of neighboring molecule with C10−H10···Br5 = 2.766 Å. The H17 of the phenolate ring is involved in H-bonding with O13 of Et2O (C17−H17···O13 = 2.465 Å).35,36 These two H-bonding interactions (C10−H10··· Br5 and C17−H17···O13) gives rise to a 1D chainlike structure (Figure S2). On the other hand, Br1 of other phenolate participates in H-bonding interaction with H5B of the −CH2− group, which is connected to the phenolate of another hexamer unit with C5−H5B···Br1 = 2.996 Å (Figure S3). This intermolecular H-bonding connects the aforementioned 1D chains in such a way that a helical network results (Figure S4). All H-bonding parameters for 1 are given in Table S4. Description of the Structure of [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2). A crystallographic study reveals that the asymmetric unit of 2 consists of an FeIII4 core (Figure 3) with two L3− ligands. Metrical parameters (Table 3 and Table S5) and BVS values (Table 2)

Figure 3. (a) Perspective view of metal coordination environment, (b) schematic representation, and (c) FeIII4 core in [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2).

confirm the presence of only FeIII ions. Four iron(III) centers are bridged by phenoxo, acetato, and hydroxo groups. A schematic presentation of the FeIII4 core is displayed in Figure 3. Unlike the case for 1, here the hydroxo group coordinates in a μ3-bridging mode. Selected metric parameters are given in Table 3. Two dimeric units involving Fe1, Fe2 and Fe3, Fe4 are connected through hydroxo and acetato bridges. Each iron(III) center attains a distorted-octahedral geometry. Fe1 and Fe2 centers are bridged by phenolato O4 with Fe1−O4− Fe2 = 93.7(5)° and hydroxo O13 with Fe1−O13−Fe2 = 103.8(2)°. Fe1 and Fe2 centers are further bridged by another exogenous acetate through a μ2-1,3 syn-syn bridging mode. As a result, the Fe1 and Fe2 centers are held at a distance of 3.036 Å. The Fe1 center is further coordinated by pyridyl N2, tertiary amine N5, phenolato O5 from L3−, and an acetate O9. Thus, E


Article

Inorganic Chemistry Table 3. Selected Bond Lengths (Å) in [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2) Fe1−O4 Fe1−O5 Fe1−O9 Fe1−O13 Fe1−N2 Fe1−N5

2.060(5) 1.945(6) 2.071(5) 1.836(5) 2.210(6) 2.223(7)


1.933(5) 2.049(5) 2.074(5) 2.099(5) 2.018(5) 1.893(5)


2.117(5) 1.920(5) 2.055(5) 1.901(5) 2.076(6) 2.000(5)

Fe4−O1 Fe4−O3 Fe4−O12 Fe4−O14 Fe4−N1 Fe4−N4

1.937(5) 2.056(5) 2.059(5) 1.837(5) 2.227(7) 2.224(6)

methylene hydrogen (H03A) attached to the bromophenolate of a neighboring tetramer with C036−H03A···Br2 = 2.753 Å and another methylene hydrogen (H03C) of ethylpyridine arm of another tetramer with C038−H03C···Br2 = 2.962 Å (Figure S7). On the other hand, Br3 is H-bonded with the methylene hydrogen (H08A) attached to the bromophenolate of a neighboring tetramer with C083−H08A···Br3 = 2.711 Å (Figure S7). All H-bonding parameters for 2 are collected in Table S6. Magnetism. For high-nuclearity coordination clusters of transition-metal ions, possibility of the occurrence of various exchange-interactions exists between the magnetic centers. Obviously, the research target involves satisfactory fitting of magnetic data with appropriate theoretical equations and providing a rationale for the observed behavior. In order to explore the magnetic properties of coordination clusters [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1) and [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2), variable-temperature magnetic susceptibility data were collected on powdered crystalline solids. [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1). A plot of χMT (χM being the magnetic susceptibility per six metal ions) vs T for 1 is displayed in Figure 4. At 300 K, the χMT value is ca. 14.8 cm3

Fe1 has an N2O4 coordination environment. It is worth mentioning here that the central phenolate O5 does not act as a bridging moiety; it is terminally bound to Fe1. Thus, tertiary amine N6 and pyridyl N8 on the other arm of the lignad remain uncoordinated and the bromophenolate O6 binds to iron center Fe3. Another bromophenolate O4 acts as a bridge between Fe1 and Fe2. Fe2 is further coordinated by hydroxo O13, two acetates O7 and O10, another bromophenolate O2 from another ligand, and hydroxo O14. Thus, Fe2 has an O6 coordination environment. The Fe2 and Fe3 centers are bridged by two hydroxo groups: O13 with Fe2−O13−Fe3 = 95.3(2)° and O14 with Fe2−O14−Fe3 = 96.1(2)°. This results in an Fe2···Fe3 distance of 2.898 Å. Fe3 is bridged with Fe1 by hydroxo O13 with Fe1−O13−Fe3 = 159.7(3)°. The distance between Fe1 and Fe3 is 3.679 Å. In addition to hydroxo O13 and O14, Fe3 is coordinated by two phenolates O3 and O6 and two acetates O8 and O11. The Fe3 and Fe4 centers are connected by bromophenolate O3 with Fe3−O3−Fe4 = 93.0(2)° and hydroxo O14 with Fe3−O14−Fe4 = 103.9(2)°. These two iron centers (O11 coordinates to Fe3 and O12 coordinates to Fe4) are bridged by an acetate group. The resultant distance between Fe3 and Fe4 is 3.025 Å. Fe4 is connected to Fe2 by an O14 hydroxo bridge with Fe2−O14−Fe4 = 158.6(3)°. Fe2 and Fe4 are 3.665 Å apart. In addition to phenolato O3 and hydroxo O14, the coordination sites of Fe4 are satisfied by another phenolate O1, acetate O12, pyridyl N1, and tertiary amine N4. Thus, Fe4 assumes an N2O4 coordination environment. As stated earlier, like O5, the central phenolate O1 of other ligand also does not act as a bridge; rather, it binds only to Fe4. As a result the tertiary amine N3 and pyridyl N9 on the other arm of L3− remain uncoordinated and the bromophenolate O2 binds to another iron center Fe2. Thus, in this structure two corners are empty (Figure S5). Such an environment could be utilized for further binding. An analysis of the packing diagram of 2 reveals that perchlorate oxygens hold three tetramers and a MeCN molecule (solvent of crystallization) through extensive C− H···O interactions35,36 (Figure S6). Perchlorate O15 is involved in an H-bonding interaction with methylene hydrogen, attached to the phenolate ring of a neighboring tetramer with C036−H03B···O15 = 2.609 Å. The O16 of perchlorate interacts with C−H (H049) of the phenolate ring of a neighboring tetramer with C049−H049···O16 = 2.555 Å. The O16 is also H-bonded to the methyl hydrogen of MeCN with C4W−H4W1···O16 = 2.622 Å. The O17 has an H-bonding interaction with the methyl hydrogen (H10C) of the acetate of the third neighboring tetramer with C103−H10C···O17 = 2.489 Å. On the other hand, O18 is involved in an H-bonding interaction with the methylene hydrogen (H04B) of the central phenolate of the first tetramer (which has an Hbonding interaction with O15) with C043−H04B···O18 = 2.662 Å. There are extensive C−H···Br interactions in 2. Br2 interacts simultaneously with two hydrogens: one from the

Figure 4. Thermal dependence of the χMT product for 1: (○) experimental data; (−) best-fit curve through eq 3a (see text). The simulation was performed with the following parameters: g(FeIII) = 2.0, g(FeII) = 2.12, Jb = −36.5 cm−1, D = −10 cm−1.

