DOI: 10.1002/ejic.201700609 Communication

Multifunctional Molecular Materials

Three Properties in One Coordination Complex: Chirality, SpinCrossover, and Dielectric SwitchingJing Ru,[a][‡] Fei Yu,[a][‡] Ping-Ping Shi,[b] Cheng-Qi Jiao,[c] Cheng-Hui Li,*[a]

Ren-Gen Xiong,*[b] Tao Liu,[c] Mohamedally Kurmoo,[d] and Jing-Lin Zuo*[a]

Abstract: In the search for multifunctional molecular materials,we designed an enantiopure pair of FeII complexes with chiral,chelating (R)/(S)-4,5-pinenepyridyl-2-pyrazine ligands exhibitingthree inherent physical properties: chirality, spin crossover, anddielectric switching. The geometrical change resulted in cou-pling of the spin-crossover transition at 187 K to dielectric

IntroductionDriven by the demands in the information and energy-storageindustries, multifunctional materials, especially those exhibitingcoupled properties that can be controlled externally by temper-ature, pressure, light, or an electric or magnetic field, are highlydesirable. The last 20 years has witnessed the design and char-acterization of numerous bifunctional materials that werepurely inorganic (e.g., metal oxides) or organic in nature or thatwere metal–organic hybrids.[1] However, reports on materialswith three or more properties are still sparse in the literature.[2,3]

Combining multiple properties into a single material is quitechallenging, as some properties are based on molecular struc-ture (e.g., chirality, spin crossover, luminescence), whereas oth-ers are based on crystal structure (e.g., porosity, chirality, dielec-tricity, ferroelectricity, ferroelasticity, and ferromagnetism), orboth. Therefore, judicious integration of functional buildingblocks and precise control of their molecular self-assembly areboth crucial. In this study, we aim to combine molecular-levelproperties (e.g., chirality and spin crossover) with crystal-struc-

[a] State Key Laboratory of Coordination Chemistry, School of Chemistryand Chemical Engineering, Collaborative Innovation Center of AdvancedMicrostructures, Nanjing University,Nanjing 210093, ChinaE-mail: chli@nju.edu.cn

zuojl@nju.edu.cnhttp://hysz.nju.edu.cn/zuojl/

[b] Ordered Matter Science Research Center and Jiangsu Key Laboratory forScience and Applications of Molecular Ferroelectrics, Southeast University,Nanjing 211189, ChinaE-mail: xiongrg@seu.edu.cn

[c] State Key Laboratory of Fine Chemicals, Dalian University of Technology,2 Linggong Rd., Dalian 116024, China

[d] Institut de Chimie de Strasbourg, CNRS UMR7177,Université de Strasbourg,4 rue Blaise Pascal, 67070 Strasbourg, France

[‡] These authors contributed equally to this work.Supporting information and ORCID(s) from the author(s) for this article areavailable on the WWW under https://doi.org/10.1002/ejic.201700609.

Eur. J. Inorg. Chem. 2017, 3144–3149 © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3144

switching. Both compounds were found to display light-in-duced spin crossover that was thermally unstable above ca.40 K. The coupling of two hysteretic properties, electric andmagnetic, renders these compounds as potential switching de-vices.

ture-related properties (e.g., dielectric switching) into a single-component coordination complex (see Scheme 1). Such a com-bination is very important, as the existence of spin crossoverand dielectric switching would open interesting prospects, asthey are two physical principles widely used for informationstorage and processing, whereas magnetic bistability coupledto chirality would lead to a magnetochiral dichroic effect.[4]

Scheme 1. Choice of components for targeted molecule with combinedchirality, spin crossover, and dielectricity.

