12266
J. Phys. Chem. B 2001, 105, 12266-12271
Pressure-Induced Spin State Conversion in Antiferromagnetically Coupled Fe(II) Dinuclear Complexes Vadim Ksenofontov,† Ana Bele´ n Gaspar,‡ Jose´ Antonio Real,*,‡ and Philipp Gu1 tlich*,† Institut fu¨ r Anorganische Chemie und Analytische Chemie, Johannes-Gutenberg UniVersita¨ t, Staudinger Weg 9, D-55099 Mainz, Germany, and Departament de Quı´mica Inorga` nica/Institut de Cie` ncia Molecular, UniVersitat de Vale` ncia, Doctor Moliner 50, 46100 - Burjassot (Vale` ncia), Spain ReceiVed: May 3, 2001; In Final Form: October 5, 2001
The effect of pressure on the thermal dependence of the magnetic susceptibility has been investigated in the 2,2′-bipyrimidine (bpm)-bridged iron(II) complexes of formula {[Fe(L)(NCX)2]2bpm} (X ) S and Se; L ) bpm and bt (2,2′-bithiazoline)) and [Fe(bpm)(NCX)2]n. At ambient pressure, the compound {[Fe(bpm)(NCS)2]2bpm} (A) is paramagnetic in the temperature region 4-300 K and shows the intramolecular antiferromagnetic interaction. At P ) 8.9 kbar, A undergoes spin transition, Tc ≈ 150 K, involving 50% of iron atoms and ultimately causing the disappearance of antiferromagnetic coupling. At intermediate pressures the interplay between the intramolecular interaction and spin transition was studied. At 1 bar, {[Fe(bpm)(NCSe)2]2bpm} (B) undergoes a 50% spin transition, Tc ≈ 120 K, similar to that of A at 9.8 kbar. As pressure was increased, B showed the occurrence of a two-step spin transition which is complete at P ) 10.3 kbar at low temperatures. Our study gives the evidence of formation of HS-LS species from HS-HS pairs in dinuclear compounds during the thermal or pressure-induced spin crossover, which is reflected in the buildup of a plateau in the middle part of the two-step continuous curve. Magnetization versus magnetic field curves of A and B at 1 bar at 1.9 K give support for the occurrence of almost 100% of mixed spin pair LS-HS state species in the low-temperature plateau corresponding to the 50% spin transition for B. The compounds [Fe(bpm)(NCS)2]n and {[Fe(bt)(NCSe)2]2bpm} have similar thermal dependences of the magnetic properties under pressure as those of A and B, respectively. In the particular case of dinuclear compounds, it is shown that application of external hydrostatic pressure discloses common features of the whole class of these compounds, which usually can only be revealed by variation of chemical composition.
Introduction The development of molecular switching units capable of storing and transferring information is currently one of the most attractive research areas in molecular science. The goal is the construction of molecular devices from an assembly of these individual electronic components into arrays. Six-coordinate iron(II) spin crossover compounds represent an important class of switchable molecular systems. They change reversibly from a low-spin (LS) diamagnetic state to a high-spin (HS) paramagnetic state by the action of temperature, pressure, or light irradiation.1 This so-called spin crossover phenomenon (SC) is observed both in solution and in solid state. In solution, the process is essentially molecular due to the absence of interactions between spin-changing molecules, whereas in solid state, cooperative intermolecular interactions can lead to phase transitions. Cooperativity stems from the large difference in the metal-to-ligand bond lengths (ca. 0.2 Å) of the spin changing molecules. Cooperativity is a measure of how efficiently the intramolecular changes are transmitted far into the crystal. Hence, it is the reason of the observed discontinuity in the magnetic and optical properties and eventually of thermal hysteresis, which confers potential memory effect to the SC systems.2 * Corresponding authors. E-mail,
[email protected]; Fax, +49 6131 39 22990. E-mail,
[email protected]; Fax, +34 96 386 48 59. † Johannes-Gutenberg Universita ¨ t. ‡ Universitat de Vale ` ncia.
