Spin-Crossover Behavior of Hofmann-Type-Like Complex Fe ( 4 , 4 ’-bipyridine ) Ni ( CN ) 4 ̈ n H 2 O Depending on Guest Species

A newly prepared metal complex Fe(4,4’-bipyridine)Ni(CN)4 ̈ nH2O, which was estimated to have a structure similar to the Hofmann-type clathrate host, changed its color from orange to deep orange and yellow on exposure to ethanol and acetone vapor, respectively, and the respective samples showed thermally induced two-step and one-step spin transitions.


Introduction
Spin-crossover Fe II complexes are a promising source for new functional molecular switches because they can alternate between a high-spin (HS) and a low-spin (LS) state depending on the circumstances and show variations in magnetism, color and structure [1][2][3][4][5][6].Many spin-crossover Fe II complexes, especially those with polymeric structures, have been intensively studied because their polymeric linked structures enhance cooperative effects which work among the spin-crossover species resulting in spin transitions with hysteresis [7][8][9][10][11][12][13][14][15][16][17].If such Fe II complexes gain another physical or chemical property besides spin-crossover, they would be even more fascinating as multi-functional materials.We now report such an example, an Fe II complex that shows a spin transition and vapochromism.
As 2D polymeric structure complexes, the Hofmann-type clathrate host complexes M II (NH 3 ) 2 M' II (CN) 4 (M = Mn, Fe, Co, Ni, Cd; M' = Ni, Pd, Pt) are known, in which octahedral M II and square planar M' II are alternately linked by cyanide bridges to form a 2D [M' II (CN) 4 M II ] 8 bimetallic cyanide network [18][19][20][21].At M II , the four N ends of the cyanide bridges and two ammines coordinate to the four equatorial and two axial sites of M II , respectively, to complete an M II N 6 octahedral coordination sphere.In the Hofmann-type clathrate, the cyanide networks are stacked to form a layered structure that works as a host for small organic molecules such as benzene, pyrrole, aniline, etc.As a modification of this complex, Hofmann-pyridine-type complexes M II (py) 2 Ni II (CN) 4 (py = pyridine) are known [22][23][24].These have no ability as a clathrate host but, in the case of M = Fe, it shows a spin transition with thermal hysteresis [25].Further structural development from a 2D to a 3D structure by replacing the unidentate pyridine with bidentate pyrazine generated the complex Fe II (pyrazine)M' II (CN) 4 ¨nH 2 O, where a spin transition with a higher transition temperature and a wider hysteresis loop were achieved [26][27][28].Based on this history, we tried to synthesize a new Hofmann-type-like Fe II complex using a larger bidentate ligand 4,4'-bipyridine(bpy), expecting the resultant complex to function as a host and also possess spin-crossover properties.

Results and Discussion
Our new complex Fe II (bpy)Ni II (CN) 4 ¨nH 2 O(n = 2.5)(1) was obtained as powder from a mixture of Mohr's salt, K 2 [Ni(CN) 4 ] H 2 O, 4,4'-bipyridine (bpy), water and acetone.Its crystallinity was poor and no crystalline sample has been obtained.Because the single crystal X-ray diffraction method was inapplicable, the structural information was obtained from indirect methods such as EXAFS (Extended X-ray Absorption Fine Structure), IR spectroscopy and powder X-ray diffraction.