Spin crossover (SCO) is a well-known phenomenon in whichd4 to d7 transition-metal ions can change their spin states as afunction of temperature, light, pressure, magnetic or electricfield, or charge flow if the coordinating atoms provide a moder-ate crystal field.[5] In addition to a SCO transition, dramaticchanges in properties (e.g., magnetism, color, fluorescence, di-electric constant, and structural parameters) are frequently ob-served.[6] Materials displaying such properties have attracted in-terest because of their applications in sensing, data storage,information processing, and display devices.[7] Although bifunc-tional SCO materials are known, a three-in-one molecular mate-rial with chirality, spin crossover, and dielectric switching hasbeen elusive.[8]

In 2006, photoswitching of the dielectric constant was re-ported for the SCO complex [Fe(L)(CN)2]·H2O {L = 2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo(12.3.1)octadeca-1(18),2,12,14,16-pentaene}, which is an important example of a photo-, electro-

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, and magnetoresponse multifunctional material.[9] More re-cently, chiral FeII tetrahedral cages with spin-crossover close toroom temperature were also studied.[10] Enantiopure SCO com-pounds may offer new physical properties, for example, magne-tochiral dichroism.[4] Thus, to achieve this goal one method ofdesigning chiral SCO compounds is to construct coordinationcomplexes from paramagnetic centers with a d4 to d7 electronicconfiguration and suitable chiral ligands.[11] We therefore em-ployed pinene groups as chiral sources, as they were proven tobe effective for deriving chiral functional materials.[12] On theother hand, octahedral coordination compounds of Fe2+ withintermediate crystal-field ligands (often involving Fe–N bond-ing) generally exhibit a change from high spin (HS) (S = 2) athigh temperature to low spin (LS) (S = 0) at low temperature.[13]

For example [FeII(L)(bpz)2] [L = 2,2-bipyridine or 1,10-phen-anthroline, bpz = bis(1-pyrazolyl)borohydride] was shown to ex-hibit SCO properties.[14] In addition, SCO-active derivativesbased on the bipyridyl ligand with radical or photoactive pen-dant groups were also reported.[15] These findings inspired thepresent study on the design of new FeII materials in the searchfor coupled physical properties. Herein, a pair of chiral FeII com-plexes, [Fe{(R)-L1}(bpz)2] (1R) and [Fe{(S)-L1}(bpz)2] (1S), weresynthesized from enantiomerically and optically pure (R)/(S)-L1

ligands (Figure 1a), Kbpz, and FeSO4·7H2O. Subsequently, thespectroscopic, magnetic, and dielectric properties of complexes1R and 1S were investigated. The two chiral compounds werefound to exhibit coupled spin-crossover transition with dielec-tric switching and a metastable excited high-spin state below40 K. In addition, we constructed an isostructural pair, 2R and2S (Figure S1 in the Supporting Information), by replacing (R)/(S)-4,5-pinenepyridyl-2-pyrazine with (R)/(S)-4,5-pinenepyridyl-2-pyridine [(R)/(S)-L2] (Figure 1a). This subtle modification tunedthe ligand field around the Fe centers and led to drastic conse-quences in the SCO properties.

Figure 1. (a) (R)/(S)-4,5-pinenepyridyl-2-pyrazine [(R)/(S)-L1] and (R)/(S)-4,5-pinenepyridyl-2-pyridine [(R)/(S)-L2]. (b) Overlap crystal structures of com-plexes Fe{(R)-L1}(bpz)2 (1R) and Fe{(S)-L1}(bpz)2 (1S) at 123 K (green) and296 K (pink); H atoms are omitted for clarity.

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Results and Discussion

Crystal Structures

Compounds 1R and 1S are a pair of enantiomers and crystallizein the same chiral, lowest-symmetry P1 space group at both123 and 296 K (Table S1). Their asymmetric units contain twoalmost-identical molecules, each with one Fe, two bpz, and one4,5-pinenepyridyl-2-pyrazine. Compounds 2R and 2S are iso-structural with closely related parameters. There is no latticesolvent molecule. Selected bond lengths, bond angles, andstructural parameters for 1R and 1S are given in Tables S2 andS3.