Most of the SC compounds consist of monomeric units held together by noncovalent interactions and, consequently, are difficult to control. Allowing SC centers to communicate via covalently bound chemical bridges, i.e., oligomeric or polymeric species, is an important strategy to explore and enhance cooperativity of a given system. Consequently, molecular distortions accompanying the spin transition should be efficiently distributed throughout the whole crystal lattice. In this way, the family of dinuclear compounds of the common formula {[Fe(L)(NCX)2]2bpm} (X ) S and Se; L ) bpm(2,2′-bipyrimidine) and bt (2,2′-bithiazoline) represents one of the most unique systems in the SC field because they are the simplest examples of polynuclear SC complexes exhibiting a rich variety of magnetic behavior.3-5 In these compounds, dinuclear units consist of two Fe(II) ions bridged by the bisbidentate ligand 2,2′-bipyrimidine (Figure 1a). In {[Fe(bpm)(NCS)2]2bpm} (hereafter A) as well as in its polymeric [Fe(bpm)(NCS)2]n6 modification (see Figure 1b), a similar magnetic behavior in the HS state characterized by weak intramolecular antiferromagnetic interaction between two HS iron(II) ions in the so-called HS-HS dinuclear units with typical values of magnetic coupling constant J ≈ -4.5 cm-1 and g ≈ 2.18 at ambient pressure was observed. The thermally induced spin transition is not observed at ambient pressure for these compounds.3 By contrast, {[Fe(bpm)(NCSe)2]2bpm} (B) shows a one-step transition at Tc ≈ 120 K, which involves about 50% of the iron(II) sites,4 whereas an almost complete two-step spin
10.1021/jp0116961 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/17/2001
Antiferromagnetically Coupled Fe(II) Complexes
J. Phys. Chem. B, Vol. 105, No. 49, 2001 12267
Figure 2. Temperature dependence of χmT for {[Fe(bpm)(NCS)2]2bpm} (compound A) at different pressures. The solid lines, together with estimated concentrations of LS-HS and HS-HS species, correspond to the calculations using the Hamiltonians 1 and 2.
Experimental Section
Figure 1. Schematic drawings of the dinuclear molecule of {[Fe(bpm)(NCS)2]2bpm} (compound A) (a) and the fragment of structure of onedimensional polymeric compound [Fe(bpm)(NCS)2]n (compound C) (b).
transition in the range between 160 and 210 K takes place for {[Fe(bt)(NCS)2]2bpm}.5 This behavior was interpreted in terms of the occurrence of the following spin pairs: HS-HS T LSHS T LS-LS upon lowering temperature.5 The stabilization of the LS-HS mixed-spin pair was assigned to the synergistic effect between intramolecular and cooperative intermolecular interactions. Considering the variety of spin crossover properties of this family of compounds two questions arise: (i) is the spin transition regime influenced by the dinuclear nature of the complexes? (ii) is HS-LS a stable spin pair state? To answer these questions, we have investigated the temperature dependence of magnetic susceptibility under applied hydrostatic pressure as a way of modifying the properties. As it is wellknown, hydrostatic pressure favors the low spin state of the separate spin crossover molecules.1c This should also be valid for the dinuclear spin crossover molecule, however, with its own specific pressure dependence. Separate or simultaneous “switching” of active centers in the dinuclear unit can have direct relation and different influence on the form of the transition curve. A second aspect which can be explored during the pressure experiments is the interplay between the intramolecular interaction in dinuclear units and the spin transition. As a continuation of the study of this singular family of compounds, we report here for the first time the effect of pressure on the magnetic properties of the {[Fe(bpm)(NCS)2]2bpm} (A), {[Fe(bpm)(NCSe)2]2bpm} (B), and {[Fe(bt)(NCSe)2]2bpm} (D) dinuclear complexes and the related [Fe(bpm)(NCS)2]n (C) quasi-one-dimensional polymeric compound.