mol−1 K. This value is smaller than the spin-only value expected for (ca. 20.8 cm3 mol−1 K, with g = 2.0) four magnetically noninteracting high-spin FeII (S = 2) ions and two high-spin FeIII (S = 5/2) ions. This fact, together with the continuous decrease of χMT upon lowering the temperature for 1, indicates the existence of an important antiferromagnetic interaction among the paramagnetic centers, a feature which is commonly found for phenoxo- and/or hydroxo-bridged FeII2, FeIII2, and FeIIFeIII centers.31,37 However, in spite of having an F


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Inorganic Chemistry

Scheme 2. Simplified Magnetic-Exchange Pathways in 1a

even number of paramagnetic centers, the χMT value does not become zero, even at 2.0 K. In fact, χMT exhibits an incipient plateau in the temperature range 35−15 K. Below 15 K, χMT decreases again to ca. 3.3 cm3 mol−1 K at 2.0 K. The value of χMT at the plateau (ca. 4.5 cm3 mol−1 K) is greater than that expected for a low-lying S = 2 spin state (ca. 3.0 cm3 mol−1 K) but smaller than that expected for an S = 3 spin state (6.0 cm3 mol−1 K), suggesting a possible thermal population of these spin states, which would be closer in energy. An inspection of the molecular structure of 1 indicates the presence of two different paramagnetic ions, Fe(II) (Fe1 and Fe3) and Fe(III) (Fe2), with several exchange pathways (Scheme 1): (a) the paramagnetic Fe1a/Fe2a, Fe2a/Fe3a,

a

See legend to Scheme 1 for numbering.

magnetic properties of 1 can be described through the spin Hamiltonian given by eq 3a:

Scheme 1. Magnetic-Exchange Pattern in 1a

̂ + S2a ̂ S3a ̂ + S1b ̂ S2b ̂ + S2b ̂ S3b ̂ ) Ĥ exchange = −Ja (S1â S2a ̂ + S2a ̂ S1b ̂ + S2a ̂ S3b ̂ + S3a ̂ S2b ̂ )− − Jb (S1â S2b ̂ + S3a ̂ S3b ̂ ) − J (S2a ̂ S2b ̂ ) − Jc (S1â S1b d

(3a)

The presence of high-spin iron(II) ions requires incorporation into the general Hamiltonian of terms that consider zero-field splitting (zfs) of these ions (eq 3b) and Zeeman factor (eq 3c) for each type of metal ion (eq 3d). Ĥ zfs = D(Sz21a + Sz21b + Sz23a + Sz23b − 8)

a

Dashed lines represent a bridging syn-syn carboxylate; see Table 4. The values of αna,mb correspond to Fe−O−Fe angles involved in the magnetic-exchange pathways (see Table 4). Fe1a and Fe1b stand for Fe1 and Fe1*, respectively, Fe2a and Fe2b stand for Fe2 and Fe2*, respectively, and Fe3a and Fe3b stand for Fe3 and Fe3*, respectively, as per X-ray structure labeling.

(3b)

̂ + S1b ̂ + S3b ̂ ) + g (S2a ̂ + S2b ̂ ) Ĥ Zeeman = βH[gA (S1â + S3a B (3c)

Ĥ = Ĥ exchange + Ĥ zfs + Ĥ Zeeman

(3d)

The coexistence of several exchange pathways with a large number of parameters governing them makes the analysis of the experimental magnetic behavior difficult and ambiguous. The use of additional tools is essential in such cases. DFT calculations have proven to be very useful in the past to identify the nature and quantitatively determine the magnitude of the magnetic couplings in polynuclear complexes. The presence of several high-spin FeII ions with low-lying excited states makes it very difficult to find the most stable electronic configuration for each one of the proposed spin configurations through the process of evaluating the magnetic coupling

Fe1b/Fe2b, and Fe2b/Fe3b pairs are connected through phenoxo, oxo, and syn-syn acetate bridges, (b) oxo and phenoxo bridges occur for the Fe3a/Fe3b and Fe1a/Fe1b pairs, (c) the Fe2a/Fe2b pair is connected through two oxo groups, and (d) the Fe2a/Fe3b, Fe3a/Fe2b, Fe1a/Fe2b and Fe2a/Fe1b pairs are linked only by an oxo group. A summary of the more relevant structural parameters is given in Table 4. One can see therein the existence of more than three different magnetic couplings, as shown in Scheme 2. In light of this, the

Table 4. Selection of More Relevant Geometrical Parameters of 1 Involved in the Magnetic Coupling Together with Calculated J Values coupling

metal ions III

II

J1a,2b/J1b,2a J2a,3b/J2b,3a J2a,2b

Fe /Fe FeIII/FeII FeIII/FeIII

J1a,2a/J1b,2b

FeIII/FeII

J2a,3a/J2b,3b

FeIII/FeII

J1a,1b

FeII/FeII

J3a,3b

FeII/FeII

pathway

d(M···M) (Å)

α(MOM) (deg)

Ja

Jb

nAnBJb,c

μ-O μ-O μ-O μ-O μ-O μ-OPh μ-O2CMe μ-O μ-OPh μ- O2CMe μ-O μ-OPh μ-O μ-OPh

3.756 3.783 2.968

134.2 135.9 95.8 100.1 98.9 94.7

−49.8 −38.8 +20.9

−48.4 −38.1 +17.1

−968.0 −762.0 +427.5

+9.5

+9.1

+182.0

3.040

96.2 91.9

+6.7

+8.0

+160.0

3.208

101.1 103.3 100.0 104.5

−4.6

−3.3

−52.8

−2.2

−3.3

−52.8

3.097

3.287

a

Short atomic basis set (see Experimental Section). bExtended atomic basis set (see Experimental Section). cnA and nB are the number of unpaired electrons at the paramagnetic sites A and B involved in each magnetic coupling G


Article

Inorganic Chemistry

Even though the molecular symmetry imposes seven different magnetic couplings, they are grouped in four sets according to the type and number of bridging ligands involved in them, as specified above. Because the values of J within each set are similar, the pattern of the magnetic interactions in 1 can be roughly simplified, reducing significantly the degree of complexity of the system and making possible the analysis of its magnetic data. In this sense, the applied conditions are Ja = J1a,2a = J1b,2b = J2a,3a = J2b,3b, Jb = J1a,2b = J1b,2a = J2a,3b = J2b,3a, Jc = J1a,1b = J3a,3b, Jd = J2a,2b, gA = g1a = g3a = g1b = g3b, and gB = g2a = g2b. The predominant magnetic coupling (Jb) would lead to two S = 3/2 units that will interact through the other exchange pathways at low temperatures. This is an idealized picture because Jb is not much stronger than other magnetic interactions, but it is useful for a qualitative discussion. Jd is ferromagnetic and stronger than Ja and Jc, and it would stabilize a low-lying S = 3 spin state. However, the ferromagnetic Ja and antiferromagnetic Jc interactions play an antagonistic role versus Jd promoting a low-lying singlet spin state. The subtle interplay among these three weaker magnetic couplings establishes the ground spin state. As an example, the weakest Jc coupling can be considered negligible and only Jd and Ja would tune the ground spin state. Independent of the size of Jb, and as a function of the Ja/Jd ratio, there are values for this quotient where a singlet or a septuplet state is more stable, but it exists in a narrow range of values of this quotient, where both states are intercrossed (Figure 5 and Figure S8).

