MnIIIâFeIII Heterometallic Compounds within ... - ACS Publications

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

MnIII−FeIII Heterometallic Compounds within Hydrogen-Bonded Supramolecular Networks Promoted by an [Fe(CN)5(CNH)]2− Building Block: Structural and Magnetic Properties David Aguilà, Olivier Jeannin, Marc Fourmigué, and Ie-Rang Jeon* Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) − UMR 6226, 35000 Rennes, France S Supporting Information *

ABSTRACT: The reaction of [Fe(CN)6]3− and [Mn(acacen)]+ (H2acacen = N,N′-bis(acetylacetone)ethylenediamine) building units in the presence of supramolecular cations, [(F-Anil)(18crown-6)]+ (F-Anil+ = 3-fluoroanilinium) or [(Me-F-Anil)(18crown-6)]+ (Me-F-Anil+ = 3-fluoro-4-methylanilinium), affords two new bimetallic compounds, [(F-Anil)(18-crown-6)][Mn(acacen)Fe(CN)5(CNH)]·MeOH (1) and [(Me-F-Anil)(18crown-6)][Mn(acacen)(MeOH)Fe(CN)5(CNH)]·MeOH (2), respectively. Compound 1 exhibits a one-dimensional topology, while compound 2 is a dinuclear discrete system due to the coordination of a MeOH molecule at the axial position of the [Mn(acacen)]− unit. For both systems, the acidity of the corresponding supramolecular cation triggers the protonation of the FeIII moiety as [Fe(CN)5(CNH)]2−. Moreover, the resulting −CNH ligand induces hydrogen bonding interactions connecting the chains for 1 or the molecules for 2 into higher dimensional supramolecular networks. Magnetic properties of compounds incorporating these [Fe(CN)5(CNH)]2− building blocks were, for the first time, thoroughly investigated, indicating a threedimensional antiferromagnetic order of single-chain magnets for 1 and an antiferromagnetically interacting S = 3/2 spin ground state for 2.

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INTRODUCTION The design of molecule-based magnetic materials is in continuous development, largely driven not only by their fundamental interest but also by their potential use in technological applications.1 This fruitful research field, which falls between coordination chemistry and condensed matter physics, has witnessed the discovery of systems behaving, for example, as magnets (high-TC long-range magnetic order,2 single-chain magnet,3 or single-molecule magnet4 behaviors), magnetic refrigerants,5 quantum information processing units,6 or photoswitchable entities.7 The main advantage of these molecule-based materials is that their design and topology can be deliberately manipulated in order to fine-tune the magnetic properties. A possible route for such a rational synthetic design is the use of organic ligands that enable encapsulating metal ions in a specific fashion.8 In contrast, one can also rationally assemble preformed magnetic molecular entities into the final material.9 This latter strategy, so-called the building-block approach, is versatile to prepare compounds in different topologies including high-nuclearity molecules,9d,10 chains,11 grids,12 or three-dimensional networks.13 Here, the coordination topology, which directly controls the electronic configuration of a metal ion and the magnetic exchange interactions, is the preliminary parameter governing the magnetic properties of the system.14 Nevertheless, the presence of weak interactions between building blocks that leads to the formation of a © XXXX American Chemical Society

supramolecular network can be also critical to significantly modulate magnetic properties.15,16 Among the diverse compounds prepared through the building-block approach, an important part derives from the assembly of the hexacyanoferrate anion, [Fe(CN)6]3− (Scheme 1, top left) with MnIII complexes featuring a tetradentate Schiff Scheme 1. Schematic Representation of the Metallic Building Blocks (Top) And the Supramolecular Cations Employed for 1 (Bottom, Left) and 2 (Bottom, Right)

Received: April 12, 2018

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DOI: 10.1021/acs.inorgchem.8b00983 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry base (SB) ligand, [Mn(SB)]+. Such systems have 2-fold interests. First, the FeIII building block assures efficient magnetic superexchange while providing diverse structure types depending on the nature of the Schiff base ligand, including discrete molecules17 and extended solids.18−20 Second, the MnIII building unit brings significant magnetic anisotropy stemmed from the axial zero-field splitting due to the tetragonal ligand field imposed by Jahn−Teller distortion.21 Moreover, the axially disposed coordination sites likely align the magnetic easy axes of MnIII units in the final compounds. Within this family of compounds, we have focused our attention on FeIII−CN−MnIII systems consisting [Fe(CN)6]3− and [Mn(acacen)]+ (Scheme 1, top right) units.22 Particularly, 1D arrangement of these building blocks have been shown to exhibit slow relaxation of the magnetization.22c Here, the incorporated cation turned out to play an important role to induce a slight distortion on the chain topology, leading to a drastic modification in the magnetic relaxation dynamics of the chain. Taking into account these results, we decided to further explore how the cation-directed crystal engineering governs the self-assembly of [Fe(CN)6]3− and [Mn(acacen)]+ building blocks, their supramolecular arrangements, and the magnetic properties of the resulting systems. Specifically, we have chosen [(m-halogenated-anilinium)(18-crown-6)]+ supramolecular cations (Scheme 1, bottom), which have also been shown to promote dielectric properties in relation with the order− disorder motion of the supramolecular unit.23 Herein, we report synthesis, crystal structures, and magnetic and thermal properties of two new compounds derived from this strategy: [(F-Anil)(18-crown-6)][Mn(acacen)Fe(CN)5(CNH)]·MeOH (1) and [(Me-F-Anil)(18-crown-6)][Mn(acacen)(MeOH)Fe(CN)5(CNH)]·MeOH (2), respectively. The structural characterizations demonstrate that the supramolecular cations direct the assembly of the building blocks into different dimensionality, depending on the substituents on the m-halogenatedanilinium. More importantly, these cations promote protonation of the iron building block as [Fe(CN)5(CNH)]2−, which triggers the formation of hydrogen-bonded supramolecular networks and directly influences their magnetic properties.

