Abrupt Spin Transition and Chiral Hydrogen-bonded One-dimensional Structure of Iron(iii) Complex [fe Iii (him) 2 (hapen)]sbf 6 (him = Imidazole, H 2 Hapen = N,n′-bis(2-hydroxyacetophenylidene)ethylenediamine)

Solvent-free spin crossover (SCO) iron(III) complex, [Fe III (Him)2(hapen)]SbF6 (Him = imidazole, H2hapen =


Introduction
Spin crossover (SCO) is a representative phenomenon of molecular bistability and an inter-conversion between high-spin (HS) and low-spin (LS) states can occur by external physical perturbations [1].While SCO originates from single molecule, steepness, multi-step, and hysteresis in a SCO profile are brought from the cooperative effect operating between the SCO sites [1].Among the interactions leading to a cooperative effect, it is well known that the hydrogen bond plays a crucial and effective role [2].
We have focused on SCO molecules exhibiting intermolecular hydrogen bonds [3,4] and investigated how the hydrogen bond affects the SCO properties.One of the molecular systems is an Fe III complex with a salen-type N2O2 ligand and imidazole [Fe III (Him)2(salen-type)] + (Scheme 1).Nishida [5,6] reported the first such SCO Fe III complex as a simple model compound of the Fe sites in some heme proteins.Later, Matsumoto [7][8][9], Murray [10,11], and Real [12] reported the analogous Fe III complexes, and their works revealed the relevant structural factors to determine the SCO properties of these types of complexes [13].Previously, we reported an abrupt SCO Fe III complex [Fe III (Him)2(hapen)]PF6 [14] and a hysteresis SCO Fe III complex [Fe III (Him)2(hapen)]AsF6 [15], where SCO Fe III compounds, in general, exhibit gradual spin equilibrium and show no thermal hysteresis [1].The unusual SCO property of the SCO Fe III complex is due to the molecular distortion of the complex-cation [Fe III (Him)2(hapen)] + and the chiral one-dimensional structure constructed by the inter-cation O•••HN hydrogen bond between a phenoxo oxygen and an imidazole nitrogen.Unfortunately, only the structure in the high-spin HS state is determined for the PF6 and AsF6 salts.In this study, we have synthesized SbF6 salt in which the spin transition temperature shifted to a higher temperature.We could determine the structures at the HS and LS states and know the molecular distortion of the complex-cation during the spin transition.Further, we could consider the abrupt SCO profile and the shift of the spin transition temperature within the frame work of a 1D structure.

Synthesis and Characterization
The [Fe III (Him)2(hapen)]SbF6 complex was prepared by the method adopted for the PF6 and AsF6 salts [14,15].The mixed solution of precursor Fe III complex [Fe III Cl(hapen)]•0.5CH3OH,imidazole, and NaSbF6, in a 1:10:1 molar ratio in methanol, precipitates black plate-like crystals.The SbF6 salt exhibits a thermochromism, both in the solution and solid states.The red color of the dilute ethanol solution at ambient temperature changes to green at the temperature of liquid nitrogen, where red and green are typical colors for the HS and LS Fe III complexes with the Schiff-base ligands with N4O2 donor atoms [16].In the solid state, the black color of the ground sample at room temperature changes to green at the temperature of liquid-nitrogen.The thermochromism demonstrates that the complex is a SCO complex, both in the solid and solution states.

Magnetic Properties
The molar magnetic susceptibilities (χM) of the polycrystalline sample were measured in the temperature range of 5-300 K, under an applied magnetic field of 0.5 T at a sweep rate of 0.5 K min −1 .The magnetic susceptibility was measured by lowering the temperature from 300 to 5 K as the first run, and, subsequently, by elevating the temperature from 5 to 300 K as the second run.The plots of χMT vs. T for the present SbF6 salt (blue), together with those of the PF6 (red) and AsF6 (green) salts were measured at the same scan rate and as reported previously [14,15], are shown in Figure 1.Three compounds show a similar, complete abrupt spin transition between the HS (S = 5/2) and LS (S =1/2) states, though Fe III SCO compounds generally exhibit gradual spin equilibrium.The magnetic profile of the SbF6 salt is described briefly.On lowering the temperature from 300 to 5 K, the χMT value keeps the upper plateau value in the temperature region higher than 150 K, decreases abruptly around ca. 110 K, and then reaches the lower plateau value in a temperature region lower than 95 K.The spin-transition temperature of the SbF6 salt is evaluated to be T1/2 = 105 K.

