Spin Crossover in Bipyridine Derivative Bridged One-Dimensional Iron(III) Coordination Polymer

: Herein, the syntheses, solid-state molecular structures, and characterization of two types of one-dimensional Fe III coordination polymers showing thermally induced spin crossover are reported. The reaction of [Fe(acen)Cl] (acen 2− = N , N’ -ethylenebis(acetylacetonylideneaminate) with 3,3′-bpy or 4,4′-bpy (bpy = bipyridine) produced zigzag and linear one-dimensional chain complexes, [Fe(acen)(3,3′-bpy)][BPh 4 ] ( 1 ) or [NEt 3 H][Fe(acen)(4,4′-bpy)][BPh 4 ] 2 ·0.5(4,4′-bpy) ( 2 ), respectively, as confirmed by single crystal X-ray diffraction analysis. Variable-temperature single crystal X-ray diffraction measurements, continuous-wave X-band electron paramagnetic resonance (EPR) spectra, 57 Fe Mössßauer spectra, and DC magnetic susceptibility data revealed that complex 1 exhibited a gradual and complete spin crossover at a transition temperature of 212 K, while complex 2 undergoes an incomplete spin crossover even at 400 K.

The most extensively studied SCO moieties are Fe II complexes with a 3d 6 electron configuration because of a distinct spin transition from a paramagnetic HS state (S = 2) to a diamagnetic LS state (S = 0), or vice versa, which generally show abrupt spin transitions and high cooperativity in the solid state [1][2][3][4][5][6][7][8][9][10][11]. In contrast, Fe III SCO complexes have not been as extensively reported as Fe II SCO complexes, mostly due to the lack of cooperativity associated with Fe III SCO complexes [27,28]. To overcome this obstacle, several fascinating Fe III SCO complexes that exhibit high cooperativity have been realized by introducing relatively weak supramolecular contacts, including hydrogen bonding and π-π stacking interactions between SCO active centers in the packed crystal. These have also been shown to result in abrupt spin transition, multistep spin transition, wide thermal hysteresis loop, and light-induced excited spin state trapping effect [27,28]. However, previously reported almost Fe III SCO complexes are discrete mononuclear and dinuclear complexes. To further expand the basic understanding of magnetostructural correlations in highly cooperative Fe III SCO complexes, the development of Fe III SCO coordination polymer candidates with structurally high dimensionality is necessary. As an initial attempt to create novel Fe III SCO coordination polymers, a one-dimensional (1D) coordination polymer structure was proposed.
To date, SCO-active Fe III complexes have been explored using Schiff base ligands with a N4O2 coordination environment [27,28]. From this perspective, constructing coordination polymers can be achieved by crystal engineering via combination of an Fe III precursor with a macrocyclic-type Schiff base ligand furnished with a N2O2 coordination donor set, as a building block and a N-donor bridging ligand.
Both 1 and 2 showed characteristic Fourier-transform infrared (FTIR) peaks, indicating the presence of BPh4 − (~700 and 730 cm -1 ) and a Fe III -coordinated imine moiety (~1580 cm −1 ), as indicated in Figure S1. The phase purity of 1 and 2 was confirmed via powder X-ray diffraction (PXRD) measurements. Experimental PXRD patterns of polycrystalline 1 and 2 were in agreement with the calculated patterns based on SCXRD data for the complexes. Therefore, no complexes exhibited crystal polymorphism and both polycrystalline samples adopted a uniform crystalline phase ( Figure  S2). The purities of 1 and 2 were confirmed by elemental analyses (Experimental Section). Thermogravimetric analyses (TGA) indicated that polycrystalline samples 1 and 2 are thermally stable up to ~420 K ( Figure S3).

