Synthesis and spin-crossover characteristics of polynuclear 4-(2′-hydroxy-ethyl)-1,2,4-triazole Fe(II) molecular materials

C. R. Acad. Sci. Paris, t. 1, S6rie II c, p. 523-532, 1998 Chimie inorganique mol6culaire/Molecular inorganic chemistry

Synthesis and spin-crossover characteristics of polynuclear 4-(2"-hydroxy-ethyl)-l,2,4- triazole Fe(ll) molecular materials Yann GARCIA a, Petra J. van KONINGSBRUGGEN a'b, Ren~ L A P O U Y A D E a, Louis RABARDEL a, Olivier KAHN a*, Maciej WIECZOREK ~, Robert BRONISZ ~, Zbigniew CIUNIK ~, Mikolaj F. RUDOLF c

~ Laboratoire des sciences mol&ulaires, Institut de chimie de la mati~re condens& de Bordeaux, UPR CNRS 9048, avenue du Dr-A.-Schwekzer, 33608 Pessac, France E-mail: [email protected]

b Department of Chemistry HEB2124, University of Utah, Salt Lake City, UT 84112-0850, USA

~ Faculty of Chemistry, University ofWroclaw, 14F. Joliot-Curie, 50-383 Wrodaw, Poland

(Received 18 March 1998, accepted 19 May 1998)

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~ili ~ !ii~il !~ ~ ~ Abstract - A new series of Fe(II) spin-crossover materials of tbrmula [Fe(hyetrz)3](Anion)2.xH20, where hyetrz = 4-(2'-hydroxy-ethyl)-l,2,4-triazole and Anion = CI-, NO3-, Br-, F, BF4-, C104-, PF6-, have been prepared and the spin transition characteristics studied. 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. Most of the hydrated compounds show non-classical spin-crossover behaviour associated with the removal of lattice water molecules, which initially stabilize the low-spin state. However for two ofthern, the perchlorate and the iodide compounds, the transition temperature is shifted to higher temperatures by dehydration. For the corresponding dehydrated compounds, the transition temperature Tl/2 increases with decreasing anion radii. [Fe(hyetrz)3]I 2 represents one of the few Fe(II) spin-crossover materials showing a spin transitian in the close vicinity of room temperature (291 K) accompanied by a thermal hysteresis (12 K). © Acaddmie des sciences/Elsevier, Paris

spin transition / hysteresis / anion / size / iron (II)

R 6 s u m 6 - Synth6se et caract6risation de nouveaux mat6riaux mol6culaires ~ transition de spin base de 4-(2"-hydroxy-6thyl) - l ,2 ,4- tr iazole . Une nouvelle sdrie de compos& du Fe(II) ~ transition de spin, de formule [Fe(hyetrz)3](Anion)2.xH20, oh hyetrz = 4-(2'-hydro'cy-&hyl)-1,2,4-triazole et Anion = CI-, NO3-, Br-, I-, BF4-, CIO4-, PF 6 , a dtd prdpar& et &udi&. Ces composds sont constituds de cha/nes lindaires dans lesquelles les ions Fe 2÷ sont relids par trois ponts form& par les amtes N1 et N2 des ligands 1,2,4-triazole. La plupart des compos& hydrat& prdsentent une transition de spin non classique, gouvernde par la perte des mold- cules d'eau qui, initialement, stabilisaient l'dtat bas spin. Deux d'entre eux (CIO 4- et I-), cependant, pr&entent une tempdrature de transition ldg&ement augment& par ddshydratation. Lorsque ces composds ont dt~ ddshydrat& par chauffage, leur tempdrature de transition Tl/2 augmente quasi lindairement avec le rayon de l'anion. Lorsque l'anion utilisd est l'iodure, le composd correspondant prdsente une transition de spin proche de la temp&ature ambiante ( Tl/2~" = 297 K et T1/25 = 285 K) accompagn& d'une hyst&&is thermique (12 K), ce qui a rarement dtd observd avec les compos& du Fe(II) ~ transition de spin. © Acaddmie des sciences/Elsevier, Paris

transition de spin / hysteresis / anion / rayon / fer (lI)

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Communicated by Olivier KAHN.

* C o r r e s p o n d e n c e a n d r e p r i n t s .

1 2 5 1 - 8 0 6 9 / 9 8 / 0 0 0 1 0 5 2 3 © Acad~mie des sciences/Elsevier, P a r i s 5 2 3