Crystal Structure of [Fe III (Him)2(hapen)]SbF6 at the HS and LS States
The crystal structures at the HS and LS states were determined at 296 and 100 K. On the basis of the magnetic susceptibility measurements, the populations of HS:LS at 296 and 100 K are evaluated by the equation of χM(T) = (1 − x) χM(HS) + xχM(LS), where χM(HS) = 4.7 cm 3 K mol −1 and χM(LS) = 0.7 cm 3 K mol −1 are used as the limiting values, and χM(100) = 1.9 cm 3 K mol −1 .The populations of HS:LS at 296 and 100 K were evaluated to be 1:0 and 0.3:0.7,respectively.The structures at 296 and 100 K have an isomorphous structure, demonstrating no phase transition during the spin transition.The crystallographic data are listed in Table 1.Relevant coordination bond distances, angles, and hydrogen-bond distances are given in Table 2.
The crystallographic unique unit consists of one cation [Fe III (Him)2(hapen)]  and one counter anion SbF6 − .The molecular structure of the [Fe III (Him)2(hapen)]  part at 296 K with the selected atom numbering scheme is shown in Figure 2a,b.The Fe III ion has an octahedral coordination environment with N2O2 donor atoms of electronically di-negative tetradentate ligand hapen at the equatorial sites, and N2 donor atoms of two imidazoles at the two axial positions.Figure 2b shows the orientations of the two imidazole rings, in which one imidazole ring bisects the angles defined by two N-Fe-O diagonals, and other imidazole ring is oriented near to along the N-Fe-O diagonal.The saturated five-membered chelate ring, involving an ethylenediamine moiety, assumes a gauche conformation, in which C9 and C10 atoms deviate by −0.22 and +0.40 Å from the plane, defined by Fe, N1, and N2 atoms.For the gauche conformation, chiral δ and λ conformations are possible, thus, the complex-cation is either a δ-and λ-[Fe III (Him)2(hapen)]  chiral molecule.The dihedral angle between the N2O2 coordination plane and the benzene ring (see Figure 2b) is −9.9° and 22.8° for benzene ring A and B, respectively, in which the B ring bound to O2 is quite tilted.At 296 K, the four Fe-N bond distances are in the range 2.116(8)-2.150(9)Å, in which the Fe-N (imidazole) distance is slightly longer than the Fe-N (imine) distance.The Fe-O1 distance of 1.882(7) Å is slightly shorter than the Fe-O2 distance of 1.908(10) Å, where O2 is hydrogen-bonded to N6* of the imidazole group of the adjacent complex-cation and O1 is free from hydrogen bonding.These Fe-N and Fe-O coordination bond distances are consistent with the bond distances reported for HS Fe III complexes with similar Schiff-base ligands [10][11][12][13][14][15].The O1-Fe-O2 bond angle of 102.5(4)° is indicative of the HS spin state on the basis of the structural parameters of the analogous [Fe III (Him)2(hapen)] + [10][11][12][13][14][15].
At 100 K, four Fe-N bond distances are in the range 1.935(7)-2.005(6)Å, in which the Fe-N (imidazole) distance is slightly longer than the Fe-N (imine) distance.The Fe-O1 distance of 1.879(5) Å and the Fe-O2 distance of 1.879(6) Å are very close.These Fe-N and Fe-O coordination bond distances are consistent with the bond distances reported for LS Fe III complexes with similar Schiff-base ligands [10][11][12][13][14][15].The averaged Fe-N bond distances decrease by 0.16 Å from 2.132 Å at 296 K to 1.968 Å at 100 K.The O1-Fe-O2 bond angle of 89.1(3)° is indicative of the LS spin state at 100 K on the basis of the structural parameters of the analogous [Fe III (Him)2(hapen)] + , where HS compounds have a large angle around 104°, while SCO compounds have smaller angle around 101° [10][11][12][13][14][15].The octahedral coordination geometry changes to a regular octahedron; especially, a large change is found in the O1-Fe-O2 bond angle, which decreases from 102.5(4)° at 296 K to 89.1(3)° at 100 K, a change of 13.4°.The five-membered chelate ring at 100 K assumes a gauche conformation, in which C9 and C10 atoms deviate by −0.20 and +0.41 Å from the plane defined by Fe, N1, and N2 atoms.The dihedral angle between the N2O2 coordination plane and the benzene moiety are −14.02°and 24.47° for benzene rings A and B, respectively, of which values are compared with −9.94° and 22.81° at 296 K.The molecular distortion of the complex-cation is quite tilted in the LS state, or, in other words, the LS state requires such a large distortion of [Fe III (Him)2(hapen)] + .
As described above, and as shown in Table 2, noticeable structural deviations are found in the coordination bond distances and angles of Fe-N and O-Fe-O, and in the dihedral angle between the coordination plane and benzene rings.Figure 3 shows the minimized overlay of the complex-cation [Fe III (Him)2(hapen)]  in the HS state at 296 K and the LS state at 100 K, showing that large structural deviations are possible for the present complex-cation of [Fe III (Him)2(hapen)] + due to the steric repulsion between the methyl group of hapen and the five-membered chelate ring.Figure 4a shows the one-dimensional (1D) zigzag-chain structure of [Fe III (Him)2(hapen)] + at 296 K, while the analogous compounds have various assembly structures, including the 1D structure [14][15][16].The 1D structure running along the b-axis is constructed by inter-cation hydrogen bonding between a phenoxo oxygen O2 and an imidazole nitrogen N6* of the adjacent complex-cation, in which hydrogen bond distance is O2···N6* = 2.95(1) Å.At 100 K, N6* of the imidazole group is bifurcated and hydrogen-bonded to O1 and O2, as shown in Figure 4b.As described above, there are difference between two Fe-O bond distances at 296 K, but not at 100 K.The adjacent complex-cations, linked by the hydrogen bonds, are related by a two-fold screw axis along the b-axis (* 1/2 − x, 1/2 + y, 1/2 − z), and the O2···N6* (O1···N6*) hydrogen bond is repeated to form a 1D structure running along the b-axis.As the complex-cation is a chiral species with δ-[Fe III (Him)2(hapen)] + or λ-[Fe III (Him)2(hapen)] + due to the δ-and λ-gauche conformation, the 1D chain is a chiral chain.The adjacent chains with opposite chiralities are arrayed alternately.The spin states are examined by the structural parameters.The unit cell volume decreases from 5596.5( 7) Å 3 at 296 K to 5314.As shown in Figure 4c, the remaining imidazole nitrogen atom N4 of the two imidazoles is close to the F3 and F9 atoms of the SbF6 − ion with N4•••F3 = 3.10(3) and N4•••F9 = 3.00(2) Å.On the other hand, the corresponding hydrogen bond distance is not found for the PF6 and AsF6 salts.