constant. The task becomes easier by delimiting the number of paramagnetic centers to only those involved in each magnetic coupling. However, a reduction of the molecular entity can induce electronic effects, particularly in 1 where there are some monatomic pathways connecting the metal ions. As demonstrated in the past, a satisfactory settlement is replacing those metal ions which are not involved in the studied magnetic coupling by similar but diamagnetic metal ions. In the case of 1, ZnII and GaIII ions are good alternatives to replace the FeII and FeIII ions, respectively, keeping the value of the overall charge of the polynuclear complex invariant. The computed values of the magnetic coupling constants in 1 through this approach are given in Table 4. One problem to rationalize the theoretical results resides in the coexistence of FeII and FeIII ions in 1, leading to three possible combinations of them in pairs related to the magnetic couplings. It is understandable that d(z2)- and d(x2−y2)-type orbitals for six-coordinate FeII and FeIII ions are often the main orbitals involved in magnetic interaction. A comparison between different values of J in 1 can roughly be done by using the nAnBJ product, where nA and nB are the number of unpaired electrons on each paramagnetic site involved in a magnetic coupling.1 Let us consider two observations. First, for polynuclear complexes with some 3d metal ions, it is wellknown that the magnetic couplings through μ-oxo, μ-hydroxo, and μ-alkoxo/μ-phenoxo (μ-OR) bridges follow similar pathways; their nature and the magnitude depend on the angle at the bridgehead atom (α). Second, ferromagnetic couplings are usual when this angle is small (close to 90°). However, the nature of the coupling becomes antiferromagnetic when it exceeds a value known as the magic angle. This angle is different depending on the metal ions involved. In both cases, the magnetic coupling is strengthened as the value of α departs from the magic angle. These features that were mainly observed in systems with pathways mediated by two μOR groups should also occur in systems with only one μ-OR magnetic pathway, although in the latter case, no examples are known with small α values. In this sense, just what one could expect for the J1a,2b, J1b,2a, J2a,3b, and J2b,3a magnetic couplings through a μ-oxo pathway and very large α values is strong antiferromagnetic interactions (Table 4). Two monatomic oxygen bridges in J2a,2b, J1a,1b, and J3a,3b contribute to the exchange pathway, making possible less obtuse values of the α angle. This feature leads to a weak antiferromagnetic interaction for J1a,1b and J3a,3b and an even smaller value of α would turn it to ferromagnetic. This last case would be enhanced in J1a,2a, J1b,2b, J2a,3a, and J2b,3b because they exhibit the smallest values for α. Nevertheless, the presence of a third bridge (syn-syn carboxylato) would change the situation. This type of carboxylate coordination is known to mediate antiferromagnetic coupling; however, as its effect is weaker than those of the μ-oxo and μ-phenoxo bridges, a weak ferromagnetic coupling results. Finally, the theoretical results have a physical meaning and provide us with an adequate set of values of J to be used as starting parameters to analyze the experimental magnetic behavior. The energy diagram for 1, obtained through the magnetic coupling constants found with a more extended basis set (seventh column in Table 4) points to a S = 0 ground state with very close low-lying excited states (S = 1, 2, and 3 at 11.7, 36.0, and 74.6 cm−1, respectively), making it possible to exchange ground and excited states by moderate or slight modifications of the values of J values.

Figure 5. Detail of the low-lying spin levels of the energy spectrum of 1 (see Figure S8). Data were obtained assuming values for Jb, Jc, and Jd of −30, 0, and +10 cm−1, respectively.

In addition to the large number of variable parameters (g factors for FeII and FeIII ions and four magnetic coupling constants), our attempts to simulate the experimental data lead to a low-lying singlet spin state in order to reproduce the drop of χMT at low temperature. Evidently, the iron(II) ion has an axial zfs (D) that can account for the behavior below 40 K. Given the fact that χMT tends to a nonzero value at 2.0 K and keeping in mind that the high-spin FeII ion shows an integer S value, only a negative D value should be envisaged, and so, a situation with S = 0, 1, 2, and 3 quasi-degenerate ground spin states would be expected. Even when a negative D parameter is included in the model, the fitting process leads to situations H


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Inorganic Chemistry

susceptibility measurements, in the presence of a dc magnetic external field (Figures S11 and S12). Despite the fact that the expected maxima could not be observed down to 1.9 K (the lowest temperature of our susceptometer), this behavior is typical of a SMM.38 Thus, under the assumption that the SMM relaxation has just one characteristic time corresponding to a Debye relaxation process driven by one activation energy (Ea), the relaxation time (τ) may be written in terms of the Arrhenius law τ = τ0 exp(Ea/kT).39 Taking into account that χ′′M/χ′M = 2πντ, where ν is the experimental ac field exciting frequency, and considering that the adiabatic susceptibility is zero, one obtains eq 4, which allows a rough evaluation of the values of Ea and τ0.

with low-lying singlet spin states. This fact is a consequence of the difficulty in catching those cases with other ground states or a nearly degenerate situation, being so limited in the range of the Jd/Ja ratio. Additionally, there is also a partial correlation between Jb and Jd. The inclusion of the Jc coupling in this discussion makes it very complicated to obtain a clear-cut answer. Whatever it would be, we can conclude that the presence of a paramagnetic ground state or a degenerate scenario and a local axial zfs for six-coordinate FeII ion are ensured. More conclusive assertions are precluded, as it is possible to simulate the experimental data with values for the parameters qualitatively similar to those obtained by the theoretical calculations (Ja = 0.0 cm−1, Jb = −27.9 cm−1, Jc = +2.7 cm−1, and Jd = +11.4 cm−1) or include in the model only the stronger intratrinuclear Jb magnetic coupling together with a local D parameter of around −10 cm−1 for the FeII ion (Figure 4). Although the bridging ligands in 1 and 2 (see below) are basically the same, their magnetic exchange parameters (J) are not strictly comparable, because they represent different metal ions. However, the values obtained for 2 (see below) follow the same trend observed and discussed for 1. The magnetization data of 1 increase with the magnetic field and attain ca. 4.1 μB at 5 T and 2.0 K. This value is very close to that expected for a spin quintuplet (4.24 μB, with g = 2.12). Nevertheless, the experimental curve deviates significantly from that calculated through the Hamiltonian of eq 2 (solid line in Figure S9). This behavior is characteristic of the existence of zfs and close excited states with different S values or at least one of them. Both situations occur in 1. Thus, we were unable to determine unambiguously the magnitude of this zfs due to the overparameterization and the large correlation with the values of J. The existence of this zfs and the consequent anisotropy of the ground spin state, possibly S = 2 or S = 3, must be responsible for the apparent slow magnetic relaxation observed in 1 below 2 K. Figure 6 shows the out-of-phase (χ′′M) alternating-current (ac) magnetic susceptibility measurements for different frequencies in the absence of any external magnetic field. The corresponding in-phase (χ′M) magnetic susceptibility is displayed in Figure S10. As one can see, both χ′M and χ′′M are frequency-dependent below 4.0 K. It should be noted that similar behavior was observed for ac magnetic

ln(χ ″M /χ ′M ) = ln(2πντo) + Ea /kT

(4)

This method has already been applied to the evaluation of these factors in several SMMs in the literature.40 Figure 7

Figure 7. Logarithm of the χ′′M /χ′M ratio vs 1/T plot for 1 at several frequencies (from Figure 6 and Figure S10). The solid lines are the best-fit curves (see text).