cations that are obtained by stoichiometric reaction of 18crown-6 and (Me-F-Anil)(BF4) in methanol. Brown, rodshaped crystals of [(Me-F-Anil)(18-crown-6)][Mn(acacen)(MeOH)Fe(CN)5(CNH)]·MeOH (2) suitable for X-ray analysis were obtained in 15% yield. The infrared spectra of compounds 1 and 2 feature characteristic ν(CN) bands in the range of 2150−2060 cm−1. Each compound shows three peaks in this region, 2145, 2127, and 2067 cm−1 for 1 and 2147, 2120, and 2069 cm−1 for 2, which deviate from that of the starting material (Bu4N)3[Fe(CN)3] at 2117 cm−1. The observed shift and splitting of CN stretching vibrations indicate the presence of three different chemical environments of cyanide ligands on the FeIII building block, presumably those uncoordinated, coordinated to MnIII ions, and engaged in hydrogen-bonded networks through the protonation (see below). Structural Description. The crystal structures of compounds 1 and 2 were determined by single-crystal X-ray diffraction analyses. Compound 1 crystallizes in the triclinic P1̅ space group (Table 1). The crystal structure consists of a one-

RESULTS AND DISCUSSION Synthesis. In order to introduce supramolecular cations, a solution of (Bu4N)3[Fe(CN)6] in methanol was first reacted with 4 equiv of [(F-Anil)(18-crown-6)]+ cations that are obtained by reacting an equimolar amount of 18-crown-6 and (F-Anil)(BF4) in methanol. The assembly reaction of FeIII and MnIII building blocks was then performed by treating the resulting yellow solution with a methanolic solution of [Mn(acacen)(H2O)2](PF6). Careful layering of diethyl ether onto the final brown solution afforded brown, block-shaped crystals of [(F-Anil)(18-crown-6)][Mn(acacen)Fe(CN)5(CNH)]·MeOH (1) in 30% of yield, suitable for single-crystal X-ray diffraction analysis. The relatively low yield is likely due to the side role of the supramolecular [(FAnil)(18-crown-6)]+ cation as an acid to protonate the FeIII building block (see below). However, attempts to optimize the reaction by increasing equivalents of supramolecular cations were unsuccessful. To investigate the steric effect of the mhalogenated-anilinium in the final supramolecular system, we have also targeted the incorporation of methyl substituent on the para-position of F-Anil+ cation. The reaction was carried out similar to that of 1, using [(Me-F-Anil)(18-crown-6)]+

a

Table 1. Crystallographic and Refinement Parameters for the Structures of Compounds 1 and 2

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compound

1

2

formula FW (g mol−1) crystal system space group T (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) dcalc (g cm−3) R1 (I > 2σ(I))a wR2 (all)b GoF

C37H54FFeMnN9O9 898.68 triclinic P1̅ 100 11.4325(10) 13.4363(13) 14.1764(12) 92.440(3) 97.505(3) 91.918(3) 2155.4(3) 1.385 0.0454 0.0839 1.024

C39H60FFeMnN9O10 944.75 triclinic P1 150 8.6803(11) 10.8962(15) 13.6361(15) 102.388(5) 93.979(5) 108.325(4) 1182.7(3) 1.326 0.0417 0.1124 0.852

R1 = Σ||F0| − |FC||/Σ|F0|. bwR2 = [Σw(F02 − FC2)2/Σw(F02)2]1/2.

dimensional anionic system where both metallic building blocks are alternatively linked through one cyanide ligand. The asymmetric unit of the chain contains one [Mn(acacen)]+ molecule and two halves of [Fe(CN)6]-containing moieties (Figure 1). The Mn center resides in a distorted octahedral coordination environment. The equatorial plane is composed of two nitrogen and two oxygen donor atoms from the tetradentate acacen2− ligand and the axial positions are occupied by nitrogen donor atoms from two cyanide ligands in trans positions of the Fe building block. As shown in Table S1, the average Mn−Nacacen and Mn−Oacacen distances (1.982 and 1.915 Å, respectively) are significantly shorter than the mean Mn−NCN distance (2.295 Å), as expected for a Jahn− Teller distortion effect of the high-spin MnIII ion.21 The observed distances exhibit values close to the ones reported for related compounds.22 The [Fe(CN)6]-containing moiety reveals a marginally distorted octahedral environment with the average Fe1−C and Fe2−C distances of 1.933 and 1.932 Å, respectively. In particular, the Fe1−C14 and Fe2−C18 distances are 1.912(2) and 1.906(2) Å, respectively, which B