Materials. The compounds have been synthesized and characterized according to procedures described elsewhere.3-6 Magnetic Susceptibility Measurements under Hydrostatic Pressure. The variable-temperature magnetic susceptibility measurements were performed using the PAR 151 Foner-type magnetometer, equipped with a cryostat operating in the temperature range 2-300 K. The hydrostatic pressure cell made of hardened beryllium bronze with silicon oil as the pressure transmitting medium operates in the pressure range 1 bar < P < 13 kbar and has been described elsewhere.7a Hydrostaticity was established in our previous studies of spin crossover compounds.7b-d Cylindrically shaped powder sample holder dimensions are 1 mm in diameter and 5-7 mm in length. The pressure was measured using the pressure dependence of the superconducting transition temperature of a built-in pressure sensor made of high purity tin. Experimental data were corrected for diamagnetism using Pascal’s constants. Results and Discussion The magnetic properties of A-D have been measured at ambient pressure in the range of temperature 4.2-300 K. The χmT product versus T, χm being the magnetic susceptibility (corrected for diamagnetic contributions) and T the temperature, is displayed in Figures 2-5 for A-D, respectively. These data match very well with the original ones published earlier.3-6 Compound A does not reveal the spin crossover behavior and the two iron(II) ions being in the HS-HS state antiferromagnetically coupled, with the spin singlet ground state completely populated at low temperatures. This behavior remains as pressure is increased up to 3.8 kbar. A maximum in the χm versus T curve is observed at around 12 K in this range of pressures. When pressure is increased to 6.3 kbar, a thermal spin crossover appears around Tc ≈ 100 K. In the slow cooling and heating mode with a rate of 10-1 K min-1 providing the thermodynamic equilibrium conditions, the transition is accompanied by a 5 K wide thermal hysteresis (open triangles, Figure 2). After quenching the sample at P ) 6.3 kbar from 300 to 4.2 K with a cooling rate of approximately 100 K s-1, the substantial part of the HS centers does not convert into the LS state. Subsequent slow heating reveals that iron centers being trapped in the HS state at low temperatures demonstrate the antiferromagnetic coupling exhibited by the compound (closed
12268 J. Phys. Chem. B, Vol. 105, No. 49, 2001
Figure 3. Temperature dependence of χmT for {[Fe(bt)(NCS)2]2bpm} and for {[Fe(bpm)(NCSe)2]2bpm} (compound B) at different pressures.
Figure 4. Temperature dependence of χmT for [Fe(bpm)(NCS)2]n (compound C) at different pressures.
triangles Figure 2). As the temperature goes up, thermal excitation prevails over the trapping barrier, and at approximately 70 K, the sample starts to relax from the metastable to the equilibrium state. The characteristic relaxation time of χmT in this region is several hours, whereas the system needs approximately 10 min to reach the thermodynamic equilibrium at T e 70 K and T g 90 K. A similar result has been reported recently when {[Fe(bt)(NCS)2]2bpm} at 10 K, where the two iron atoms are in the LS state, is irradiated with red light and transforms into HS-HS antiferromagnetically coupled in the metastable state.8 As pressure attains 8.9 kbar, the spin conversion curve transforms into a typical one-step transition without hysteresis with 50% of molecules remaining in the HS state. The relative high transition temperature Tc ≈ 150 K probably does not allow to observe noticeable quenching effects at this pressure. The curve χmT versus T is interesting in that a wide plateau is reached between ca. 25 and 120 K. Further increase of pressure to 11 kbar shifts the transition curve until Tc ≈ 200 K, with the main features of the transition at P ) 8.9 kbar being retained. After the pressure-induced 50%-type of transition, the magnetic behavior of the compound A resembles completely the behavior of B at ambient pressure (Figure 3). The 50% transition for B takes place up to 4.5 kbar. At this pressure, the transition shifts to higher temperatures and the conversion curve becomes less steep, but the completeness of the transition still
Ksenofontov et al.
Figure 5. Temperature dependence of χmT for {[Fe(bt)(NCSe)2]2bpm} (compound D) at different pressures.
