Magnetic studies on mu-azido polynuclear nickel(II) compounds with the 222-tet ligand. Crystal structure of (mu-N-3)(2)[Ni(222-tet)](2)(BPh4)(2)(222-tet=triethylenetetramine) and EXAFS structural characterization of the triangular compounds (mu-N-3)(3)[Ni(222-tet)](3)(X)(3) (X=PF6,(-), ClO4,(-))

The nickel(II) dinuclear 1,3-azido-bridged compound (mu-N-3)(2)[Ni(222-tet)](2)(BPh4)(2) (1) and the trinuclear compounds (mu-N-3)(3)[Ni-3(222-tet)(3)](PF6)(3) (2) and (mu-N-3)(3)[Ni-3(222-tet)(3)](ClO4)(3) (3) were synthesized and characterized. 222-tet is the tetraaminate ligand triethylenetetramine. The crystal structure of 1 was solved by X-ray diffraction. 1 crystallizes in the monoclinic system: space group P2(1)/n, a = 10.639(4) Angstrom, b = 19.770(7) Angstrom, 13.609(6) Angstrom, beta = 97.78(3)degrees, Z = 2, formula C60H76B2N14Ni2. In the absence of single crystals of 2 and 3, we carried out an EXAFS study of 1-3 at the nickel K-edge, using compound 1 as a model, in order to obtain structural information for compounds 2 and 3. The analysis of XANES and EXAFS spectra of compounds 1-3 reveals the occurrence of azido-bridged trinuclear nickel(II) compounds for 2 and 3 with Ni-Ni separations of 5.16 and 5.12 Angstrom, respectively. Each nickel(II) atom is placed in an octahedral NiN6 environment: four nitrogen atoms of the amine and two nitrogen atoms of two azido bridges. The magnetic properties of the three compounds were studied by susceptibility measurements at variable temperatures (300-4 K). From the spin Hamiltonian H = -JS(1)S(2), the calculated J value for 1 is -83.6 cm(-1), in good agreement with the expected value. From the spin Hamiltonian H = -J(1,2)(S1S2 + S1S3) -J(3)(S2S3), the obtained J values are J(1,2) = -72(3) cm(-1), J(3) = -36(3) cm(-1)for 2 and J(1,2) = -60.3(3) cm(-1), J(3) = -29.4(2) cm(-1) for 3.