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Inorganic Chemistry

distance of 8.505 Å between FeIII ions of adjacent chains, and accordingly, the MnIII units are also close with the Mn···Mn distance of 8.619 Å. These metallic grids are separated by a layer consisting of MeOH molecules (interacting through hydrogen bond interactions as seen above) and the [(FAnil)(18-crown-6)]+ units (Figure 2, bottom). The latter are intercalated between metallic layers, with the aromatic rings from the F-Anil+ moieties featuring π···π interactions in a parallel displaced configuration. The fluorine atoms of these two anilinium units are pointing in two opposite directions (Figure S2). Moreover, each of the F-Anil+ aromatic rings faces the 18-crown-6 molecule from an adjacent supramolecular unit as well as the metallic array, resulting in a steric hindrance for the anilinium. Compound 2 crystallizes in the P1 space group (Table 1). The crystal structure exhibits an asymmetric unit that consists of a dinuclear complex resulting from the assembly of the MnIII and the FeIII building blocks, one [(Me-F-Anil)(18-crown-6)]+ cation and one uncoordinated MeOH molecule (Figures 3 and S3). One cyanide ligand is coordinated to the MnIII center and one MeOH molecule occupies the trans-position to the bridging CN group, resulting in a discrete [FeIII−CN−MnIII]− species. The MnIII adopts a Jahn−Teller elongated octahedral coordination environment comprising the four N2O2 donor atoms from the acacen2− ligand, one apical nitrogen atom from the cyanide ligand, and one apical oxygen atom from the MeOH molecule. The average Mn−Nacacen and Mn−Oacacen distances are 1.977 and 1.894 Å, respectively, close to values observed in 1 (Table S2). Accordingly, Mn−NCN and Mn− OMeOH distances are 2.277(5) and 2.299(4) Å, respectively, featuring the elongated Jahn−Teller axis. The mean Fe−C distance of 1.933 Å, with short Fe1−C2 and Fe1−C4 distances of 1.912(7) and 1.916(7) Å, is consistent with the values observed in 1, likely indicating the protonation of a cyanide. The Fe−C−N and the Mn−N−C angles are 176.0(5) and 154.9(4)°, respectively, a feature which separates two metallic centers by 5.223 Å. The crystal lattice of 2 additionally contains one [(Me-FAnil)(18-crown-6)]+ cation and one interstitial MeOH molecule. The hydrogen bonding between the −NH3+ group of the anilinium cation and the oxygen atoms from the 18crown-6 is evidenced by the average N···O distances (2.954 Å). At 150 K, the crown ether is disordered in two different positions, with occupancies of 70 and 30%, respectively. As for 1, the presence of only one [(Me-F-Anil)(18-crown-6)]+ molecule in the asymmetric unit suggests the monoanionic character of the dinuclear moiety. This was again confirmed by the protonation of one of the cyanide ligands, which was localized crystallographically. In this case, the electronic density was found closer to N2, in agreement with the short C−N bond distance obtained for −C2N2 (1.142(7) Å) compared to the adjacent cyanide group −C4N4 (1.148(6) Å). Taken together, these observations lead to a configuration for the anionic complex in 2 of [Mn(acacen)(MeOH)Fe(CN)5(CNH)]−. The protonated −CNH ligand interacts through hydrogenbonding with a cyanide acceptor group from an adjacent [Mn(acacen)(MeOH)Fe(CN)5(CNH)]− anion (Figure 4, top), promoting the formation of a comb-like one-dimensional supramolecular network of the [MnIIIFeIII]− dinuclear complex with an intermolecular separation between the metallic centers of 8.680 and 8.680 Å for FeIII and MnIII, respectively. In the lattice, hydrogen bonds between the axially coordinated MeOH

Figure 1. X-ray crystal structure of the anionic unit in 1, collected at T = 100 K. Brown, magenta, red, blue, and gray ellipsoids, shown at the 50% probability level, represent Fe, Mn, O, N, and C atoms, respectively; H atoms, except for H4 and H8 (shown in white spheres) are omitted for clarity. Nonlabeled atoms are generated by symmetry operations.