Y. Garcia et al.

Version frangaise abr6g6e

L'intdret portd aux matdriaux moldculaires du Fe(II) [,our lesquels on observe une transition de l'dtat haut spin (HS, S = 2) vers l'&at bas spin (LS, S = 0), sous l'effet d'un refroidissement, d'une aug- mentation de pression ou d'une irradiation lumineuse n'a cessd de croltre durant ces dix derni~res anndes [1-8]. La famille des compos& du Fe(II)/l base de d&ivds du 1,2,4-triazole substituds par un groupe R sur l'azote 4, a dtd tr~s dtudide pour leur propridtds physiques qui pourraient permettre des applications en dlectronique moldculaire [5, 8-10]. Dans ce travail, nous avons prdpard et dtudid une nouvelle famille de composds de formule [Fe(hyetrz)3](Anion)2"xH20 oh hyetrz = 4-(2'-hydroxy- &hyl)-l,2,4-triazole (schdma 1) et Anion = Cl-, NO3-, Br-, I-, BF4-, CIO4-, PF 6- oh la chalne hydroxydthyl sur l'azote 4 a dtd choisie afin de favoriser la formation de contacts inter- et intra- moldculaires et ainsi accroltre la coopdrativitd. Tousles compos& prdpar& dans une solution alcoo- lique, puis hiss& h l'air, forment des hydrates. A tempdrature ambiante, leur dtat de spin, et donc leur couleur, est fonction de l'anion associd. Les compos& incluant les anions chlorure (figure 2), nitrate, bromure et hexafluorophosphate pr&entent une transition de spin au-dessus de la temp&ature ambiante et sont roses, alors que les composds incluant les anions iodure, t&rafluoroborate (figure 4), et perchlorate apparaissent blancs, car leur transition s'effectue en dessous de la tempdrature ambiante. Les premiers prdsentent ce que l'on a appeld une transition de spin non dassique lide au ddpart des mol&ules d'eau du rdseau [11, 19], alors que les seconds montrent une transition de spin sans modification de la composition chimique au cours de la mesure. Par ailleurs, une relation quasi lindaire a dtd trouv& entre les temp&atures de transition des compos(is d&hydratds et les rayons ioniques des anions correspondants (figure 5). Lorsque l'anion est l'iodure, le composd pr&ente une transition de spin proche de la temp&ature ambiante (7"1/21" = 297 K et T1/2,[, = 285 K), accompagnde d'une hystdrdsis thermique (12 K), ce qui a rarement dtd observd avec les composds du Fe(II) fi transition de spin (figure3).

1. Introduction

Nowadays there is an increasing interest in new bistable iron(II) spin-crossover compounds, which show a transition from the high- spin state (HS, S = 2) to the low-spin state (LS, S = 0) on cooling, upon increasing pressure, or by light irradiation [1-8].

In particular, extensive research efforts have been directed towards polynudear Fe(II) coordination compounds containing 4-R-substituted derivatives of 1,2,4-triazole, because of the favourable characteristics of their spin- crossover behaviour involving very abrupt transitions, as well as important thermal hysteresis

OH I ~H2 o½

bl N 1 2

S c h e m e 1.4-(2 '-hydroxy-ethyl)-1,2,4-triazole (= hyetrz).

Schema 1.4-(2'-hydroxy-dthyl)-l.2,4-triazole (= hyetrz).

effects accompanied by a thermochromic response, making them the compounds of choice for various applications in molecular electro- nics [5, 8-10] or as temperature sensors [8, 11].

In this paper, we report and discuss the spin transition characteristics of polynuclear iron (II) compounds of formula [Fe(hyetrz)3](Anion) 2 "xH20 (Anion = CI-, NO3-, Br-, I-, BF4-, CIO4-, PF6-), where hyetrz is the ligand 4-(2"- hydroxy-ethyl)-l,2,4-triazole (see scheme l), selected in order to increase the dimensionality through hydrogen bonding interactions.

Typically, Fe(II) compounds of 4-R-1,2,4-triazole appear as fine microcrystalline powders. Therefore, EXAFS was the only method available to probe directly the local structure centered around the metal ion. Furthermore, the detailed analysis of the multiple scattering EKAFS signal displayed at the double metal- metal distance allowed for a detection of the metal alignment in these compounds [12, 13]. I~ fact, for [Fe(Htrz)2(trz)] (BF4) and [Fe(Htrz) 3](BF4)2"H20 (Htrz = 1,2,4-4H-triazole; trz = 1,2,4-triazolato) EXAFS studies pointed out that the compounds consist of linear chains with typical Fe-Fe separations of 3.65 • in the low-spin state [12]. Recently

524

Polynuclear 4-(2"-hydroxy-ethyl)-l,2,4-triazole Fe(ll) molecular materials

N24' C 23~:f/~,,~25 ' N14 13 N54 N64'

C 5 3 - ~ ' - CS3'

N24

Hg~e 1. Projection showing the structure of [Cu(hyetrz)3](CIO4)2.3H20. Hydroxy-ethyl groups and hydrogen atoms have been omitted for clarity. Primed atoms are generated by the symmetry operations 1 - x ,

1 -y, 1 - zand 1 - x, 1 - y , - z (taken from [14]).

Figure 1. Projection montrant la structure de [Cu(hyetrz)3l(CIO4)2°3H20. Les groupes hydroxy-&hyl et les atomes d'hydrog~ne ont dtd omis dans un soucis de clartd. Les atomes not& avec un prime sont gdndr~s par les opdrations de sym&rie 1 - x, 1 - y, 1 - z et 1 - x , 1 - y, - z

(d'apr& [14]).