shows the fit of the experimental data by using eq 4. The values obtained were τ0 = 1.9(2) × 10−7 s−1 and Ea = 5.1(3) cm−1. They lie within the range of those expected for a SMM.40,41 Assuming that the Ea corresponds to the energy splitting of the ground spin state, S = 2, by zfs, Ea = 4|D|, D being the axial zfs parameter, D ≈ − 1.3 cm−1 may be estimated. Two relevant examples showing SMM behavior are FeII9 (S = 14) coordination cluster, bridged by hydroxo and acetate groups,13d and metal−metal-bonded cluster Fe3 (S = 11/2).11i To our knowledge, 1 provides first example of a mixed-valent FeII4FeIII2 coordination cluster exhibiting SMM behavior. [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2). Figure 8 displays an χMT vs T plot (χM being the magnetic susceptibility per four metal ions) for 2. χMT at 300 K is ca. 3.3 cm3 mol−1 K, a value which is well below the spin-only value expected for four magnetically noninteracting high-spin FeIII centers (17.5 cm3 mol−1 K with S = 5/2). Upon cooling, χMT decreases and approaches zero below 20 K. These features suggest the presence of a prominent antiferromagnetic interaction. Although the structures and magnetic properties of several FeIII clusters have been the subject of previous reports, a clear rationalization of a possible magnetostructural correlation is still lacking. In this context, further studies on FeIII multinuclear complexes are relevant to understand the structural

Figure 6. Frequency dependence of the out-of-phase (χ′′M) component of the ac susceptibility for 1 in zero static field under a ±4 G oscillating field. I


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Inorganic Chemistry

These results point out that the interaction between Fe2 and Fe3 is ferromagnetic and the remaining interactions are antiferromagnetic. The crystal structure shows that Fe2 and Fe3 are bridged by two hydroxo moieties with values of 95.3 and 96.1° for Fe2−O13−Fe3 and Fe2−O14−Fe3, respectively. Since these angles are close to 90°, the orthogonality between the magnetic orbitals of Fe2 and Fe3 would be achieved.1 Whereas this orthogonality tends toward ferromagnetism,43 the syn-syn acetate bridge favors antiferromagnetism.44,45 As a consequence, a moderate ferromagnetic coupling (+4.5 cm−1) results. Fe1 and Fe2 are bridged by syn-syn acetate and hydroxo bridges (O13) with Fe1−O13−Fe2 = 103.8° (which is well above 90°) and a phenoxo (O4) with Fe1−O4−Fe2 = 93.7(2)°, which is also close to 90°. Now, the combined effect of the syn-syn acetate and hydroxo bridges overcomes the contribution through the phenoxo bridge and a significant antiferromagnetic coupling (−56.5 cm−1) occurs. A similar situation is expected for Fe3 and Fe4 centers. Finally, the values of the bridgehead O13 (between Fe1 and Fe3) and O14 (between Fe2 and Fe4) atoms are 159.7 and 158.6°, respectively. These angles are well above 90°, thus leading to an antiferromagnetic coupling (−71.6 cm−1). Given that the values of the Fe1−O13−Fe3 and Fe2−O14−Fe4 angles are quite close to 180°, in comparison to the bridging angles between Fe1 and Fe2 or Fe3 and Fe4, JB would be antiferromagnetic in nature and stronger than that of JA, as found through the fit of the magnetic data.

Figure 8. Temperature dependence of magnetic susceptibility data for [FeIII4(L)2(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2).

features which govern the magnetic interactions between the FeIII centers. Annalysis of the molecular structure of 2 suggests the presence of three types of exchange pathways (Scheme 3): (a) Scheme 3. Schematic Presentation of FeIII4 Core in 2

■

With a phenol-based binucleating ligand in its deprotonated form L 3− , the successful syntheses of mixed-valent [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1) and [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2) coordination clusters have been achieved. The coordination cluster 1 with a butterfly-like topology has μ4-O and acetate bridges, in addition to phenoxo bridges (both bridging and terminal) from L3−. To our knowledge, 1 provides the first example of a mixed-valent FeIIFeIII (≥Fe3)32 coordination cluster, supported by phenoxo, oxo, and carboxylato bridges. The coordination cluster 2 has μ3-OH, acetato, and terminal phenoxo bridges from L3−. In 2, two diiron(III) units involving Fe1, Fe2 and Fe3, Fe4 are connected through hydroxo and acetato bridges, giving rise to a FeIII4 core. Magnetic studies reveal that the FeII4FeIII2 coordination cluster exhibits SMM behavior with an S = 2 spin ground state. The existence of zero-field splitting and the consequent anisotropy of the ground spin state (S = 2) must be responsible for the apparent slow magnetic relaxation observed for 1 below 2 K. To our knowledge, 1 is the first mixed-valent FeII4FeIII2 coordination cluster to exhibit SMM behavior. A magnetostructural analysis demonstrates that the nature and mode of bridging and FeIII−O(H)−FeIII angle(s) connecting FeIII centers are key factors affecting the effective magnetic properties. An analysis of structural parameters of 2 reveals the following: (i) J values approach ferromagnetic, when the bridging angle(s) are close to 90°, (ii) an increase in FeIII···FeIII separation leads to an increased antiferromagnetic interaction, and (iii) when iron centers are singly hydroxo bridged, significant orbital overlap occurs, which in turn leads to effective antiferromagnetic interaction, in comparison to double/triple hydroxo bridges.

Fe1 and Fe2 as well as Fe3 and Fe4 are connected through phenoxo, hydroxo, and syn-syn acetate bridges; (b) two hydroxo and a syn-syn acetato group act as bridges between Fe2 and Fe3; (c) Fe1 and Fe3 as well as Fe2 and Fe4 are connected through a single hydroxo bridge. An inspection of the Fei−O−Fej angles (αij) allows us to assume that α12 ≈ α34 and α13 ≈ α24, and so, the pattern of magnetic interactions in 2 can be described by the spin Hamiltonian of eq 5 H = −JA (S1̂ S2̂ + S3̂ S4̂ ) − JB (S1̂ S3̂ + S2̂ S4̂ ) − JC (S2̂ S3̂ ) 4

+ gβH ∑ Si i=1

SUMMARY AND CONCLUDING REMARKS

(5)

where J12 = J34 = JA, J13 = J24 = JB, J23 = JC and g1 = g2 = g3 = g4 = g. However, despite this great simplification three exchange parameters (JA, JB, and JC) are too many to unambiguously describe the χMT curve, whose dependence on the temperature is almost linear. The set of physically reasonable best-fit parameters obtained through this Hamiltonian and using fullmatrix diagonalization, as implemented in the VPMAG program package,42 are JA = −56.5(3) cm−1, JB = −71.6(4) cm−1, JC = +4.5(2) cm−1, and g = 1.99(1). These values have to be considered with great caution because of the aforementioned reasons. J