are significantly short compared to the rest. Given that the average Fe−C distance of known coordination compounds containing a linear [−(SB)Mn−NC−Fe(CN)5−] repeating unit falls between 1.939 and 1.958 Å,17a,b,d,f,18a,d,22a,c the observed short Fe−C distances in 1 likely stem from the protonation of the corresponding cyanide ligand (see below). The Fe−C−N angles for Fe1 and Fe2 are 176.5(2) and 177.7(2)°, respectively, thus very similar and close to linearity. In contrast, the two Mn−N−C deviate significantly from the linearity angles (147.4(2) and 145.3(2)°), resulting in a corrugated chain. The distances between the metal ions along the chain are Mn1···Fe1 = 5.165 Å and Mn1···Fe2 = 5.100 Å, respectively. The crystal structure of 1 reveals the presence of one [(FAnil)(18-crown-6)]+ cation and one MeOH molecule in the lattice (Figure S1). In the [(F-Anil)(18-crown-6)]+ cations, the halogenated anilinium molecules are inserted within the 18crown-6 host. Hydrogen bonding interactions between the ammonium group of F-Anil+ and the oxygen atoms of the crown, with an average N···O distance of 2.889 Å, ensure the stability of the supramolecular entity as previously reported.23 The OH proton of the methanol solvent molecule is engaged in a hydrogen bond interaction with a cyanide ligand of the iron unit (O9···N5 = 2.899 Å, Figure S1, blue lines). Intriguingly, only one molecule of [(F-Anil)(18-crown-6)]+ was found to compensate the negative charges produced by the asymmetric metallic array. Consequently, the monoanionic character of the latter suggests the protonation of one of the cyanide ligands in the Fe III building block, as [Fe(CN)5(CNH)]2−. Each of the −CNH units acts as a Lewis acid and interact with the facing acceptor cyanide ligand from an adjacent chain (Figure 2, top). The short distance between the corresponding nitrogen atoms (N4−N8 = 2.553 Å) evidences the presence of a rather strong hydrogen bonding interaction, thus producing a two-dimensional supramolecular network. The corresponding proton, which was identified in Fourier difference map, was found to be disordered between −C14N4 and −C18N8. The similar C−N bond distances of these two CN ligands connected through the protonation (1.144(3) and 1.145(3) Å, respectively) also agree with the proton being disordered between both cyanide groups. The presence of [Fe(CN)5(CNH)]2− unit is further coherent with the spectroscopic and structural data discussed above, three different CN vibration stretches in FT-IR and the shorter Fe−C distances than those in known [Fe(CN)6]3− units. The formation of this supramolecular grid forces the shortest C


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Inorganic Chemistry

Figure 2. X-ray crystal structure of the anionic complex, [Mn(acacen)Fe(CN)5(CNH)]−, in 1, highlighting two-dimensional network formed by the interchain −CNH···NC− hydrogen bonding interactions represented as blue dashed lines. For clarity, only one position of this proton is shown (top). Crystal packing in 1, revealing the alternating layered disposition of the anionic and the cationic parts (bottom). Brown, magenta, green, red, blue, gray, and white spheres represent Fe, Mn, F, O, N, C, and H atoms; other H atoms are omitted for clarity.

all the [(Me-F-Anil)(18-crown-6)]+ molecules in 2 point to the same direction. However, the additional methyl group of the mhalogenated-anilinium unit does not particularly avoid steric hindrance around the supramolecular cation. In this case, the Me-F-Anil+ molecule is stacked between both hydrogenbonded layered architectures and an 18-crown-6 molecule from an adjacent supramolecular cation (Figure S4). To our best knowledge, 1 and 2 represent the very first example of [M−CN−M′] coordination complexes featuring

molecules and the cyanide ligands from adjacent supramolecular chains allow the extension of the compound in two dimensions (O3···N1 = 2.754 Å, Figure 4, bottom). The space between the generated supramolecular layers is occupied by the MeOH solvent molecules and the supramolecular cations. The noncoordinated MeOH solvent molecules is also engaged in a hydrogen bonding with one of the free cyanide ligands (−C3N3), as evidenced by O10···N3 distance of 2.823 Å (Figure S3). In contrast to compound 1, the fluorine atoms of D


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Inorganic Chemistry

that this type of molecular architecture was obtained only with m-halogenated anilinium derivatives as cations. Several attempts to reproduce this system using less acidic anilinium-derivatives, such as 4-methoxyanilinium and 2,6-diisopropylanilinium, were unsuccessful. This observation suggests that the acidity of the cation is indeed responsible for the formation of the −CNH ligands. Magnetic Properties. To probe and compare magnetic exchange interactions, variable-temperature dc magnetic susceptibility measurements were carried out on solid samples under an applied dc field of 0.1 T. The resulting plots of χT versus T (χ being the molar susceptibility) for 1 and 2 are shown in Figure 5. At 300 K, the compounds exhibit a value of χT = 3.3 and 3.5 cm3 K mol−1, for 1 and 2, respectively, which agree well with the expected value of 3.375 cm3 K mol−1 for magnetically noninteracting S = 1/2 (FeIIILS) and S = 2 (MnIIIHS) ions assuming g = 2. Upon lowering temperature, the χT value of 1 decreases reaching a minimum of 2.9 cm3 K mol−1 at 75 K, indicating the presence of an intrachain antiferromagnetic interaction between FeIII and MnIII ions via

Figure 3. X-ray crystal structure of the anionic asymmetric unit in 2, collected at T = 150 K. Brown, magenta, red, blue, and gray ellipsoids, shown at the 30% probability level, represent Fe, Mn, O, N, and C atoms, respectively; H atoms, except for H2 (shown in a white sphere), are omitted for clarity.

