Non-classical FeII spin-crossover behaviour in polymeric iron(II ) compounds of formula [Fe(NH2trz)3]X2 xH2O (NH2trz=4-amino-1,2,4-triazole; X=derivatives of naphthalene sulfonate) Petra J. van Koningsbruggen, Yann Garcia, Epiphane Codjovi, Rene� Lapouyade, Olivier Kahn,* Leopold Fourne`s and Louis Rabardel L aboratoire des Sciences Mole�culaires, Institut de Chimie de la Matie`re Condense�e de Bordeaux, UPR CNRS no. 9048, Avenue du Docteur Schweitzer, F-33608 Pessac, France A new series of iron(II) spin-crossover materials of formula [Fe(NH2trz)3]X2 xH2O [X=1-naphthalene sulfonate (1ns), 2-naphthalene sulfonate (2ns), 4-hydroxy-1-naphthalene sulfonate (4OH-1ns), 4-amino-1-naphthalene sulfonate (4NH2-1ns) and 6-hydroxy-2-naphthalene sulfonate (6OH-2ns)] are reported. The structure of these compounds consists of linear chains in which the Fe(II) ions are linked by triple N1,N2-1,2,4-triazole bridges.All compounds show non-classical spin-crossover behaviour. Optical and magnetic measurements recorded upon heating show an abrupt low-spin to high-spin transition accompanied by a pronounced thermochromic eVect occuring between 330 and 340 K depending on the anion.Thermogravimetric analyses show that the transition is induced by the removal of the two lattice water molecules, which initially stabilized the low-spin state. The dynamical character of this transition has been monitored by extended 57Fe Mo� ssbauer spectroscopic studies on [Fe(NH2trz)3](2ns)2 xH2O. Upon cooling, the dehydrated modifications show classical spin-crossover behaviour with hysteresis at much lower temperatures, ranging from 229 to 297 K depending on the anion.[Fe(NH2trz)3](2ns)2 represents one of the first iron(II) spin-crossover materials showing a spin transition in the close vicinity of room temperature (290 K) accompanied by hysteresis (14 K). Nowadays there is an increasing interest in new bistable iron(II) ing in the following order: ClO4-, I-, Br-, BF4-, NO3-.25,27 spin-crossover compounds, which show a transition from the EXAFS studies on the low-spin forms have shown that the high-spin state (HS, S=2) to the low-spin state (LS, S=0) on FeMN bond distances tend to decrease with the abovecooling, upon increasing pressure, or by light irradiation.1–10 mentioned order of substitution of the anions.25,26 Recently, the use of such materials as molecular-based memory Furthermore, 57Fe Mo� ssbauer spectroscopy studies27 and devices and displays has been investigated.8,11,12 The require- analyses of the X-ray fluorescence spectra24 confirm that this ments for this type of application can be formulated as follows: decrease in interatomic distances correlates with the covalence the compound must exhibit abrupt transitions with large of the FeMN bond in the same series of anions.Therefore, hysteresis (ca. 50 K), the middle of which should preferably be Lavrenova and coworkers concluded that this increase in situated close to room temperature. To allow the material to transition temperatures appears to be associated with the act as a display, the spin transition should be accompanied by increasing anion–cation interactions, and the thereby arising a change in color (thermochromism). Finally, the compound increasing ‘electrostatic pressure’ which causes a subsequent should be stable under normal operating conditions. 2,7,9,11,13,14 increasing compression of the FeN6 octahedron.25 Moreover, A promising class of materials meeting these criteria are the it appears that a rather low transition temperature (T c, where linear chain iron(II) compounds of general formula 50% high-spin FeII and 50% low-spin FeII are present) is [Fe(NH2trz)3]X2 xH2O (NH2trz=4-amino-1,2,4-triazole; accompanied by a rather small hysteresis loop, whereas an X=NO3-,9,15–17 ClO4-,18 BF4-,11,18 I-,19 Br-,11,18 increase in T c leads to an increase in hysteresis width.28 CH3SO3-,20 tosylate21). Although no X-ray crystallographic Within this family of 4-amino-1,2,4-triazole compounds the data are available on these iron(II ) compounds, it is obvious derivative containing tosylate21 shows a rather peculiar FeII that their structure must be comparable to that of spin-crossover behaviour involving a large apparent hysteresis [Cu(NH2trz)3](ClO4)2 0.5H2O.22 The EXAFS study on of about 80 K, which could be ascribed to the synergy between the related materials [Fe(Htrz)2(trz)](BF4) and the FeII spin-crossover behaviour and a dehydration process.