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Inorganic Chemistry

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(4) (a) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. Single-Molecule Magnets. MRS Bull. 2000, 25, 66−71. (b) Gatteschi, D.; Sessoli, R. Quantum Tunneling of Magnetization and Related Phenomena in Molecular Materials. Angew. Chem., Int. Ed. 2003, 42, 268−297. (c) Affronte, M.; Troiani, F.; Ghirri, A.; Carretta, S.; Santini, P.; Corradini, V.; Schuecker, R.; Muryn, C.; Timco, G.; Winpenny, R. E. Molecular Routes for Spin Cluster Qubits. Dalton Trans. 2006, 2810−2817. (d) Bogani, L.; Wernsdorfer, W. Molecular Spintronics using Single-Molecule Magnets. Nat. Mater. 2008, 7, 179−186. (e) Gatteschi, D.; Cornia, A.; Mannini, M.; Sessoli, R. Organizing and Addressing Magnetic Molecules. Inorg. Chem. 2009, 48, 3408−3419. (f) Aromí, G.; Aguilà, D.; Gamez, P.; Luis, F.; Roubeau, O. Design of Magnetic Coordination Complexes for Quantum Computing. Chem. Soc. Rev. 2012, 41, 537−546. (g) Bunting, P. C.; Atanasov, M.; Damgaard-Møller, E.; Perfetti, M.; Crassee, I.; Orlita, M.; Overgaard, J.; van Slageren, J.; Neese, F.; Long, J. R. A Linear Cobalt(II) Complex with Maximal Orbital Angular Momentum from a Non-Aufbau Ground State. Science 2018, 362, No. eaat7319. (h) Guo, F.; Day, B. M.; Chen, Y.; Tong, M.; Mansikkamäki, A.; Layfield, R. A. Magnetic Hysteresis up to 80 K in a Dysprosium Metallocene Single-Molecule Magnet. Science 2018, 362, 1400−1403. (5) (a) Leuenberger, M. N.; Loss, D. Quantum computing in molecularmagnets. Nature 2001, 410, 789−793. (b) Urdampilleta, M.; Klyatskaya, S.; Cleuziou, J. P.; Ruben, M.; Wernsdorfer, W. Supramolecular Spin Valves. Nat. Mater. 2011, 10, 502−506. (c) Cornia, A.; Seneor, P. The Molecular Way. Nat. Mater. 2017, 16, 505−506. Cinchetti, M.; Dediu, V. A.; Hueso, L. E. Activating the Molecular Spinterface. Nat. Mater. 2017, 16, 507−515. (6) Arora, H.; Mukherjee, R. Coordination Polymers Using (2Pyridyl)alkylamine-Appended Carboxylates: Magnetic Properties. New J. Chem. 2010, 34, 2357−2365 and references therein. . (7) (a) Arora, H.; Lloret, F.; Mukherjee, R. One-Dimensional CoII and CuII Coordination Polymers and a Discrete CuII4 Complex of Carboxylate-Appended (2-Pyridyl)alkylamine Ligands: Spin-Canting and Anti-/Ferromagnetic Coupling. Inorg. Chem. 2009, 48, 1158− 1157. (b) Arora, H.; Lloret, F.; Mukherjee, R. One-Dimensional Coordination Polymers of MnII, CuII, and ZnII Supported by Carboxylate-Appended (2-Pyridyl)-alkylamine Ligands − Structure and Magnetism. Eur. J. Inorg. Chem. 2009, 2009, 3317−3325. (c) Arora, H.; Lloret, F.; Mukherjee, R. Molecular Squares of NiII and CuII: Ferromagnetic Exchange Interaction Mediated by Syn−Anti Carboxylate-Bridging. Dalton Trans. 2009, 9759−9769. (d) Arora, H.; Cano, J.; Lloret, F.; Mukherjee, R. Unprecedented Heptacopper(II) Cluster with Body-Centred Anti-Prismatic Topology. Structure, Magnetism and Density Functional Study. Dalton Trans. 2011, 40, 10055−10062. (e) Barman, S. K.; Lloret, F.; Mukherjee, R. A Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced Hydrolysis of Carboxyesters under Ambient Conditions. Inorg. Chem. 2016, 55, 12696−12706. (f) Arora, H.; Cano, J.; Lloret, F.; Mukherjee, R. A Carboxylate-Bridged NiII8 Cluster with a Distorted Cubane Topology: Structure, Magnetism and Density Functional Studies. Dalton Trans. 2016, 45, 14174−14183. (8) (a) Tshuva, E. Y.; Lippard, S. J. Synthetic Models for Non-Heme Carboxylate-Bridged Diiron Metalloproteins: Strategies and Tactics. Chem. Rev. 2004, 104, 987−1012. (b) Sazinsky, M. H.; Lippard, S. J. Correlating Structure with Function in Bacterial Multicomponent Monooxygenases and Related Diiron Proteins. Acc. Chem. Res. 2006, 39, 558−566. (9) (a) Oshio, H.; Nakano, M. High-Spin Molecules with Magnetic Anisotropy toward Single-Molecule Magnets. Chem. - Eur. J. 2005, 11, 5178−5185. (b) Coronado, E.; Dunbar, K. R. Preface for the Forum on Molecular Magnetism: The Role of Inorganic Chemistry. Inorg. Chem. 2009, 48, 3293−3295. (c) Brooker, S.; Kitchen, J. A. NanoMagnetic Materials: Spin Crossover Compounds vs. Single Molecule Magnets vs. Single Chain Magnets. Dalton Trans. 2009, 7331−7340. (d) Powell, A. K. A Bridge to Higher Ground. Nat. Chem. 2010, 2, 351−352. (e) Pedersen, K. S.; Bendix, J.; Clérac, R. Single-Molecule

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00828. 1 H NMR spectrum of H3L1, C−H···Br interactions present in 1, view of empty space present in the structure of 2, C−H···O and C−H···Br interactions present in 2, energy spectrum of 1, magnetization of 1, frequency dependence of the in-phase (χ′M) component of the ac susceptibility for 1 in zero static field and under a ±4 G oscillating field, frequency dependence of the out-of-phase components of ac susceptibility for 1 under an external field of 1000 G and an oscillating field of 4 G, frequency dependence of the in-phase components of ac susceptibility for 1 under an external field of 1000 G and an oscillating field of 4 G, X-ray structure determination parameters for 1 and 2, selected metric parameters for 1 and 2, and H-bonding parameters for 1 and 2 (PDF) Accession Codes

CCDC 1849001 and 1849015 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*R.M.: e-mail, [email protected]; tel, +91-512-2597437; fax, +91512-2597436. ORCID

Rabindranath Mukherjee: 0000-0003-0739-5896 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a J.C. Bose fellowship grant by the Department of Science & Technology (DST), Government of India and Ministerio Español de Ciencia e Innovación (Projects CTQ2016-75068P and CTQ2016-75671P), Unidad de Excelencia Marı ́a de Maetzu (MDM-2015-0538). R.M. sincerely thanks the DST for a J.C. Bose fellowship. S.K.B. acknowledges the award of an SRF by the CSIR. We sincerely thank Dr. Arunava Sengupta for his help in the synthesis of 1 and its X-ray structure determination. Comments of the reviewers were very helpful at the revision stage.

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DEDICATION Dedicated to Prof. V. Chandrasekhar on the occasion of his 60th birthday. REFERENCES

(1) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (2) Winpenny, R. E. P. High Nuclearity Clusters: Clusters and Aggregates with Paramagnetic Centres: Oxygen and Nitrogen Bridged Systems. In Comprehensive Coordination Chemistry II: From the Molecular to Nanoscale: Synthesis and Properties; McCleverty, J. A., Meyer, T. J., Fujita, F., Powell, A., Creutz, C., Eds.; Elsevier/ Pergamon: Amsterdam, 2004, Vol. 7, pp 125−175. (3) Gatteschi, D.; Sessoli, R.; Villain, J.; Molecular Nanomagnets; Oxford University Press: 2008. K


