Spin-Crossover Phenomenon in a Pentanuclear Iron(II) Cluster Helicate

Article pubs.acs.org/IC

Spin-Crossover Phenomenon in a Pentanuclear Iron(II) Cluster Helicate Zheng Yan,†,‡,§ Wei Liu,†,§ Yuan-Yuan Peng,† Yan-Cong Chen,† Quan-Wen Li,† Zhao-Ping Ni,*,† and Ming-Liang Tong*,† †

Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P. R. China ‡ College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, P. R. China S Supporting Information *

ABSTRACT: The first spin-crossover cluster helicate [{Fe II (μbpt) 3 } 2 Fe II 3 (μ 3 -O)][Fe II 2 (μ-Cl)(μ-bpt)(NCS) 4 (H 2 O)]·2H 2 O·C 2 H 5 OH [Hbpt = 3,5-bis(pyridin-2-yl)-1,2,4-triazole] was reported, in which one spincrossover unit and one low-spin triple-stranded [FeII(bpt)3]− unit wrapped a rare planar [FeII3(μ3-O)]4+ core to form a bis(triple-helical) cation with trigonal-bipyramidal topology. The origin of different magnetic behaviors of two [FeII(bpt)3]− units could be ascribed to their different intermolecular interactions.

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[{FeII(μ-L) 3}2FeII3(μ3-O)](NCS) 2·10H2O [L− = 3,5-bis(pyridin-2-yl)pyrazolate], in which a rare HS [FeII3O]4+ core was wrapped by two LS [FeIIL3]− units.10d By using 3,5bis(pyridin-2-yl)-1,2,4-triazole (Hbpt) as the ligand, we constructed four similar cluster helicates with trigonalbipyramidal topology in 2010.11 The spin states of two apical [FeII(bpt)3]− units could be tuned to the HS state by a [Fe2OCl6]2− anion, while there were still LS states with NCS−, ClO4−, and I− anions. This interesting phenomenon prompted us to introduce the SCO property into cluster helicates to explore multifunctional materials. Herein, we report the first SCO cluster helicate, [{FeII(μ-bpt)3}2FeII3(μ3-O)][FeII2(μCl)(μ-bpt)(NCS)4(H2O)]·2H2O·C2H5OH (1), in which one apical [FeII(bpt)3]− unit exhibits SCO behavior while the other [FeII(bpt)3]− unit remains the LS state.

INTRODUCTION The development of spin-state switching for advanced devices has attracted great attention.1 Toward this end, iron(II) spincrossover (SCO) materials are typical examples that can switch between high-spin (HS; S = 2 for 5T2) and low-spin (LS; S = 0 for 1A1) states under external stimuli (temperature, pressure, light, guest, etc.).2 Hence, they have potential for application in data storage, displays, and sensors. To date, most of them are mononuclear iron(II) compounds in zero-dimensional SCO systems.3 Meanwhile, a number of di- and tetranuclear iron(II) complexes were also studied to explore the multistep SCO effect.4 However, SCO clusters with higher iron(II) nuclearity are rare. To the best of our knowledge, there are only two examples of such clusters: a distorted rhombic dodecahedron, [FeII6{CuI(Tp4‑py)}8] {(Tp4‑py)− = tris[3-(4′-pyridyl)pyrazol-1yl]hydroborate},5 and a six-capped body-centered cubic, [FeII9{ReV(CN)8}6].6 As a switching individual unit, such a nanoscale SCO cluster can be potentially coated on a solid surface toward practical application.7 The construction of multimetallic helicates is a fascinating topic in the field of supramolecular coordination chemistry, which has been pursued as components of functional materials, devices, and machines.8 Apart from some linear and circular helicates,8b,c,9 there are only a few examples of cluster helicates, in which the metal centers form a polyhedron around the helical axis.8e,10 This new variety has the possibility of introducing the novel properties of clusters into the helicates, as pointed out by Bermejo et al. in 2005.10b In 2006, Kawata et al. reported the first pentanuclear bis(triple-helical) compound, © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials and General Procedures. The Hbpt ligand was purchased from Jinan Camolai Trading Company. The other reagents and solvents were commercially available and were used without further purification. Synthesis of 1. An ethanol solution (10 mL) of FeCl2·4H2O (20 mg, 0.10 mmol), Hbpt (23 mg, 0.10 mmol), and KNCS (30 mg, 0.30 mmol) was sealed in a 25 mL Teflon-lined reactor. It was heated at 160 °C for 3 days and cooled to room temperature at 5 °C/h. Red crystals of 1 were obtained in 57% yield based on iron. IR (KBr, cm−1): 3407 (s, br), 3070 (w), 2059 (m, NCS−), 1600 (s), 1498 (w), Received: February 19, 2016