[Fe(Htrz)3](BF4)2 H2O (Htrz=1,2,4-4H-triazole; trz=1,2,4- In fact, this type of synergy appears to be a general feature triazolato) confirms that a similar structure can indeed be particular to FeII linear chain compounds containing this obtained with FeII.23 Very recently, a detailed EXAFS study type of planar aromatic anions.Recently, we have observed finally allowed the acquisition of direct information on the the same phenomenon in [Fe(hyetrz)3]X2 3H2O (hyetrz=4- eVect of FeII spin transition on the spatial and electronic (2¾-hydroxyethyl)-1,2,4-triazole; X=3-nitrophenylsulfonate), structure of [Fe(NH2trz)3]X2 (X=NO3-, BF4-, Br-, ClO4-) which gave rise to an unprecedented apparent hysteresis and the magnetically diluted phases [FexZn1-x width of 270 K.29 The structure of the copper(II) analogue (NH2trz)3](NO3)2.24–26 In these compounds the FeII ions are [Cu(hyetrz)3](ClO4)2 3H2O has been elucidated and indeed linked by triple N1,N2-1,2,4-triazole bridges.This direct linkage consists of linear chains in which the CuII ions are linked by of the iron(II) centers results in a large cooperativity of the triple N1,N2-1,2,4-triazole bridges.30 It is worth noting that the spin-crossover behaviour, and therefore considerable hysteresis large thermal hysteresis (60 K) of the mononuclear compound has been found in these materials.7,9 Variation of the non- [Fe{HB(pz)3}]2 [HB(pz)3=tris(1-pyrazolyl)borate]31 is not coordinated anion in [Fe(NH2trz)3]X2 xH2O leads to comdue to the mechanism mentioned above.In fact, the transition pounds with significantly diVerent spin-crossover characterfrom low-spin to high-spin FeII occurring at about 400 K is istics. The temperature of the transition to the high-spin state increases when the radius of the anion diminishes upon chang- associated with a crystallographic phase transition, whereby J.Mater. Chem., 1997, 7(10), 2069–2075 2069the initially well formed crystals shatter into extremely small Synthesis of [Fe(NH2trz)3]X2 2H2O (X=1ns, 2ns, 4OH-1ns, 4NH2-1ns and 6OH-2ns) fragments.31 To study the role of planar aromatic anions in further detail A methanolic solution (40 ml ) containing 2 mmol (1.2 g) of in the 4-amino-1,2,4-triazole systems we have selected the [Fe(H2O)6](2ns)2 and a small amount of ascorbic acid was sulfonate derivatives of naphthalene. This choice allows a added under stirring to a methanolic solution (15 ml ) containsystematic variation of the anion.The sulfonate group can be ing 6 mmol (0.5 g) of 4-amino-1,2,4-triazole.Instantaneously, in two diVerent positions, and other substituents capable of a white precipitate formed, which was filtered and dried in air. forming hydrogen bonds (such as the hydroxy and the amino During the drying the compound turned pink. Yield: 1.04 g group) may be attached to the naphthalene ring system. Here (69%).Elemental analyses. Calc. for C26H30S2O8FeN12 we report on the spin-crossover behaviour of the {[Fe(NH2trz)3](2ns)2 2H2O}: C, 41.17; H, 3.99; N, 22.16; S, [Fe(NH2trz)3]X2 xH2O derivatives containing the mono- 8.45; Fe, 7.36. Found: C, 39.61; H, 3.88; N, 22.77; S, 8.19; valent anions 1-naphthalene sulfonate (abbreviated as 1ns), 2- Fe, 7.20%. naphthalene sulfonate (2ns), 4-hydroxy-1-naphthalene sulfon- The compounds containing other naphthalene sulfonate ate (4OH-1ns), 4-amino-1-naphthalene sulfonate (4NH2–1ns) anions have been prepared analogously.The elemental analyses and 6-hydroxy-2-naphthalene sulfonate (6OH-2ns) (see Fig. 1). of these compounds suggest the same composition as for the 2ns derivative. However, in all cases the C and N analyses indicate a small deficit of ligand.This feature has been generally Experimental ob this type of polynuclear iron(II ) compounds and may be attributed to the presence of a mixture of linear triply Materials N1,N2-1,2,4-triazole bridged compounds diVering in chain Commercially available solvents were used without further length.10 The terminal FeII ions of such a chain are supposed purification.FeCl2·4H2O, 4-amino-1,2,4-triazole and the to have one or more water ligands; consequently, these ions sodium salts of 1-naphthalene sulfonic acid and 6-hydroxy-2- remain in the high-spin state over the whole temperature naphthalene sulfonic acid were purchased from Aldrich. The range. Furthermore, it may not be excluded that the polymeric sodium salts of 2-naphthalene sulfonic acid, 4-hydroxy-1- nature of these compounds gives rise to particular diYculties naphthalene sulfonic acid and 