Slow Self-Assembly Favors Hysteresis above RoomTemperature for an Iron(II) 1D-Chain Spin-CrossoverComplex
René Nowak,[a] Wolfgang Bauer,[a] Tanja Ossiander,[a] andBirgit Weber*[a]
Keywords: Spin crossover / Coordination polymers / Magnetic properties / N,O ligands / Iron
The X-ray structures and magnetic properties of three newiron(II) coordination polymers with the general formula[FeL1(Lax)]n are presented. L1 is the tetradentate N2O2
2–
Schiff-base-like ligand (E,E)-{dimethyl-2,2�-[1,2-phenylene-bis(iminomethylidyne)]bis(3-oxobutanato)(2–)-N,N�,O3,O3�}.For the axial coordination, 4,4�-bipyridine (bipy), 1,2-bis(4-pyridyl)ethane (bpea), and 1,2-bis(4-pyridyl)ethene (bpee)were used. Spin-transition behavior was observed for thebpea and the bipy complexes. In both cases, two different
IntroductionThere is a continuing interest in iron(II) spin-crossover
(SCO) complexes. The SCO phenomenon results from thepossibility to realize two different electronic states for oneand the same compound as a function of external param-eters such as the temperature or the pressure.[1–3] The spintransition is accompanied by a change of the properties ofthe complex. This can be the magnetism [for iron(II) fromdiamagnetic, S = 0, in the low-spin (LS) state to paramag-netic, S = 2, in the high-spin (HS) state], the size, or thecolor.[2] The latter is so pronounced, it can be detected eas-ily with the naked eye, and possible applications of suchcompounds as temperature or pressure sensors with an easyoptical readout are discussed.[4] In diluted systems, the spintransition is always gradual and takes place over a widetemperature or pressure range. In the solid state, coopera-tive effects can be observed, which lead to stepwise orabrupt spin transitions and sometimes even to bistability(hysteresis) over a wide temperature region. Depending onthe requirements of the desired application, one of the dif-ferent types of spin transition is needed, and reproduciblesynthetic protocols are necessary. This has to be emphas-ized, as polymorphism is frequently observed for SCO com-pounds.[5] Detailed investigations that show how small
[a] Inorganic Chemistry II, Universität Bayreuth,Universitätsstraße 30, NW I, 95440 Bayreuth, GermanyFax: +49-921-55-2157E-mail: [email protected]: http://www.ac2-weber.uni-bayreuth.deSupporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejic.201201051.
polymorphs were isolated, showing different magnetic prop-erties. In the case of the bipy complex 1, different preparationmethods were tested to elucidate which conditions favor onepolymorph over another. It appears that a slow precipitationof the coordination polymer favors a cooperative spin transi-tion above room temperature, whereas a fast precipitationresults in a gradual spin transition at lower temperatures. Inthe case of the bpea complex, an incomplete spin transitionis observed, which can be explained by the crystal packing.
changes in the synthetic route influence the spin-transitionproperties are necessary in those cases. This deserves specialattention, if the synthesis of nanoparticles or composite ma-terials is considered – a necessary step for an application ofspin crossover. In this paper, we present three new iron(II)1D coordination polymers, two of which show SCO proper-ties. The polymers were obtained by combining a Schiff-base-like equatorial ligand L1 with an N2O2 coordinationsphere with 4,4�-bipyridine (bipy), 1,2-bis(4-pyridyl)ethane(bpea), or 1,2-bis(4-pyridyl)ethene (bpee) as bidentate,bridging axial ligands. For both SCO complexes, differentpolymorphs were obtained, depending on the used syn-thetic procedure. Different synthetic routes were exploredfor the purposeful synthesis of the two polymorphs of thebipy complex.