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DOI: 10.1021/acs.inorgchem.6b00416 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 1457 (w), 1417 (m), 1349 (vs), 1261 (w), 1193 (w), 1020 (w), 908 (w), 788 (w), 728 (w), 713 (m). MS (positive ESI; CH3OH): m/z 814.73 ([{FeII(μ-bpt)3}2FeII3(μ3-O)]2+; Figure S1, Supporting Information). Elem anal. Calcd: C, 46.39; H, 2.94; N, 23.44. Found: C, 46.04; H, 2.71; N, 23.42. Physical Measurements. Fourier transform infrared spectroscopy was performed on a Bio-Rad FTS-7 spectrometer at room temperature in the range of 4000−400 cm−1. Electrospray ionization mass spectrometry (ESI-MS) for 1 in methanol was carried out on a Thermo-Finniga LCQ DeCA XP quadrupole ion-trap mass spectrometer with an electrospray ionization source. Data were analyzed by the spectrometer software MassLynx NT (version 3.4). Elemental analysis was measured with an Elementar Vario-EL CHN elemental analyzer. The kinds and amounts of solvents were determined under a N2 atmosphere with a heating rate of 10 K/min by a thermogravimetric mass spectrometer (STA449 F3 Jupiter-QMS 403C aedo). 57Fe Mössbauer spectra of 1 were collected by the transmission geometry using a Mössbauer spectrometer operating at a constant acceleration mode and equipped with a 50 mCi 57Co(Rh) source. Magnetic data were measured with a Quantum Design superconducting quantum interference device (SQUID) magnetometer (MPMS-XL) under a magnetic field of 1000 Oe with a sweeping rate of 2 K/min. The diamagnetic correction was obtained from Pascal’s constants. Meanwhile, the background of the sample holder was experimentally determined. Crystal Structure Determination. Single-crystal X-ray diffraction (XRD) data were collected on a Rigaku R-Axis Spider IP diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 150(2) and 293(2) K. Their structures were solved by direct methods in the SHELXTL program, and all non-hydrogen atoms were anisotropically refined by least squares on F2. Hydrogen atoms on the organic ligands were introduced geometrically in calculated positions and then refined on a riding model.12a The disordered water and ethanol molecules at 293 K and disordered water at 150 K could not be modeled properly; thus, we used the program SQUEEZE,12b a part of the PLATON package of crystallographic software, to calculate the disordered area and remove their contributions to the overall intensity data. CCDC 999094 (1-150 K) and 999095 (1-293 K) contain the 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.

Table 1. Crystal Data for 1 value/comment

a

parameter

150(2) K

293(2) K

chemical formula Mr cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Rint Z μ(Mo Kα)/mm−1 R1 [I > 2σ(I)]a wR2 (all data)b indep reflns indep reflns [I > 2σ(I)] GOF

C90H64Fe7N39O3ClS4 2294.44 triclinic P1̅ 16.9133(9) 17.0550(8) 18.8694(11) 87.496(2) 79.802(2) 63.501(1) 4790.4(4) 0.0610 2 1.218 0.0858 0.2743 21095 11872 1.095

