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

C.P. Raptopoulou et al. / Polyhedron 24 (2005) 711–721 717the spectrum exhibits fairly narrow lines suggesting thatthe iron sites in molecules 1a and 1b are characterized bysimilar Mössbauer parameters and especially DE Q . Thesituation for complex 2 is similar and the O 5 N coordinatediron atom [Fe(1)] with a larger DE Q is assignedto site II, despite its more symmetric octahedral coordinationgeometry.At 4.2 K and in the absence of external magnetic fieldsa slight broadening of the doublets for both complexes isobserved (Fig. S3). No magnetic ordering is evidenced at4.2 K. The broadening can be attributed to the onset ofmagnetic relaxation effects. Alternatively, structuralchanges may occur leading to three distinct sites.Although the deconvolution of the spectra is not uniqueboth can be simulated assuming two sites at 2:1 ratio asin the room temperature case with the parameters quotedin Table 3 (Fig. S3). The average isomer shift for bothcompounds at 4.2 K is 0.54 mm s 1 and the differencein the isomer shifts with the room temperature spectrais due to the second order Doppler effect [38].Fig. 3. Room temperature Mössbauer spectra from solid powdersamples of 1 and 2. Solid lines are theoretical spectra assuming twodifferent iron sites with the parameters quoted in Table 3.Table 3Mössbauer parameters for 1 and 2Complex Site Temperature(K)d a,b,cDE Qa,cC/2 a,d Area (%) d1 I 300 0.42(1) 0.56(1) 0.15 674.2 0.54(1) 0.61(1) 0.19II 300 0.41(1) 1.24(1) 0.15 334.2 0.53(1) 1.15(1) 0.192 I 300 0.42(1) 0.70(1) 0.14 674.2 0.55(1) 0.79(1) 0.19II 300 0.41(1) 1.29(1) 0.14 334.2 0.54(1) 1.31(1) 0.19a In mm s 1 .b Isomer shifts are reported relative to iron metal at roomtemperature.c Number in parentheses represent errors in the last digit.d Fixed.3.6. Magnetic behaviourFor complex 1, the v M T product at 300 K is3.94 cm 3 mol1 K(Fig. 4), significantly lower than thetheoretically expected value for three non-interactingS = 5/2 spins (13.14 cm 3 mol1 K), suggesting antiferromagneticinteractions. Upon cooling, the v M T productdecreases to 0.31 cm 3 mol1 K at 2 K, without extrapolatingto zero as the temperatures tends to zero, whichagrees with the interplay of antiferromagnetic interactions,and a magnetic ground state.As analyzed in the description of the structures, complex1 actually consists of two independent molecules(1a and 1b), with two rather different geometries; isosceles(for 1a) and scalene (for 1b, see description of structures).Due to the different geometries it is expected thatmolecule 1a, and by Fe(5), Fe(6) in molecule 1b, with theshortest bond being the Fe–(l 3 -O 2 ) in each case. Thecorresponding trans effect experienced by atoms Fe(1)and Fe(4) is insignificant (see Table 2). In terms of donoratoms, there are two iron atoms with asymmetric O 5 Nchromophores [Fe(1) and Fe(4)] and four atoms withmore symmetric O 6 chromophores [Fe(2), Fe(3), Fe(5)and Fe(6)]. The Mössbauer spectra, therefore, indicatethat the O 5 N coordination should be considered responsiblefor the larger DE Q value of site II.The resolution of the spectrum of complex 1 does notallow us to distinguish the two crystallographically differentmolecules 1a and 1b. We observe however thatFig. 4. v M T vs. T experimental data for complex 1, and theoreticalcurves based on the Hamiltonian of Eq. (3), using the parameters ofsolution A.