Given that the two pairs of compounds have the same struc-tures related by a mirror reflection, only the crystal structure of1R will be described in detail (Figure 1b). The central FeII ion hasa distorted octahedral geometry with six coordinating nitrogenatoms from two chelating bpz anions and one chelating (R)-L1

ligand. Both molecules within the asymmetric unit adopt the Δ

and Λ configurations for 1R and 1S, respectively. The averageFe–N bond length at 123 K is 1.991 Å, which corresponds toFeII in its low-spin (LS) state. At 296 K, this length increases to2.18 Å, and the increase of 0.189 Å for each molecule is compa-rable to that (0.2 Å) found for a complete LS-to-HS transforma-tion (Table S2).[16] Besides the change in the Fe–N bond lengths,the coordination environment of FeII is significantly modifiedduring the spin transition. At both 123 and 296 K, several struc-tural parameters, including the continuous shape measures(CShMs) relative to the ideal octahedron obtained by using theSHAPE 2.1 program,[17a] the average trigonal distortionΦ,[17b,17c] and Σ,[17d] were calculated and are listed in Table 1.All the CShMs and Σ and Φ parameters at 296 K are signifi-cantly larger than those at 123 K, which indicates more distinctdistortion of the coordination octahedron at 296 K. Thechanges in these parameters are consistent with the SCO transi-tion and are also in line with the spin-state assignments ofthese two compounds found in their magnetic properties (seebelow). The crystal packing can be described as layers of neu-tral molecules lying in the 011 plane (Figure S2).

Table 1. Selected bond lengths [Å] and structural parameters of 1R and 1Sat various temperatures.

1R 1R 1S 1S(296 K) (123 K) (296 K) (123 K)

Fe1–N1 2.223(9) 1.899(13) 2.193(9) 1.929(7)Fe1–N3 2.191(9) 1.992(12) 2.173(9) 1.950(7)Fe1–N4 2.210(10) 1.977(12) 2.221(9) 2.014(7)Fe1–N6 2.153(9) 1.992(12) 2.163(9) 1.991(7)Fe1–N8 2.205(9) 2.035(13) 2.181(10) 2.045(7)Fe1–N10 2.159(10) 2.051(13) 2.163(9) 2.025(8)Fe–Nav 2.19 1.99 2.18 1.99CShM[a] 0.860 0.353 0.841 0.310ΣFe[b] 52.5 36.84 51.13 37.70Φ[c] 83.49 64.02 84.25 61.24

[a] CShM: continuous shape measure relative to ideal octahedron. [b] ΣFe:the sum of the deviation from 90° of the 12 cis angles of the FeN6 octa-hedron. [c] Φ: the sum of the deviation from 60° of the 24 trigonal angles ofthe projection of the FeN6 octahedron onto its trigonal face.

Careful analysis of the supramolecular interactions within thecrystal structure of 1R was performed, as their types and

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strengths are responsible for the presence of hysteresis in theSCO transition. The neighboring molecules of 1R (123 K) areweakly connected into pseudodimers through the C54···N2contact (3.127 Å, ∠C54–H54···N2 97.90°) and π···π interactionsbetween the pyridyl pyrazine (Figure S3a). These pseudodimersare then linked to each other through C–H···π interactions(dC52···Cg′ = 3.322 Å, ∠C52–H52···Cg′ = 107.54°, in which Cg′ de-notes the centroid of the pyrazine) (Figure S3a). As expected,the C–H···N and π···π interactions become weaker upon switch-ing from LS to HS (some related parameters in the HS stateare listed in Table S4). Although the intermolecular distancesbetween the nearest neighbors are short, peripheral overlap ofthe molecules would result in a very weak interaction (Fig-ure S3b). Therefore, we can consider that these weak interac-tions are enough to hold the molecules in the crystal but arenot enough to influence each other electronically. Because ofthe limited supramolecular interactions, the hysteresis of theSCO transition is very narrow (see below).