[Fe(CN)6−x(CNH)x]n− as a building block, although the protonation of hexacyanometallates have been studied spectroscopically and crystallographically in few cases.24 Note

Figure 4. Representation of the crystal structure of the anionic complex, [Mn(acacen)(MeOH)Fe(CN)5(CNH)]−, in 2, revealing the onedimensional supramolecular architecture formed due to the intermolecular −CNH···NC− hydrogen bonding (represented by blue dashed lines, top). Crystal packing of 2 highlighting the layered disposition of the chains, the supramolecular cations and the MeOH molecules (bottom). Brown, magenta, green, red, blue, gray, and white spheres represent Fe, Mn, F, O, N, C, and H atoms; other H atoms are omitted for clarity. E


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Inorganic Chemistry

antiferromagnetic nature of the intramolecular exchange interaction. Moreover, the unsaturated high-field magnetization at 1.8 K, in conjunction with the splitting of isofield curves in the M versus H/T plot, confirms the presence of magnetic isotropy in the system. The data were thus modeled considering a magnetic exchange interaction J and the axial magnetic anisotropy brought by MnIII, DMn, based on the following Hamiltonian using the program Magprop:27 2 H = − 2J(SMn·SFe) + DMn(Sz,Mn −

1 SMn(SMn + 1)) 3 (2)

The best fit was obtained for g = 2.04, J/kB = −2.3 K, and DMn/kB = −2.8 K (Figure S6). The value of J confirms the antiferromagnetic interaction between the FeIII and the MnIII ions, and the value of DMn is in agreement with the one observed in other related compounds.28 Nevertheless, the simulation of the χT versus T data considering only the obtained J and DMn values did not perfectly reproduce the low temperature data (Figure S7, green solid line), suggesting that the magnetic properties in this temperature regime are governed not only by the magnetic anisotropy but also by intermolecular interactions (zJ′). In order to avoid overparameterization, the magnetic susceptibility data was then modeled using an isotropic Heisenberg model with the following spin Hamiltonian:

Figure 5. Variable-temperature dc magnetic susceptibility (χ = M/H per mole of compound) data for 1 (red sphere) and 2 (blue sphere), shown as a plot of χT vs T, collected under an applied field of 1 T from 400 to 100 K and 0.1 T below 100 K. Inset: ln(χT) vs T−1 plot of 1, with solid black line being the best exponential fit described in the text.

the cyanide bridging ligand. As temperature is further decreased, the χT product shows an abrupt increase to a maximum of 19.2 cm3 K mol−1 at 6 K, then a precipitous drop to 5.2 cm3 K mol−1 at 1.8 K. This thermal behavior is indicative of a ferrimagnetically arranged one-dimensional system that is interacting antiferromagnetically with the adjacent ones in three dimensions. The temperature dependence of the χT product was modeled between 15 and 300 K using an alternating chain model of quantum spins (SFe) and classical spins (SMn) considering the following Hamiltonian:25

H = − 2J(SMn·SFe)

and a mean-field approximation treatment for the intermolecular interactions.29 A fit to data between 1.8 and 300 K affords J/kB = −2.3 K, zJ′/kB = −0.1 K, and g = 2.04 (Figure S5 and red solid line in Figure S7). While the value of J is in agreement with the one derived from the magnetization measurements, the value obtained for zJ′ is most likely overestimated due to the effect of the magnetic anisotropy. Almost an order of magnitude decrease in the magnetic exchange from 1 to 2 may be explained by the difference of ca. 7° in the Mn−N−C angles and the possible overestimation of the magnetic exchange in 1. Remarkably, the presence of intramolecular antiferromagnetic interactions contrasts with the ferromagnetic exchange observed in the previously reported [Fe(CN)6Mn(acacen)]2− systems22 or in other compounds containing a linear [MnIII−NC−FeIII(CN)4−CN−MnIII] moiety.17a,b,d,f,18a,d,30 Taking into account that the values of the Mn− N−C and Fe−C−N angles of these systems are similar to the ones observed in 1 and 2, one would also expect a ferromagnetic exchange. A possible explanation of this behavior is probably the different chemical environment brought by the protonated [Fe(CN)5(CNH)]2− unit, which influences the degree of magnetic orbital overlap between MnIII and FeIII ions, and thus the character of the exchange. This observation reveals the effect of the generated −CNH ligand not only in the formation of the supramolecular hydrogen-bonded network and therefore the mediation of intermolecular antiferromagnetic interactions but also in the nature of the FeIII−MnIII magnetic coupling itself. In order to ascertain the one-dimensional magnetic character of 1, the ln(χT) versus T‑1 plot was closely examined. As shown in the inset of Figure 5, the 1D correlation length (ξ), that is proportional to the χT product,31 shows an exponential increase upon decreasing temperature between 20 and 10 K, therefore evidencing the 1D nature in 1. A linear fit to data yields Δξ/kB = 17.3 K, which is the energy to create a domain

N

H = − 2J ∑ ((SMn, i + SMn, i + 1) ·SFe, i) i=1

(3)

(1)

The best fit was obtained for J/kB = −24.7 K, gMn = 2.1, and gFe = 2.2, evidencing the antiferromagnetic intrachain superexchange (Figure S5). The obtained g values for MnIIIHS and FeIIILS are in the reasonable range compared to literature examples,17b and comparable J values have been also found in similar systems.26 Despite the good reproduction of experimental data above 15 K, it is worth mentioning that the analytically available single J model is not the most suitable for 1, given the presence of two independent Fe sites in the crystal asymmetric unit. Such a feature, in conjunction with a possible 2D correlation via a hydrogen-bonded network, may overestimate the magnitude of the magnetic exchange in 1. In contrast, the χT product of 2 shows a fairly constant value down to 100 K and then a decrease to reach a minimum value of 1.6 cm3 K mol−1 at 1.8 K, upon decreasing temperature. This temperature dependence suggests the presence of an antiferromagnetic interaction between S = 1/2 and S = 2 spin carriers, resulting in an S = 3/2 spin ground state. However, the data does not show a plateau at 1.875 cm3 K mol−1 expected for an S = 3/2, indicating a possible role of intermolecular antiferromagnetic interactions (consistent with the effective hydrogen-bonded network observed in the crystal structure) and/or effects of the potential magnetic anisotropy derived from the MnIII ion. In order to further evaluate the low temperature magnetic behavior, variable-field magnetization measurements were carried out (Figure S6). The magnetization value reaches 3 μB under 5 T at 1.8 K, in agreement with the F


