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

The synthesis, crystal structures, spectroscopic and magnetic properties studies of four trinuclear oxo-centered carboxylate complexes are reported and discussed: [Cr-3(mu(3)-O){O2C(OH)-Ph-2}(6)(H2O)(3)](NO3)center dot 2H(2)O center dot 3Me(2)CO (1 center dot 2H(2)O center dot 3Me(2)CO), [Cr-3(mu(3)-O)(O2CPh)(6)(H2O)(3)](NO3)center dot 1.9MeCN center dot 0.5H(2)O (2 center dot 1.9MeCN center dot 0.5H(2)O), [Fe-3(mu(3)-O)(O2C(OH)Ph-2)(6),(H2O)(3)](NO3)center dot 5.69H(2)O (3 center dot 5.69H(2)O), and [Fe-3(mu(3)-O)(O2CPh)(6)(H2O)(3)](NO3)center dot 3MeCN (4 center dot 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 Cr-III clusters (1, 2), with the magnetic symmetry resembling that of an almost equilateral triangle. For the Fe-III clusters the magnetic symmetry resembles that of an isosceles triangle. Solid-state H-1 NMR studies for 1, through the measurements of the temperature variation of the relaxation rate 1/T-1, reveal that the first excited spin state with S-T = 3/2 is well above (ca. 30 cm(-1)) the ground state with S-T = 1/2, consistent with the magnetic susceptibility data, Mossbauer spectroscopic studies give a larger quadrupole splitting, Delta E-Q, 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 Cr-III 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. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).