4-amino-1-naphthalene sulfonic in the determination of the elemental analyses.acid were purchased from Acros. The iron(II) salts of the various naphthalene sulfonates were prepared by mixing aque- Results and Discussion ous solutions of iron(II ) chloride and the corresponding naphthalene sulfonate sodium salt.The compounds [Fe(NH2trz)3]X2 xH2O (X=1ns, 2ns, 4OH- 1ns, 4NH2-1ns, 6OH-2ns) appear as purple–pink powders. This pink color is due to the 1A1g�1T1g d–d transition of low- Measurements spin FeII occurring at 520 nm. The color of the compounds changes to white upon heating to ca. 340 K. This white color Elemental analyses were performed by the Service Central is due to the fact that the spin-allowed d–d transition of lowest d’Analyse (CNRS) in Vernaison, France. 57Fe Mo� ssbauer energy of the compound in high-spin state, 5T2g�5Eg, occurs measurements were performed using a constant acceleration at the limit of the visible and IR regions. Halder-type spectrometer with a room temperature 57Co source (Rh matrix) in a transmission geometry.All isomer Optical measurements shifts reported in this work refer to natural iron at room temperature. The spectra were fitted to the sum of Lorentzians Since these compounds are highly thermochromic, the FeII by a least-squares refinement. Thermogravimetric measure- spin transition has been studied optically using a home-made ments were carried out with a Setaram apparatus in the device.7,21 This technique provides an accurate determination temperature range 300–400 K under ambient atmosphere.of the transition temperatures, however it does not give any Magnetic susceptibilities were measured in the temperature information on the percentage of FeII ions involved in the spin range 77–400 K with a fully automatized Manics DSM-8 transition. Since the possible use of these materials in molecular susceptometer equipped with an Oxford instruments DN170 electronics requires stability of the physical behaviour of the continuous-flow cryostat and a Bruker BE15f electromagnet compounds (i.e.retaining of the hysteresis loop) the spinoperating at ca. 0.8 Tesla. Data were corrected for magnetizcrossover behaviour has been investigated during at least three ation of the sample holder and for diamagnetic contributions, thermal cycles.In all cases the temperature has firstly been which were estimated from the Pascal constants. Optical raised to 400 K followed by additional cooling and heating measurements have been carried out using the device described experiments. The results of these optical measurements are previously.7,21 shown in Table 1.All compounds virtually show the same physical properties. It is worth noting that the spin-crossover behaviour shows a paramount analogy with that of the tosylate derivative.21 For Table 1 Results of the optical measurements for [Fe(NH2trz)3]X2 xH2Oa first cycle second cycle third cycle X Tc( Tc3 Tc( Tc3 Tc( Tc3 1ns 330 229 235 229 235 229 4OH-1ns 332 230 240 230 240 230 4NH2-1ns 340 225 235 225 235 225 SO3 – SO3 – SO3 – OH NH2 SO3 – HO SO3 – Fig. 1 Structures of 1-naphthalene sulfonate (1ns), 4-hydroxy-1- 2ns 340 283 297 283 297 283 6OH-2ns 334 260 270 265 270 265 naphthalene sulfonate (4OH-1ns), 4-amino-1-naphthalene sulfonate (4NH2-1ns), 2-naphthalene sulfonate (2ns), and 6-hydroxy-2-naphthalene sulfonate (6OH-2ns) aTc/K has been taken at the half intensity height. 2070 J. Mater. Chem., 1997, 7(10), 2069–2075instance, [Fe(NH2trz)3](1ns)2 xH2O (see Fig. 2) shows upon Interestingly, under these conditions we observed that the compound gradually transforms to its high-spin state. heating a very abrupt low-spin�high-spin transition occurring at 330 K. Decreasing the temperature reveals a very smooth Apparently, even at temperatures lower than 340 K, the temperature at which the low-spin to high-spin transition is found high-spin�low-spin transition with Tc3=229 K.A second heating of the sample shows a rather smooth low-spin�high- in the heating mode while heating at 1 K min-1, the compound is capable of exhibiting spin-crossover behaviour. This feature spin transition at Tc(=235 K.Subsequent heating and cooling cycles indicate that this small hysteresis (Tc(=235 K, Tc3= may be explained by the mechanism associated with this spin transition in the first heating mode, which will be analyzed in 229 K) is retained. Compounds containing the related anions 2ns, 4OH-1ns, 4NH2-1ns and 6OH-2ns show a similar behav- detail in the