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Inorganic Chemistry wall in the chain, i.e., the interface between two domains with an opposite magnetization orientation.31c,32 In the Ising limit, the theoretical value can be estimated to Δξ = 4|J|SFeSMn.3c Given the J/kB value obtained from the fit of the χT versus T, this equivalence would give ca. 100 K, confirming that the compound is far from being in the Ising limit. Upon further decreasing the temperature, the ln(χT) value drops significantly, suggesting the presence of a 3D antiferromagnetically ordered phase. This magnetic behavior reflects well the presence of the supramolecular network established through strong hydrogen bonding interactions, as discussed above. In order to further assess the magnetic anisotropy and the long-range antiferromagnetic order in 1, M versus H measurements were carried out (Figure S8). The magnetization is not saturated even at high fields (5 T) and very low temperatures (1.8 K), reaching a value of 2.4 μB, implying a significant magnetic anisotropy in the system. The linear extrapolation of the magnetization to the expected saturation value is 10.2 T that correspond to an anisotropy energy, KA = 10.3 K.16b At low fields, magnetization curves showed an inflection point (an Sshape curve, Figure S8), confirming the existence of antiferromagnetic interactions between chains which can be compensated by an applied magnetic field. The temperature variation of this characteristic field (HC) was examined by taking the maximum values of field-dependence of magnetization (dM vs dH, Figure S9) and temperature-dependence of susceptibility (χ vs T, Figure S9) data, in order to construct the (T, H) phase diagram (Figure 6). This phase diagram

Figure 7. Field dependence of the magnetization for 1 at 1.8 K measured with an average sweep-rate of about 580 Oe min−1. Solid line is a guide.

3D antiferromagnetic ground state. Given the 1D nature of the system, as confirmed by the analysis of ln(χT) versus T−1, this magnetic hysteresis suggests that 1 may join the short list of an antiferromagnetic ordered phase manifesting intrinsic dynamics of the chain component.16 To confirm the dynamics of the magnetization stemming from the 1D system, variable-temperature and -frequency ac magnetic susceptibility data were collected. As expected for an antiferromagnetic ground state, in-phase ac susceptibility shows a frequency-independent maximum at 6 K while no response in out-of-phase susceptibility was observed (Figure 8). However, below 5 K, both in- and out-of-phase components become frequency-dependent without an applied magnetic field, revealing a slow relaxation of the magnetization in this regime (Figures 8 and S10). The characteristic relaxation times (τ) at each temperature were estimated using the generalized Debye model34 and plotted as τ versus 1/T (Figure 9, filled circles). The thermally activated regime was fitted to an Arrhenius law to obtain an energy barrier of Δτ/kB = 48.9 K with τ0 = 1.3 × 10−9 s. It should be noted that for an antiferromagnetically ordered phase the relaxation times are expected to increase close to the antiferromagnetic−paramagnetic phase transition line at which the interchain antiferromagnetic coupling is overcome.16a,b,d Thus, variable-field ac susceptibility measurements between 0 and 2000 Oe were performed in order to evaluate the influence of an external dc magnetic field (Figure S11). The characteristic time was estimated from the out-of-phase component, affording the slowest relaxation of the magnetization at 1000 Oe in agreement with the magnetic phase diagram. Therefore, ac susceptibility data were collected as a function of ac field frequency under an external dc field of 1000 Oe (Figure 10). The derived semilogarithmic plot of τ versus 1/T exhibits two relaxation regimes above and below T* = 3.6 K, in contrast to a single relaxation time observed under zero applied field. Such a behavior, which is often observed for single-chain magnets, suggests that the chain of 1 can be viewed as an infinite-size above 3.6 K.3,35 It is important to note that this crossover was not observed in the plot of ln(χT) versus T−1 owing to the occurrence of antiferromagnetic order. The two slopes observed in the Arrhenius plot are directly related to the relaxation barrier of the infinite-size (Δτ1) and finite-size (Δτ2) regimes. Linear regression fits to data for the different relaxation regimes give Δτ1/kB = 64.8 K and Δτ2/kB = 48.8 K with τ0 = 5.4 × 10−11 and 3.4 × 10−9 s, respectively (Figure 9).

