Synthesis,Crystal Structure,and Magnetic Studies of Oxo-Centered Trinuclear Chromium(III)Complexes:[Cr3(mu_3-O)(mu_2-PnCOO)_6(H2O)3]NO3· 4H2O·2CH3OH,a Case of Spin-Frustrated System,and [Cr3(mu_3-O)-(mu_2-PhCOO)2(mu_2-OCH2CH3)2(bpy)2(NCS)3],a New T

摘要

The synthesis,crystal structure,and magnetic properties of two trinuclear oxo-centered carboxylate complexes are reported and discussed:[Cr3(mu_3-O)(mu_2-PhCOO)6(H2O)3]NO3·4H2O·2CH3OH(1)and [Cr_3(mu_3-O)(mu_2-PhCOO)2(mu_2-OCH2CH3)2(bpy)2(NCS)3](2).For both complexes the crystal system is monoclinic,with space group C2/c for 1 and P_1 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(mu_3-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 =-2J_(12)S_1S_2-2J_(13)S_1S_3-2J_(23)S_2S_3(J_(12)= J_(13)= J_(23))with J_(ij)= -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:G_(zz)= 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 =-2J_1(S_1S_2 + S_1S_3)-2J_2(S_2S_3)with J_1 =-7.44 cm~(-1),J_2 =-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.