Circular Dichroism and Vibrational Circular Dichroism

The circular dichroism (CD) spectra in KBr pellets confirm theoptical activity and enantiomeric nature of 1R and 1S (Fig-ure S4). The CD spectrum of 1R (R enantiomer) exhibits a nega-tive Cotton effect at λmax = 299 nm and a positive signal cen-tered at 330 nm, whereas 1S (S enantiomer) shows a positiveCotton effect and a negative signal at the same respectivewavelengths. Extending the CD into the infrared region of thespectrum to study the vibrational transitions, vibrational circulardichroism (VCD), as a powerful technique in the structural anal-ysis of chiral molecules, was employed.[18] The IR spectra of thetwo enantiomers are almost the same, whereas the VCD spectraare nearly mirror images (Figure 2) of each other. The mid-IRregion is dominated by C–H deformations (1500, 1474, 1402,

Figure 2. VCD (top) and absorption (bottom) infrared spectra of 1R (purple)and 1S (green).

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1293, 1200 cm–1 for 1R and 1S) and are of opposite rotation inthe corresponding VCD spectra.

Magnetic Properties

The temperature dependence of the magnetic moment repre-sented as the product of the susceptibility and temperature,�MT, for 1R (Figure 3) and 1S (Figure S5) are displayed. The valueof �MT, weakly temperature dependent from 3.85 cm3 K mol–1

at 300 K to 3.70 cm3 K mol–1 at 250 K, indicates a HS state.Below this temperature, �MT decreases much more quickly andattains a value of 0.01 cm3 K mol–1 at 100 K. This value is con-sistent with all molecules being in the LS (S = 0) state. In theheating mode, the magnetic behavior does not exactly matchthat of the cooling mode, and a narrow hysteresis loop ca. 2 Kwide is observed (T1/2↓ = 187 K, T1/2↑ = 189 K) at a sweep rateof 10 K min–1. The hysteresis width narrows for slower tempera-ture sweep rates of 2 and 5 K min–1 (Figure S6). The smallhysteresis is a clear indication of a weak intermolecular inter-action, as discussed above, and consequently, the changes inspin state and consequent size are governed by each moleculeand not by an ensemble of molecules. Therefore, it is easy tochange the spin reversibly. The heat involved is therefore ab-sorbed/released by the sum of those of each individual mol-ecule. It is important to note that the purity and crystallinity ofthe bulk samples were confirmed by using powder X-ray diffrac-tion (Figure S7).

Figure 3. The �MT versus T plot for 1R under 1 kOe dc field and after lightirradiation (red squares). The decrease in the �MT value with decreasing tem-perature indicates spin crossover from FeII(HS) (S = 2) to FeII(LS) (S = 0). In-verted triangle (green) and triangle (blue) denote �MT values with decreasingand increasing temperatures, respectively. The dielectric constant (ε′) versusT plot for 1R is shown (purple line).

Similar measurements for 2R reveal weak dependence of themoment with temperature and no anomaly. At low tempera-ture, there is a gradual decrease that may be associated with achange in the spin state or a weak antiferromagnetic inter-action. Details of the magnetic data are given in Figure S8.

Photoexcitation measurements on 1R (Figure 3) and 1S (Fig-ure S5) indicate the occurrence of the LIESST (light-induced ex-cited spin-state trapping) effect.[19] The samples were cooled to10 K and were irradiated with a 532 nm laser (P = 30 mW cm–2)for 1.5 h. After reaching photoexcited magnetic saturation, the

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laser was switched off. Under such treatment, some LS FeII ionsare converted into their HS* FeII states.[20] Upon heating thesample from 2 K after irradiation, the �MT value increases ab-ruptly and reaches its maximum value (2.1 cm3 mol–1 K for 1Rand 2.7 cm3 mol–1 K for 1S) at 21 and 25 K, which are 57 and73 % of the values (3.75 cm3 mol–1 K) measured at 300 K. Asthe temperature is increased further, the �MT value decreasesgradually and reaches 0.1 and 0.2 cm3 K mol–1 at 48 and 50 K,respectively, owing to relaxation of the HS* to LS FeII ions.[20]

According to the first derivative of the �MT versus T curves,T(LIESST) is 48 and 50 K, respectively.[21] The measurements indi-cate that magnetization can be increased by light irradiationand can be recovered by thermal treatment.

