Syntheses, structural, and physical studies of basic CrIII and FeIII benzilates and benzoates: Evidence of antisymmetric exchange and distributions of isotropic and antisymmetric exchange parameters

Abstract

The synthesis, crystal structures, spectroscopic and magnetic properties studies of four trinuclear oxo-centered carboxylate complexes are reported and discussed: [Cr3(μ3-O)(O2C(OH)-Ph 2)6(H2O)3](NO3) ·2H2O·3Me2CO (1·2H 2O·3Me2CO), [Cr3-μ3-O) (O2CPh)6(H2O)3](NO 3)·1.9MeCN·0.5H2O (2·1.9MeCN· 0.5H2O), [Fe3(μ3-O){O2C(OH) Ph2}6(H2O)3](NO3) ·5.69H2O (3·5.69H2O), and [Fe 3(μ3-O)(O2CPh)6(H 2O)3](NO3)·3MeCN (4·3MeCN). From a crystallographic point of view, the three metal ions form an isosceles triangle in 1 and 3 (benzilato complexes) and an almost equilateral one in 2 and 4 (benzoato complexes). Magnetic susceptibility measurements reveal metal-dependent isotropic exchange, for the CrIII clusters (1, 2), with the "magnetic symmetry" resembling that of an almost equilateral triangle. For the FeIII clusters the "magnetic symmetry" resembles that of an isosceles triangle. Solid-state 1H NMR studies for 1, through the measurements of the temperature variation of the relaxation rate 1/T1, reveal that the first excited spin state with S T = 3/2 is well above (ca. 30 cm-1) the ground state with ST = 1/2, consistent with the magnetic susceptibility data. Mössbauer spectroscopic studies give a larger quadrupole splitting, ΔEQ, value for 3 than for 4, because of a larger charge asymmetry in 3 attributed to the different nature of the carboxylato ligands. EPR experiments at X-band reveal the presence of antisymmetric exchange within the CrIII clusters and distributions of both isotropic and antisymmetric exchange interactions. In the case of 2 the magnitude and distribution of these interactions are greater. EPR spectroscopy for 3 suggests weak interactions between neighboring units, which could be described assuming a dimer of trimers. For complex 4 the EPR spectra indicate a more complicated picture. © Wiley-VCH Verlag GmbH & Co. KGaA, 2006.