Structural Descriptions
The SCXRD measurements of 1 and 2 were performed at 100 and 300 K (Table S1). At both temperatures, complex 1 crystallized in a monoclinic space group P21/c, with the asymmetric unit containing [Fe(acen)(3,3′-bpy)] + and a BPh4anion (upper part of Figure 1a). The Fe III center approximated a distorted octahedral geometry (octahedral distortion parameter, Σ [31]; Σ = 35.82(6) and 36.81(7)° at 100 and 300 K), with an equatorial plane occupied by a N2O2 coordination donor from an acen 2− ligand, while each axial site was bridged via two N ends of a 3,3′-bpy ligand in a trans configuration with dihedral angles between pyridine rings of 36.35 and 32.68° at 100 and 300 K, forming a zigzag 1D chain (lower part of Figure 1a). At 100 K, the average Fe-Oequatorial, Fe-Nequatorial, and Fe-Naxial distances were consistent with those in the LS Fe III complexes with similar coordination environments of related Fe III (acen)-type complexes (Table S2) [32][33][34][35][36][37][38]. In contrast, the corresponding distances at 300 K were significantly longer than those observed at 100 K, suggesting a thermally induced SCO between 100 and 300 K (Tables 1, 2 and S2). The changes of these coordination bond distances are comparable with those previously reported for Fe III (acen)-type SCO complexes (Tables 2 and S2). No lattice solvent molecules were present in the crystal packing and the intrachain Fe III ···Fe III separations were determined to be 9.5895(7) and 9.8189(7) Å at 100 and 300 K, respectively. Each zigzag 1D chain of 1 propagated along the crystallographic c axis in the crystal packing, separated by bulky BPh4 − anions, with the closest interchain Fe III ···Fe III separation of 9.3105(5) and 9.6025(8) Å at 100 and 300 K, respectively (Figure 2a).

Continuous-Wave X-band Electron Paramagnetic Spectroscopy
Continuous-wave X-band electron paramagnetic resonance (CW X-band EPR) spectroscopy is useful, because Fe III complexes with octahedral coordination geometry are Kramers systems, and are generally EPR-active at X-band frequencies (~0.3 cm −1 ) under magnetic fields (~1 T), regardless of the electron configuration of the Fe III center (i.e., LS (S = 1/2) or HS (S = 5/2)). However, it should be noted that for the HS Fe III center effective g values were obtained, which are only useful in a qualitative sense. The lack of quantitation arises from the dependence on the magnitude of the zero-field splitting energies for the HS state of S = 5/2 [39]. The CW X-band EPR spectra of polycrystalline 1 and 2 were collected at 90-280 K (Figure 3).
At low temperatures, the EPR spectra of 1 can be described as a complete and well-resolved LS state, S = 1/2, indicating highly rhombicity of the gx, gy, and gz values (1.919, 2.086, and 2.408, respectively), and an average g value of 2.138 (Figure 3a). The observed g values are characteristic of LS Fe III complexes with similar coordination environments [40][41][42][43][44][45][46][47]. With the increasing temperature, the EPR signals centered at the average g value of 2.138 for 1 gradually broadened from rhombic to isotropic symmetry, and became steadily indistinct. Instead, a new signal centered at approximately g = 4.3 appeared and steadily grew, corresponding to a gradual change from the LS to HS state with highly rhombicity (E  D/3; where D and E are the axial and rhombic zero-field splitting parameters, respectively) [48], ultimately becoming very broad dominated by the HS signal at 280 K.
The low temperature EPR spectra of 2 showed isotropic signals centered at g = 2.137, which is considerably similar to the average g value of 2.138 observed for the LS state of 1 (Figure 3b). With increasing temperature, the EPR signals of 2 showed slight line broadening, while a strongly rhombic HS signal at g  4.3 appeared only at the high temperature region and was not overly dominant. Therefore, the Fe III center of 2 remained in a quasi-LS state throughout the temperature range measured.
Therefore, both 1 and 2 undergo thermally induced SCO from LS to HS in the solid-state. These results also suggest that 1 exhibited a gradual and complete SCO, while 2 displayed incomplete SCO over the temperature range measured herein.

Magnetic Properties
To further prove thermally induced SCO for 1 and 2, the temperature dependence of molar DC magnetic susceptibility (χM), measurements were performed using a SQUID magnetometer on the polycrystalline samples over the temperature range of 2-400 K, under an applied field of 0.5 T at a scan rate of 2 K min −1 (Figure 4).