Results and Discussion
Synthesis and General Characterisation
In Scheme 1, the structures of the used equatorial andaxial ligands are given together with the general syntheticroute of the presented 1D iron(II) coordination polymers.They were prepared by a self-assembly approach, in whichthe precursor [FeL1(MeOH)2] was treated with the respec-tive axial ligand until the coordination polymer precipi-tated. As parameters, the reaction temperature [reflux orroom temperature (r.t.)], the reaction time (direct mixing orslow diffusion), and the solvent were varied. For the dif-fusion setup, a Schlenk tube was used, which was, up to a
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certain height, parceled by a glass wall into two chambers.The starting materials were each placed at the base of onechamber, and the solvent was carefully filled up just highenough to allow little diffusion between the chambers. Forthe reflux setup, the starting materials and the solvent weremixed and then heated at reflux. For the reaction at roomtemperature, the starting materials were dissolved in the de-sired solvent, and then the two solutions were mixed. InTable 1, the different approaches for the syntheses of allcomplexes discussed in this paper are summarized togetherwith the used abbreviations. All samples were fully charac-terized by elemental analysis, IR spectroscopy, mass spec-trometry, and magnetic susceptibility measurements. Ac-cording to the different analytic techniques, the composi-tion of the different samples of one complex is identicalregardless of the solvent used.
Scheme 1. Top: Schematic representation of the equatorial and ax-ial ligands discussed in this work together with the used abbrevi-ations. Bottom: General synthesis of the 1D coordination polymersby self-assembly.
Table 1. Summary of the presented compounds in the context ofthe methods used for their preparation.
Compound Solvent Method Number
[FeL1(bipy)]n MeOH reflux 1MeOH slow diffusion 1kMeOH stirring at r.t. 1aEtOH stirring at r.t. 1b
MeOH/THF stirring at r.t. 1cDMF stirring at r.t. 1d
toluene stirring at r.t. 1e[FeL1(bpea)]n MeOH reflux 2
Crystals suitable for X-ray diffraction structure analysisof the three 1D chain coordination polymers were obtainedby slow-diffusion techniques. In Figure 1, the asymmetricunits of 1k and 3k are given. The asymmetric unit of 2k ispresented in Figure 2. In Table 2, selected bond lengths andangles are given, and in Table S1, the crystallographic dataof the three complexes are summarized. 1k and 3k crys-tallize in the space group P21/c with one iron center in theasymmetric unit. In the case of 1k, the iron center is clearlyin the LS state with average Fe–N bond lengths of 1.90 Å(Fe–Neq), 1.93 Å (Fe–Oeq), and 1.99 Å (Fe–Nax). These val-ues and the Oeq–Fe–Oeq angle of 86.5° agree with valuesreported previously for octahedral iron(II) LS complexes of
Figure 1. (a) ORTEP drawing of the asymmetric unit of 1k (250 K,LS) and (b) ORTEP drawing of the asymmetric unit of 3k (200 K,HS) with atom labels (anisotropic displacement ellipsoids drawn atthe 50% probability level). H atoms are omitted for clarity. Theasymmetric unit corresponds to the monomer unit [FeL1(Lax)] ofthe polymer strand.
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Table 2. Selected bond lengths [Å] and angles [°] of 1k (LS), 2k (HS), and 3k (HS).
this ligand type.[3,6] For 3k, the bond lengths [average2.09 Å (Fe–Neq), 2.01 Å (Fe–Oeq), and 2.29 Å (Fe–Nax)]and angles (Oeq–Fe–Oeq 109.0°) are in regions typical foroctahedral iron(II) complexes of this ligand type in the HSstate.[3,6] For 2k, the situation is more complex. This com-plex crystallizes in the space group P1̄, and five iron centersare observed in the asymmetric unit. The bond lengths andangles given in Table 2 indicate that all iron centers are inthe HS state with average bond lengths and angles of 2.17 Å(Fe–Neq), 2.02 Å (Fe–Oeq), 2.26 Å (Fe–Nax), and 110.9°(Oeq–Fe–Oeq) for all five complex molecules. The sub-scripted abbreviations refer to the axial (ax) or equatorial(eq) bonding situation.
Figure 2. ORTEP drawing of the asymmetric unit of 2k (250 K,HS, anisotropic displacement ellipsoids drawn at the 25% prob-ability level). H atoms are omitted for clarity. The asymmetric unitconsists of five monomer units [FeL1(Lax)] of five polymer strands.