discussion.iour, diVering only in the position of the transition temperatures. Thermogravimetry Among these compounds the 2ns derivative shows very Thermogravimetric analysis (see Fig. 4) carried out with the interesting characteristics of the stable spin-crossover behavsame velocity of heating (1 K min-1) as for the optical iour occurring in the second and further heating and cooling measurements has been carried out for the 1ns and 2ns experiments (see Fig. 3). A first heating (1 K min-1) results in derivatives. These measurements reveal a continuous loss of an abrupt low-spin�high-spin transition at 340 K. Upon mass starting at room temperature. This decrease in mass cooling a rather abrupt high-spin�low-spin transition occurs proceeds rapidly in the temperature range 315–350 K, after at Tc3=283 K.Heating the compound once more reveals which it continues in a much smoother fashion. It is important another abrupt low-spin�high-spin transition, now situated to notice that at the transition temperatures T c, 330 and 340 K at 297 K. Additional heating and cooling experiments show for 1ns and 2ns, respectively, the mass% lost corresponds to that this hysteresis of 14 K centered in the close vicinity of the removal of 0.5 and 1.3 molecules of lattice water for 1ns room temperature (290 K) remains stable.and 2ns, respectively. If we assume that the measuring con- The rate of heating has a paramount eVect on the spinditions are really identical in the optical and thermogravimetric crossover behaviour. In fact, when the optical measurements measurements, this would imply that the spin transition already for the 2ns derivative are carried out at a heating rate of 0.1 K starts while there are still water molecules present in the sample min-1, the low-spin to high-spin transition proceeds in an and proceeds in a moderately abrupt way until all water abrupt fashion at 330 K.The increasing of the heating rate molecules have been released.This is in sharp contrast to above 1 K min-1 does not aVect the spin-crossover charac- [Fe(hyetrz)3](3-nitrophenylsulfonate)2 3H2O,29 where under teristics with respect to the measurements carried out at identical measuring conditions the spin transition has been 1 K min-1: in all cases, the transition takes place abruptly found to take place in a very abrupt ‘explosive’ fashion at the at 340 K.In addition, optical measurements of moment all water molecules have been removed from the [Fe(NH2trz)3](2ns)2 xH2O have also been carried out by sample. The measurement cell used in the thermogravimetric keeping the sample at a fixed temperature lower than 340 K. analyses is in direct contact with the air. Upon cooling to In a typical experiment the temperature was fixed at 326 K.room temperature an increase in the mass of the 1ns and 2ns samphich indicates that the compounds are being rehydrated: the 1ns compound then has recovered 1.35 molecules of water per formula unit, while the 2ns derivative has only been rehydrated by 1 molecule of water per FeII ion. On the contrary, [Fe(hyetrz)3](3-nitrophenylsulfonate) 2 3H2O has been found not to rehydrate under these experimental conditions.29 57Fe Mo� ssbauer spectroscopy 57Fe Mo�ssbauer spectra have been recorded for [Fe(NH2trz)3](2ns)2 xH2O in the heating mode in the temperature range 293–400 K.Representative Mo�ssbauer spectra are shown in Fig. 5, whereas detailed values of the Mo�ssbauer parameters resulting from the least-squares fitting procedure are listed in Table 2.The area fractions have been calculated Fig. 2 Optical measurement (intensity vs. temperature; recorded at 1 K min-1) for [Fe(NH2trz)3](1ns)2 xH2O Fig. 4 Thermogravimetric analysis (recorded at 1 K min-1) for Fig. 3 Optical measurement (intensity vs. temperature; recorded at [Fe(NH2trz)3](1ns)2 xH2O (dotted line) and [Fe(NH2trz)3](2ns)2 xH2O (full line) 1 K min-1) for [Fe(NH2trz)3](2ns)2 xH2O J.Mater. Chem., 1997, 7(10), 2069–2075 2071Fig. 6 Temperature dependence of the high-spin area fraction xHS as determined by 57Fe Mo� ssbauer spectroscopy for [Fe(NH2trz)3](2ns)2 xH2O. Data obtained from the first (#), second (2) and third (6) heating experiment are included. NO3-, BF4-, Br-, I- and ClO4-.27 This supports the fact that we are dealing with polynuclear compounds in which the active iron(II) spin-crossover sites are provided by FeII ions in a six nitrogen environment.The spin transition is neither complete at low temperatures nor at higher temperatures. The