Temperature-Dependent UV/Vis Absorption

As the temperature is lowered, solutions of 1R and 1S changecolor slowly from deep blue to green and the UV/Vis absorptionspectra exhibit an increase in the intensity of the bands at410 nm (band I) and 670 nm (band II), which are assigned tothe 1A1→1T2 and 1A1→1T1 transitions on the FeII

LS (S = 0) site,respectively (Figure 4).[19d] Therefore, the transition from thehigh-T form to the low-T form in the �MT versus T plots is dueto spin crossover from FeII

HS to FeIILS. Crystals of 2R and 2S

and their solutions are red in color at room temperature. Thedifference indicates a change in the crystal-field strength frompyrazine to pyridine.[5]

Figure 4. Temperature dependence of the UV/Vis absorption spectrum from293 to 178 K. Inset: schematic illustration for the ground and excited statesof FeII. Arrows indicate the d–d transitions of band I (1A1→1T2) and band II(1A1→1T1) on FeII(LS).

Differential Scanning Calorimetry and Dielectric

The associated heats of transformation with the thermally in-duced SCO transitions in 1R and 1S (Figure S9) were measuredby differential scanning calorimetry (DSC) at a sweep rate of10 K min–1. The temperature dependence of the resultant ex-cess of heat capacity, ΔCp, in the cooling and heating modesfor 1R is shown in Figure S9. The enthalpy changes (ΔH) andthe critical temperatures in the cooling and heating modes are

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5.12 kJ mol–1 (T1/2↓ = 187 K) and 5.37 kJ mol–1 (T1/2↑ = 189 K).Accordingly, the entropy changes (ΔS) are estimated to be27.44 J mol–1 K–1 upon cooling and 28.53 J mol–1 K–1 upon heat-ing, given that ΔS = ΔH/T1/2. These observed peaks representa reversible phase transition with a thermal hysteresis of 2 K, inagreement with the magnetic measurements.

The temperature dependence of the real part (ε′) of the com-plex dielectric constant (ε = ε′ – iε′′; ε is the complex dielectricconstant, whereas ε′ and ε′′ are the real and imaginary parts ofε) of compound 1R reveals an anomaly at approximately 200 Kupon cooling at high frequencies (1 MHz), which indicates thata structural phase transition occurs near this temperature as aresult of spin crossover. The structure change is confirmed X-ray crystallography (Table 1) and the DSC analysis stated above.As shown in Figure 3 for 1R and Figure S5 for 1S, the dielectricconstant (ε′) in the LS state is smaller than that in the HS statewith Δε′ = ε′HS – ε′LS = 0.58, which indicates a very interestingand important dielectric bistable existence. As we are aware,this phenomenon observed in 1R makes it unprecedentedmagnet-electric bi-bistable.[22] No clear shifts to higher temper-atures are observed upon increasing the frequency, which indi-cates this phase-transition character should be of the no-relaxa-tion-type behavior. The change in the dielectric constant of SCOmaterials results from structural modifications. Table S5 summa-rizes the parameters characteristic of the spin transitions as de-tected by dielectric spectroscopy of the reported complexes,[6e]

that is, transition temperatures (Tc↓ and Tc↑), width of the ther-mal hysteresis (ΔT), and change in dielectric constant (Δε′)upon spin transition (Δε′ = ε′HS – ε′LS). For most of the investi-gated SCO compounds, the polarizability is mainly composedof electronic contributions that are due to the absence of spon-taneous polarization.[6e,6f ] However, for homochiral compound1R containing a polar ligand, dipolar reorientation can contrib-ute significantly to the dielectric response. Hence, unlike otherknown SCO complexes, in this case both spin transition andchirality play crucial roles in the larger Δε′ of complex 1R. Thecrystallographic data reveal an increase in the thermal parame-ters and higher anisotropy for the pinene moieties of the twomolecules of the asymmetric unit in each case than that ob-served for the other moieties. Their magnitudes are reducedupon cooling the crystals from 296 to 123 K. The dielectrichysteresis is more significant than the magnetic hysteresis, asshown in Figure 3. This is due to the fact that structural relaxa-tion and dipolar reorientation may take a longer time than spintransition. Moreover, the different temperature sweep rates inmeasuring the dielectric (20 K min–1) and magnetic properties(10 K min–1) can also be a reason for this difference, as thewidth of magnetic hysteresis changes with different sweeprates (Figure S6).