Article

Inorganic Chemistry Magnet Engineering: Building-Block Approaches. Chem. Commun. 2014, 50, 4396−4415. (10) (a) Milios, C. J.; Raptopoulou, C. P.; Terzis, A.; Lloret, F.; Vicente, R.; Perlepes, S. P.; Escuer, A. Hexanuclear Manganese(III) Single-Molecule Magnets. Angew. Chem., Int. Ed. 2004, 43, 210−212. (b) Tasiopoulos, A. J.; Vinslava, A.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Giant Single-Molecule Magnets: A {Mn84} Torus and Its Supramolecular Nanotubes. Angew. Chem., Int. Ed. 2004, 43, 2117− 2121. (c) Soler, M.; Wernsdorfer, W.; Folting, K.; Pink, M.; Christou, G. Single-Molecule Magnets: A Large Mn30 Molecular Nanomagnet Exhibiting Quantum Tunneling of Magnetization. J. Am. Chem. Soc. 2004, 126, 2156−2165. (d) Aliaga-Alcalde, N.; Edwards, R. S.; Hill, S. O.; Wernsdorfer, W.; Folting, K.; Christou, G. Single-Molecule Magnets: Preparation and Properties of Low Symmetry [Mn4O3(O2CPh-R)4(dbm)3] Complexes with S = 9/2. J. Am. Chem. Soc. 2004, 126, 12503−12516. (11) (a) Zipse, D.; Abboud, K. A.; Dalal, N. S. An Fe4 Cluster with a Slowly Relaxing Paramagnetic Excited State: [Fe4O(OH)5(tacn)4]I7· 2.5H2O. J. Appl. Phys. 2003, 93, 7086−7088. (b) Oshio, H.; Hoshino, N.; Ito, T.; Nakano, M. Single-Molecule Magnets of Ferrous Cubes: Structurally Controlled Magnetic Anisotropy. J. Am. Chem. Soc. 2004, 126, 8805−8812. (c) Gregoli, L.; Danieli, C.; Barra, A.-L.; Neugebauer, P.; Pellegrino, G.; Poneti, G.; Sessoli, R.; Cornia, A. Magnetostructural Correlations in Tetrairon(III) Single-Molecule Magnets. Chem. Eur.-J. 2009, 15, 6456−6467. (d) Ako, A. M.; Mereacre, V.; Lan, Y.; Wernsdorfer, W.; Clérac, R.; Anson, C. E.; Powell, A. K. An Undecanuclear FeIII Single-Molecule Magnet. Inorg. Chem. 2010, 49, 1−3. (e) Jeon, I.-R.; Park, J. G.; Xiao, D. J.; Harris, T. D. An Azophenine Radical-Bridged Fe2 Single-Molecule Magnet with Record Magnetic Exchange Coupling. J. Am. Chem. Soc. 2013, 135, 16845−16848. (f) Zhu, Y.-Y.; Cui, C.; Qian, K.; Yin, J.; Wang, B.-W.; Wang, Z.-M.; Gao, S. A Family of Enantiopure FeIII4 Single Molecule Magnets: Fine Tuning of Energy Barrier by Remote Substituent. Dalton Trans. 2014, 43, 11897−11907. (g) Hernández Sánchez, R.; Betley, T. A. Meta-Atom Behavior in Clusters Revealing Large Spin Ground States. J. Am. Chem. Soc. 2015, 137, 13949−13956. (h) Botezat, O.; van Leusen, J.; Kravtsov, V. Ch.; Ellern, A.; Kö gerler, P.; Baca, S. G. Iron(III) Carboxylate/Aminoalcohol Coordination Clusters with Propeller-Shaped Fe8 Cores: Approaching Reasonable Exchange Energies. Dalton Trans. 2015, 44, 20753− 20762. (i) Hernández Sánchez, R.; Bartholomew, A. K.; Powers, T. M.; Ménard, G.; Betley, T. A. Maximizing Electron Exchange in a [Fe3] Cluster. J. Am. Chem. Soc. 2016, 138, 2235−2243. (12) (a) Waldmann, O. A Criterion for the Anisotropy Barrier in Single-Molecule Magnets. Inorg. Chem. 2007, 46, 10035−10037. (b) Gatteschi, D.; Sorace, L. Hints for the Control of Magnetic Anisotropy in Molecular Materials. J. Solid State Chem. 2001, 159, 253−261. (13) (a) Oshio, H.; Hoshino, N.; Ito, T. Superparamagnetic Behavior in an Alkoxo-Bridged Iron(II) Cube. J. Am. Chem. Soc. 2000, 122, 12602−12603. (b) Lee, D.; Sorace, L.; Caneschi, A.; Lippard, S. J. Hydroxo-Bridged Cubane-Type Tetrairon(II) Clusters Supported by Sterically-Hindered Carboxylate Ligands. Inorg. Chem. 2001, 40, 6774−6781. (c) Kiskin, M. A.; Fomina, I. G.; Aleksandrov, G. G.; Sidorov, A. A.; Novotortsev, V. M.; Shvedenkov, Y. G.; Eremenko, I. L.; Moiseev, I. I. First Triangular Carboxylate Cluster with the Fe(II)Fe(II)Fe(II) Metal Core. Inorg. Chem. Commun. 2004, 7, 734−736. (d) Boudalis, A. K.; Donnadieu, B.; Nastopoulos, V.; Clemente-Juan, J. M.; Mari, A.; Sanakis, Y.; Tuchagues, J.-P.; Perlepes, S. P. A Nonanuclear Iron(II) Single-Molecule Magnet. Angew. Chem., Int. Ed. 2004, 43, 2266−2270. (e) Boudalis, A. K.; Sanakis, Y.; Clemente-Juan, J. M.; Donnadieu, B.; Nastopoulos, V.; Mari, A.; Coppel, Y.; Tuchagues, J.-P.; Perlepes, S. P. A Family of Enneanuclear Iron(II) Single-Molecule Magnets. Chem. - Eur. J. 2008, 14, 2514− 2526. (f) Malassa, A.; Agthe, C.; Görls, H.; Podewitz, M.; Yu, L.; Herrmann, C.; Reiher, M.; Westerhausen, M. Synthesis, Structures, and Magnetic Properties of N-Trialkylsilyl-8-amidoquinoline Complexes of Chromium, Manganese, Iron, and Cobalt as well as of Wheel-Like Hexanuclear Iron(II) and Manganese(II) Bis(8-amido-

