Salicylaldoxime (H2salox) in iron(III) carboxylate chemistry ...

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718 C.P. Raptopoulou et al. / Polyhedron 24 (2005) 711–7211a and 1b have different magnetic properties. However,consideration of two different magnetic entities for theanalysis of the bulk magnetic susceptibility data wouldlead to a large number of parameters with a small degreeof reliability. Therefore, we have analyzed the dataassuming one single trinuclear species.Initial attempts to simulate the magnetic properties ofthe complex by considering a single exchange constant,J did not yield satisfactory results. It was, therefore,concluded that a model taking into account a second exchangeconstant, J 0 , should be considered. The Hamiltonianused was^H ¼ 2Jð^S 1^S 3 þ ^S 2^S 3 Þ 2J 0^S 1^S 2 : ð3ÞThe fitting process yielded two solutions of comparablequality (with g fixed to 2.0), as is usually the case forcomplexes containing the [Fe 3 O] 7+ core, with a S = 1/2ground state [39,40]. The best-fit parameters were, A:J = 27.3 cm 1 , J 0 = 42.7 cm 1 , g = 2.0 (R = 1.2 ·10 3 ) and B: J = 34.7 cm 1 , J 0 = 24.6 cm 1 , g = 2.0(R = 1.7 · 10 3 ). This is better depicted by an error surfaceplot of J versus J 0 , which reveals the existence oftwo minima (Figure S4). Both these solutions imply aS = 1/2 ground state.In order to verify the validity of these results, a simulationof the M versus H curve at 2.5 K was carried outwith the MAGPACK program package [41,42], using thebest-fit parameters of solutions A and B. These reproducedthe experimental curves very nicely, producingpractically superimposable curves, without the use of aparamagnetic impurity fraction (Fig. 5).The situation is similar in the case of complex 2, withthe v M T product decreasing smoothly from 3.65cm 3 mol1 K at 300 K down to 0.28 cm 3 mol1 K at2 K, without extrapolating to zero (Fig. 6). For the analysisof the data we assumed the same model as above.The fit shown in Fig. 6 was carried out over the 8–Fig. 6. v M T vs. T experimental data for complex 2, and the theoreticalcurve based on the Hamiltonian of Eq. (3) (solution B).300 K temperature region. Two solutions of comparablequality were determined, their parameters being, A:J = 35.9 cm 1 , J 0 = 29.8 cm 1 , g = 2.0 (R = 3.7 ·10 4 ) and B: J = 31.3 cm 1 , J 0 = 41.2 cm 1 , g = 2.0(R = 4.3 · 10 4 ). Fitting over the 2–300 K temperaturerange yielded small discrepancies at low temperatures.These discrepancies are attributed to the presence ofweak intermolecular interactions.The existence of two minima in the fitting procedureof the magnetic susceptibility data is an inherent propertyof trinuclear complexes [40]. It would be reasonableto correlate the value of the exchange coupling constantswith the metric characteristics of the complexes. This approachhowever may not be realistic in the present case.First, complex 1 consists of two different molecules andthe unambiguous determination of the individual magneticproperties is not feasible from the present bulkmagnetic susceptibility measurements. Second, even inthe simpler case of equilateral complexes the necessityof at least two different exchange coupling constantsfor the reproduction of the magnetic susceptibility measurementsindicate that magnetostructural correlationsare not straightforward and other factors have to be takeninto account. We observe however that on average,the derived values for the exchange constants for solutionsA and B for both complexes fall well within therange of values usually found in trinuclear complexes[43–47].3.7. EPR spectroscopyFig. 5. Variation of the magnetization M vs. the applied field H at2.5 K for complex 1. The curves calculated according to the parametersof solutions A or B are superimposable.EPR spectra from powder samples for both complexesat 4.2 K are shown in Fig. 7. For compound 1the spectra reveal a feature at g 2.0 accompanied bybroad features at lower magnetic fields. For compound2 an additional broad feature is observed at higher magneticfields. The magnetic susceptibility data indicatethat the clusters are characterized by a S = 1/2 groundstate, however the EPR signals cannot be attributed to