stays at 50%. At a pressure of 5.8 kbar, the shape of the transition curve starts to develop toward the two-step transition, which is complete at P ) 10.3 kbar. The spin crossover reveals no hysteresis both in the upper and in the lower part of the two-step curve. As the pressure grows up, the plateau between the two steps smears out and decreases indicating the conversion of the transition into a continuous one. At this stage, the similarity of the χmT curve with that of {[Fe(bt)(NCS)2]2bpm} at ambient pressure becomes evident (Figure 3). It is important to note that the plateau reached in the case of A at P > 8.9 kbar has a constant value of χmT and that the ratio between the saturation value and the plateau is close to 0.55 for each of the curves. The natural explanation of this fact is that one-half of the initial iron(II) ions in the HS state has converted to the LS state. Two possible channels of HS-HS pairs transformation at decreasing temperatures may be proposed: HS-HS T LS-HS or HS-HS T LS-LS. The observation that χmT between 50 and 110 K is temperatureindependent and that no maximum of susceptibility occurs at temperatures below 50 K suggests that no intramolecular antiferromagnetic coupling in pairs remains. In fact, the magnetization curves of A and B at ambient pressure and 1.9 K displayed in Figure 6 clearly indicate the difference in nature of the spin pairs for both compounds at low temperature. The M versus H curve for B varies linearly with the field up to ca. 8 kOe and then progressively tends to saturation. Nevertheless, it is obvious that the experimental M values are smaller than the theoretical ones calculated with the Brillouin function for an S ) 2 ground state with g ) 2 (dashed line), indicating that zero-field splitting (ZFS) occurs in this state. In contrast, the magnetization curve for A increases linearly as the field increases up to ca. 30 kOe. In the range 35-50 kOe, the slope of the M versus H curve increases significantly. For A, the M values are significantly smaller than those for B. All these facts are consistent with an antiferromagnetic S ) 0 ground-state arising from the HS-HS species in A. The increase in the slope observed for the M versus H curve at fields greater than 35 kOe are due to the crossing of the Ms ) 0 and the Ms ) (1 belonging to the ground and first excited states of the antiferromagnetically coupled HS-HS species. The solid line, which is the fit of the experimental data of B, has been obtained from Hamiltonian (2) (see below) with J ) -4 cm-1, D ) 8 cm-1, and g ) 2.2. Direct monitoring of HS-HS, LS-HS, and LSLS pairs in this family has been recently established by recording
Antiferromagnetically Coupled Fe(II) Complexes
J. Phys. Chem. B, Vol. 105, No. 49, 2001 12269
Figure 6. Magnetization curves of {[Fe(bpm)(NCS)2]2bpm} (compound A) and {[Fe(bpm)(NCSe)2]2bpm} (compound B) measured at T ) 1.9 K and ambient pressure. The theoretical curve (dashed line) was calculated with the Brillouin function for an S ) 2 ground state with g ) 2 without considering the zero-field splitting. The solid line is the fit of the experimental data of B.
field dependent Mo¨ssbauer spectra and confirm the results presented here.9 The experimental curves of A at 1, 3.8, and 8.9 kbar have been simulated with a full-matrix diagonalization routine using the Hamiltonian (1) for the HS-HS coupled species a and b (P ) 1 bar and 3.8 kbar) and the Hamiltonian (2) for the LS-HS uncoupled species in the axial ZFS approximation with S ) 2
H ˆ ) -JSˆ aSˆ b + Sˆ aDaSˆ a + Sˆ bDbSˆ b + β(Sˆ aga + Sˆ bgb)H (1) H ˆ ) D[Sˆ z2 - (1/3)S(S + 1)] + gβHSˆ
(2)
where J is the coupling constant characteristic of the HS-HS species, Sˆ , Sˆ a, and Sˆ b and Da ) Db ) D are the local spin operators and anisotropy tensors, respectively. The best set of parameters which match the experimental data at 1 and 3.8 kbar are J ) -4.5 cm-1 and D ) 8 cm-1. The g value is slightly pressure dependent being 2.18 at P ) 1 bar and 2.16 at P ) 3.8 kbar. The best fit to the experimental data at 8.9 kbar corresponds to D ) 10 cm-1 and g ) 2.19. The solid lines in Figure 2 correspond to the calculated data with these parameters. A linear combination of two extreme behaviors at 3.8 and 8.9 kbar has been used to simulate the magnetic behavior at 6.3 kbar both in the normal regime and in the quenching experiment (solid line in Figure 2). The estimated concentrations of LSHS species were around 55% and 25%, respectively. The essential details in the behavior of C are very similar to that of the dinuclear analogue A, with the difference that spin transition appears at higher pressures at comparable Tc (Figure 4). The curve measured at 9.8 kbar shows no irregularity; at 11.8 kbar, however, there is a spin transition between 100 and 150 K. The transition is complete around 50-60% HS. Measurements performed under equilibrium conditions reveal a small hysteresis with a width between 2 and 4 K. Quenching the sample from T ) 200 K leads to trapping of approximately 10% of HS species in addition to the amount detected at low temperatures in the slow cooling mode. Like in the case of the compound A, the low-temperature behavior of the χmT elucidates the type of “frozen” species, which are of HS-HS type. The antiferromagnetic coupling in the additional HS-HS pairs