C88H58Fe7N39O2ClS4 2248.37 triclinic P1̅ 16.9293(12) 17.1187(11) 19.0528(14) 87.996(2) 79.695(2) 63.911(2) 4873.1(6) 0.0645 2 1.195 0.0862 0.2731 21237 8561 0.978

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

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RESULTS AND DISCUSSION Crystal Structure. Single-crystal XRD measurements reveal that 1 crystallizes in the triclinic P1̅ space group at 293 and 150 K (Table 1). At a glance, the asymmetric unit at 293 K contains one [{FeII(μ-bpt)3}2FeII3(μ3-O)]2+ cation and one [FeII2(μCl)(μ-bpt)(NCS)4(H2O)]2− anion (Figure 1). The disordered water and ethanol molecules at 293 K and the disordered water at 150 K were removed by SQUEEZE to achieve reasonable refinements, which were determined by thermogravimetry− mass spectroscopy (TG−MS) and elemental analysis (Figure S2, Supporting Information). The geometry of a pentanuclear cation is similar to those of our previous works, in which two terminal triple-stranded [FeII(bpt)3]− units wrapped a rare planar [FeII3(μ3-O)]4+ core to form a bis(triple-helical) complex with trigonal-bipyramidal topology. The rigid bisbidentate bpt− ligand links one apical ion and one equatorial iron(II) ion in a cis bridging mode. Six bpt− ligands wrap around the pentanuclear cluster with offset face-to-face π−π interactions, giving rise to two homochiral [FeII(bpt)3]− units. The apical Fe1 and Fe2 ions are located in distorted octahedral environments with six nitrogen atoms from three bpt− ligands. The equatorial Fe3−Fe5 ions are all in the distorted N4O trigonal-pyramidal geometry, where four nitrogen atoms come from the remaining coordination sites of two bpt− ligands. The

Figure 1. Structure illustrations of the pentanuclear cation (left) and binuclear anion (right) at 293 K. Color code: orange, HS FeII; brown, LS FeII; blue, N; gray, C; yellow, S; green, Cl. Hydrogen atoms are omitted for clarity.

values of the trigonality index τ [=(θ1 − θ2)/60,13 where θ1 and θ2 are defined as the two largest L−Fe−L angles in the coordination geometry] are 0.61, 0.57, and 0.73 for Fe3−Fe5 at 293 K, respectively (Table S1, Supporting Information). Theoretically, the τ values for a perfect trigonal bipyramid and a square pyramid are 1 and 0, respectively.13 The Fe6 and Fe7 ions of the dinuclear anion have different coordination numbers and are bridged by the bpt− ligand and a chloride ion. The Fe6 ion is located in a distorted N4Cl square-pyramidal environment (τ = 0.45 at 293 K) formed by one bridging chloride ion, one bpt− ligand, and two NCS− ligands. Meanwhile, the Fe7 ion adopts an N4ClO octahedral geometry defined by one water and one NCS− ligand at axial positions and the other NCS− ligand and the bridging bpt− and Cl− ligands in the equatorial plane. B


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Inorganic Chemistry Two dinuclear anions with symmetry codes ′ = x, 1 + y, z and ″ = 1 − x, 1 − y, −z are linked by offset face-to-face π···π interactions between two bpt− ligands (the shortest distance is C80′···C76″ = 3.37 Å at 293 K) and hydrogen bonding between the bpt− and NCS− ligands (C76′···N38″ = 3.38 Å) to form an anion dimer. Meanwhile, a pair of cationic enantiomers that have opposite chirality are present on either side of the anion dimer to form a racemic tetramer unit (Figure 2). The

for the Fe2 ion as the temperature is decreased from 293 to 150 K. Meanwhile, it displays relatively small Σ values for the Fe1 ion. They are consistent with the formation of a more regular octahedron in the LS state. Magnetic Properties. Magnetic susceptibility measurements for 1 were performed in the temperature range of 2−300 K (Figure 3a). The χMT value at 300 K is 14.26 cm3·K/mol,