EXAFS data of these Fe(II) derivatives could be compared with those of the Cu(II) analogue [Cu(hyetrz)3](C104)2,3H20, the crystal structure of which has been solved, confirming that both metal ions form one-dimensional compounds [14]. The structure of [Cu(hyetrz) 3] (C104)2"3H20 ( s e e f i g u r e 1 ) shows Cu(II) ions linked by triple N 1 , N 2 - 1 , 2 , 4 - t r i a z o l e bridges yielding a slightly alternating chain with C u l - C u 2 and Cu2-Cu3 distances, 3.853(2) fit and 3.829(2) fli respectively. It is important to notice that even though the Cu(II) ions are in Jahn-Teller distorted octahedra, the chain shows only a relatively small deviation from linearity. There is still some controversy con- cerning whether the corresponding Fe(II) polymers would be linear in both the low-spin and high-spin states, or if this spin transition would be accompanied by a change in structure involving a linear alignment of metal ions in the low-spin state and a "zig-zag" deformation in the high-spin state [14]. This implies that it is not yet clear whether the quite abrupt spin transitions encountered in these Fe(II) compounds are associated with a crystallographic phase transition, as has been discussed for [Fe (hyetrz) 3] (3-nitro phenyls ulfo nate) 2 "xH20 [27]. Up to now, it is believed that the spin transition is mainly driven by the gain in vibra- tional entropy on going from the low-spin to the high-spin state. In polynuclear Fe(II) compounds, strong direct links of the active spin- crossover sites by bridging ligands allow those

molecular deformations to be spread out all along the chains in a cooperative fashion. Moreover, this cooperativity may even be enhanced by inter- and intramolecular interactions mediated by solvent molecules and/or non-coordinated counterions. Renovitch and Baker as early as 1967 reported that the transition temperature for [Fe(2-pic)3]X 2 (2-pic = 2-picolylamine) strongly depended on the nature of the anion X- (CI-, Br-, I-) [15]. Later on, Giitlich and coworkers found a pronounced solvent effect for the same system [16]. More recently, several studies pointed out the influence of non-coordinating anions and solvents [3a, 17, 18]. Going back to the poly- meric system, it is interesting to notice that the crystal structure of [Cu(hyetrz)3](C104) 2 • 3 H 2 0 did not reveal any direct intermolecular interactions between neighbouring Cu(II) chains [14]. From the studies on [Fe(NH2trz) 3] (Anion)2"xH20 (NH2trz = 4-amino-l,2,4-triazole; Anion = derivatives of naphthalene sul- fonate) [19], we previously proposed that the size and shape of the non-coordinated anionic group,'; influence the pore size in these poly- meric compounds. It appeared that such a structure enclosing cavities allows the incorporation of lattice water molecules, which can relatively easily be removed by heating. Intere:~tingly, this process also involves a low- spin to high-spin transition induced by the removal of these lattice water molecules, which initially stabilized the low-spin state. In fact, we have already observed this type of non-classical spin-crossover behaviour for [Fe(hyetrz)3](3- nitrophenylsulfonate)2°3H20 leading to an unprecedented extremely large apparent thermal hysteresis of 270 K [11]. In order to further investigate the effect of lattice trapped water molecules on the spin-crossover properties and the rei.ation between the anion size and T m , we report on series of [Fe(hyetrz) 3] (Anion)i°xH20 polymers (Anion = CI-, NO3-, Br-, I-, BF4-, C104--, PF6-) where the anion size can in a first approximation be represented by the radius of the equivalent sphere. Moreover, the effect of the inclusion and removal of lattice water molecules on the spin-crossover behaviour is also discu, sed.

2. Experimental 2 . 1 . ~ I a t e r i a l s

Commercially available solvents were used without further purification. Fe(BF4)2.6H20

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Y. Garcia e l al.

and Fe(CIO4)2°6H20 were purchased from Mdrich. FeCI2°4H20, KI, KPF 6, KNO 3, etha- nolamine, triethylorthoformate and formic acid hydrazide were purchased from Acros. FeBr 2 was purchased from Strem.

2.2. Preparation

The ligand 4-(2'-hydroxy-ethyl)-l,2,4-triazole has been prepared as already described [14]. The [Fe(hyetrz)3](Anion)2"xH20 compounds studied have been obtained by two different procedures.

2.2.1. Synthesis o f [Fe(hyetrz)3] (Anion) 2

• x H 2 0 (Anion = CI- Oc = 3), NQ3- (x = 2),

Br- (x = 3), I- (x = i), BF 4- (x = 2), ClO 4-

Oc = 2), PF 6- Oc = 2)). Method A.

The compounds with Anion = CI-, Br-, BF 4- and C10 4- have been prepared in methanol using the corresponding Fe(II) salts. The compounds with Anion = NO3-, I- and PF 6- have been obtained from ethanolic solutions using the corresponding potassium salts and FeC12 "4H20.

An alcoholic solution (5-15) cm 3 of Fe(Anion)2°xH20 (5 mmol) and a small amount of ascorbic acid was added to a stirred alcoholic solution (10 cm 3) of 4-(2'-hydroxy- ethyl)-l,2,4-triazole (15 mmol). Instantane- ously pink-purple precipitates are formed for Anion = C1-, Br-, NO 3- and white precipitates for Anion = CIO4-, BF4-, PF 6 . The compound with I as counteranion is mauve. The precipitates were filtered and left on air. After several hours it appeared that the eventually lattice trapped alcohols have been substituted by water molecules. During this process, all the compounds keep their initial colour except for the hexafluorophosphate derivative which turns pink upon the substitution of ethanol by water molecules. Only for Anion = I- the reaction mixture was cooled to facilitate the precipita- tion of the product and the hydrate obtained was stored in a dessicator in vacuo to prevent its decomposition.

