Magnetic Studies on μ-Azido Polynuclear Nickel(II) Compounds with the 222-tet Ligand. Crystal Structure of (μ-N₃)₂[Ni(222-tet)]₂(BPh₄)₂ (222-tet = Triethylenetetramine) and EXAFS Structural Characterization of the Triangular Compounds (μ-N₃)₃[Ni(222-tet)]₃(X)₃ (X = PF₆^-, ClO₄^-)

The nickel(II) dinuclear 1,3-azido-bridged compound (μ-N₃)₂[Ni(222-tet)]₂(BPh₄)₂ (1) and the trinuclear compounds (μ-N₃)₃[Ni₃(222-tet)₃](PF₆)₃ (2) and (μ-N₃)₃[Ni₃(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 P2₁/n, a = 10.639(4) Å, b = 19.770(7) Å, c = 13.609(6) Å, β = 97.78(3)°, 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 Å, 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 = −JS₁S₂, 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) − J₃(S₂S₃), the obtained J values are J_1,2 = −72(3) cm^-1, J₃ = −36(3) cm^-1 for 2 and J_1,2 = −60.3(3) cm^-1, J₃ = −29.4(2) cm^-1 for 3.