quinoline). Eur. J. Inorg. Chem. 2010, 2010, 1777−1790. (g) Steinert, M.; Schneider, B.; Dechert, S.; Demeshko, S.; Meyer, F. Spin-State Versatility in a Series of Fe4 [2 × 2] Grid Complexes: Effects of Counteranions, Lattice Solvent, and Intramolecular Cooperativity. Inorg. Chem. 2016, 55, 2363−2373. (h) Fan, K.-H.; Huang, Q.; Fang, X.-Y.; Zhu, L.-W.; Yan, Z. Tuning the Spin States of Two Apical Iron(II) Ions in a Pentanuclear Iron(II) Cluster Helicate through the Choice of Anions. Crystals 2018, 8, 119. (14) (a) Cañada-Vilalta, C.; O’Brien, T. A.; Brechin, E. K.; Pink, M.; Davidson, E. R.; Christou, G. Large Spin Differences in Structurally Related Fe6 Molecular Clusters and Their Magnetostructural Explanation. Inorg. Chem. 2004, 43, 5505−5521. (b) Boudalis, A. K.; Aston, R. E.; Smith, S. J.; Mirams, R. E.; Riley, M. J.; Schenk, G.; Blackman, A. G.; Hanton, L. R.; Gahan, L. R. Synthesis and Characterization of the Tetranuclear Iron(III) Complex of a New Asymmetric Multidentate Ligand. A Structural Model for Purple Acid Phosphatases. Dalton Trans. 2007, 5132−5139. (c) Mason, K.; Prescimone, A.; Schau-Magnussen, M.; Piligkos, S.; Tasker, P.; Brechin, E. Salicylaldoxime-Supported Nona- and Tetrametallic FeIII Cages. Curr. Inorg. Chem. 2013, 3, 76−85. (d) Majumdar, A.; Apfel, U.-P.; Jiang, Y.; Moënne-Loccoz, P.; Lippard, S. J. Versatile Reactivity of a Solvent-Coordinated Diiron(II) Compound: Synthesis and Dioxygen Reactivity of a Mixed-Valent FeIIFeIII Species. Inorg. Chem. 2014, 53, 167−181. (e) Abhervé, A.; Clemente-Juan, J. M.; ClementeLeón, M.; Coronado, E.; Boonmak, J.; Youngme, S. Tuning the Nuclearity of Iron(III) Polynuclear Clusters by Using Tetradentate Schiff-Base Ligands. New J. Chem. 2014, 38, 2105−2113. (f) Anwar, M. U.; Dawe, L. N.; Parsons, S. R.; Tandon, S. S.; Thompson, L. K.; Dey, S. K.; Mereacre, V.; Reiff, W. M.; Bunge, S. D. Oligonuclear Fe Complexes (Fe, Fe4, Fe6, Fe9) Derived from Tritopic Pyridine BisHydrazone Ligands - Structural, Magnetic, and Mössbauer Studies. Inorg. Chem. 2014, 53, 4655−4668. (g) Mitchell, K. J.; Abboud, K. A.; Christou, G. Magnetostructural Correlation for High-Nuclearity Iron(III)/Oxo Complexes and Application to Fe5, Fe6, and Fe8 Clusters. Inorg. Chem. 2016, 55, 6597−6608. (15) (a) Stamatatos, T. C.; Rentschler, E. Organic Chelate-Free and Azido-Rich Metal Clusters and Coordination Polymers from the Use of Me3SiN3: A New Synthetic Route to Complexes with Beautiful Structures and Diverse Magnetic Properties. Chem. Commun. 2019, 55, 11−26. (b) Alexandropoulos, D. I.; Vignesh, K. R.; Stamatatos, T. C.; Dunbar, K. R. Rare “Janus”-faced {FeII7} Single-Molecule Magnet Exhibiting Intramolecular Ferromagnetic Interactions. Chem. Sci. 2019, 10, 1626−1633. (16) (a) Bechlars, B.; D’Alessandro, D. M.; Jenkins, D. M.; Iavarone, A. T.; Glover, S. D.; Kubiak, C. P.; Long, J. R. High-spin Ground States via Electron Delocalization in Mixed-Valence ImidazolateBridged Divanadium Complexes. Nat. Chem. 2010, 2, 362−368. (b) Hernández Sánchez, R.; Zheng, S.; Betley, T. A. Ligand Field Strength Mediates Electron Delocalization in Octahedral [(HL)2Fe6(L’)m]n+ Clusters. J. Am. Chem. Soc. 2015, 137, 11126− 11143. (c) Sánchez, R. H.; Zheng, S.-L.; Betley, T. A. Thermally Persistent High-Spin Ground States in Octahedral Iron Clusters. J. Am. Chem. Soc. 2018, 140, 16792−16806. (17) Borovik, A. S.; Papaefthymiou, V.; Taylor, L. F.; Anderson, O. P.; Que, L., Jr. Models for Iron-Oxo Proteins. Structures and Properties of FeIIFeIII, ZnIIFeIII, and FeIIIGaIII Complexes with (μPhenoxo)bis(μ-carboxylato)dimetal Cores. J. Am. J. Am. Chem. Soc. 1989, 111, 6183−6195. (18) Hagen, K. S. Iron(II) Triflate Salts as Convenient Substitutes for Perchlorate Salts: Crystal Structures of [Fe(H2O)6](CF3SO3)2 and Fe(MeCN)4(CF3SO3)2. Inorg. Chem. 2000, 39, 5867−5869. (19) (a) Sheldrick, G. M. SHELXL-2014, Program for Crystal Structure Refinement; University of Göttingen: Göttingen, Germany, 2014. (b) Sheldrick, G. M. Crystal Structure Refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, C71, 3−8. (c) Farrugia, L. J. WinGX ver. 2014.1, An Integrated Systems of Windows Programs for the Solution, Refinement and Analysis of SingleCrystal X-ray Diffraction Data; Department of Chemistry, University of Glasgow: 2014. L


Article

Inorganic Chemistry (20) (a) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339− 341. (b) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A. Mercury CSD 2.0 - New Features for the Visualisation and Investigation of Crystal Structures. J. Appl. Crystallogr. 2008, 41, 466−470. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; J. Cioslowski, D.; Fox, D. J. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (22) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785− 789. (23) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (24) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (25) Schafer, A.; Horn, H.; Ahlrichs, R. Fully Optimized Contracted Gaussian-Basis Sets for Atoms Li to Kr. J. Chem. Phys. 1992, 97, 2571−2577. (26) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (27) Ruiz, E.; Rodríguez-Fortea, A.; Cano, J.; Alvarez, S.; Alemany, P. About the Calculation of Exchange Coupling Constants in Polynuclear Transition Metal Complexes. J. Comput. Chem. 2003, 24, 982−989. (28) Ruiz, E.; Cano, J.; Alvarez, S.; Alemany, P. Broken Symmetry Approach to Calculation of Exchange Coupling Constants for Homobinuclear and Heterobinuclear Transition Metal Complexes. J. Comput. Chem. 1999, 20, 1391−1400. (29) Ruiz, E.; Cano, J.; Alvarez, S.; Alemany, P. Magnetic Coupling in End-On Azido-Bridged Transition Metal Complexes: A Density Functional Study. J. Am. Chem. Soc. 1998, 120, 11122−11129. (30) Tomasi, J.; Mennucci, B.; Cancès, E. The IEF Version of the PCM Solvation Method: An Overview of a New Method Addressed to Study Molecular Solutes at the QM Ab Initio Level. J. Mol. Struct.: THEOCHEM 1999, 464, 211−226. (31) Lambert, E.; Chabut, B.; Chardon-Noblat, S.; Deronzier, A.; Chottard, G.; Bousseksou, A.; Tuchagues, J.; Laugier, J.; Bardet, M.; Latour, J. Synthesis, Structural, Magnetic, and Redox Properties of Asymmetric Diiron Complexes with a Single Terminally Bound Phenolate Ligand. Relevance to the Purple Acid Phosphatase Enzymes. J. Am. Chem. Soc. 1997, 119, 9424−9437. (32) (a) Papadaki, I.; Malliakas, C. D.; Bakas, T.; Trikalitis, P. N. Molecular Supertetrahedron Decorated with Exposed Sulfonate Groups Built from Mixed-Valence Tetranuclear Fe33+Fe2+(μ3-Ο)(μ3SO4)3(-CO2)3 Clusters. Inorg. Chem. 2009, 48, 9968−9970. (b) Saalfrank, R. W.; Scheurer, A.; Reimann, U.; Hampel, F.; Trieflinger, C.; Büschel, M.; Daub, J.; Trautwein, A. X.; Schünemann, V.; Coropceanu, V. Homo-/Heterotrinuclear Mixed-Valent OxoCentered Iron/Nickel Clusters−Mö ssbauer Studies on Internal Electron-Exchange Processes. Chem. - Eur. J. 2005, 11, 5843−5848.