percentage of high-spin FeII detected at lower temperatures may be attributed to ‘defects’ in the crystal lattice, for instance, caused by terminal FeII ions having water molecules in the coordination sphere.Indeed, it has already been reported for this type of iron(II) compounds that the residual high-spin FeII fraction is generally rather high.10 Fig. 6 shows the temperature dependence of the relative Fig. 5 Selected 57Fe Mo� ssbauer spectra for [Fe(NH2trz)3](2ns)2 high-spin area fraction (AHS) as obtained from accumulation xH2O of the spectra for about two days.The data obtained from three heating experiments have been included. Interestingly, all measured data fall upon the same curve showing the assuming identical Lamb–Mo�ssbauer factors for the high-spin and the low-spin state. At 293 K, where according to the recovery of the sample after each heating and subsequent cooling.From these data it may be deduced that the transition optical data the spin transition has not yet started, the spectrum is characterized by a central doublet with a small quadrupole temperature would be 326 K, a value significantly lower than the 340 K obtained from the optical measurements. On first splitting (DEQ) of 0.187(3) mm s-1 and an isomer shift (d) of 0.432(3) mm s-1.The spectral contribution for this doublet is sight these findings seem to be in contradiction. However, in order to interpret all data recorded during the spin transition 92%. This doublet may be assigned to FeII ions in low-spin state. Furthermore, a doublet with a significantly larger quad- it is important to focus on the mechanism of the present FeII spin crossover.In fact, we are dealing with a non-classical rupole splitting [2.70(4) mm s-1] and d of 1.07(4) mm s-1 (spectral contribution=8%) has been observed, which can be spin-crossover behaviour which is induced by the removal of water molecules (vide infra). Clearly, this loss of water molecules attributed to FeII in high-spin state. At increasing temperatures this FeII low-spin doublet gradually decreases in intensity, is governed by a kinetics that is rather slow at lower temperatures, and increases at higher temperatures. Consequently, the while the doublet with larger quadrupole splitting gains intensity.At 373 K, only 13% of FeII in low-spin state is present temperature at which the equilibrium value of 50% high-spin and 50% low-spin FeII is detected significantly depends on the [DEQ=0.341(3) mm s-1 and d=0.18(3) mm s-1]. The fraction of FeII in high-spin state is 87% and the doublet is characterized measuring conditions.If the increase of temperature is rather fast, e.g. 1 K min-1, in the optical measurements, the transition by DEQ=2.532(3) mm s-1 and d=0.986(3) mm s-1. Similar values for these Mo�ssbauer parameters have also been reported temperature is found at 340 K.By Mo�ssbauer spectroscopic measurements a time-averaged spectrum is collected over a in an extended study on a series of linear iron(II ) chain spincrossover compounds of 4-amino-1,2,4-triazole with the anions much longer period of time. This allows the dehydration of Table 2 Mo�ssbauer parameters (mm s-1) for [Fe(NH2trz)3](2ns)2 xH2Oa T /K d(LS) DEQ(LS) C/2(LS) d(HS) DEQ(HS) C/2(HS) AHS(%) 293 0.432(3) 0.187(3) 0.274(3) 1.07(4) 2.70(4) 0.43(7) 8 316 0.422(4) 0.191(4) 0.261(4) 1.06(3) 2.65(3) 0.39(6) 15 321 0.415(7) 0.178(7) 0.284(7) 1.01(6) 2.67(6) 0.6(1) 18 326 0.413(7) 0.173(7) 0.318(9) 1.017(4) 2.700(4) 0.294(7) 51 331 0.40(1) 0.16(3) 0.34(2) 1.011(3) 2.680(3) 0.284(5) 72 335 0.39(2) 0.12(2) 0.35(3) 1.006(6) 2.664(6) 0.29(1) 80 345 0.26(3) 0.30(3) 0.47(9) 1.002(6) 2.651(6) 0.26(1) 78 354 0.24(4) 0.25(4) 0.44(5) 0.995(2) 2.603(2) 0.278(4) 85 364 0.22(4) 0.30(3) 0.41(8) 0.988(5) 2.561(5) 0.280(9) 87 398 0.18(2) 0.29(2) 0.32(3) 0.971(2) 2.434(2) 0.284(4) 87 a=isomer shift, DEQ=quadrupole splitting, C/2=half-width of the lines, AHS=area fraction of the high-spin doublets. 