For each complex, the bridging axial ligand connects theiron centers to infinite chains, as illustrated in Figure 3. Thechains are best described as linear, as the steps induced bythe bpea and bpee ligands are not very pronounced. For 1kand 3k, the chains are aligned parallel in the crystal pack-ing, as observed for most of the 1D-chain coordinationpolymers of this ligand type, and propagate along [101] for1k and along [100] for 3k. A top view of the chains in the
crystal packing is illustrated in Figure 4 for both complexes.Several short intermolecular contacts are observed, the de-tails of which are given in Table 3. For both molecules, thesecontacts result in the formation of a 3D network of linkedpolymer strands. In the case of 2k, the situation is different.Of the five iron centers in the asymmetric unit, three centers(Fe2, Fe3, Fe4) form parallel strands, which propagatealong [100], whereas the other two (Fe1, Fe5) form 1Dchains, which are perpendicular to those three strands andpropagate along [010]. In Figure 5, the crystal packing of2k projected along these two directions of propagation isgiven. As for the other two complexes, several short inter-molecular contacts are observed in the crystal packing of2k, the details of which are summarized in Table 3. For thesake of clarity, these contacts are not depicted in Figure 5.
Figure 3. Illustration of the polymer chain structures of (a) 1k,(b) 2k, and (c) 3k.
Such a network of cross-linked chains was previously re-ported for a 1D-chain complex of this ligand type with 4,4�-azopyridine as the bridging ligand.[7] The cross-linked ar-rangement led to a two-step spin transition with a very wideintermediate plateau. However, as that complex crystallizedin the temperature region of the plateau with two in-equiva-lent iron centers, a final decision about whether the non-equivalent iron centers or the cross-linked arrangement ofthe 1D chains are responsible for the step with the wideplateau was not possible.[7] For the five different iron cen-ters in the asymmetric unit of 2k, the bond lengths andangles within the first coordination sphere are all in the
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Figure 4. (a) Crystal packing of 1k projected along [101]. (b) Crys-tal packing of 3k projected along [100]. H atoms are omitted forclarity, except for those involved in the short contacts given inTable 3.
Table 3. Distances [Å] and angles [°] of all intermolecular contactsin 1k, 2k and 3k that are shorter than the sum of the van der Waalsradii by 0.2 Å.
Figure 5. Crystal packing of 2k, projected along (a) [010] and along(b) [100].
same order of magnitude. The most pronounced differenceis the arrangement of the two pyridine rings of the bpealigand with regard to each other, which varies from nearlycoplanar (Fe1) to almost perpendicular (Fe5). Because ofthe more rigid nature of the bipy and the bpee ligand, thearrangement of the two pyridine rings is close to coplanarin both cases.
Magnetic Susceptibility Data
Magnetic susceptibility measurements in the temperaturerange between 10 and 400 K were done for all complexesdiscussed in this work. Firstly, we compare the results ob-tained for the crystalline samples 1k, 2k, and 3k with thoseof the corresponding powder samples 1, 2, and 3, whichwere prepared in methanol under reflux conditions. In Fig-ure 6, the temperature dependence of the product χMT isgiven for the pairs 1/1k (a) and 2/2k (b). Of the six samples,three (2, 3, and 3k) remain in the HS state over the entiretemperature range with χMT in the region of3.3 cm3 Kmol–1 – a typical value for an iron(II) complex inthe HS state. For all three samples, the magnetic momentdecreases at temperatures below 25 K, which can be attrib-uted to the zero-field splitting. At the bottom of Figure 6,the results for 2 (squares) are given as a typical example forthose three complexes.
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Figure 6. Plot of the χMT product over T for (a) 1 (squares) and1k (open circles) and for (b) 2 (squares) and 2k (open circles).
For the complexes 1, 1k, and 2k, spin-crossover behavioris observed. In the case of 1, the spin transition is gradualin the temperature range between 270 and 110 K with aT1/2 value of 204 K. The room-temperature value of theχMT product of 3.3 cm3 K mol–1 is typical for an iron(II) HScomplex, and it is χMT = 0.26 cm3 K mol–1 at 50 K, which istypical for an iron(II) LS complex. Thus, the spin transitionis complete. In the high-temperature region, a small differ-ence is observed between the transition curves in the heat-ing and cooling mode, resulting in an approximately 10 Kwide hysteresis loop. For the crystalline sample 1k, a com-pletely different behavior is observed. At room temperature,the χMT product of 0.14 cm3 Kmol–1 is typical for aniron(II) LS complex. Upon heating, this value remains con-stant up to 330 K. Above this temperature, an abrupt spintransition takes place with TC = 336 K and a χMT productof 3.26 cm3 Kmol–1 at 350 K. In the cooling mode, the spintransition is shifted to slightly lower temperatures with TC
= 330 K, which reveals a 6 K wide thermal hysteresis loop,which can be repeated several times. The room-temperatureχMT product of 2k of 3.27 cm3 K mol–1 is similar to that ofother HS complexes discussed in this work. Upon cooling,
Table 4. Summary of the magnetic properties for the different samples of [FeL1(bipy)] discussed in this work.
