Mössbauer Spectroscopic Study on Hofmann-like Coordination Polymer Fe ( 4-Clpy ) 2 [ Ni ( CN ) 4 ]

57Fe Mössbauer spectroscopic results on the 2D Hofmann-like coordination polymer Fe(4-Clpy)2Ni(CN)4 (sample 1) indicate that iron(II) ions are in a high spin state both at R.T. and T = 80 K, showing that the compound exhibits no spin crossover (SCO) behavior. Mössbauer spectra at R.T. and T = 80 K show one major doublet which corresponds to the HS1 is due to high spin iron(II) sites coordinated by four N atoms of cyano groups and two N atoms of 4-Clpy ligand in the sample 1. Two minor doublets coexisted in sample 1. One minor doublet with a large quadrupole splitting can be explained by the presence of defects in the coordination polymer frameworks. Another minor doublet is with a small quadrupole splitting, indicating the presence of iron(III) high spin species or iron(II) low spin species.


INTRODUCTION
PIN state switching behavior in iron(II) coordination polymer compounds have become most attractive in recent years and are extensively investigated by chemists and physicists. [1]Special attention is currently much paid to Hofmann-like coordination polymer iron(II) spin crossover (SCO) compounds such as FeL2M(CN)4 (L = monodentate pyridine derivatives, M = Ni II , Pd II , Pt II ) and FeLM(CN)4 (L = bridging pyradine and bi-pyridine derivatives, M = Ni II , Pd II , Pt II ) acting cooperative behavior due to sensory and memory functions.  Some fmann-like coordination polymer compound are SCO compounds, however others are not due to properties of pyridine derivatives and environments of octahedral iron(II) ions. 57Fe Mossbauer spectroscopy is a very powerful tool for investigations on iron(II) SCO compounds and related materials whose spin states are high spin states under all temperature ranges.Recently 57 Fe Mössbauer spectroscopy has provided very powerful information on re-appearance of cooperativity in ultrasmall spin crossover Hofmann-type Fe(pz)[Ni(CN)4] nanoparticles. [2] 57Fe Mössbauer spectroscopy and SQUID technique in 1996 revealed that the original Hofmann pyridine coordination polymer Fe(py)2Ni(CN)4 shows iron(II) SCO with one step transition containing hysteresis. [7]It should be noted that iron(II) ions in the Hofmann-type clathrate Fe(NH3)2Ni(CN)4 • 2C6H6 are in high spin states at R.T. and T = 78 K, showing the clathrate exhibits no SCO behavior. [7]Details of history about Hofmann-type and Hofmann-related compounds before the discovery of SCO Fe(py)2Ni(CN)4 were reviewed by Iwamoto. [8,9] brief history after the discovery of SCO Fe(py)2Ni(CN)4 is listed in Table 1.Pressure tuning Raman spectroscopy of the prototype compound Fe(py)2Ni(CN)4 has been reported, indicating the pressure induced SCO behavior. [10]Emission Mössbauer spectroscopic study on 57 Co-labelled analogous 57 Co(py)2Ni(CN)4 demonstrates the nuclear-decay-induced excited spin state trapping (NIESST), indicating NIESST is also important in Hofmann-type compounds. [11]Recently the investigation for the prototype Fe(py)2Ni(CN)4 using temperature-dependent EXAFS has been carried out. [12]The effects on the SCO behaviors of Cl and CH3 replacements located at different positions on the S pyridine ring were also found. [13,14] 57Fe Mössbauer spectroscopic study of the SCO polymer complex, Fe(3-Clpy)2Ni(CN)4 was reported with the SQUID data. [13]ue to the physical and chemical properties of 2D and 3D frameworks, the related Hofmann-like SCO coordination polymer compounds have been developed extensively.  Some ohem contain 3D pillared Hofmann-type-organic frameworks with guest molecules associated with SCO behaviors.[22][23][24][25][26][27][28]41] Recently an interconversion of nuclear-spin isomers for hydrogen molecule H2 adsorbed in a Hofmanntype porous coordination polymers (PCPs), by the temperature dependence of Raman spectra has been reported.