Figure 2. (a) Abundant intermolecular interactions yielding a tetramer in the structure of 1 at 293 K. The Δ,Δ-configuration (right-handed, P) and Λ,Λ-configuration (left-handed, M) are shown in yellow and blue strands, respectively. (b) Space-filling representations of the two enantiomers present in 1 at 293 K. Hydrogen atoms are omitted for clarity.

pentanuclear cation links one anion through hydrogen bonding (N10···O1W′ = 2.84 Å and C71···S1′ = 3.86 Å) and offset faceto-face π···π interactions between two bpt− ligands (the shortest distance is C15···C77′ = 3.17 Å) and the other anion through hydrogen bonding (C47···S2″ = 3.89 Å and C2···Cl1″ = 3.77 Å) and C3···N37″ contact (3.17 Å) (Table S2, Supporting Information). The tetramer units were further packed into a three-dimensional racemic supramolecular structure through various intermolecular hydrogen bondings and π···π interactions (Figure S3 and Table S2, Supporting Information). The details of the bond distance and distortion parameter around an octahedral iron(II) ion are effective to construct the magnetostructural relationships (Table 2).2d,14 At 293 K, the

Figure 3. (a) Plots of χMT versus T for 1 (black squares) and the [FeII3(μ3-O)]4+ core (gray squares). The latter are the magnetic data of [{FeII(μ-bpt)3}2FeII3(μ3-O)](NCS)2·10H2O. The purple line represents the χMT value of 1 after subtraction of the contribution of the [FeII3(μ3-O)]4+ core. The purple curve between 30 and 110 K is fitted by a dinuclear S = 2 model and extrapolated to the higher-temperature region, which represents the contribution of the dinuclear anion (cyan line). (b) χMT value of the Fe2 ion obtained by subtraction of the contributions of the [FeII3(μ3-O)]4+ core and the dinuclear anion from 1.

Table 2. Fe−N Bond Lengths (Å) and Octahedral Distortion Parameters Σ (deg) for 1 at 150 and 293 K 1

Fe1−Nav

ΣFe1

Fe2−Nav

ΣFe2

150 K 293 K change

1.99 2.01 0.02

60.9 65.8 4.7

2.01 2.17 0.16

63.7 93.7 30.0

which is smaller than the value (18 cm3·K/mol) expected for six free HS ions and one LS iron(II) ion. On the whole, the χMT value smoothly decreases upon cooling with two little bulges around 210 and 70 K and then descends more steeply to 0.84 cm3·K/mol at 2 K, suggesting obvious antiferromagnetic coupling. Such magnetic behavior is repeatable in the subsequent heating mode (Figure S4, Supporting Information). As mentioned above, X-ray crystal structure analyses clearly indicate that only the Fe2 ion undergoes the SCO process. To make this SCO behavior apparent, the χMT contributions of the [FeII3(μ3-O)]4+ core and dinuclear anion can be subtracted by the following two steps. First, the experimental χMT value of the [FeII3(μ3-O)]4+ core can be obtained from a similar cluster, [{FeII(μ-bpt)3}2FeII3(μ3-O)](NCS)2·10H2O, in which two apical iron(II) ions are in the LS state.11 Second, after the

average Fe1−N and Fe2−N bond distances are 2.01 and 2.17 Å, corresponding to the LS and HS states, respectively. At 150 K, they decrease to 1.99 and 2.01 Å, respectively, suggesting that both are in the LS states. It clearly indicates that the Fe1 ion remains the LS state while the Fe2 ion undergoes the SCO process. The value of the octahedral distortion parameter Σ, defined as the sum of absolute deviations between real and ideal values of 12 cis N−Fe−N angles,15 changes from 93.7° to 63.7° C