Figure 6. H vs T magnetic phase diagram for 1. The filled and empty spheres are respectively experimental points deduced from the dM vs dH and χ vs T data (Figure S9).

unambiguously proves the presence of an ordered antiferromagnetic ground state below TN = 6.0 K. Note that the topology of the diagram indicates that 1 is an antiferromagnet with a metamagnetic behavior, likely owing to the magnetic anisotropy brought by the MnIII ions. In this case, the interchain interaction (zJ′/kB) can be estimated from the HC value extrapolated at 0 K (H0C = 1000 Oe) using the following expression:33 gμB HC0ST = 2|zJ ′|ST2

(4)

providing zJ′/kB = −0.046 K. This value is about half of the one obtained for compound 2 using the mean-field approximation, confirming the presence of the combined effect of both intermolecular interactions and magnetic anisotropy in the low temperature magnetic properties of 2. Remarkably, 1 exhibits a magnet-type behavior (hysteresis of the magnetization) at 1.8 K with a coercive field of 860 Oe as shown in Figure 7, despite the G


Article

Inorganic Chemistry

Figure 8. Temperature dependences of the real (χ′) and imaginary (χ″) parts of the ac susceptibility at selected frequencies for 1 in zero dc field with a 3 Oe ac field. Solid lines are guides.

suggesting again that compound 1 belongs to a non-Ising system.16b As such, further evaluation of anisotropic energy as a function of the local magnetic parameters was not possible analytically. It is worth mentioning that variable-frequency ac magnetic susceptibility measurements performed in compound 2 did not evidence any signal of magnetic dynamics, neither at zero field nor under an external dc magnetic field (Figure S12). Thermal Properties. The particular cations incorporated in compounds 1 and 2 are known to function as a molecular stator-rotator in solid-state, as mentioned in the introduction.23 To explore the presence of such an order−disorder motion of the [(F-Anil)(18-crown-6)]+ and [(Me-F-Anil)(18-crown-6)]+ cations in 1 and 2, differential scanning calorimetry (DSC) measurements were performed between 300 and 420 K in heating and cooling modes at 10 K min−1 (Figure S13). 1 and 2 exhibit a nonreversible transition at 410 and 390 K, respectively, and none of these processes were found to be reversible once the temperature was decreased back to 300 K. These processes are likely related to the decomposition of the compounds to an amorphous phase, as revealed by the PXRD measurements (Figure S14). The absence of molecular motion in compounds 1 and 2 was nevertheless not surprising, taking into account the steric impediments observed for each supramolecular cation within the corresponding magnetic hosts.

Figure 9. Plots of magnetization relaxation time (τ) vs T−1 for 1 under zero dc field (filled circle) and 1000 Oe (empty circle). Solid lines are best fits of the experimental data to the Arrhenius laws discussed in the text.

To further investigate the intrinsic behavior of the chain component in 1, the obtained values of the relaxation barrier were compared with the theoretical description. For any singlechain magnet, the relationships between the energy gaps can be expressed as Δτ1 = 2Δξ + ΔA and Δτ2 = Δξ + ΔA, where ΔA is the anisotropy energy gap.3c The calculated Δξ for 1 from the two relaxation energy gaps amounts to 16 K, which agrees well with the one obtained from the plot of ln(χT) versus T−1 (17.3 K). This further confirms that the dynamic magnetic property of 1 stems from the chain component of the compound undoubtedly. Furthermore, the above relationship gives ΔA = 30.3 K, which is about three times the value of KA (10.3 K),

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CONCLUSIONS The foregoing results describe synthesis and characterizations of two new MnIII−FeIII bimetallic systems, involving the

Figure 10. Frequency dependences of the real (χ′) and imaginary (χ″) parts of the ac susceptibility for 1 under an applied dc field of 1000 Oe with a 3 Oe ac field, at every 0.2 K between 2.4 (blue) and 4.4 K (red). Solid lines are guides. H


Article

Inorganic Chemistry [Fe(CN) 5 (CNH)] 2− building unit. The reaction of [MnIII(acacen)]+ and [FeIII(CN)6]3− building blocks in the presence of [(F-Anil)(18-crown-6)]+ or [(Me-F-Anil)(18crown-6)]+ supramolecular cations allowed the preparation of two compounds featuring a chain and a discrete dinuclear molecule, respectively. The acidity of these stator-rotator units was found to promote, in both cases, the protonation of one of the cyanide ligands of the FeIII building block, leading to a [Fe(CN)5(CNH)]2− moiety. The generated −CNH allowed the connection through hydrogen bonding of the metallic arrays in 1, and the discrete dinuclear entities in 2, producing two- and one-dimensional supramolecular hydrogen-bonded networks, respectively. In addition, the formation of [Fe(CN)5(CNH)]2− was found to affect the magnetic exchange coupling between the iron and manganese units, mediating an antiferromagnetic interaction between both metal centers. For compound 1, a typical single-chain magnet-like behavior was observed, even within the 3D antiferromagnetic order triggered by the intensive hydrogen bonding interactions between the chains. Compound 2 stabilizes an S = 3/2 spin ground state, which interacts antiferromagnetically. In both compounds, the supramolecular cation inserted within the magnetic host features steric hindrance, which impedes its molecular motion as suggested by structural characterization and DSC measurements. In summary, this work shows for the first time the incorporation of the [Fe(CN)5(CNH)]2− unit within a heterometallic magnetic material, highlighting its potential to promote higher dimensional supramolecular materials through −CNH···NC− hydrogen bonding.