2.2.2. Synthesis o f [Fe(hyetrz)3] (Anion) 2

oxH20 (Anion = CI- Oc = 3), BF 4-

Oc = O, I), CIO 4- (x = 0)). Method B.

Some of the studied [Fe(hyetrz)3](Anion) 2 °xH20 polymers have been synthesized, in the

526

scale as given above, under strongly anaerobic conditions starting with aqueous solutions of substrates. The resulting cloudy solutions have been evaporated in a stream of deoxygenated N 2 or Ar to ~ 6 cm 3, the formed solids filtered, washed twice with small portions of water, finally twice with 2 cm 3 of ethanol and dried in wlcuo/P4Oj0 for 12h.

[Fe(hyetrz)3]Cl2o3H20 (Method A): Anal. for C12H27N9CI206Fe: Calc.(found): C, 27.72(28.09); H, 5.19(4.90); N, 24.25(24.79); CI, 13.64(13.27); Fe, 10.74(11.08). Yield: 68 %.

[Fe(hyetrz) 3]C12°3H20 (Method B): Anal. for C12H27N9C1206Fe: Calc.(found): C, 277.72(28.16); H, 5.19(4.95); N, 24.25(24.65); CI, 13.64(13.87); Fe, 10.74(10.98). Yield: 62 %.

[Fe(hyetrz)3]Br2°3H20 (Method A): Anal. for C12H27N9Br206Fe: Calc.(fbund): C, 23.66(24.08); H, 4.47(4.44); N, 20.7O(2O.65); Br, 26.24(24.80); Fe, 9.17(10.19). Yield: 90 q6.

[Fe(hyetrz)3](CIO4)2°2H20 (Method A): Anal. for C12H25N9C12 O 13 Fe: Calc.(found): C, 22.87(23.41); H, 4.00(3.77); N, 20.01(20.30); C1, 11.25(11.15); Fe, 8.86(8.99). Yield: 77 96.

[Fe(hyetrz)3](C104) 2 (Method B): Anal. for CI2H21N9CI2OllFe: Calc.(found): C, 24.26(24.2O); H, 3.54(3.82); N, 21.23(20.92); Fe, 9.40(9.51). Yield: 73 %.

[Fe(hyetrz) 3](BF4)2°2H20 (Method A): Anal. for CI2H25NgB2F805Fe: Calc.(found): C, 23.83(24.07); H, 4.17(4.02); N, 20.84(20.3O); B, 3.57(3.58); Fe, 9.23(9.33). Yield: 59.70 %.

[Fe(hyetrz) 3](BF4)2°H20 (Method B): Anal. for C12H23N9B2FsO4Fe: Calc.(found): C, 24.57(24.77); H, 3.92(3.82); N, 21.49(21.35); Fe, 9.23(9.33). Yield: 59 %.

[Fe(hyetrz) 3](BF4) 2 (Method B): Anal. for C12H21N9B2F803Fe: Calc.(found): C, 25.34(25.35); H, 3.69(3.56); N, 22.17(22.08); Fe, 9.82(9.68). Yield: 90 %.

[Fe(hyetrz)3]I2"H20 (Method A): Anal. for C12H23N91204Fe: Calc. (found): C, 21.61(23.61); H, 3.48(3.77); N, 18.90(19.88); Fe, 8.37(8.30). Yield: 30.3 %.

The water content of all compounds includ- ing [Fe(hyetrz)3] (NO3)2°2H20 (Yield: 27.2 %) and [Fe(hyetrz)3] (PF6)2°2H20 (Yield: 42.8 %) have been verified by thermogravimetric analyses.

Caution! Although no problems were encountered in the preparation of the following corn-


pounds as perchlorate salts, two heavy detonations occurred in the past while handling small amounts of [Fe(NH2trz)3}(ClO4)2"xH20. Suitable care should be taken when handling such potentially hazardous compounds.

2.3. Physical m e a s u r e m e n t s

Elemental analyses were performed by the Service Central d'Analyse (CNRS) in Vernal- son, France and with a Perkin Elmer 2400 C H N Elemental Analyzer (U.Wr.). Thermo- gravimetric measurements were carried out with a Setaram apparatus in the temperature range 300-400 K at a heating rate of 1 K min q under ambient atmosphere. Differential scanning calorimetric measurements were carried out on a Perkin Elmer DSC 7 Differential Scanning Calorimeter. Infrared spectra were carried out with a Perkin Elmer Paragon 1000 FT-IR spectrophotometer using KBr pellets and with a Specord M80 IR spectrophotometer in Nujol and Fluorolube mulls. Optical measurements have been carried out using the device described previously [4, 20] with a heating rate o f 1 K min -1 in the temperature range 77-400 K. Magnetic susceptibilities were measured in the range 5-300 K with a fully automated Manics DSM-8 susceptometer equipped with a TBT continuous-flow cryostat and a Drusch EAF 16 UE electromagnet opera- ting at ca. 1.7 T. Data were corrected for magnetization of the sample holder and for dia- magnetic contributions, which were estimated from the Pascal constants. M6ssbauer spectra were recorded with a conventional transmission geometry with a 25 mCi 57Co/Rh source (at RT). The sealed samples were mounted in an insert type bath cryostat (78-300 K). Na2Fe(CN)5NO'2H20 at RT was used as a reference.

loss of water starts at about 310 K, and at 400 K no water molecules are retained. There does not appear to be a correlation between the size of the anions and the number of water molecules per iron(II) atom incorporated in the crystal lattice. The anions are not involved in coordination, as could be confirmed by IR spectroscopy for the polyatomic anions: absorptions for BF4-at 522 cm q (V1) and 1050 cm -1 (v2), for CIO4- at 623 cm -1 (v 1) and 1096 cm q (v2), for N O s - a t 1407 cm -1 and for PF 6- at 844 cm i (v3) and 559 cm -1 (v 4) [21] inde- pendent on the synthesis route of the [Fe(hyetrz) 3] (Anion)2-xH20 polymers.