While Temperature-dependent EXAFS study for spin crossover complex Fe(py)Ni(CN) 4 was reported [29], the information about the coordination structures of Fe II and Ni II was obtained from EXAFS spectra.The EXAFS functions of 1 and a Hofmann-type-like complex Fe(py) 2 Ni(CN) 4 , whose crystal structure is shown in Figure 1a, after the Fourier conversion were shown in Figure 2. Similarity between 1 and Fe(py) 2 Ni(CN) 4 in Fe K-edge and Ni K-edge EXAFS spectra are clear.These findings strongly suggest the coordination structures of Fe II and Ni II atoms of both compounds are very similar.The two intense peaks of 1 in the Ni K-edge EXAFS spectra come from the multiple-scattering from the linearly arranged Ni with a square planar form, C and N atoms, namely Ni-C-N linear linkage [29,30].The third small peak around ca. 4.5 Å arise from the Fe atom bound to the N end of the Ni-C-N linkage.The figure seeing the linkage from the Fe end is the Fe K-edge EXAFS spectrum.The remarkable three peaks in the Fe K-edge EXAFS spectrum come from the linear arrangement of Fe-N-C-Ni linkage.The Fe has an octahedral configuration by six atoms so that the first peak is more intense than that in the Ni K-edge spectrum.These situation well agree with the network structure of the Hofmann-type host complex Fe(ligand) 2 Ni(CN) 4 as shown in Figure 1b.The EXAFS spectra of 1 which are almost identical to those of Fe(py) 2 Ni(CN) 4 strongly suggest that 1 has the net structure of the Hofmann-type host complex.In IR spectrum of 1, a sharp absorption band was observed at 2153 cm ´1 (Figure S1), assignable to the stretching mode of cyanide.This finding shows that there is only one kind of cyanide ligand in 1. Considering the results of the EXAFS spectra all cyanides in 1 work as bridging ligands to form a bimetallic cyanide network.This is the picture of the cyanide ligand seen in the Hofmann-type host complexes [22,31,32].
In many Hofmann-type-like complexes, a characteristic powder X-ray diffraction pattern coming from a well-regulated layered structure of the cyanide networks is observed [33].The intensity of the powder pattern measured for 1 was rather weak.However, a series of diffraction peaks suggesting a layered structure with a space distance of ca.11.3 Å was observed (Figure S2).This distance agrees with that of 11.5 Å observed in a known spin cross over (SCO) complex [Fe(bpy)(biim) 2 ](ClO 4 ) 2 ¨2C 2 H 5 OH (biim = 2-(2-pyridyl)imidazole)), which contains bpy working as a bridge between two Fe(II) atoms [34].Based on the above information, the basic structure of 1 is the layered structure similar to that of the Hofmann-type host complex and the layers are linked by bridging bpy ligands.In this structure, there is void space between the layers and the bpy ligands acting as pillars supporting the layers.Figure 1c illustrates a structure estimated for 1 from the above information.The water molecules are trapped in this space and there is a possibility that the water molecules are replaceable with other molecules.
1 showed vapochromic behavior [35][36][37][38][39][40][41].The orange powder of 1 displayed a color change on exposure to certain organic compounds, such as ethanol, acetone, 2-propanol, etc., in the gas phase.In this paper, we report the cases of ethanol and acetone.From the contact of 1 with ethanol and acetone vapor at room temperature and under ambient pressure, deep orange (2) and yellow (3) powders were obtained, respectively.Accompanying these color changes, the weight of 1 increased by 5.0%-8.0%and 10%-30%, respectively.These color changes were reversible. 2 and 3 were very labile in a guest-free atmosphere.They quickly released the guests and returned to the orange powder 1. Diffuse reflectance spectra of 1, 2 and 3 are shown in Figure 3.Although clear change was observed visually between 1 and 2, the difference in diffuse reflectance spectrum between them was little.Between 1 and 3, clear change was recognized in their spectra and the spectrum of 3 returned to that of 1 by standing 3 in air.These findings suggest that the coordination environment in 1 receives some influence from the absorption of acetone molecules.Magnetic susceptibility measurements revealed that 1 did not show any spin-crossover phenomenon but the guest-containing samples 2 and 3 did show spin-crossover.The χ M T vs. T plots are shown in Figure 4a.In the range between 50 K and 300 K, the χ M T value of 1 was almost constant at ca. 3.3 emu K mol ´1.This value is close to the expected value of 3.0 emu K mol ´1 for a Fe II HS(S = 2) state.This confirms that the Ni II maintains the diamagnetic square planar [Ni(CN) 4 ] 2´m oiety, which is the structural base of Hofmann-type-like cyanide networks [18][19][20][21].In contrast to 1, 2 showed a two-step spin transition with thermal hysteresis, the critical temperatures of which on cooling (T C Ó) were 213 and 187 K, and on warming (T C Ò) were 223 and 197 K.These T C values are close to those observed in the Hofmann-pyridine-type complex Fe(py) 2 Ni(CN) 4 .[25,42] In 3, a spin transition was observed as a one-step transition with hysteresis, the T C Ó and T C Ò of which were 125 and 145 K, respectively.The T C values of 3 are considerably lower than those of Fe(py) 2 Ni(CN) 4 .In both cases, the spin transitions were not complete.Below 100 K, 20%-25% of Fe II remained in the HS state.