Spin Transition Profile and Temperature
Three compounds of the PF6, AsF6, and AsF6 salts of [Fe III (Him)2(hapen)]Y have an isomorphous structure and show a one-step and abrupt SCO profile.(1) The complex-cation of [Fe III (Him)2(hapen)] + has a favorable molecular structure that can easily adapt the molecular distortion associated with the spin transition.During the spin transition from the HS to the LS state, the average Fe-N distance and O-Fe-O angle decrease by 0.16 Å and 13.4°, respectively, accompanying the molecular change of the coordination geometry from distorted octahedron to regular octahedron.(2) The abrupt SCO profile should be due to the hydrogen-bonded chiral one-dimensional assembly structure.Within a chiral chain, molecular expansion and contraction associated with the spin transition can be transmitted along the chain without resistance.(3) The spin transition temperature may be related to the cation-anion interaction and inter-chain interaction.The spin-transition temperature of the PF6 salt is 78 K and that of the AsF6 salt is T1/2↑ = 74.0K and T1/2↓ = 69.4K with 4.6 K thermal hysteresis.The spin-transition temperature of the present SF6 salt is 105 K.The geometry of the anions, PF6, AsF6, and AsF6, are the same and the difference is found in the anion size.The molecular volumes evaluated by the molecular mechanics calculation are PF6 = 73.0Å 3 , AsF6 = 78.5 Å 3 , and SbF6 = 88.7 Å 3 [17].The order of the spin transition temperature is AsF6 < PF6 < SbF6, while the order of the anion size is PF6 < AsF6 < SbF6.There are many studies on the influence of the anion on the SCO properties for a series of SCO complexes [1,2,13].The majority of the studies revealed that the larger the size of the anion, the lower the spin transition temperature.Our series of the complexes shows the reverse tendency; the larger the size of the anion, the higher the spin transition temperature.The remaining imidazole nitrogen atom N4 of the two imidazoles is close to the F3 and F9 atoms of the SbF6 − ion.On the other hand, the corresponding hydrogen bond is not found for the PF6 and AsF6 salts.The anion-cation interaction may be related to the spin transition temperature.