At room temperature, all studied compounds generally appear as pink-purple or white powders depending on the spin state of the Fe(II) centres The pink colour is due to the IA.

1 ' t g ---> Wig d -d transition occurring at 520 nm for the low-spin Fe(II) sites, whereas the white colour is due to the fact that the spin-allowed lowest energy d -d transition (5T2g-+SEg) for the high-spin sites occurs in the near infrared (NIR). Since these compounds are highly thermochromic, the spin transitions have been studied for most samples optically using a horne.-made device [4, 20]. For most compounds, the spin-crossover behaviour has been investigated by this optical method during at least three thermal cycles, the results of which are l!isted in table I. The spin-crossover

Table I. Results of optical measurements for [Fe(hyetrz) 3] (Anion)2 .xH20. *

Tableaa I. Rdsultats des mesures optiques pour [Fe(hyetrz)3 ] (Anion) 2oxH20.

Cycle First cycle Second cycle Third cycle

Anion TII2"~ + T1/2-~ T1/2"~ 7"11 ~ ,l~ Tlt2~T" TlI2,[,

3. Results and discussion

After having been left in contact to the air for several hours all compounds synthesized according to method A are converted to hydrates. The number of water molecules per formula unit varies from 1 to 3, which has been determined by elemental analyses, as well as by thermogravimetric measurements. The results of both techniques are in fair agreement and yield the compositions as mentioned in the experimental section. Thermogravimetric measurements show for all compounds that the

CI- 364 301 314 301 314 301

NO~ 355 298 315 298 315 298

Br 312 285 294 285 294 285

I 285 285 297 285 297 285

BF 4 260 215 225 215 225 215

CIO~ 249 245 255 245 255 245 PF~ 322 195 205 195 205 195

* T1/2 given in [K] has been taken at half intensity height.

+ T1/2 ~:or the I , BF 4 and CIO 4- derivatives has been obtained from a first heating from 77 K to 400 K, followed by several 77-400 K temperature cycles. For the other compounds, the temperature has firstly been raised from raom temperature to 400 K, followed by several 77-400 K temperature cycles.

527

Y. Garcia et al.

[Fe(hyetrz)3](Anion)2°xH20 polymers according to method B have been studied by magnetic, M6ssbauer and DSC methods. Generally for those samples, the spin transition temperatures derived from magnetic susceptibility and M6ssbauer studies do not differ by more than 2 K. The results obtained for poly- mer samples synthesized according to method B are listed in table II. The transition temperature t"1/2 is defined as the temperature at which equal (50 %) populations of the high- spin Fe(II) and low-spin Fe(II) sites are observed.

Table II. Results of M6ssbauer measurements for [Fe(hyetrz)3] (Anion) 2"xH20.*

Tableau II. Rdsultats des mesures M6ssbauer pour [Fe(hyetrz) 3] (Anion) 2°xH20.

First cycle Second cycle

Anion x T1/2"~ Tt/2,~ T1/2 ~ TI/2~

BF~ 1 252 239 251 239 BF~ O* gradual incom-

plete CIO; 0 238 232.5 239 233

* 71/2 given in [K] has been taken at XHs = 0.5. Subsequent 300-100-300 K cooling-heating cycles applied.

t Sample obtained from dehydration of [Fe(hyetrz) 3] (BF4)2°H20 at 380 K for 15 min.

For the low-spin [Fe(hyetrz)3](Anion) 2 • x H 2 0 (Anion = C1- (x=3), N O 3- (x=2), Br- (x=3), PF 6- (x--2)), isolated according to method A as pink powders, the temperature was firstly raised to 400 K and subsequently lowered to 77 K in the first thermal run, followed by consecutive up-down thermal cycles. As a representative example, the optical data for the chloride derivative are displayed in figure 2. This compound shows upon heating a very abrupt low-spin --+ high-spin transition occurring at 364 K. Decreasing the temperature reveals a high-spin ---> low-spin transition with T1/25 = 301 K. A second heating of the sample shows a low-spin --> high-spin transition at T1/25 = 314 K. Subsequent heating-and- cooling cycles indicate that this hysteresis of 13 K (T~/2~ = 314 K, T1/2~ = 301 K) is retained. All these hydrated compounds display a first low-spin to high-spin transition always taking place at temperatures (i.e. between 312 and 364 K) higher than the temperature at which the release of water molecules has been

528

= Ii

(D

{7, 0.8

>.,

0.6

c~ 0.4

-~ {).2

:= 0 I

240 280 320 360 400 T / K

Figure 2. Optical measurement (intensity vs. temperature; recorded at 1 K rain q) for [Fe(hyetrz)3]Cl2"xH20.