Figure 4b,c shows the results of samples processed with deuterated ethanol (C 2 D 5 OD) and acetone ((CD 3 ) 2 CO).The sample including C 2 D 5 OD showed small shift to higher temperature region as often observed in deuterated SCO complexes.However, in the sample containing (CD 3 ) 2 CO, a remarkable increase in the hysteresis width was observed [42].Such increase in hysteresis width coming from deuteration is rare [42].Table 1 lists the transition temperatures including those obtained from the deuterated samples.In the case of the influence of hydrogen bonding on the hysteresis width in iron(II) spin crossover complexes, Weber el al. reported an increase of hysteresis width after deuteration [43].This is much more pronounced and remarkable for the presence case with (CH 3 ) 2 CO.Table 1.Spin transition temperatures of 2 and 3 determined by magnetic susceptibility measurements.

Sample
T c Ó a (K) The spin states of 1, 2 and 3 are confirmed by 57 Fe Mössbauer spectra shown in Figure 5. Mössbauer parameters obtained from the spectra are listed in Table 2.In all spectra, two kinds of absorption, one major and one minor, were observed.The main absorption of 1 comprised quadrupole doublets corresponding to Fe II in the HS state, the isomer shift (δ) and quadrupole splitting (∆E Q ) of which were 1.00(3) mm s ´1 and 0.63(3) mm s ´1 at 298 K, and 1.13(3) mm s ´1 and 1.10(3) mm s ´1 at 77 K, respectively.No remarkable change was observed between the spectra of 1 at 298 K and 77 K.In 2, the dominant absorption observed at 298 K was a doublet with δ of 1.04(2) mm s ´1 and ∆E Q of 0.69(2) mm s ´1, which corresponds to Fe II in the HS state.At 77 K this signal almost completely converted into an absorption peak assignable to Fe II in the LS state with δ of 0.52(2) mm s ´1 and ∆E Q of 0.31(2) mm s ´1.The spectral change observed in 3 was similar to the case of 2. The doublet of the major absorption at room temperature, whose δ and ∆E Q were 1.070(1) mm s ´1 and 1.190(2) mm s ´1, respectively, converted into a singlet peak with δ of 0.490(2) mm s ´1 and ∆E Q of 0.217(6) mm s ´1 at 77 K.These observations on the major components agree with the spin transition observed in the magnetic susceptibility measurement.Comparing the Mössbauer parameters with those obtained in Fe(py) 2 Ni(CN) 4 [25,42] and its related complexes [44][45][46][47][48][49][50][51], this major signal is considered to come from the octahedral Fe II N 6 species in the cyanide network.On the other hand, the minor signals, which always appeared as a doublet assignable to Fe II in the HS state in 1, 2, and 3, showed no large change over all measurements.This invariant HS state corresponds to the residual magnetization observed in the χ M T vs T plot of 2. Its larger ∆E Q value suggests that the coordination environment around the HS Fe II is more distorted than that of the major component.Although one imaginable case is that some Fe II ions are coordinated by H 2 O molecules, no effective method for its confirmation is available.The whole spectral intensity of 1 was weak.On the other hand, in the spectra of 2 and 3, their intensities were rather strong.The absorption of the guest molecules is considered to improve the stiffness of the host lattice and recoilless fraction.