Figure 4.
Plots of the χMT versus temperature over the temperature range 2-400 K in an applied dc field of 0.5 T at scan rate of 2 K min -1 for 1 (black) and 2 (blue).
At 2 K, the χMT (χM times temperature) value for 1 was determined to be 0.52 cm 3 K mol −1 , which is consistent with the expected χMT value at g > 2 for an essentially complete LS state for the Fe III center. With the increasing temperature, the χMT values increased steadily, with a more rapid increase in the χMT data between 120 and 300 K, which is indicative of thermally induced SCO. This thermal spin transition (T1/2) occurred at ~210 K without thermal hysteresis, as confirmed by differential scanning calorimetry (DSC; Figure S5). At temperatures of >120 K, the χMT values again gradually increased to 4.24 cm 3 K mol −1 at 400 K, characteristic of a fully HS state for the Fe III center. The χMT value for 2 was approximately 0.60 cm 3 K mol −1 at 2 K, remaining constant up to 200 K, and then gradually increases to 2.78 cm 3 K mol −1 at 400 K. In contrast to 1, the Fe III center of 2 remained in a near LS state even at 400 K (inflection point at ~380 K), undergoing incomplete SCO.
To quantify the energy separation (ΔE) between the zero-point levels of the LS ( 2 T2) and HS ( 6 A1) states in 1, which exhibited complete SCO, the temperature dependence of χMT data was fitted to the following molecular vibrational partition function (Equation (1)) [27,49,50]: where T is the absolute temperature, g is the Landé g factor, C is the molecular partition function ratio in the LS and HS states, and ζ is the spin-orbit coupling. The adequate parameters determined from fitting the data obtained from 1 are summarized in Table 3 ( Figure S6). Compared to values previously reported for related Fe III (acen)-type SCO complexes, those of 1 are similar in magnitude, whereas those of 2 are relatively large in magnitude. These large values of 2, in contrast to 1, likely result from crystal packing differences brought about by the dense assembly. In fact, the difference in the crystal packing between 1 and 2 are to be reflected in the thermodynamic parameters (vide infra).  1 Ligand abbreviations: 4-Mepy = 4-methylpyridine, 3,4-Me2py = 3,4-dimethylpyridine, and bpyp = 1,3-bis(4-pyridyl)propane. 2 Values obtained from EPR spectra, which were fixed in the simulation.
To determine the thermodynamic parameters associated with the thermally induced SCO for 1, the enthalpy (ΔH) and entropy (ΔS) changes were calculated using the temperature dependence of γHS (molar fraction of the HS state), which also used the following model proposed by Slichter and Drickamer (Equation (2) where T is the absolute temperature, Γ is the cooperativity evaluation parameter (C = Γ/2RT1/2), and R is the gas constant (8.314 J K −1 mol −1 ). The obtained parameters are listed in Table 4 ( Figure S7). The calculated ΔS values were larger than those expected for SCO between the HS and LS states (Rln(2SHS + 1)/(2SLS + 1) = 9.134 J K −1 mol −1 ), indicating a vibrational contribution to ΔS associated with bond softening (especially in the ligand-metal bonds) via SCO transition [52,53]. The value of Γ for 1 and 2 was absent, indicative of a less cooperative SCO.  [37]. However, to date, no structurally characterized example of an Fe III (acen)-type SCO complex has been shown to exhibit abrupt and complete SCO, as well as thermal hysteresis. The parameters associated with SCO for the previously reported Fe III (acen)-type SCO complexes with six-coordinate geometry are summarized in Table 5.  (4-pyridyl)isonicotinamide, bpyp = 1,3-bis(4-pyridyl)propane, and bimb = 1,4-bis(imidazolyl)butane. 2-4 n.a., g, a, c, and ic denote not analyzed, gradual, abrupt, complete, and incomplete, respectively. 4 The C2N2 backbone conformation of the acacen 2− ligand.

57 Fe Mössßauer Spectroscopy
For further spectroscopic verification of complete SCO in 1, variable-temperature 57 Fe Mössßauer spectra were collected on the polycrystalline solids at select temperatures ( Figure 5a) and Table 6). 57 Fe Mössßauer spectral data of 2 were not collected as the Fe III center remained the LS state throughout the measured temperature range. Despite undergoing complete SCO, each 57 Fe Mössßauer spectrum of 1 at different temperatures contained only a single symmetric doublet. The observation of the single doublet in the 57 Fe Mössßauer spectra of Fe III SCO complexes arises from a rapid electronic relaxation between the LS and HS states. Thus, the electronic relaxation of 1 between the two spin states is faster than the time scale of 57 Fe Mössßauer spectroscopy (~10 −7 s), and the 57 Fe Mössßauer spectra report the average between the LS and HS states [49,50]. At high temperatures, the respective 57 Fe Mössßauer spectra exhibit an asymmetric quadrupole doublet, due to the spin-lattice relaxation effect [54]. The considerable broadening of the 57 Fe Mössßauer spectra with increasing temperature was attributed to the Debye-Waller factor [54]. Indeed, absorption at 300 K is no more than ~1% and all spectra could be fitted with a single quadrupole doublet featuring a single line width. The resulting parameters (isomer shifts, δ, quadrupole splittings, ΔEQ, and line widths, Γ) are listed in Table 2 and shown in Figure 5b. The estimated δ, ΔEQ, and Γ parameters indicated a notable thermal dependence, consistent with the variable-temperature X-band EPR spectra and magnetic susceptibility data (vide supra).   (6) 1 The δ is referenced to α-iron at 300 K.