Figure 2. Mesure optique (intensitd en fonction de la tempdrature, enregistrde /t 1 K min q) pour [Fe(hyetrz)3] CI2*xH20.

found to start by thermogravimetric analyses (310 K). These features are typical for the phe- nomenon that we have previously called a non- classical spin-crossover driven by the release of lattice water molecules [11, 19]. Thermogravi- metric measurements performed under identical experimental conditions as in the optical measurements, confirm that after a first heating to 400 K the compounds have been completely dehydrated. Examining the temperatures at which the solvent induced non-classical spin transition has been found to take place for the various compounds, reveals that these do not seem to be correlated to the lattice water content or to the radius of the anion. Upon record- ing subsequent thermal cycles the features of a stable classical spin-crossover behaviour involving very sharp low-spin - high-spin transitions have been observed for the dehydrated con]- pounds.

Thus, all these compounds show non-classical spin-crossover behaviour as has already been described for [Fe(hyetrz)3](3-nitro-phe- nylsulfonate)2°3H20 [11]. However, in the present series of compounds, the apparent hysteresis (63 K, 57 K, 27 Kand 127 Kfor the CI-, NO3-, Br- and PF 6- derivatives, respectively) is much smaller than the extremely large apparent thermal hysteresis of 270 K encountered for the 3-nitrophenylsulfonate derivative [11].

In contrast for the white, high-spin [Fe(hyetrz) 3](Anion)2"2H20 (Anion = ClO4-, BF4-) as well as the mauve-colored


[Fe(hyetrz)3 ]I2"H20 compounds, spin transitions at ambient or low temperatures have been found. [Fe(hyetrz)3]I2°H20 shows transitions with T1/2"~ = 285 K and T1/25 = 270 K. The transition temperatures of [Fe(hyetrz)3]I 2 • H 2 0 derived from magnetic studies are Trot = 274 K and T1/2,1, = 266 K and are indicative for a fairly complete spin-crossover. The value for ZM T, where ZM stands for the molar magnetic susceptibility per iron(II) ion, at 292 K is 3.62 cm 3 K mol -I corresponding to high-spin Fe(II) ions, whereas the observed value of 0.09 cm 3 K mol q at 19 K is typical for low-spin Fe(II) ions.

For [Fe(hyetrz)3](BF4)2"2H2 O, abrupt transitions in the heating (T1/2"t = 260 K) and cooling mode ( 7"1/2,1, = 247 K) have been observed. This behaviour is confirmed by temperature dependent magnetic susceptibility measurements, where Tv2"~ = 260 K and 7"1/2,1, = 250 K have been found. ZM T is equal to 3.38 cm 3 K mol q at 290 K which corresponds to a quintet spin state for Fe(II), whereas the value at 150 K of 0.71 cm 3 K mo1-1 is indicative of a singlet state.

For [Fe(hyetrz) 3] ( C L O 4 ) 2 ° 2 H 2 0 similar spin- crossover properties have been found with T1/2"~ = 249 K and 7"1/2,1, = 240 K from optical studies. The magnetic data indicate T1/2"~ = 245 K and 711/2,[, = 240 K. The ZM T value at 290 K of 3.38 cm 3 K tool -1 is typical for Fe(II) in the high-spin state, whereas the ZMTvalue at 50 K of 0.43 cm 3 K mo1-1 corresponds to Fe(II) ions in the low-spin state.

For the three compounds discussed above, the hysteresis loop can be reproduced several times if the temperature of the sample does not exceed 300 K. The slight discrepancies between the transition temperatures determined by optical and magnetic methods are most likely related to different sample-thermal response/ temperature detector geometries in both applied techniques. The optical measurements focus on the colour, i.e. the surface of the sample, whereas the magnetic data reflect the physical behaviour of the bulk material.

It has been possible to determine the optical data for [Fe(hyetrz) 3] (Anion) 2 (Anion = C104-, BF4-, I-) by heating the hydrated forms up to 400 K, followed by several 77-400 K temperature cycles. [Fe(hyetrz)3](BF4) 2 shows T 1 / 2 [ = 225 K and Tv2,[, = 215 K, whereas the transition temperatures for [Fe(hyetrz)3](CIO4) 2 are T1/25 = 255 K and Tv2,[, =-245 K.

=E ~ 0.8

O.4

~ 0.2

0 260 280 300 320 340

T / K

Hgure 3. Optical measurement (intensity vs. temperature; recorded at 1 K min -]) for [Fe(hyetrz)3]I 2.

Hgure 3. Mesure optique (intensit6 en fonction de la tem- p&atme, enregistrde ~i 1 K min -I) pour [Fe(hyetrz)3]I 2.

[Fe(hyetrz)3]I 2 shows Tv2"~ = 297 K and Tv2,1, = 285 K (see figure 3).

Except for [Fe(hyetrz)3]Cl2"3H2 O, syntheses according to method B allowed for the isolation of the monohydrate [Fe(hyetrz)3](BF4)2°H20, completing the series x = O, 1,2, as well as of the non-hydrated [Fe(hyetrz)3](C104) 2 which was synthesized independently by dehydratation of the corresponding dihydrate (from method A) at 400 K. The reproducible hysteresis loop deduced from M/Sssbauer experiments of [Fe(hyetrz) 3] ( B F 4 ) 2 " H 2 0 is shown on .figure 4.