due to the quenching leads to stronger temperature dependence of χmT versus T at low temperatures in comparison with the dependence provided by ZFS. The tendency of the pressure influence on the compound C is directed toward the one-step state of the compound A at high pressure and low temperatures, which means the 100% of HS-LS pairs. The significantly higher pressures required to trigger the spin transition in C with respect to A is rather surprising in view of the molecular structure of both compounds. The molecular unit of A is sketched in Figure 1a and consists of centrosymmetric bpm-bridged iron(II) dinuclear species. The intramolecular iron-iron distance is 6.050(6) Å. Two NCS- ligands are in cis position, and a peripheral bpm acting as didentate ligand completes the octahedron around each metal ion. The Fe-N(CS) bond distances [Fe-N(2) ) 2.051(7) Å and Fe-N(1) ) 2.078(6) Å] are significantly smaller than those of Fe-N(bpm) [Fe-N(3) )2.200(6) Å, Fe-N(14) ) 2.211(6) Å, Fe-N(16) ) 2.316(20) Å, and Fe-N(20) ) 2.223(6) Å]. The structure of C is made of one-dimensional polymeric units in which the iron(II) ions are bridged by bpm in a non centrosymmetrically fashion. The intramolecular iron-iron distance is 5.961(6) Å. The chains are homochiral with all the coordination octahedrons running along the z axis [(001) direction]. The Fe-N(CS) bond distances are Fe-N(5) ) 2.084 Å and Fe-N(6) ) 2.028 Å. The Fe-N(bpm bond distances are Fe-N(1) ) 2.217 Å, FeN(2) ) 2.260 Å, Fe-N(3) ) 2.226 Å, and Fe-N(4) ) 2.221 Å (Figure 1b). The average Fe-N bond distance can be considered the same in both compounds (2.180 and 2.179 Å for A and C, respectively), and consequently, the strength of the ligand field is expected to be very similar in both compounds. Hence, the difference in sensitivity to pressureinduced spin conversion should be attributed to the different packing of molecules in the crystal lattice. Finally, compound D undergoes an almost complete twostep spin transition at ambient pressure. The transition curve is similar to one of the compound B measured at 10.3 kbar. The big shift of the transition curve of ca. 50 K is observed yet at a relatively low pressure of 3.7 kbar (Figure 5). Nature of the Plateau The common peculiarity of the spin crossover process in all dinuclear compounds so far studied is a plateau in the spin transition curve. From the analysis of pressure experiments, it follows that its nature is determined by the formation of HSLS pairs and intermolecular interactions which provide the stability of crystal areas (domains) with HS-LS species. Indeed, in the absence of intermolecular interactions, the increase of pressure should decrease the amount of the HS fraction. The pressure-induced low-temperature state of the compound A consisting almost entirely of the HS-LS units is stable, and corresponding to this state, a plateau is observed at least up to 11 kbar. For compound B, a pressure of 4.5 kbar shifts the transition temperature T1/2 by ca. 50 K upward without increasing the amount of the LS species. Both experimental facts underline the role of long-range interactions in the stabilization of the hypothetical “chess board”-like structure consisting of HS-LS units. Only further increase of pressure causes a decrease of the amount of HS complexes in HS-LS units in favor of LS-LS species. The transition curve transforms to a continuous one but contains in the middle part the fragment of a plateau, which indicates the existence of clusters with HSLS units. A similar conclusion may be drawn for the compound D at ambient and elevated pressure. In a theoretical consideration, the observation of a plateau during the spin crossover in
12270 J. Phys. Chem. B, Vol. 105, No. 49, 2001 dinuclear compounds was explained as being due to a synergetic effect of intramolecular interactions favoring HS-LS pairs and intermolecular interactions favoring domains of like-species.5 In this study, we have followed experimentally the pressureinduced evolution of the plateau starting from the known initial state, consisting almost completely of HS-LS entities. In our opinion, the main conclusion may be extended to the class of mononuclear compounds. After a series of publications10 devoted to the study of correlations in the distribution of spin states in spin crossover compounds, it was concluded that the step in the thermal transition curve of some mononuclear compounds is a direct consequence of a competition of shortrange interactions with long-range interactions. By Monte Carlo simulations the “chess board”-like structure of domains consisting of alternating ...