Conclusions

In summary, a design concept was realized in the search tocombine three molecular properties, chirality, spin crossover,and dielectric switching, which rely on the coordination of FeII

to chiral ligands. The homochiral FeII SCO complexes,[Fe{(R)L1}(bpz)2] and [Fe{(S)-L1}(bpz)2], were synthesized and

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structurally characterized. Their chirality was verified by circulardichroism of the UV/Vis and vibrational spectra. These com-plexes exhibited a weakly hysteretic one-step complete spincrossover at 187 ± 2 K that was accompanied by a majorchange in the dielectric constant. The low-spin state could beoptically pumped to an excited HS* (LIESST), which was stableup to 40 K. The structural change associated with the ligandfields coupled these two properties. By replacing the pyrazinewith a pyridine to give (R)/(S)-4,5-pinenepyridyl-2-pyridine[(R)/(S)-L2], the SCO transition was moved to lower temperaturesfor the corresponding complexes. We are working to show thecoupling of chirality by using circular polarized light for theLIESST experiments. We expect the effect should depend onthe handedness of the excitation pump probe and its reso-nance with the absorbing molecule.

Experimental SectionMaterials: All reactions were performed under an inert N2 atmos-phere by using standard Schlenk techniques. All chemicals werecommercially available and were used as received. Potassium bis(1-pyrazolyl)borohydride (Kbpz),[23] (R)-4,5-pinenepyridyl-2-pyrazine[(R)-L1], and (S)-4,5-pinenepyridyl-2-pyrazine [(S)-L1] were preparedaccording to published procedures.[24]

Measurements: Elemental analyses were performed with a Perkin–Elmer 240C analyzer. IR spectra (KBr pellets) were recorded with aVector22 Bruker Spectrophotometer in the range of 400 to4000 cm–1. CD spectra were recorded with a Jasco J-810 spectropo-larimeter. IR and VCD spectra in the region of 800 to 1800 cm–1

were recorded with a VERTEX 80v Fourier-transform infrared spec-trometer equipped with a PMA 50 VCD/IRRAS module (Bruker, Ger-many) by using previous procedures.[25] Mass spectra were recordedwith a LCQ Fleet instrument for ESI-MS. X-ray powder diffraction ofthe complexes was performed with a Bruker D8 ADVANCE diffrac-tometer at room temperature. Calculated PXRD patterns were gen-erated by using Mercury 3.8. Magnetic susceptibility measurementsof the polycrystalline samples were performed with a Quantum De-sign MPMS-SQUID-VSM magnetometer in 1 kOe between 2 and300 K. DSC was recorded with a TA Instruments DSC Q20 calorime-ter at a heating rate of 10 K min–1. The complex permittivity wasmeasured by using a Tonghui TH2828A LCR meter with an ac volt-age of 1 V from 130 to 300 K at a sweep rates of 20 K min–1. Thepressed pellets with electrodes attached using silver conductingglue were used for dielectric studies.

[Fe{(R)-L1}(bpz)2] (1R): A mixture of FeSO4·7H2O (83 mg, 0.3 mmol)and Kbpz (111 mg, 0.6 mmol) in methanol (15 mL) was stirred atroom temperature under a N2 atmosphere for 1 h, which was fol-lowed by filtration of the white precipitate. The filtrate was addedto (R)-L1 (80 mg, 0.3 mmol) to obtain a blue solution. After stirringfor 30 min, a dark-blue powder was obtained by filtration. The pow-der was dissolved in acetonitrile, and the solution was concentratedslowly to obtain 1R (180 mg, 0.3 mmol, 74 %) as a blue powder. MS(ESI): m/z calcd. for C32H39B2FeN13Na: 706 [M + 2MeCN + Na]+;found 706. C28H33B2FeN11 (601.1): C 55.95, H 5.53, N 25.63; found C56.20, H 5.35, N 25.43. IR (KBr): ν̃ = 2937 (s), 2393 (m), 2277 (w),1599 (s), 1500 (m), 1474 (s), 1403 (s), 1293 (s), 1201 (s), 1050 (m),759 (m), 670 (s) cm–1.