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source and PHOTON100 detector at the Centre de Diffratométrie (CDFIX, ISCR). The data obtained were integrated using Bruker APEX3 v. 201538 and corrected from absorption using SADABS.39 Both structures were solved and refined with SHELXL40 using OLEX41 interface. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were assigned to ideal positions and refined isotropically using a riding model except for those on nitrogen and oxygen atoms which were localized on the difference Fourier map and refined using DFIX and DANG restraints. The hydrogen atom involved in the [Fe(CN)5(CNH)]2− unit of compound 1 was found to be disordered in two positions (H4 and H8) with 0.445:0.555 occupancies. The crystal structure of 2 contains one MeOH molecule in the lattice featuring a disordered O atom that was refined using ISOR instruction, as well as one disordered C atom from the acacen2− ligand (C15). The 18-crown-6 molecule from the [(Me-F-Anil)(18crown-6)]+ supramolecular cation is disordered in two positions with relative occupancies 0.693:0.307 (A:B). The carbon and oxygen atoms from the latter position (B) were further refined using DFIX and EADP instructions, respectively. The Flack parameter obtained (0.41(2)) suggests the crystal as a racemic twin. Magnetic Measurements. Magnetic measurements were carried out with an MPMS-XL Quantum Design SQUID magnetometer on polycrystalline samples of 1 (16.98 mg) and 2 (17.0 mg), previously introduced in a sealed polypropylene bag (17.3 and 17.8 mg for 1 and 2, respectively). Prior to measurements, the field-dependent magnetization was measured at 100 K in order to confirm the absence of any bulk ferromagnetic impurity. The dc susceptibility data were corrected for diamagnetic contributions from the core diamagnetism of each sample and those from the sample holder.42 Other Physical Measurements. Elemental analysis was performed by microanalysis service at the Institut de Chimie des Substances Naturelles (CNRS UPR 2301, France). Differential scanning calorimetry was carried out using a NETZSCH DSC 200 F3 instrument equipped with an internal cooler at 10 K min−1.

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EXPERIMENTAL SECTION

Syntheses. H2acacen ligand, (F-Anil)BF4, and (Me-F-Anil)BF4 were prepared following literature procedures.36,23b [Mn(acacen)(H2O)2](PF6) was prepared by reacting Mn(CH3COO)3·2H2O, H2acacen, and KPF6 following a modified literature method.37 All other reagents were commercially available and used as received. [(F-Anil)(18-crown-6)][Mn(acacen)Fe(CN)5(CNH)]·MeOH (1). A colorless solution of 18-crown-6 (70.0 mg, 0.26 mmol) and (FAnil)BF4 (35.0 mg, 0.18 mmol) in methanol (2 mL) was added on a yellow solution of (Bu4N)3[Fe(CN)6] (41.4 mg, 0.044 mmol) in methanol (2 mL). The final yellow solution was left stirring for 1 h and was then treated dropwise and under stirring with a 2 mL methanolic brown solution of [Mn(acacen)(H2O)2](PF6) (15.0 mg, 0.035 mmol). The final brown solution was filtered through diatomaceous earth (Celite) and layered with diethyl ether. After 1 week, brown, blockshaped crystals of compound 1 were obtained in 30% yield. Anal. calcd (found) for 1·0.8H2O: C 48.67 (48.57), H 6.14 (6.01), N 13.81 (13.87). IR (cm−1): 2914 m, 2656 m, 2145 s, 2127 m, 2067 m, 1581 s, 1503 s, 1432 s, 1394 s, 1348 s, 1281 w, 1267 m, 1135 m, 1090 s, 951 s, 832 w, 790 w. [(Me-F-Anil)(18-crown-6)][Mn(acacen)(MeOH)Fe(CN)5 (CNH)]· MeOH (2). A colorless solution of 18-crown-6 (70.0 mg, 0.26 mmol) and (Me-F-Anil)BF4 (38.0 mg, 0.18 mmol) in methanol (2 mL) was added to a yellow solution of (Bu4N)3[Fe(CN)6] (41.4 mg, 0.044 mmol) in methanol (2 mL). The final yellow solution was left stirring for 1 h and was then treated dropwise under stirring with 2 mL of a methanolic brown solution of [Mn(acacen)(H2O)2](PF6) (15.0 mg, 0.035 mmol). The final brown solution was filtered using diatomaceous earth and layered with diethyl ether. After 1 week, brown, rodshaped crystals of compound 2 were obtained in 15% yield. Anal. calcd (found) for 2·0.9H2O: C 48.75 (48.00), H 6.48 (6.06), N 13.12 (13.92). IR (cm−1): 2915 m, 2660 m, 2147 s, 2120 m, 2069 m, 1586 s, 1502 s, 1431 s, 1391 s, 1351 s, 1280 w, 1244 m, 1090 s, 956 s, 833 w. X-ray Crystallography. Single crystals of 1 and 2 were mounted on a MicroMounts rod coated by Paratone-N oil and collected on a Bruker D8 Venture diffractometer equipped with Mo Kα radiation

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00983. Additional crystallographic, magnetic, DSC, and PXRD data for complexes 1 and 2 (PDF) Accession Codes

CCDC 1835843−1835844 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

*E-mail: [email protected]. ORCID

Ie-Rang Jeon: 0000-0001-5509-169X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the French National Research Agency Grant ANR 16-ACHN-0007 and the SAD Research Program from the Région Bretagne. We thank the CDFIX and the physical measurement platforms (Th. Guizouarn) in Rennes for the use of their X-ray diffractometer and SQUID magnetometer. I


Article

Inorganic Chemistry

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