1 G – 204 – –1k H 6 – 336 3301a H 10 – 341 3311b G + H 7 209 338 3311c H 10 – 340 3301d H 8 – 338 3301e G + H 13, 17[d] 206 337, 332[d] 324, 315[d]
[a] H indicates a thermal hysteresis, while G stands for gradual transition. [b] Critical temperature at heating. [c] Critical temperature atcooling. [d] First and second cycle, respectively.
it remains constant down to 210 K. Below this temperature,an abrupt decrease of the magnetic moment is observedwith TC = 195 K and a χMT value of 1.33 cm3 Kmol–1 at100 K. This value corresponds to approximately three outof five iron centers in the LS state, whereas the other tworemain in the HS state. Upon heating, the magnetic mo-ment remains constant up to 195 K. Above this tempera-ture, it increases abruptly with TC = 209 K; a 14 K widethermal hysteresis loop is observed. Interestingly, the spin-transition properties reflect the asymmetric unit determinedby X-ray structure analysis. It appears that the three ironcenters forming parallel strands undergo a spin transition,whereas the other two remain in the HS state. A similarbehavior was observed for 1D-chain compounds of this li-gand type with azopyridine as a bridging ligand and across-linked arrangement of the 1D chains.[7] Differentreasons are conceivable for stepwise spin transitions (e.g.,symmetry breaking).[8] It was not possible to determine theX-ray structure at low temperatures (crumbling of the crys-tals possibly because of a phase transition). Thus, no fur-ther insights could be obtained with regard to the incom-plete spin transition of 2k.
As the chemical composition of the pairs 1/1k and 2/2kis identical, the different magnetic properties indicate thattwo different polymorphs were obtained in both cases. Todetermine which parameters influence the formation of thedifferent polymorphs, different reaction conditions weretested for the synthesis of 1. In a first try, methanol waskept as the solvent, but instead of refluxing the reactionmixture, it was stirred at room temperature. Magnetic mea-surements of the resulting complex 1a show a thermal hys-teresis above room temperatures with similar parameters asfor the crystalline sample. Interestingly, a slight increase ofthe hysteresis width is observed (Table 4). It appears thatthe room-temperature approach favors the cooperative spintransition, which indicates that longer reaction times arenecessary for the formation of a sample with a cooperativespin transition. Subsequently, different solvents were tested,as the varying solubility of the starting materials and thefinal product may also influence the reaction time necessaryfor the precipitation of the product. Indeed, with ethanol(1b) and toluene (1e), for which a lower solubility of thecoordination polymer is observed, a gradual spin transitionis detected in the magnetic measurements, as illustrated inFigure 7 for the ethanol sample 1b.
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As can be seen in Figure 7, 1b reaches its maximum χMTvalue above 350 K and shows a combination of a gradualspin transition in the low-temperature region and a thermalhysteresis loop above room temperature. In the area be-tween approximately 275 and 375 K, hysteresis is observed,which disembogues below 275 K into a gradual transitionuntil all molecules have passed over to the LS state at100 K. Overall, the magnetic susceptibility ranges from0.09 cm3 K mol–1 at 100 K to 3.26 cm3 Kmol–1 at 350 K.
Figure 7. Plot of χMT over T in the temperature range 10–375 Kfor samples of the compound [FeL1(bipy)]n prepared from meth-anol/THF (1c, open triangles) and ethanol (1b, circles). The solidblue line corresponds to calculated values for a mixture of 1 (75%)and 1k (25%), which agree very well with the spin-transition curveof 1b.
