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

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