2072 J. Mater. Chem., 1997, 7(10), 2069–2075the sample to occur at a much lower temperature.Therefore, it might not be correct to directly compare these d values, since the variation observed in these values is extremely small a lower transition temperature of 326 K has been observed. Indeed, these findings are in agreement with the optical and may fall within the uncertainty range. Furthermore, it is certainly not clear whether the present compound would obey measurements performed with a heating rate of 0.1 K min-1, where a transition temperature of 330 K has been found.this relation, since its non-classical spin-crossover mechanism involving the release of lattice water molecules certainly diVers Furthermore, the optical measurements carried out at a fixed temperature below 340 K (326 K) have shown that the from those of the compounds investigated by the Russian group. compound is also capable of exhibiting the spin-crossover behaviour at these lower temperatures.In order to confirm the dynamical character of this non- Magnetic measurements classical spin transition, additional Mo�ssbauer spectroscopic Variable-temperature magnetic susceptibility measurements measurements have been performed.In an experiment, the have been carried out in order to obtain additional infor- temperature has been fixed at 326 K and aMo� ssbauer spectrum mations on the completeness of the non-classical spin transition has been recorded every 30 minutes. As the measurement of [Fe(NH2trz)3](2ns)2 xH2O. At 307 K the xT value [x being proceeds at this fixed temperature, the compound starts to the magnetic susceptibility per iron(II) ion and T the tempera- transform to the high-spin state.Probably, the compound ture] is 0.248 cm3 K mol-1. Upon increasing the temperature, slowly starts to loose its lattice water molecules. The xT attains a value of 3.512 cm3 K mol-1 at 364 K, which Mo�ssbauer data listed in Table 3 show the evolution of the roughly corresponds to the value expected for a quintet state.percentage of FeII in high-spin state as a function of time. The percentages listed correspond to the accumulation of the timeaveraged spectra over the total measuring time indicated. After Concluding remarks 30 min 40.08% of FeII in high-spin state is observed. As the time proceeds, the high-spin FeII fraction gradually increases.Here we have described a new series of polynuclear iron(II) spin-crossover materials based on the family [Fe(NH2- Finally, the spectrum recorded after accumulation of the data over 300 min shows Interestingly, trz)3]X2 xH2O (X=1ns, 2ns, 4OH-1ns, 4NH2-1ns and 6OH-2ns). All these compounds which contain derivatives of the same percentage of high-spin FeII ions (51%) has been found at 326 K, while accumulating the Mo�ssbauer data for naphthalene sulfonate as non-coordinating anions show similar characteristics in their spin-crossover behaviour as the pre- about two days.Apparently, to reach the equilibrium ratio of high-spin and low-spin FeII under these experimental con- viously reported tosylate derivative.21 In particular, the physical properties of the compounds are determined by the synergy ditions and at this temperature a measuring time of about 300 min is required, after which this ratio remains stable.These between the FeII spin-crossover behaviour and a dehydration– rehydration process. For all the compounds, the mechanism measurements clearly indicate the non-classical character of this spin transition.We may then interpret the identical describing the physical properties is as follows: at room temperature the thermodynamically stable state for the physical behaviour observed during the three heating experiments (see Fig. 6) by a complete rehydration taking place upon hydrated compound [Fe(NH2trz)3](naphthalene sulfonate derivative)2 xH2O is the low-spin state.Evidently, this low- cooling of the sample, which resides in a measurement cell open to the air. spin state is stabilized by the hydrated nature of this modifi- cation. Indeed, studies on mononuclear iron(II ) spin-crossover In addition, the significant line broadening of the high-spin absorption observed in the spectrum recorded at 321 K [C/2= compounds have already revealed that the low-spin state may be stabilized by interactions with lattice water molecules.32–35 0.6(1) mm s-1] may also be indicative of the presence of highspin FeII ions in diVerent FeN6 environments.Upon heating, the compound starts to lose its lattice water molecules, and consequently the stabilization of the low-spin For a series of related iron(II) compounds exhibiting classical spin transitions Varnek and Lavrenova have observed a state ceases. For the present series of naphthalene sulfonate compounds, it appears that the destabilization of the low-spin decrease in the isomer shift value for the low-spin state (determined either at 78 K or at 295 K) as a function of state already occurs while the compounds are not yet completely dehydrated.At first sight this may seem to be