IR spectrum of 1 was measured by the Nujol mull method using JASCO FTIR-350 spectrometer (JASCO Corp., Hachioji, Tokyo, Japan).Powder X-ray diffraction pattern of 1 was measured using a Rigaku Multi-Flex diffractometer (Rigaku Corp., Akishima, Tokyo, Japan) with graphite monochromated Cu Kα radiation (λ = 1.5406Å).Thermogravimetric analysis (TG) was carried out using TA instruments TGA 2950 thermobalance (TA Instruments, New Castle, DE, USA) at 10 K min ´1 scan rate under N 2 gas flow.The results of IR, XRD and TG are shown in Figures S1, S2 and S3, respectively.
Diffuse reflectance spectra of 1, 2 and 3 were measured on a JASCO V-570 UV/VIS spectrometer (JASCO Corp., Hachioji, Tokyo, Japan) equipped with a JASCO ISN-470 integrating sphere (JASCO Corp., Hachioji, Tokyo, Japan) at room temperature.The powder samples were diluted with BaSO 4 powder and sealed in a cell with quartz window.
The Fe and Ni K-edge EXAFS spectra of 1 and Fe(py) 2 Ni(CN) 4 were measured at BL-9C station of the Photon Factory (PF) in High Energy Accelerator Research Organization (KEK, Tsukuba, Japan).The powder samples were pressed to disks and the measurements were carried out in the transmission mode at ambient temperature.The radiation was monochromatized with Si(111) double crystals.The spectra were recorded over the range between 6604.2-8205.2eV for Fe K-edge spectra and 7826.7-9425.7 eV for Ni K-edge spectra.The data reduction from the absorption spectra to EXAFS functions of k 3 χ(k) and Fourier transformed functions FT|k 3 χ (k)| were carried out using the program Athena/Ifeffit (Ver.0.8.061,CARS(the Consortium for Advanced Radiation Sources), Chicago, IL, USA, 2009) [52].For the spectra of Fe(py) 2 Ni(CN) 4 the curve fitting was performed based on the structural information [25] over the real space range of R = 1-8 Å using the program Artemis/Ifefffit [52] and FEFF8 (Ver.8.40, FEFF Project, Seattle, WA, USA, 2008) [53], including multiple-scattering paths.This analysis confirmed that the characteristic three peaks come from the linear atom arrangements of Fe-N-C-Ni and Ni-C-N-Fe as mentioned in the Result and Discussion Section.The fitting results and parameters used are shown in Figure S4 and Table S1, respectively.
Magnetic susceptibility measurements were carried out for 1, 2, and 3 between 10 and 300 K using a Quantum Design MPMS-5 superconducting quantum interference device (SQUID) magnetometer (Quantum Design Inc., San Diego, CA, USA).The samples of 2 and 3 were sealed into an Al cell to prevent the release of the guest molecules.External magnetic field of 1000 Oe was applied and scan rate of 0.5 K min ´1 was used. 57Fe Mössbauer experiments were carried out on powder samples of 1, 2 and 3, which were sealed into a cell made of poly(methyl methacrylate), at 298 K and 77 K using a Wissel Mössbauer spectrometer system consisting of an MDU-1200 driving unit (Wissenschaftliche Elektronik GmbH, Starnberg, Germany) and an MVT-100 velocity transducer (Wissenschaftliche Elektronik GmbH, Starnberg, Germany), incorporating with a Seiko Model 7800 multi-channel analyzer (Seiko EG&G Co.,Ltd., Chuo-ku, Tokyo, Japan).The temperature control of the samples was done with a Heli-Tan LT-3 gas-flow cryostat (Advanced Research System Inc., Allentown, PA, USA) with a 9620 digital temperature controller (Science Instruments, Inc., West Palm Beach, FL, USA) from Scientific Instruments, and the 57 Co(Rh) source (Ritverc GmbH, Petersburg, Russia) was kept at room temperature.δ values are given with respect to α-iron foil at room temperature.

a
and b: transition temperature in the heating and cooling mode, respectively.c: hysteresis width.

Figure 5 .
Figure 5. Mössbauer spectra of 1, 2 and 3 at 298 K and 77 K: (a,b) 1 at 298 K and 77 K; (c,d) 2 at 298 K and 77 K; and (e,f): 3 at 298 K and 77 K.The red and the blue line show the major and the minor component, respectively.The black line shows the sum of both components.