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Inorganic Chemistry

core, HS2 and HS3 are assigned to iron(II) ions in the [FeII2(μCl)(μ-bpt)(NCS)4(H2O)]2− anion, and LS1 corresponds to two apical iron(II) ions. At 290 K, an additional peak appears in the Mössbauer spectrum. Then the spectrum is fitted by five quadrupole doublets (HS1−HS4 and LS1 in red, orange, green, pink, and blue, respectively). The peak area ratio of HS1, HS2, HS3, HS4, and LS1 is fixed at 3:1:1:1:1 to avoid overparameterization. The parameters of four doublets (HS1, HS2, HS3, and HS4) are δ = 0.90 and ΔEQ = 2.31 mm/s, δ = 1.22 and ΔEQ = 1.84 mm/s, δ = 0.79 and ΔEQ = 2.40 mm/s, and δ = 0.99 and ΔEQ = 0.84 mm/s, respectively. Meanwhile, the doublet LS1 has Mössbauer parameters of δ = 0.40 and ΔEQ = 0.20 mm/s. Consequently, HS1 corresponds to the [FeII3(μ3O)]4+ core, while HS2 and HS3 correspond to the [FeII2(μCl)(μ-bpt)(NCS)4(H2O)]2− anion. LS1 and HS4 can be assigned to apical LS Fe1 and SCO-active Fe2 ions, respectively. Thus, the Mössbauer measurements also confirm the SCO behavior of 1, as suggested by structural and magnetic studies. Discussion. To investigate the origin of different spin states for two apical [FeII(bpt)3]− units, the structure−function relationships should be explored in greater detail. It is wellknown that the SCO behavior is dependent on the nature of the ligands, uncoordinating counterions, solvent molecules, and crystal packing.2d,14,17 Although these two apical iron(II) ions are coordinated by the same three bpt− ligands, our previous work had already shown that their spin states could be tuned by different counterions. Both of them are in the HS states with a [Fe2OCl6]2− anion and in the LS states with NCS−, ClO4−, or I− anions. Here, two apical [FeII(bpt)3]− units have different spin states at room temperature. It provides a good example to explore the influence of crystal packing on the SCO behavior in one crystal,17 although two disordered water molecules cannot be solved for complete investigation. As shown in Table S2 (Supporting Information), they indeed exhibit different crystal packing with different intermolecular interactions around two apical [FeII(bpt)3]− units. At 293 K, the most significant interaction is a strong intermolecular O−H···N hydrogen bonding (O1W′···N10 = 2.84 Å) between a water molecule coordinated to Fe7 in the anion unit and a triazole group in a bpt− ligand coordinated to Fe1. This bpt− ligand also involves strong offset face-to-face π···π interactions with the bpt− ligand in the anion unit (the shortest distance is C15···C77′ = 3.17 Å). Meanwhile, the intermolecular hydrogen bonding (C2···Cl1″ = 3.77 Å) and short contacts (C3···N37″ = 3.21 Å and C27···S2 = 3.40 Å) are observed between the other two bpt− ligands around Fe1 and the respective nearby anion units. Moreover, the pyridine parts in three bpt− ligands coordinated to the [FeII3(μ3-O)]4+ core also involve intermolecular interactions, as shown in Table S2 (Supporting Information). However, their influence on the crystal-field strengths should be weaker than those of the pyridine and triazole parts coordinated to Fe1 as mentioned above, whereas completely different intermolecular interactions are observed around the Fe2 units. The bpt− ligand, which is parallel to the bpt− ligand coordinated to Fe1 with strong hydrogen bonding and π···π interactions, involves short contact [C62···S1 (−x, 1 − y, 1 − z) = 3.77 Å] with the nearby anion and offset face-to-face π···π interactions [the shortest distance is C66···C70 (−x, 2 − y, 1 − z) = 3.17 Å] with the nearby cation. The other two bpt− ligands involve intermolecular hydrogen bonding [C40···S3 (−x, 1 − y, −z) = 3.63 Å, C49···S1 (−x, 1 − y, 1 − z) = 3.74 Å, and C50···Cl1 (−x, 1 − y, 1 − z) = 3.78 Å] with the nearby anions and offset