-HS-LS-... mononuclear species in the range of the plateau has been predicted.11 Utilizing the advantage of dinuclear compounds in the interpretation of the plateau nature, we can confirm this conclusion. Dynamics of HS-HS T HS-LS Interconversion Quenching experiments with A and C specimens revealed that HS-HS entities can be “frozen” in the metastable state at low temperatures. Heating the sample above ca. 60 K leads to reentrance to the stable state, which consists mostly of HS-LS dinuclear species. Two main factors, namely, antiferromagnetic intramolecular interactions and elastic interactions, should be taken into account to clarify the reason that stabilizes the metastable state. Considering the low value of J ≈ -4.5 cm-1 of the former one in comparison with the reentrance temperature T ≈ 60 K and the unusually slow kinetics of the reentrant process with relatively fast kinetics of an equilibrium spin transition taking place at higher temperature, one can conclude that the reentrance is an essentially thermally activated process and that the crystal lattice is substantially involved. The kinetics of the spin transition in dinuclear compounds confirms, that this is the structural rearrangement, i.e., that the structure is responsible for the stability of the 50% phase and not the magnetism. If it was the magnetic interaction which traps the metastable state, the dynamics of the reentrant process would be much faster. In other words, elastic interactions rather than magnetic coupling drives the transformation of HS-LS T HSHS under pressure. The competition between magnetic interaction and spin transition in spin crossover compounds resembles the coexistence of magnetism and superconductivity in superconductors. Spin crossover and magnetic interactions can probably coexist if they belong to different sublattices of a crystal. Conclusion The mechanism of spin conversion in compounds with dinuclear units has not yet been extensively studied and in most cases is still unclear. One can assume that if one Fe(II) ion undergoes the transition, the other Fe(II) of the pair with equal coordination sphere has a high probability of undergoing the transition, too, due to intramolecular interaction. This is why the authors of the paper3 considered the formation of HS-LS species as questionable. Contrary to this expectation and according to more recent reports,6,7our study gives decisive evidence of formation of HS-LS species from HS-HS pairs in dinuclear compounds during the thermal or pressureinduced spin crossover. The earlier finding by heat capacity measurements and mixing entropy determination on the one hand and results of Monte Carlo simulations on the other hand, demonstrate that short-range interactions lead to a
Ksenofontov et al.
Figure 7. Schematic representation of different cases of synergism between AF coupling and thermal spin crossover, initiated by pressure or chemical influence on ligand field strength in dinuclear iron(II) compounds. χmT ) χmT/(χmT)max vs T dependencies are normalized to the maximum χmT values for the different cases: (1) The two iron(II) centers are in HS state (Sa ) Sb ) 2) and are antiferromagnetically coupled over the whole temperature range. (2) One iron(II) center Sa ) 2 undergoes thermal spin crossover and thereby AF coupling with Sb ) 2 disappears. χmT vs T merges into a value reflecting the existence of mainly HS-LS pairs. A further decrease of χmT at low temperatures arises from zero-field splitting. (3) The two iron(II) centers being in HS state at high temperatures undergo successive thermal spin transition HS-HSfHS-LS (plateau) f LS-LS. (4) Applying pressure increases the ligand field strength, shifts the spin transition to higher temperatures and smears the plateau.
...-HS-LS-HS-LS-... “chessboard” pattern during spin transition in mononuclear complexes. According to the present study, the “chessboard” structure also may be observed in dinuclear compounds. HS-LS pair formation is reflected in the built-up of a plateau and ultimately causing the disappearance of antiferromagnetic coupling. In Figure 7, we illustrate schematically four different cases of thermal spin transitions in dinuclear compounds as a consequence of chemical modification or application of pressure both causing higher ligand field strength at the iron(II) centers. The present dinuclear compounds have shown to be very fortunate systems to study the role of short range and long-range interactions in spin crossover processes. Further work along this line is in progress, which includes the light-induced effects in dinuclear compounds. In the present case of dinuclear compounds, it is shown how external hydrostatic pressure reveals common features of the whole class of compounds, which usually can be obtained by variation of chemical composition only. Acknowledgment. We dedicate this work to Professor Dieter Sellmann on the occasion of his 60th birthday. We are grateful for financial support from the European Commission for granting the TMR_Network “Thermal and Optical Switching of Molecular Spin States (TOSS)”, Contract No. ERB-FMRXCT98-0199EEC/TMR. We thank the financial assistance from the Spanish DGICYT through Project PB97-1397. We also acknowledge the financial help from the Fonds der Chemischen Industrie and the Materialwissenschaftliches Forschungszentrum der UniVersita¨ t Mainz. References and Notes (1) (a) Goodwin, H. A. Coord. Chem. ReV. 1976, 18, 293. (b) Gu¨tlich, P. Struct. Bonding (Berlin) 1981, 44, 83. (c) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2024. (d) Ko¨nig, E.; Ritter, G.; Kulshreshtha, S. K. Chem. ReV. 1985, 85, 219. (e) Ko¨nig, E. Struct. Bonding (Berlin) 1991, 76, 51. (f) Gu¨tlich, P.; Garcia, Y.; Woike, T. Coord. Chem. ReV. 2001, 219-221, 839. (2) (a) Kahn, O.; Martinez, C. J. Science 1998, 279, 44. (b) Real, J. A.; Andre´s, E.; Mun˜oz, M. C.; Julve, M.; Granier, T.; Bousseksou, A.; Varret, F. Science 1995, 268, 265.