In the course of cooling, the HS state remains stabledown to 335 K. Then, a partial but sudden transitionemerges, decelerating to an HS–LS plateau in the region320–260 K. The χMT product in this region(2.46 cm3 Kmol–1) indicates that about three quarters of theiron centers are still in the HS state. Thence, the completeLS state is entered gradually. During heating up, a similarbehavior is observed with slightly shifted transition tem-peratures in the high-temperature region for both the grad-ual and the abrupt transition. The critical temperatures forthe heating and cooling processes of the cooperative transi-tion are 338 K for TC� and 331 K for TC�, and a 7 Kwide thermal hysteresis loop is observed. Those parametersare in good agreement with the spin transition observed forthe crystalline sample 1k. For the gradual transition withT1/2 = 209 K, both the shape of the curve progression andthe transition parameters are in good agreement with thoseof the powder sample 1. This identicalness becomes evenmore obvious if the solid blue line in Figure 7 is taken intoconsideration. This curve was obtained by adding up theweighted χMT values of 1 and 1k with the assumption that75% of the sample 1b is of the same polymorph as 1 and25% of the sample is the same polymorph as 1k. An excel-lent agreement between the blue curve and the black dotsin Figure 7 is obtained, thus, both polymorphs are observedin the sample 1b. The magnetic measurements of this sam-ple can be repeated several times without any changes inthe curve progression. Thus, both polymorphs are stableand cannot be converted into one another by thermal treat-ment. The samples obtained from DMF and from a meth-anol/THF mixture show the same magnetic properties asthe crystalline sample – an abrupt spin transition with hys-
teresis above room temperature. As a typical example, thetransition curve of 1c is given in Figure 7. Within the mea-sured range, the magnetic susceptibility χMT reaches from3.39 cm3 Kmol–1 at 400 K to 0.08 cm3 Kmol–1 at 100 K. Atroom temperature, compound 1c is diamagnetic. During thecooling mode starting at 400 K, the HS state remains stabledown to approximately 333 K. An entire LS state emergesat 320 K. By heating up, the LS � HS switch appears ataround 338 K. The characteristic points are TC� = 340 Kand TC� = 330 K, resulting in a hysteresis width of around10 K. As for the methanol sample, a slight increase in thehysteresis loop width is observed compared to the crystal-line sample.
The powder-diffraction data of the different samples of1 and 2 were collected to obtain a deeper insight into thepolymorphism of these complexes. In Figures 8c and d thepowder diffraction pattern of 1c is compared with the calcu-lated pattern of the single crystals of 1k. As can be clearlyseen from Figure 8, both patterns are identical. The samediffraction pattern is also observed for the samples 1a and
Figure 8. X-ray powder diffraction patterns of 1 (a), 1b (b), and1c (d) at room temperature and a pattern calculated from the sin-gle-crystal X-ray diffraction analysis of 1k at 250 K (c). The greenline in (b) corresponds to a pattern calculated for a mixture of 1(75%) and 1k (25%).
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1d – the same polymorph is obtained in all cases (Fig-ure S2). The line widths of the Bragg peaks are all in thesame order of magnitude, and no conclusion can be drawnwith regard to the slightly different hysteresis widths. Forthe sample 1b (Figure 8b), another set of reflections is ob-served, as this pattern is clearly dominated by the othermodification of the complex. This pattern is very similar tothat of powder sample 1 (Figure 8a) with its gradual spintransition. By using the same procedure as for the magneticmeasurements, a calculated powder diffraction pattern wasobtained by assuming a contribution of 75% of 1 and of25 % of 1k. This pattern is given as a green line in Figure 8b.A very good agreement between the green and the blackpattern in Figure 8b is obtained, which supports the con-cept that a mixture of two different polymorphs is obtainedin sample 1b.
The magnetic properties of the toluene sample 1e arevery similar to those of the ethanol sample 1b. Again, acombination of the gradual spin transition below roomtemperature and the abrupt spin transition with hysteresisabove room temperature is observed. However, in this case(Figure S1) a small shift of the spin-transition parametersis observed for the second cycle, and the small hysteresisobserved for the gradual part is missing. The reason for thisdifference is not clear. The powder diffraction patterns of1b and 1e are very similar (Figure S3). Probably, a smallamount of solvent is included in the powder sample 1e, anamount too small to be detected by elemental analysis. Thehysteresis in the high-temperature region is wider thanthose observed for the other samples, but this might be dueto the loss of included solvent molecules. The powder dif-fraction patterns of 2 and 2k, shown in Figure S4 confirmthe assumption that two different polymorphs are observedfor this sample. This explains the different magnetic proper-ties.
