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Figuerola A, Tangoulis V, Ribas J, Hartl H, Brudgam I, Maestro M, Diaz C: Synthesis, crystal structure, and magnetic studies of oxo-centered trinuclear chromium (III) complexes: [Cr (3)(mu (3)-O)(mu (2)-PhCOO)(6)(H (2) O)(3)] NO (3). 4H (2) O.2CH (3) OH, a case of spin-frustrated system, and [Cr (3)(mu (3)-O)- (mu (2)-PhCOO)(2)(mu (2)-OCH (2) CH (3))(2)(bpy)(2)(NCS )(3)], a new type of [Cr (3) O] core. Inorg Chem. 2007 Dec 24;46(26):11017-24. Epub 2007 Nov 16.
The synthesis, crystal structure, and magnetic properties of two trinuclear oxo-centered carboxylate complexes are reported and discussed: [Cr3 (mu3-O)(mu2-PhCOO) 6 (H2O) 3] NO3.4H2O.2CH3OH (1) and [Cr3 (mu3-O)(mu2-PhCOO) 2 (mu2-OCH2CH3) 2 (bpy) 2 (NCS) 3] (2). For both complexes the crystal system is monoclinic, with space group C2/c for 1 and P1/n for 2. The structure of complex 1 consists of discrete trinuclear cations, associated NO3- anions, and lattice methanol and water molecules. The structure of complex 2 is built only by neutral discrete trinuclear entities. The most important feature of 2 is the unusual skeleton of the [Cr3O] core due to the lack of peripheral bridging ligands along one side of the triangular core, which is unique among the structurally characterized (mu3-oxo) trichromium (III) complexes. Magnetic measurements were performed in the 2-300 K temperature range. For complex 1, in the high-temperature region (T > 8 K), experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -2J12S1S2 - 2J13S1S3 - 2J23S2S3 (J12 = J13 = J23) with Jij = -10.1 cm (-1), g = 1.97, and TIP = 550 x 10 (-6) emu mol (-1). The antisymmetric exchange interaction plays an important role in the magnetic behavior of the system, so in order to fit the experimental magnetic data at low temperature, a new magnetic model was used where this kind of interaction was also considered. The resulting fitting parameters are the following: Gzz = 0.25 cm (-1), delta = 2.5 cm (-1), and TIP = 550 x 10 (-6) emu mol (-1). For complex 2, the experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -2J1 (S1S2 + S1S3) - 2J2 (S2S3) with J1 = -7.44 cm (-1), J2 = -51.98 cm (-1), and g = 1.99. The magnetization data allows us to deduce the ground term of S = 1/2, characteristic of equilateral triangular chromium (III) for complex 1 and S = 3/2 for complex 2, which is confirmed by EPR measurements. |
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