Antiferromagnetically Coupled Fe(II) Complexes (3) Real, J. A.; Zarembowitch, J.; Kahn, O.; Solans, X. Inorg. Chem. 1987, 26, 2939. (4) Real, J. A.; Castro, I.; Bousseksou, A.; Verdaguer, M.; Burriel, R.; Castro, M.; Linares, J.; Varret, F. Inorg. Chem. 1997, 36, 455. (5) Real, J. A.; Bolvin, H.; Bousseksou, A.; Dworkin, A.; Kahn, O.; Varret, F.; Zarembowitch, J. J. Am. Chem. Soc. 1992, 114, 4650. (6) De Munno, G.; Julve, M.; Real, J. A.; Lloret, F.; Scopelliti, R. Inorg. Chim. Acta 1996, 250, 81. (7) (a) Baran, M.; Dyakonov, V.; Gladczuk, L.; Levchenko, G.; Piechota, S.; Szymczak, H. Phys. C 1995, 241, 383. (b) Garcia, Y.; van Koningsbruggen, P. J.; Lapouyade, R.; Fourne`s, L.; Rabardel, L.; Kahn, O.; Ksenofontov, V.; Levchenko, G.; Gu¨tlich, P. Chem. Mater. 1998, 10, 2426. (c) Ksenofontov, V.; Levchenko, G.; Spiering, H.; Gu¨tlich, P.; Le´tard, J. F.; Bouhedja, Y.; Kahn, O. Chem. Phys. Lett. 1998, 294, 545. (d) Ksenofontov, V.; Spiering, H.; Schreiner, A.; Levchenko, G.; Goodwin, H. A.; Gu¨tlich, P. J. Phys. Chem. Solids 1999, 60, 393.
J. Phys. Chem. B, Vol. 105, No. 49, 2001 12271 (8) Le´tard, J. F.; Real, J. A.; Moliner, N.; Gaspar, A. B.; Capes, L.; Cador, O.; Kahn, O. J. Am. Chem. Soc. 1999, 121, 10630. (9) (a) Ksenofontov, V.; Spiering, H.; Gaspar, A. B.; Real, J. A.; Gu¨tlich, P. Proceedings of the International Conference on Molecule-Based Magnets, San Antonio, Texas A&M University, 2000. (b) Ksenofontov, V.; Spiering, H.; Reiman, S.; Garcia, Y.; Gaspar, A. B.; Moliner, N.; Real, J. A.; Gu¨tlich, P. Chem. Phys. Lett., in press. (10) (a) Ko¨ppen, H.; Mu¨ller, E. W.; Ko¨hler, C. P.; Spiering, H.; Meissner, E.; Gu¨tlich, P. Chem. Phys. Lett. 1982, 91, 348. (b) Jakobi, R.; Spiering, H.; Gu¨tlich, P. Phys. Chem. Solids 1992, 53, 267. (c) Ko¨hler, C. P.; Jakobi, R.; Meissner, E.; Wiehl, L.; Spiering, H.; Gu¨tlich, P. J. Phys. Chem. Solids 1990, 51, 239. (d) Boinnard, D.; Bousseskou, A.; Dworkin, A.; Savariault, J. M.; Varret, F.; Tuchagues, J. P. Inorg. Chem. 1994, 33, 271. (11) Spiering, H.; Kohlhaas, T.; Romstedt, H.; Hauser, A.; Bruns-Yilmaz, C.; Kucz, J.; Gu¨tlich, P. Coord. Chem. ReV. 1999, 190-192, 629.