increasing transition temperatures.27 This linear correlation showed a much steeper dependence at 78 than at 295 K. These in contrast with [Fe(hyetrz)3](3-nitrophenylsulfonate)2·xH2O, where optical and thermogravimetric measurements carried authors have concluded that this corresponds to a higher degree of covalency of the FeMN bonds as the transition out at an identical heating rate (1 K min-1) have shown that no lattice water molecules are present in the sample as the temperature increases.The d value at 293 K of 0.432(3) mm s-1 for the present compound is somewhat higher than the low-spin to high-spin transition occurs in an extremely abrupt way at 370 K.29 However, recently, new experiments on values for the series of compounds reported by the Russian group, which range from 0.427 to 0.416 mm s-1.This would [Fe(hyetrz)3](3-nitrophenylsulfonate)2 xH2O have been carried out: the optical data recorded while fixing the temper- imply that a transition temperature lower than room temperature would be expected for the present compound. However, ature at a value below 370 K show that also at these lower Table 3 Evolution of the Mo�ssbauer parameters (mm s-1) as a function of time at 326 K for [Fe(NH2trz)3](2ns)2 xH2Oa t/min d(LS) DEQ(LS) C/2(LS) d(HS) DEQ(HS) C/2(HS) AHS(%) 30 0.41(3) 0.14(3) 0.28(3) 0.99(3) 2.71(3) 0.26(4) 40.08 60 0.41(1) 0.16(1) 0.28(2) 1.01(2) 2.69(2) 0.26(3) 40.20 90 0.41(1) 0.16(1) 0.31(2) 1.01(1) 2.69(1) 0.28(2) 41.82 120 0.41(1) 0.17(1) 0.28(1) 1.01(1) 2.03(1) 0.27(2) 43.39 150 0.41(1) 0.19(1) 0.28(1) 1.01(1) 2.70(1) 0.28(2) 43.62 180 0.41(1) 0.19(1) 0.28(1) 1.01(1) 2.71(1) 0.27(1) 44.45 210 0.412(9) 0.193(9) 0.29(1) 1.014(9) 2.711(9) 0.29(1) 46.65 240 0.414(9) 0.188(9) 0.29(1) 1.015(9) 2.708(9) 0.30(1) 47.81 270 0.41(1) 0.18(1) 0.29(1) 1.01(1) 2.701(1) 0.30(1) 48.51 300 0.413(7) 0.173(7) 0.318(9) 1.017(4) 2.700(4) 0.294(7) 51.00 ad=Isomer shift, DEQ=quadrupole splitting, C/2=half-width of the lines, AHS=area fraction of the high-spin doublets.J. Mater. Chem., 1997, 7(10), 2069–2075 2073temperatures the material slowly transforms to the high-spin completely rehydrate the dehydrated sample back to its initial state. This process of spin crossover induced by dehydration state.36 Therefore, also in that case the spin transition is associated with the loss of lattice water molecules.There are (low-spin to high-spin) and reversible by hydration (high-spin to low-spin) is entirely reproducible. In fact this process may indications that [Fe(NH2trz)3](tosylate)2 xH2O has similar characteristics for the low-spin to high-spin transition taking be regarded as a self-assisted process related to the solvateinduced change of spin state, which has already been reported place at 361 K. 21 For the series of compounds with derivatives of naphthalene sulfonate as anion, the transition temperatures for [Fe(hyetrz)3](3-nitrophenylsulfonate)2 3H2O.29 The dehydrated modifications show spin-crossover behav- as determined by optical measurements using a velocity of 1 K min-1 are in the range 330–340 K.Obviously, these lower iour at lower temperatures. The genuine spin transition with stable hysteresis is centered at temperatures ranging from 229 transition temperatures reflect the easier destabilization of the low-spin state in compounds containing naphthalene sulfonates to 297 K for the various compounds. The diVerences observed in transition temperatures within the series may be due to as compared to phenyl sulfonates. From the proposed mechanism it follows that this spin- slight structural variations induced by the anions.For [Fe(NH2trz)3](2ns)2 a stable hysteresis of 14 K centered crossover behaviour should be entirely governed by the removal of lattice water molecules. Indeed, various experiments around 290 K, i.e. in close vicinity to room temperature, has been found.Up to now, such features have been observed have shown that the observed transition temperatures depend very much on the measurement conditions, in particular the for a few compounds with 4-amino-1,2,4-triazole. For [Fe(NH2trz)3](CH3SO3)2·H2O, Bronisz et al. reported a hys- heating rate. For all hydrated naphthalene sulfonate compounds, the teresis of 26 K centered around 282 K.20 Surprisingly, these authors also reported that the dehydrated form of this com- transition temperatures for this first low-spin to high-spin transition all occur in a very narrow temperature range.This pound shows spin-crossover behaviour with a