Single Crystal X-ray Crystallography
Single crystals of 1 and 2 were coated with Nujol, quickly mounted on MicroLoops (MiTeGen LLC., Ithaca, NY, USA), and immediately cooled in a cold dinitrogen stream. The data collections were performed on a R-AXIS RAPID II IP diffractometer (Rigaku Corporation, Tokyo, Japan) with graphite-monochromated Mo-Kα radiation (λ = 0.71075 Å) and a low-temperature device. The data integration, preliminary data analysis, and absorption collections were performed on a Rigaku CrystalClear-SM 1.4.0 SP1 [55], using the CrystalStructure 4.2.2 [56] crystallographic software packages. The molecular structures were solved by the direct methods included in SIR2011 [57] and refined with the SHELXL [58] program. All non-hydrogen atoms were refined anisotropically. CCDC 2007100-2007103 for 1 and 2 contain the supplementary crystallographic data for this paper and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. All the hydrogen atoms were included in the calculated positions. Table S1 summarizes the lattice constants and structure refinement parameters for complexes 1 and 2.

Physical Measurements
The elemental analyses were performed on a J-Science Lab Micro Corder JM10 (J-Science Lab Co., Ltd., Kyoto, Japan). IR spectra were recorded on a JASCO FT/IR-6200 spectrometer equipped with an attenuated total reflectance accessory (ATR) (JASCO Corporation, Tokyo, Japan) in the range of 650-4000 cm −1 at 293 K. PXRD data were collected at 293 K with a RIGAKU MultiFlex diffractometer (Rigaku Corporation, Tokyo, Japan) (50 kV/32 mA, 1.6 kW) with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 5-50° and a step width of 0.02°. TGA were carried out on a Seiko Instruments SSC5200 Thermo Analyzer (Seiko Instruments Inc., Tokyo, Japan), with a heating rate of 10 K min −1 in the temperature range of 293-550 K, under a N2 atmosphere. For CW X-band EPR measurements, finely ground microcrystalline powders were sealed in quartz tubes of 4 mm diameter under a N2 atmosphere. EPR spectra were recorded on a Bruker EMXmicro spectrometer equipped with a continuous flow liquid N2 cryostat and a temperature controller (Bruker BioSpin Ltd., Yokohama, Japan). All EPR spectra were analyzed with the Bruker Xenon software package (Bruker BioSpin Ltd., Yokohama, Japan). All the EPR data were collected under the following experimental conditions: microwave frequency, 9.44 GHz; microwave power, 0.11 mW; modulation amplitude, 4 G; modulation frequency, 100 kHz. The magnetic data were collected using a Quantum Design MPMSXL7 SQUID magnetometer (Quantum Design Japan, Inc., Tokyo Japan). The measurements were performed with polycrystalline samples in a calibrated gelatin capsule. The dc magnetic susceptibility measurements were performed in the temperature range of 2-400 K in a dc field of 0.5 T. The obtained dc magnetic susceptibility data were corrected for diamagnetic contributions from the sample holder, as well as for the core diamagnetism of each sample, estimated from Pascal's constants [59]. DSC measurements were carried out on a Seiko Instruments EXSTAR DSC 6200 Differential Scanning Calorimeter (Seiko Instruments Inc., Tokyo, Japan) with a heating rate of 10 K min −1 in the temperature range of 150-350 K under a N2 atmosphere. The Mössbauer spectra were measured between 8.5, 100, 150, 175, 200, 250, and 300 K, respectively, and performed with a Wissel MVT-1000 Mössßauer spectrometer (Wissenschaftliche Elektronik GmbH, Starnberg, Germany), with a 57 Co/Ph source equipped with a closed-cycle He refrigerator cryostat in transmission mode. All spectra were calibrated at 300 K with α-Fe and were fitted using the MossA software package [60].

Conclusions
Two novel 3,3′-and 4,4′-bipyridine-bridged Fe III 1D chain complexes (1 and 2) were synthesized herein using a cationic mononuclear precursor, [Fe III (acen)] + , as a building unit for constructing 1D Fe III spin crossover coordination polymers. The SCXRD structures of 1 and 2 featured 1D chains, which can be considered as extended 1D zigzag and linear coordination polymer arrangements, respectively. Notably, 1 underwent thermally induced gradual and complete SCO with a transition temperature of 212 K, whereas 2 also showed thermally induced incomplete SCO even at 400 K. Current efforts are focusing on preparing coordination architectures with higher dimensionality, such as two-dimensional layers and three-dimensional MOF-like structures, which may induce cooperative SCO behavior, accompanied by abrupt and wide thermal hysteresis.

Conflicts of Interest:
The authors declare no conflict of interest.