above subtraction, the magnetic susceptibility plot between 30 and 110 K (Fe1 and Fe2 are LS states) can be fitted to the Van ̂ ·SFe7 ̂ ),16 Vleck equation by the spin Hamiltonian Ĥ = −2J(SFe6 −1 giving the parameters of JFe6−Fe7 = −3.40(5) cm , g = 2.16(1), and R2 = 0.997. Then, it is extrapolated to the highertemperature region to obtain the contribution of the dinuclear anion. Because the intramolecular magnetic coupling between the SCO-active Fe2 ion and the central [FeII3(μ3-O)]4+ core and intermolecular magnetic interactions between the anions and cations are very weak,11 they can be omitted for simplification. Moreover, the Fe1 ion remains the LS state in the whole temperature region. Thus, the magnetic susceptibility data of only the Fe2 ion are obtained and shown in Figure 3b. The χMT value is 3.77 cm3·K/mol at 300 K, corresponding to the HS state of the Fe2 ion. Upon cooling, gradual and complete SCO behavior is clearly shown with the spin transition temperature T1/2 = 193 K. 57 Fe Mö ssbauer Spectroscopy. The spin states of the iron(II) ions were further characterized by 57Fe Mössbauer spectra at 20 and 290 K (Figure 4). The Mössbauer parameters

Figure 4. 57Fe Mössbauer spectra of 1 at 20 and 290 K.

relative to iron metal were calculated and summarized in Table S3 (Supporting Information). The spectrum at 20 K is composed of four doublets (HS1−HS3 in red, green, and orange, respectively, and LS1 in blue). The peak-area ratio of HS1, HS2, HS3, and LS1 is free to fit, which is very close to 3:1:1:2. The parameters of the isomer shift δ and quadrupole splitting ΔEQ for HS1−HS3 are δ = 1.02 and ΔEQ = 2.35 mm/ s, δ = 0.97 and ΔEQ = 2.63 mm/s, and δ = 1.11 and ΔEQ = 2.34 mm/s, respectively, which are typical values for the HS iron(II) ions. Meanwhile, they are δ = 0.48 and ΔEQ = 0.24 mm/s for LS1, which are characteristic of LS iron(II) ions. Consequently, HS1 is assigned to three iron(II) ions in the [FeII3(μ3-O)]4+ D


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Inorganic Chemistry face-to-face π···π interactions [the shortest distance is C52··· N25 (−x, 2 − y, 1 − z) = 3.77 Å] with the nearby cation. The ethanol molecule in the structure at 150 K reveals that it involves hydrogen bonding (C58−H···O2 = 3.336 Å) with the pyridine part coordinated to Fe3. Obviously, the total influence of all intermolecular interactions around the Fe1 unit is stronger than that around the Fe2 unit, giving rise to the different spin states for two apical [FeII(bpt)3]− units at room temperature.

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CONCLUSIONS In summary, by the introduction of an unsymmetrical binuclear anion, [FeII2(μ-Cl)(μ-bpt)(NCS)4(H2O)]2−, to the pentanuclear cluster helicate, different magnetic behavior of the two apical [FeII(bpt)3]− units in the cationic cluster with trigonalbipyramidal topology can be obtained, in which one exhibits the SCO property while the other remains the LS state. The origin of different spin states can be explained from their different strengths of intermolecular interactions. Then, to the best of our knowledge, 1 represents the first case of a SCO cluster helicate, which provides an excellent example to introduce the SCO property into cluster helicates to explore multifunctional materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00416. ESI-MS spectrum, structural illustrations, magnetic data, TG−MS spectra, and Mössbauer parameters (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

(Z.Y., W.L.) These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “973 Project” (Grant 2012CB821704), NSFC (Grants 91122032, 21201182, 21373279, 21501067, and 21121061), and Program for Changjiang Scholars and Innovative Research Team in University of China and the Zhejiang Provincial Natural Science Foundation of China (Grant LQ15B010002).

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Spin-Crossover Phenomenon in a Pentanuclear Iron(II) Cluster Helicate

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