Conclusions
Added together, the diversity of the preparative impacton the magnetic behavior of [FeL1(bipy)]n is clearly recog-nizable. The powder sample 1 exhibits a gradual spin transi-tion similar to that of the self-assembly samples 1b and 1e,whereas all other presented self-assembly compounds exhi-bit a thermal hysteresis, as do the crystals. The thermal hys-teresis loops are all located in the same temperature range,but they are different with respect to the width of the hys-teresis. Interestingly, the hysteresis loops for the differentpowder samples are wider than that of the single crystals.This clearly illustrates that the particle size is not the onlyfactor that influences the cooperative interactions. The dif-ference in hysteresis width is mainly a result of a higherTC� for the self-assembly batches, whereas TC� remainsnearly the same for all those samples. The formation of thetwo different polymorphs can be satisfactorily explainedwith the different solubilities of the final product in the dif-ferent solvents. A fast precipitation of the product, which iseither due to the low solubility or due to a higher concen-
tration for the batch under reflux conditions, results in agradual spin transition. In contrast, if the concentration islower and more time passes until the product precipitates,the (thermodynamically more stable) product with a coop-erative spin transition is obtained. For the complex 2k, aninteresting, incomplete spin transition is observed, whichinvolves about 3/5 of the molecules. This can be explainedby the crystal packing, which features five iron centers inthe asymmetric unit with a cross-linked arrangement of the1D chains. As observed recently for an 4,4�-azopyridinecomplex of this ligand type, such a cross-linked arrange-ment obviously favors incomplete spin transitions or verywide intermediate plateaus.[7]
Experimental SectionSyntheses: If not described differently, all syntheses of iron(II) com-plexes were carried out under argon by using Schlenk-tube tech-niques. All solvents were purified as described in the literature[9]
and distilled under argon. The syntheses of the ligand H2L1 andthe precursor [FeL1(MeOH)2][10] were published before. 4,4�-Bipyr-idine was purchased from Acros Organics and was used as received.1,2-Bis(4-pyridyl)ethene and 1,2-bis(4-pyridyl)ethane were pur-chased from Aldrich and were also used as received. All complexsyntheses were reproduced at least once.
[FeL1(bipy)]n (1): A solution of [FeL1(MeOH)2] (0.23 g, 0.48 mmol)and 4,4�-bipyridine (0.38 g, 2.4 mmol) in methanol (16 mL) washeated to reflux for 0.5 h. A dark violet precipitate was formedduring heating, which was filtered off after cooling to room tem-perature, washed with methanol (2� 5 mL), and dried in vacuo(yield: 0.14 g, 50%). C28H26FeN4O6 (570.37): calcd. C 58.96, H4.59, N 9.82; found C 59.08, H 4.69, N 9.77.
[FeL1(bipy)]n (1a): [FeL1(MeOH)2] (0.21 mmol, 0.10 g)was dis-solved in MeOH (50 mL). After the addition of 4,4�-bipyridine(0.42 mmol, 0.07 g) in the form of a 0.1 m methanol solution, themixture was stirred at room temperature. After 6 d, a dark violetprecipitate was obtained, which was filtered off, washed withMeOH, and dried in vacuo (yield: 0.12 g, 98%). IR (KBr): ν̃ =1694 (COO), 1553 (CO) cm–1. MS [DEI(+), 70 eV]: m/z (%) = 414.1(100) [FeL1]+, 156.1 (100) [bipy]+. C28H26FeN4O6 (570.37): calcd.C 58.96, H 4.59, N 9.82; found C 58.74, H 4.63, N 9.82.
[FeL1(bipy)]n (1b): [FeL1(MeOH)2] (0.21 mmol, 0.10 g) was dis-solved in EtOH (50 mL). After the addition of 4,4�-bipyridine(0.42 mmol, 0.07 g) in the form of a 0.1 m ethanol solution, themixture was stirred at room temperature. After 4 d, a black precipi-tate was obtained, which was filtered off, washed with MeOH, anddried in vacuo (yield: 0.11 g, 90%). IR (KBr): ν̃ = 1695 (COO),1556 (CO) cm–1. MS [DEI(+), 70 eV]: m/z (%) = 414.1 (81)[FeL1]+, 156.1 (98) [bipy]+. C28H26FeN4O6 (570.37): calcd. C 58.96,H 4.59, N 9.82; found C 58.57, H 4.55, N 9.72.