[Fe{(S)-L1}(bpz)2] (1S): A procedure similar to that described for thesynthesis of 1R was followed, except that (S)-L1 was used in placeof (R)-L1. The resultant blue, block-shaped crystals of 1S were ob-

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tained and collected. Yield: 65 %. C28H33B2FeN11 (601.1): C 55.95, H5.53, N 25.63; found C 56.21, H 5.33, N 25.42. IR (KBr): ν̃ = 2939 (s),2393 (m), 2277 (w), 1609 (s), 1500 (m), 1474 (s), 1403 (s), 1294 (s),1201 (s), 1050 (m), 759 (m), 670 (s) cm–1.

[Fe{(R)-L2}(bpz)2] (2R): A procedure similar to that described forthe synthesis of 1R was followed, except that (R)-L2 was used inplace of (R)-L1. The resultant red, block-shaped crystals of 2R wereobtained and collected. Yield: 38 %. C29H34B2FeN10 (600.1): C 58.04,H 5.71, N 23.34; found C 58.38, H 6.02, N 23.12. IR (KBr): ν̃ = 2937(s), 2397 (m), 2277 (w), 1599 (s), 1499 (m), 1475 (s), 1437 (s), 1402(m), 1050 (m), 746 (m), 637 (s) cm–1.

[Fe{(S)-L2}(bpz)2] (2S): A procedure similar to that described for thesynthesis of 1R was followed, except that (S)-L2 was used in placeof (R)-L1. The resultant red, block-shaped crystals of 2S were ob-tained and collected. Yield: 35 %. C29H34B2FeN10 (600.1): C 58.04, H5.71, N 23.34; found C 58.39, H 5.98, N 23.11. IR (KBr): ν̃ = 2938 (s),2396 (m), 2276 (w), 1598 (s), 1498 (m), 1476 (s), 1438 (s), 1402 (m),1050 (m), 746 (m), 636 (s) cm–1.

X-ray Structure Determination

The crystal structures were determined at 123 and 296 K with aBruker SMART CCD diffractometer by using monochromated Mo-Kα

radiation (λ = 0.71073 Å). The cell parameters were retrieved byusing SMART software and were refined by using SAINT for all ob-served reflections. The data were collected by using a narrow-framemethod with a scan width of 0.3° in ω and an exposure time of 5 sper frame. The redundant data sets were reduced by using SAINT[26]

and were corrected for Lorentz and polarization effects. The absorp-tion corrections were applied by using SADABS.[27] The structurewas solved and refined by using SHELXLTL.[28] The direct methodyielded all non-hydrogen atoms, which were refined by using aniso-tropic thermal parameters. All hydrogen atom positions were calcu-lated geometrically and were set riding on their respective atoms.More details of the data collection and structure refinements aregiven in Table S1. Selected bond lengths, structural parameters, andbond angles for complexes 1R and 1S are listed in Tables S2–S4.

CCDC 1409851 [for 1R (296 K)], 1409852 [for 1R (123 K)], 1409853[for 1S (296 K)], 1409854 [for 1S (123 K)], 1524980 (for 2R), and1524981 (for 2S) contain the supplementary crystallographic datafor this paper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre.

AcknowledgmentsThis work was supported by the Major State Basic ResearchDevelopment Program (2013CB922100), the National NaturalScience Foundation of China (No. 21631006 and 91422302), theproject of the scientific and technological support program inJiangsu province (Grant No. BE2014147-2), and the Natural Sci-ence Foundation of Jiangsu Province (No. BK20151377). M. K.was funded by the Centre National de la Recherche Scientifique(CNRS), France.

Keywords: Iron · Multifunctional materials · Chirality · Spincrossover · Ligand effects

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Received: May 24, 2017