[33] Three types of 2D coordination polymer iron(II) spin crossover complexes containing 3,5-lutidine as ligands and guest molecules were reported with a general formula Fe(3,5-lutidine)2Ni(CN)4 • n(H2O)m (3,5-lutidine).[15,16] Hofmann-like spin crossover coordination compound Fe(3-Clpy)2Pd(CN)4 with thermal-, pressure-and light-induced SCO behaviors were described in details. [17,18] Recently Mössbauer spectroscopic study on Fe(3-Clpy)2Pd(CN)4 has been reported. [47] We rport here 57 Fe Mössbauer spectroscopic study on the new coordination polymer Fe(4-Clpy)2Ni(CN)4 (sample 1), which showed that this compound exhibits no SCO down to at least T = 80 K. Mössbauer spectra at R.T. and T = 80 K show one major doublet which correspond to the site1, which is a iron(II) high spin state, and two minor doublets with a large and a small quadrupole splitting, the site 2 and the site 3.In the site 1, iron(II) ions are coordinated by four N atoms of cyano groups and two N atoms of 4-Clpy ligand.The minor site 2 with the large quadrupole splitting is due to the presence of defects in the coordination polymer frameworks.The other minor site 3 with the small quadrupole splitting is probably due to the presence of iron(III) high spin states or iron(II) low spin states.

EXPERIMENTAL
We prepared the 2D coordination polymer Fe(4-Clpy)2Ni(CN)4 sample 1 in the ambient atmosphere by a method similar to that for the prototype Fe(py)2Ni(CN)4. 57Fe Mössbauer spectra were obtained using WissEl Mössbauer system consisting of MVT-1000, MR-260A, DFG-500 and CMCA-550.The temperature dependence of the spectra in the range of 80-295 K were measured by keeping the sample in a gas-flow type cryostat (Advanced Research System LT-3-100 Helitran System) and the source was kept at room temperature.The temperature of the sample was controlled by a 9620-1 digital temperature controller from Scientific Instruments.A silicon diode was used to measure the temperature.The spectra were computer-fitted using a MossWinn 3.0i software.The values of the isomer shift (δ) are given relative to α-iron foil at room temperature.
IR spectra were recorded on JASCO FT/IR-4100 in Nujol mulls between KBr windows.X-ray powder diffraction studies were performed on a RIGAKU RINT-2500VHF with Cu Kα radiation.

RESULTS AND DISCUSSION
In the present preparation of Fe(4-Clpy)2Ni(CN)4 (sample 1), citric acid and 1,3-diamonopropane were used to adjust pH of the mother aqueous solution containing Fe(NH4)2(SO4)2 • 6H2O and K2[Ni(CN)4].The precipitation rate of the sample 1 depends on the pH of the mother aqueous solution, and fine particles were precipitated due to the direct contact method.Powder XRD patterns for sample 1 (Figure 1S) are almost similar to those of Fe(4methyl-pyridine)2Ni(CN)4 which was determined by single crystal X-ray determination. [38]The proposal structure of sample 1 is shown in Figure 1.The layer consists of a square-meshed network of square-planar Ni(II) ion and octahedral iron(II) ions alternately linked by the bridged Table 1.Brief History of Hofmann-like Spin Crossover (SCO) Coordination Polymers.cyano groups.The geometry of iron(II) is accomplished by four N terminals of cyano groups and two N atoms of the 4-Clpy ligands in a trans configuration.The solid state IR for sample 1 was measured at room temperature in Nujol mulls between KBr windows (Figure 2S).The IR vibration from the Ni-CN-Fe sheet structure is due to 2148 cm -1 .This strongly suggests that the CN groups acts as bidentate bridging ligands.1593 cm -1 is due to a strong band of the pyridine ring.Considering the analytical studies, the powder XRD patterns and IR spectra of sample 1, sample 1 contains CN bridged 2D sheet framework structures as shown Figure 1.