[FeL1(bipy)]n (1c): [FeL1(MeOH)2] (0.22 mmol, 0.11 g) was dis-solved in MeOH (50 mL). After the addition of 4,4�-bipyridine(0.44 mmol, 0.07 g) in the form of a 0.1 m tetrahydrofuran solution,the mixture was stirred at room temperature. After 4 d, a dark,violet precipitate was obtained, which was filtered off, washed withMeOH, and dried in vacuo (yield: 0.12 g, 97%). IR (KBr): ν̃ =1694 (COO), 1553 (CO) cm–1. MS [DEI(+), 70 eV]: m/z (%) = 414.2(55) [FeL1]+, 156.1 (88) [bipy]+. C28H26FeN4O6 (570.37): calcd. C58.96, H 4.59, N 9.82; found C 58.68, H 4.20, N 9.78.
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[FeL1(bipy)]n (1d): [FeL1(MeOH)2] (0.24 mmol, 0.11 g) was dis-solved in DMF (50 mL). After the addition of 4,4�-bipyridine(0.48 mmol, 0.08 g) in the form of a 0.1 m solution in DMF, themixture was stirred at room temperature. After 5 d, a black precipi-tate was obtained, which was filtered off, washed with MeOH, anddried in vacuo (yield: 0.13 g, 92%). IR (KBr): ν̃ = 1696 (COO),1555 (CO) cm–1. MS [DEI(+), 70 eV]: m/z (%) = 413.6 (96)[FeL1]+, 156.1 (95) [bipy]+. C28H26FeN4O6 (570.37): calcd. C 58.96,H 4.59, N 9.82; found C 58.46, H 4.57, N 9.84.
[FeL1(bipy)]n (1e): [FeL1(MeOH)2] (0.21 mmol, 0.10 g) was dis-solved in toluene (50 mL). After the addition of 4,4�-bipyridine(0.42 mmol, 0.08 g) in the form of a 0.1 m toluene solution, themixture was stirred at room temperature. After 7 d, a black precipi-tate was obtained, which was filtered off, washed with MeOH, anddried in vacuo (yield: 0.12 g, 94 %). IR (KBr): ν̃ = 1689 (COO),1563 (CO) cm–1. MS [DEI(+), 70 eV]: m/z (%) = 414.0 (80)[FeL1]+, 156.1 (100) [bipy]+. C28H26FeN6O6 (598.38): calcd. C58.96, H 4.59, N 9.82; found C 58.07, H 4.61, N 9.55.
[FeL1(bipy)]n (1k): Crystals of [FeL1(bipy)] suitable for X-ray dif-fraction analysis were obtained from a slow diffusion process byusing a Schlenk tube, which was, to a certain height, parceled by aglass wall into two chambers. [FeL1(MeOH)2] (0.09 g, 0.19 mmol)was placed at the bottom of one chamber, and 4,4�-bipyridine(0.05 g, 0.32 mmol) was placed in the other chamber. The solvent(methanol) was carefully filled up just high enough to allow littlediffusion between the chambers. After 2 weeks, 1k was obtained inform of black crystals. C28H26FeN4O6 (570.37): calcd. C 58.96, H4.59, N 9.82; found C 58.83, H 4.63, N 9.74.
[FeL1(bpea)]n (2): A solution of [FeL1(MeOH)2] (0.29 g,0.61 mmol) and 1,2-bis(4-pyridyl)ethane (0.56 g, 3.03 mmol) inmethanol (30 mL) was heated to reflux for 1 h. After cooling, ablack precipitate was obtained, which was filtered off, washed withmethanol (2 � 5 mL), and dried in vacuo (yield: 0.23 g, 63%).C30H30FeN4O6 (598.15): calcd. C 60.21, H 5.05, N 9.36; found C60.05, H 5.12, N 9.34.