1.0

0.8 eo

0.6

E .H O.,i

'~ 0.2

0.0 180

oOC~ocC:oo o • o

1 O

o , . o #

O O •

O •

, t , I , , , I , , , I , , , I , , , I i i i [ i

200 220 240 260 280 300 T / K

Figure 4. M6ssbauer measurement (high-spin molar fraction vs temperature) for [Fe(hyetrz)3](BF4)2oH20.

Figure 4. Mesure de spectroscopie MSssbauer (fraction molaire HS en fonction de la temp&arure pour [Fe(hyctrz) 3] (BF4)2*H20.

529

Y. Garcia et al.

A rather complete spin transition with T1/25 = 252 K and Tlo,l, = 239 K (see tableII) is found. The observed transition temperatures within the series of [Fe(hyetrz) 3] (BF4)ioxH20 increase with increasing number of H20 molecules in the lattice as mostly observed. The opposite is observed for the non-hydrated polymers of formula [Fe(hyetrz)3](Anion) 2 with Anion= C104-, I-. Their transition temperatures are slightly shifted to higher temperatures if compared with those of the parent hydrates. There- fore, these compounds represent the first two examples among the [Fe(hyetrz)3](Anion) 2 series, where the inclusion of non-coordinated water molecules results in a destabilization of the low-spin state. The same effect has been observed earlier for [Fe(NH2trz) 3] (CH3SO3) 2, indicating T~/eq" = 299 K and T1/e$ between 295 and 291 K whereas [Fe(NH2trz).3] (CH3SO3)2°H20 exhibits a hysteresis loop of 26 K with T:/f~ = 295 K and Tj/2,I, = 269 K [28]. Furthermore, [Fe(Htrz)9(trz)] (C104) • H20 shows transitions at 7"1/2"[" ~ 320 K and 7"l/2,1, = 315 K, while for the non-hydrated modification T1/2~ = 266 K and T~/e$ = 263 K [22, 23]. Therefore one might conclude that the presence of lattice water molecules influ- ences the transition temperatures more through the crystalline structure it imposes on the compound, than by specific (hydrogen bonding) interactions, since the latter should have the same effect for all members of this series of iron(II) compounds based on the hyetrz ligand. Moreover, unlike in the [Fe(NH2trz)3] (Anion) 2 and [Fe(Htrz)2trz](Anion) 2 materials, the possible hydrogen bondings involving the hydro- xyl groups cannot alter the electronic effect of the ligand.

Quite pronounced differences are observed among the studied non-hydrated [Fe(hyetrz)3 ] (BF4) 2 compounds (see tables I and I/). For samples obtained by heating the dihydrate, obtained according to method A, at 400 K a nice shaped hysteresis loop with T1/e't = 225 K and T1/25 = 215 K is observed. Samples of [Fe(hyetrz)3](BF4)2"H20 (method B) heated at 380 K for 15 rain. show no DSC response in the cooling and heating modes (380-160- 380-300 K cycle) and magnetic studies reveal only a gradual spin transition. A non-hydrated sample of [Fe(hyetrz)3](BF4) 2 might be also synthesized with dry EtOH as solvent (method A). If the sample obtained is not exposed to atmospheric moisture and stored in a dessiccator over P40 l0 a gradual and incom-

530

plete transition is observed in magnetic studies. Therefore one can define for the non- hydrated [Fe(hyetrz)3](BF4) 2 lattices the presence of a synthesis route dependent memory effect.

Examining the positions of the reproducible and stable hysteresis loops with width 9-17 K for all non-hydrated samples, it becomes evi- dent that the transition temperatures have a tendency to increase in an almost linear fashion with decreasing van der Waals radii of the non- coordinated anionic groups, as is illustrated on figure 5.

Also within the [Fe(NH2trz)3](Anion) 2 °xH20 series it has been observed that the transition temperatures to the high-spin state increase with decreasing anion radii along the series of anions: CIO4-, 1-, Br-, N O r , C1- [24, 25, 26a]. EXAFS studies on the low-spin forms have shown that the Fe-N bond distances tend to decrease with the above-mentioned order of substitution of the anions [24, 26a, 26b]. Fur- thermore, 57Fe M6ssbauer spectroscopy [25] and X-ray fluorescence spectral studies [26c] confirm that a decrease ofinteratomic distances correlate with the covalency of the Fe-N bond in the same series of anions. Therefore, Lavre- nova et al. concluded that this increase of transition temperatures seems to be associated with increasing anion-cation interactions - and the

320

300

280

ad 260

b.,- 240

220

200

180

@ •

0 O@ •

O O

O @

0

o I , J , l , , ~ l ~ , t l , , , I , , , , , , I ,

1.8 2 2.2 2.4 2.6 2.8 3

Hgure 5. The transition temperatures for the spin-crossover behaviour of the dehydrated [Fe(hyetrz)3](Anion)2 compounds in the heating (O) and cooling mode (0) as function of the radius of the anion.

Figure 5. Temperatures de transition pour les composds ddshydrat& [Fe(hyetrz)3l(Anion) 2 dans les modes chauffage (e) et refroidissement (O) en fonction du rayon de l'anJon.


thereby resulting increase of an electrostatic pressure, which causes a subsequently increasing compression of the FeN 6 octahedron [24, 26a].