C.P. Raptopoulou et al. / Polyhedron 24 (2005) 711–721 719Fig. 7. EPR spectrum from a solid powder sample of 1 and 2 at 4.2 K.EPR conditions: microwave frequency, 9.41 GHz; microwave power,50 mW; modulation amplitude 10 G pp .Fig. 8. EPR spectrum from 1 and 2 in acetone glass at 4.2 K. EPRconditions: microwave frequency, 9.41 GHz; microwave power, 2 mW;modulation amplitude 10 G pp .isolated S = 1/2 systems. We attribute this behaviour tointermolecular interactions present in the solid state.This is supported by the crystal structure revealing intermolecularinteractions between neighbouring trimers.We note that the low temperature magnetic susceptibilitydata for complex 2 cannot be reproduced with isolatedtrimers and the EPR spectra support weakintermolecular interactions. For compound 1 the intermolecularinteractions appear to be weak enough, sothey do not affect the magnetic susceptibility data. However,weak dipolar interactions may substantially affectthe EPR spectra. For instance a weak dipolar interactionbetween two S = 1/2 systems of the order of0.05 cm 1 can be hardly discernible in bulk magneticsusceptibility measurements, but it will critically affectthe EPR measurements leading to spectra extending ina large magnetic field region.In order to eliminate the solid-state effects, we studiedby EPR both compounds in an acetone glass, a weaklycoordinating solvent. The 4.2 K spectra are shown inFig. 8. A strong signal is observed at g 2.0 for bothcompounds. The signal is characterized by a symmetricderivative feature with a line-shape which is better describedby a Lorenztian curve. The temperature dependenceof this signal indicates that it arises from aground state. The dependence of the signal on microwavepower and the line-shape argues against a free radicalspecies. Further, it is improbable that this signalarises from monomeric Fe III (S = 5/2) species resultingfrom decomposition of the compound upon solvationin acetone. Usually such species gives rise to signals athigher g-values (for instance at g 4.3) as a result ofzero-field splitting effects. No such signals are observedin the present case. A monomeric Fe III (S = 1/2) low spinspecies can be also excluded because in this case fairlyanisotropic signals are expected.On the other hand, it is reasonable to assume that theEPR signals of Fig. 8 arise from the S = 1/2 ground stateof isolated trimers in both compounds. The symmetricline-shape of the signals suggests an isotropic system.This is indeed expected from a trimer comprising Fe III(S = 5/2) sites antiferromagnetically coupled with theisotropic Heisenberg–Dirac–van Vleck Hamiltonian asthe main interaction. Although symmetric EPR linesare expected this is rarely met in trinuclear complexesof Fe III (S = 5/2) or Cr III (S = 3/2) [39,48–53]. Usually,in such systems, non-Heisenberg interactions such asantisymmetric exchange and/or single-ion zero-fieldsplitting terms result in an anisotropic S = 1/2 groundstate. Due to the anisotropy, broad EPR signals are observedextending to high field values with g 2.0. In thepresent case the signal is symmetric and confined at theg 2.0 region indicating that the contribution of theseterms is negligible. From this point of view compounds1 and 2 are rare cases of triferric complexes where theanticipated S = 1/2 EPR signals are indeed observed.4. ConclusionsAs part of our continuing efforts to synthesize polynucleariron(III) complexes with novel magnetic properties,we have reacted H 2 salox with iron(III) in thepresence of carboxylates. The two new neutral iron(III)complexes, [Fe 3 (l 3 -O)(O 2 CPh) 5 (salox)L 1 L 2 ] (L 1 =L 2 =MeOH (1), L 1 = EtOH, L 2 =H 2 O(2)) derived, containthe [Fe 3 O] 7+ core found in Ôbasic iron carboxylatesÕ. Incomplexes 1 and 2 however two iron atoms are in anO 6 coordination environment whereas the third is foundin an O 5 N one. A detailed characterization of complexes1 and 2 including spectroscopic and magnetic studieshas been carried out. The nonequivalence of the
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