[FeL1(bpea)]n (2k): Crystals of [FeL1(bpea)] suitable for X-ray dif-fraction analysis were obtained from a slow diffusion process.[FeL1(MeOH)2] (0.19 g, 0.40 mmol) was placed at the bottom ofone chamber of a parcelled Schlenk tube, and 1,2-bis(4-pyridyl)-ethane (0.12 g, 0.68 mmol) was placed in the other chamber. Thesolvent (methanol) was carefully filled up just high enough to allowlittle diffusion between the chambers. After 2 weeks, 2k was ob-tained in form of black crystals. C30H30FeN4O6 (598.15): calcd. C60.21, H 5.05, N 9.36; found C 59.88, H 5.10, N 9.31.
[FeL1(bpee)]n·MeOH (3): A solution of [FeL1(MeOH)2] (0.34 g,0.71 mmol) and 1,2-bis(4-pyridyl)ethene (0.65 g, 3.55 mmol) inmethanol (30 mL) was heated to reflux for 1 h. After cooling, ablack precipitate was obtained, which was filtered off, washed withmethanol (2 � 5 mL), and dried in vacuo (yield: 0.43 g, 96%).C31H32FeN4O7 (628.16): calcd. C 59.25, H 5.13, N 8.92; found C59.10, H 4.97, N 9.05.
[FeL1(bpee)]n·MeOH (3k): Crystals of [FeL1(bpee)]·MeOH suitablefor X-ray diffraction analysis were obtained from a slow diffusionprocess. [FeL1(MeOH)2] (0.17 g, 0.35 mmol) was placed at the bot-tom of one chamber of a parceled Schlenk tube, and 1,2-bis(4-pyridyl)ethene (0.11 g, 0.59 mmol) was placed in the other cham-ber. The solvent (methanol) was carefully filled up just high enoughto allow little diffusion between the chambers. After 2 weeks, 3kwas obtained in form of black crystals. C31H32FeN4O7 (628.16):calcd. C 59.25, H 5.13, N 8.92; found C 59.08, H 4.69, N 9.77.
X-ray Diffraction Analysis: The intensity data of 1k, 2k, and 3kwere collected with a Nonius Kappa CCD diffractometer by using
graphite-monochromated Mo-Kα radiation. The data were cor-rected for Lorentz and polarization effects. The structures weresolved by Direct Methods (SIR97 and SHELXS)[11,12] and refinedby full-matrix least-squares techniques against Fo
2 – Fc2 (SHELXL-
97 and SHELXH-97).[12] All hydrogen atoms were calculated inidealized positions. ORTEP-III[13] was used for the structure repre-sentation, SCHAKAL-99[14] was used to illustrate the moleculepacking, and PLATON[15] was used for the calculation of the inter-molecular distances and angles. The crystallographic data are sum-marized in the Supporting Information (Table S1). The asymmetricunit of 2k is very large, and a refinement of potentially disorderedresidual solvent molecules was not possible. A detailed discussionof the intermolecular interactions is thus not possible. CCDC-896078 (1k), -896080 (2k), and -896079 (3k) contain the supple-mentary crystallographic data for this paper. These data can beobtained free of charge from The Cambridge CrystallographicData Centre via www.ccdc.cam.ac.uk/data_request/cif. The powderdiffractograms were recorded with a STOE StadiP diffractometer(transmission geometry) by using Ge-monochromated Cu-Kα1 ra-diation and a Mythen1K detector.
Magnetic Measurements: Magnetic susceptibility data were col-lected by using a Quantum Design MPMSR-2 or an MPMS-XL5SQUID magnetometer with an applied field of 0.2 T over the tem-perature range 10–400 K in the settle mode. The samples wereplaced in gelatin capsules held within a plastic straw. The data werecorrected for the diamagnetic magnetization of the ligands, byusing tabulated Pascal constants, and of the sample holder. Themeasurements were analyzed by using the CGS system.
Supporting Information (see footnote on the first page of this arti-cle): Magnetic measurements of 1e, powder diffraction patterns ofall samples of 1 and 2, and the crystallographic data of 1k, 2k, and3k.
Acknowledgments
We thank M. Reichvilser, T. Kerscher, and P. Mayer (University ofMunich) for the collection of the single-crystal X-ray diffractiondata and W. Milius (University of Bayreuth) for the collection ofthe powder diffraction data. Support from the University of Bay-reuth, the Deutsche Forschungsgemeinschaft (WE 3546_4-1 andSFB 840/A10), and the Fonds der Chemischen Industrie is grate-fully acknowledged.
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