Another interesting feature is represented by the fact that the hysteresis width within the [Fe(hyetrz)3](Anion) 2 (Anion = Cl-, NO S, Br-, I-, BF4-, CIO4-, PF6-) series does not appear to depend on the position of the transition temperature, or alternatively, on the radius of the anion. In fact, the hysteresis widths for the non-hydrated compounds generally are at 13 -+ 4 K. Furthermore, the hysteresis width does not seem to be altered upon incorporation of water molecules within the lattice. The last point may be ilustrated by optical measurements of [Fe(hyetrz)3](BF4) 2 and [Fe(hyetrz)3](BF4)2.2H20 showing identical hysteresis widths of 10 K centered around 220 K and 255 K, respectively. In addition, a recent study has shown that magnetic measurements performed while applying an increasing external pressure on [Fe(hyetrz)3](3-nitrophenylsulfonate) 2 leads to a shift of TI/e to higher temperatures (from 100 K without applied external pressure to 270 K for 8.9 kbar) without altering significantly the hysteresis width, which remains at a constant value of 10 K [27]. Apparently, the electrostatic pressure occurring within the crystal lattice due to anion-cation interactions has a comparable effect on the spin-crossover behaviour as an applied external pressure. Both lead to considerable shifts in transition temperatures without significant influence on the hysteresis width.

From this follows that within the hyetrz family, spin-crossover behaviour with a hysteresis of about 10 K in the vicinity of room temperature may be expected if the pressure within the crystal lattice attains a certain optimal value, which can either be induced by anion- cation interactions or by applying an external pressure. The nearly ideal, for practical applications, internal crystal lattice pressure require- ments appear to be fullfilled in the non- hydrated iodide compound, which shows a spin-crossover transition involving a thermal hysteresis of 12 K centered in the vicinity of room temperature (291 K). Therefore, [Fe(hyetrz)3]I 2 represents one of the very few Fe(II) spin-crossover materials showing a spin transition with hysteresis and an associated thermochromic effect in close vicinity of room temperature [19, 20, 28, 29].

Evidently, the spin-crossover behaviour of all studied and closely related linear chain Fe(II) polymers with 1,2,4-triazole type ligands respond differently to both the incorporation and ~:emoval of non-coordinated water molecules from their lattices. Since there is still lack of information on the way and mechanism the lattice water molecules are incorporated in the crystal lattice it is hard to interpret quantita- tively the results obtained. But our recent studies indicate that the role of H-bond formation as a key factor controlling the spin-crossover parameters of triazole-based linear chain polymers of Fe(II) is overestimated [30, 31].

References

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(b) Giitlich P., Mol. Cryst. Liq. Cryst. 305 (1997) 17-40.

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[7] Haasnoot J.G., 1,2,4-triazoles as ligands for iron(II) high spin to low spin crossovers, in: Kahn O. (Ed.), blagnetism: A Supramolecular Function, Kluwer Aca- demic Publishers, Dordrecht, 1996, pp. 299-321.

[8] Kahn O., Martinez-Jay C., Science 279 (1998) 44-48.

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[11] Garcia Y., van Koningsbruggen RJ., Codjovi E., Lapou- yade R., Kahn O., Rabardel L., J. Mater. Chem. 7 (3997) 857-858.

[ 12] M ichalowicz A., Moscovici J., Ducourant B., Cracco D., Kahn O., Chem. Mater. 7 (1995) 1833-1842.

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[15] Renovitch G.A., Baker Jr. W.A., J. Am. Chem. Soc. 89 (1967) 6377-6378.

[16J Sorai M., Ensling J., Hasselbach K.M., Gfitlich E, Chem. Phys. 20 (1977) 197-208.

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[20] Codjovi E., Sonnier L., Kahn O., Jay C., NewJ. Chem. 20 (1996) 503-505.

[21] Nakamoto K., Infrared and Raman spectra of inorganic and coordination compounds, 4th ed., Wiley, New York, 1986.

[22] Lavrenova L.G., Ikorskii V.N., Varnek V.A., Oglezneva I.M., Larionov S.V., Koord. Khim. 16 (1990) 6544560.

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[25] Varuek V.A., Lavrenova L.G., J. Struct. Chem. 36 (1995) 104-111.

[26] (a) [~renburg S.B, Bausk N.V., Lavrenova L.G., Varnek V.A., Mazalov L.N., Solid State Ionics 101-103 (1997) 571-577;

(b) Erenburg S.B, Bausk N.V., Varnek V.A., Lavrenova L.G.,J. Magn. Magn. Mater. 157/158 (1996) 595-5!)6;

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[27] Garcia Y., van Koningsbruggen P.J., Lapouyade R., Fourn~s L., Rabardel L., Kahn O., Ksenofontov V., Levchenko G., Giitlich P., Chem. Mater., in press.

[28] Bronisz R., Drabent K, Polomka P., Rudolf M.E, Con- ference Proceedings, ICAME95, 50 (1996) 11-14.

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[3t] Wojcieszczyk G., Drabent K., Polomka 12., Rudolf M.E, Chem. Eur. J., submitted.

532

Documents

Synthesis and spin-crossover characteristics of polynuclear 4-(2′-hydroxy-ethyl)-1,2,4-triazole Fe(II) molecular materials