The site 2 species in 1 with large quadrupole splitting (ΔEQ) can be also explained by the presence of defects in the coordination polymer frameworks. [2]The minor site 2 parameters with a large ΔEQ value as 1.31 mm s -1 at R.T. is also probably due to the FeN5(OH2) or FeN4(OH2)2 environment.The FeN5O core consists of an octahedral iron(II)   atom coordinated by four nitrogen atoms of cyano group, one nitrogen atom of the lutidine and one oxygen atom of the water ligand.The FeN4O2 core completes an octahedral iron(II) atom by coordinating two water molecules at the axial position.However, the Mössbauer parameter ΔEQ values for Fe(H2O)2Ni(CN)4 • 2C4H8O2 which contains the FeN4(OH2)2 environment (δ = 1.16,ΔEQ = 2.56 mm s -1 at R.T. and δ = 1.28,ΔEQ = 3.30 mm s -1 at T = 80 K ) are larger than those of the site 2. [36] Taking into account of particle sizes of sample 1, the site 2 doublet may be ascribed to the iron(II) units located at the nanocrystals edge as they must have coordinative defects, that is, coordinatively unsaturated sites or coordinated terminal water molecules. [20]s listed in Table the ratio of the site 1 to the site 2 and the site 3 at R.T. is 70 : 9 : 21°, that at T = 80 K is 81 : 5 : 14, respectively.The recoil-free fraction of the site 1 is different of that of the site 2 and the site 3 due to different octahedral iron(II) environments.The other minor site 3 with a small quadrupole splitting is probably due to iron(III) high spin state species.Iron(III) high spin species would be produced by an air oxidation during the synthesis or treatment process before the Mössbauer measurements.Since iron(III) high spin and iron(II) low spin are distinguished from those Mössbauer parameters, we cannot give an unambiguous assignment. 57Fe Mössbauer spectra of the Fe[Ni(CN)4] • 2pz (= Fe(pz)2Ni(CN)4) sample which is not a SCO material contain iron(II) low spin state atoms as the minor phase. [37]NCLUSION This study shows an example of ligand-dependence of SCO behavior in Hofmann-like SCO compounds.Mössbauer study for the sample 1 indicates Fe(4-Clpy)2Ni(CN)4 does not have SCO behavior, while both 3-methyl-pyridine Hofmann complex Fe(3-Methyl-py)2Ni(CN)4 and 3-chloropyridine Hofmann complex Fe(3-Clpy)2Ni(CN)4 show incomplete spin crossover behavior. [13,14]The Fe(4-Methylpy)2Ni(CN)4 coordination polymer is not a SCO compound. [14,38]The position-3 substituent of pyridine ring is associated with SCO behavior, while the position-4 substituents of pyridine ring do not allow SCO behavior.However, Ligand 4-X pyridine (X = Cl, Br, I) dependence in Hofmanntype spin crossover complexes: Fe(4-Xpyridine)2[Au(CN)2]2 has been reported. [39]Different properties between [Ni(CN)4] 2-and [Au(CN)2] -are associated with SCO behaviors in Hofmann-like Coordination polymer systems.Ligand effects on SCO behaviors are different from guest molecule effects. [40,41]e(4-Mepy) 2.10 g (8.11 mmol) of K2[Ni(CN)4] were added into 120 ml of water.To the water solution, 3.15 g (8.11 mmol) Mohr's salt (Fe(NH4)2(SO4)2 • 6H2O), and 7.95 g of citric acid were added.Then a light white blue precipitate formed immediately.The precipitate was dissolved by adding 4.0 ml of 1,3diaminopropane dropwise, adjusting the pH of the solution to 5.2.4-Cl-pyridine was added to the mixture solution.Pale light yellow solid particles were formed after several days as the sample 1.The sample 1 was characterized by 57 Fe Mössbauer and infrared (IR), X-ray diffraction (XRD) and elemental analysis.The C, H and N elemental analysis for the sample 1 was carried out with a Perkin-Elmer Model 2400 CHNS/O analyser and suggested the formation of Fe(4-Clpy)2Ni(CN)4 (Found C, 37.16; H, 1.24; N, 18.50.Calcd.C, 37.72; H, 1.81; N, 18.86).