Experimental Section
All reagents and solvents used in this study are commercially available from Tokyo Kasei Co., Ltd., Tokyo, Japan, and Wako Pure Chemical Industries, Ltd., Osaka, Japan, and were used without further purification.All synthetic procedures were carried out in an open atmosphere.

Physical Measurements
Elemental analyses (C, H, and N) were carried out at the Center for Instrumental Analysis of Kumamoto University.Magnetic susceptibilities were measured by a Quantum Design (San Diego, CA, USA) MPMS-XL5 magnetometer in the temperature range of 5-300 K at a scan rate of 0.5 K min −1 under an applied magnetic field of 0.5 T. The magnetic susceptibilities were measured while lowering the temperature from 300 to 5 K in the first run.In the second run, the magnetic susceptibility was measured while raising the temperature from 5 to 300 K.The calibration was performed with palladium metal.Corrections for diamagnetism were applied using Pascal's constants.

X-Ray Crystallography
X-ray diffraction data were collected using a Rigaku (Tokyo, Japan) RAXIS RAPID imaging plate diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å).The temperature of the crystal was maintained at the selected temperature by means of a Rigaku cooling apparatus.The X-ray diffraction data for [Fe III (Him)2(hapen)]SbF6 were collected at 296 and 100 K.The data were corrected for Lorentz, polarization, and absorption effects.The structures were solved using a direct method, and expanded using the Fourier technique.Hydrogen atoms were fixed at the calculated positions and refined using a riding model.All calculations were performed using the CrystalStructure crystallographic software package (CrystalStructure 4.0, Tokyo, Japan).The X-ray crystallographic data in a CIF format have been deposited with CCDC (reference numbers CCDC 926685 and 926686).They can be obtained free of charge at http://www.ccdc.cam.ac.uk /conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

Conclusions
A family of the SCO Fe III complexes [Fe III (Him)2(hapen)]Y shows a one-step and abrupt SCO profile.The present work can reveal how the series of the compounds can exhibit such a SCO property.The complex-cation of [Fe III (Him)2(hapen)] + has a favorable molecular structure that can easily adapt the molecular distortion between a distorted and regular octahedron, especially found in the large change of O-Fe-O angle.The abrupt SCO profile should be due to the intermolecular imidazole…phenoxo hydrogen-bonded chiral one-dimensional assembly structure.Within a chiral chain, molecular expansion and contraction associated with the spin transition can be transmitted along the chain.The spin transition temperature may be related to the cation-anion interaction and inter-chain interaction.The spin-transition temperature of the PF6 salt is 78 K and that of the AsF6 salt is T1/2↑ = 74.0K and T1/2↓ = 69.4K with 4.6 K thermal hysteresis.The spin-transition temperature of the present SF6 salt is 105 K.The order of the spin

Figure 2 .
Figure 2. (a) An ORTEP drawing of λ-[Fe III (Him)2(hapen)]  cation of [Fe III (Him)2(hapen)]SbF6 with the selected atom numbering scheme at 296 K, showing the molecular distortions.The thermal ellipsoids were drawn at the 50% probability level; (b) View of the complex-cation projected on hapen, showing the orientations of two imidazole rings.