(c) Oshio, H.; Hoshino, N.; Ito, T.; Nakano, M.; Renz, F.; Gütlich, P. High-Spin Wheel of a Heptanuclear Mixed-Valent FeII,III Complex. Angew. Chem., Int. Ed. 2003, 42, 223−225. (e) Caneschi, A.; Cornia, A.; Lippard, S. J.; Papaefthymiou, G. C.; Sessoli, R. Magnetic Properties of Dodecanuclear Mixed Valence Iron Clusters. Inorg. Chim. Acta 1996, 243, 295−304. (33) (a) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Crystallogr., Sect. B: Struct. Sci. 1985, B41, 244−247. (b) Thorp, H. H. Bond Valence Sum Analysis of MetalLigand Bond Lengths in Metalloenzymes and Model Complexes. Inorg. Chem. 1992, 31, 1585−1588. (c) Brown, I. D. Recent Developments in the Methods and Applications of the Bond Valence. Chem. Rev. 2009, 109, 6858−6919. (34) Neve, F.; Crispini, A. C−H···Br−M Interactions at Work: Tetrabromometalates of the Bolaamphiphilic N,N’-Dodecamethylenedipyridinium Cation. Cryst. Growth Des. 2001, 1, 387−393. (35) (a) Brammer, L. In Perspectives in Supramolecular Chemistry− Crystal Design: Structure and Function; Desiraju, G. R., Ed.; Wiley: Chichester, U.K., 2003; Vol. 7, pp 1−75. (b) Balamurugan, V.; Hundal, M. S.; Mukherjee, R. First Systematic Investigation of C− H···Cl Hydrogen Bonding Using Inorganic Supramolecular Synthons: Lamellar, Stitched Stair-Case, Linked-Ladder, and Helical Structures. Chem. - Eur. J. 2004, 10, 1683−1690. (36) Mishra, H.; Mukherjee, R. Half-sandwich η6-Benzene Ru(II) Complexes of Phenolate-Based Pyridylalkylamine/Alkylamine Ligands: Synthesis, Structure, and Stabilization of One-Electron Oxidized Species. J. Organomet. Chem. 2007, 692, 3248−3260. (37) (a) Snyder, B. S.; Patterson, G. S.; Abrahamson, A. J.; Holm, R. H. Binuclear Iron System Ferromagnetic in Three Oxidation States: Synthesis, Structures, and Electronic Aspects of Molecules with a Fe2(OR)2 Bridge Unit Containing Fe(III,III), Fe(III,II), and Fe(II,II). J. Am. Chem. Soc. 1989, 111, 5214−5223. (b) Kurtz, D. M., Jr. Oxoand Hydroxo-Bridged Diiron Complexes: A Chemical Perspective on a Biological Unit. Chem. Rev. 1990, 90, 585−606. (c) Hendrich, M. P.; Day, E. P.; Wang, C.-P.; Synder, B. S.; Holm, R. H.; Münck, E. Ground-State Electronic Structures of Binuclear Iron(II) Sites: Experimental Protocol and a Consistent Description of Mössbauer, EPR, and Magnetization Measurements of the Bis(phenolate)Bridged Complex [Fe2(salmp)2]2−. Inorg. Chem. 1994, 33, 2848− 2856. (d) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee, S.-K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-S.; Zhou, J. Geometric and Electronic Structure/Function Correlations in NonHeme Iron Enzymes. Chem. Rev. 2000, 100, 235−349. (e) Korendovych, I. V.; Kryatov, S. V.; Reiff, W. M.; Rybak-Akimova, E. V. Diiron(II) μ-Aqua-μ-hydroxo Model for Non-Heme Iron Sites in Proteins. Inorg. Chem. 2005, 44, 8656−8658. (38) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press: 2008. (39) Orbach, R. Proc. R. Soc.London, Ser. A 1961, A264, 458−484. (40) (a) Ferrando-Soria, J.; Cangussu, D.; Eslava, M.; Journaux, Y.; Lescouëzec, R.; Julve, M.; Lloret, F.; Pasán, J.; Ruiz-Pérez, C.; Lhotel, E.; Paulsen, C.; Pardo, E. Rational Enantioselective Design of Chiral Heterobimetallic Single-Chain Magnets: Synthesis, Crystal Structures and Magnetic Properties of Oxamato-Bridged MIICuII Chains (M = Mn, Co). Chem. - Eur. J. 2011, 17, 12482−12494. (b) Dinca, A. S.; Vallejo, J.; Shova, S.; Lloret, F.; Julve, M.; Andruh, M. Synthesis, Crystal Structure and Magnetic Properties of a New [ZnII6DyIII6] Dodecanuclear Motif. Polyhedron 2013, 65, 238−243. (c) MartinezLillo, J.; Mastroprieto, T. F.; De Munno, G.; Lloret, F.; Julve, M.; Faus, J. Enhancing the Magnetic Coupling of Oxalato-Bridged ReIV2MII (M = Mn, Co, Ni, and Cu) Trinuclear Complexes via Peripheral Halide Ligand Effects. Inorg. Chem. 2011, 50, 5731−5739. (41) (a) Vallejo, J.; Pascual-Alvarez, A.; Cano, J.; Castro, I.; Julve, M.; Lloret, F.; Krzystek, J.; De Munno, G.; Armentano, D.; Wernsdorfer, W.; Ruiz-García, R.; Pardo, E. Field-Induced Hysteresis and Quantum Tunneling of the Magnetization in a Mononuclear Manganese(III) Complex. Angew. Chem., Int. Ed. 2013, 52, 14075− 14079. (b) Martínez-Lillo, J.; Armentano, D.; De Munno, G.; M


Article

Inorganic Chemistry Wernsdorfer, W.; Julve, M.; Lloret, F.; Faus, J. A Heterotetranuclear [NiIIReIV3] Single-Molecule Magnet. J. Am. Chem. Soc. 2006, 128, 14218−14219. (42) Cano, J. VPMAG, Revision 3.0; University of Valencia: Valencia, Spain, 2015. (43) Cervera, B.; Ruiz, R.; Lloret, F.; Julve, M.; Cano, J.; Faus, J.; Bois, C.; Mrozinski, J. Tuning the Nature of the Exchange Interaction in Out-of-Plane Oximato-Bridged Dinuclear Copper(II) Complexes. J. Chem. Soc., Dalton Trans. 1997, 395−401. (44) Ruiz-Pérez, C.; Rodríguez-Martín, Y.; Hernández-Molina, M.; Delgado, F. S.; Pasán, J.; Sanchíz, J.; Lloret, F.; Julve, M. Malonic acid: A Multi-Modal Bridging Ligand for New Architectures and Properties on Molecule-Based Magnets. Polyhedron 2003, 22, 2111−2123. (45) (a) Zhao, J.-P.; Hu, B.-W.; Lloret, F.; Tao, J.; Yang, Q.; Zhang, X.-F.; Bu, X.-H. Magnetic Behavior Control in Niccolite Structural Metal Formate Frameworks [NH2(CH3)2][FeIIIMII(HCOO)6] (M = Fe, Mn, and Co) by Varying the Divalent Metal Ions. Inorg. Chem. 2010, 49, 10390−10399. (b) Canadillas-Delgado, L.; Fabelo, O.; Rodriguez-Velamazan, J.; Lemee-Cailleau, M. H.; Mason, S. A.; Pardo, E.; Lloret, F.; Zhao, J. P.; Bu, X. H.; Simonet, V.; Colin, C. V.; Rodriguez-Carvajal, J. The Role of Order−Disorder Transitions in the Quest for Molecular Multiferroics: Structural and Magnetic Neutron Studies of a Mixed Valence Iron(II)−Iron(III) Formate Framework. J. Am. Chem. Soc. 2012, 134, 19772−19781.

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