small hysteresis (4–8 K) centered around ca. 296 K.20 Furthermore, indicates that the destabilization of the low-spin state or alternatively, the removal of the lattice water molecules occurs [Fe(NH2trz)3](tosylate)2 has been reported to have a hysteresis of 17 K around 290 K.21 Moreover, the mixed-ligand material in a similar way.This is also supported by the results of the thermogravimetric studies showing very similar dehydration [Fe(Htrz)3-3x(NH2trz)3x](ClO4)2 xH2O has been reported to exhibit hysteresis of 17 K centered around 304 K. 14 Therefore, characteristics for the 1ns and 2ns derivatives (see Fig. 4). Therefore, it may be supposed that the anions of the naphtha- [Fe(NH2trz)3](2ns)2 represents one of the very few iron(II) spin-crossover materials showing a spin transition and an lene sulfonate type also have similar capabilities for hydrogen bonding with the lattice water molecules.On the other hand, associated thermochromic eVect in close vicinity of room temperature with hysteresis. compounds containing derivatives of phenyl sulfonate generally show higher transition temperatures.21,29 Clearly, in these latter compounds the loss of water molecules is more diYcult, which may re from a diVerent way of incorporation of the References anions in the crystal lattice allowing stronger hydrogen-bond- 1 P.Gu� tlich, Struct.Bonding (Berlin), 1981, 44, 83. ing interactions with the lattice water molecules. 2 J. Zarembowitch and O. Kahn, New J. Chem., 1991, 15, 181. Furthermore, in view of the striking similarities (identical 3 P. Gu� tlich, in Mo�ssbauer Spectroscopy Applied to Inorganic transition temperature for the first low-spin to high-spin Chemistry, ed.G. J. Long, Modern Inorganic Chemistry Series, transition) between the spin-crossover behaviour of tosylate Plenum Press, New York, 1984, vol. 1. compounds containing the 4-amino-1,2,4-triazole21 and 4- 4 E.Ko� nig, Prog. Inorg. Chem., 1987, 35, 527. 5 P.Gu� tlich and A. Hauser, Coord. Chem. Rev., 1990, 97, 1.alkyl-substituted 1,2,4-triazoles,37 it may be excluded that 6 P. Gu� tlich, A. Hauser and H. Spiering, Angew. Chem., Int. Ed. hydrogen bonding interactions involving the 4-amino substitu- Engl., 1994, 33, 2024. ent and the lattice water molecules are the determining factor 7 O. Kahn and E. Codjovi, Philos. T rans. R. Soc. L ondon A, 1996, in maintaining these lattice water molecules. 354, 359. This leads us to the idea that the lattice water molecules in 8 O. Kahn,MolecularMagnetism, VCH, New York, 1993. such compounds are bound in a very loose fashion, which 9 O. Kahn, E. Codjovi, Y. Garcia, P. J. van Koningsbruggen, R. Lapouyade and L. Sommier, in Molecule-Based Magnetic might be comparable to the way water molecules are incorpor- Materials, ed.M. M. Turnbull, T. Sugimoto and L. K. Thompson, ated in zeolites. Indeed, a zeolite is an aluminosilicate with a Symp. Ser. No. 644, ACS,Washington, DC, 1996, p. 298. structure enclosing cavities occupied by large ions and water 10 J. G. Haasnoot, in Magnetism: A Supramolecular Function, ed. molecules, both having considerable freedom of movement O. Kahn, Kluwer Academic, Dordrecht, Netherlands, 1996, p. 299. which permits ion exchange and reversible dehydration. For 11 O. Kahn, J. Kro�ber and C. Jay, Adv.Mater., 1992, 4, 718. [Fe(NH2trz)3](aryl sulfonate)2 xH2O one can propose that 12 C. Jay, F. Grolie`re, O. Kahn and J. Kro�ber,Mol. Cryst. L iq. Cryst., 1993, 234, 255. the shape and size of the diVerent aryl sulfonates induce the 13 J. Kro� ber, J.-P. Audie`re, R.Claude, E. Codjovi, O. Kahn, pore size of the polymeric compounds which, in turn, lead to J. G. Haasnoot, F. Grolie`re, C. Jay, A. Bousseksou, J. Linare`s, a considerable variation in the rate of water uptake or release. F. Varret and A. Gonthier-Vassal, Chem.Mater., 1994, 6, 1404. It may be proposed that the stabilization of the low-spin state 14 J. Kro� ber, E. Codjovi, O.Kahn, F. Grolie`re and C. Jay, J. Am. of FeII by lattice water molecules results from a cooperative Chem. Soc., 1993, 115, 9810. solvation, where hydrogen bonding of these water molecules 15 L. G. Lavrenova, V. N. Ikorskii, V. A. Varnek, I. M. Oglezneva and S. V. Larionov, J. Struct. Chem., 1993, 34, 960. to the sulfonate group increases the nucleophilicity of the 16 L. G. Lavrenova, V.N. Ikorskii, V. A. Varnek, I. M. Oglezneva water oxygen atom, and hence its interaction with FeII mainand S. V. 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