Synthesis, Crystal Structures, and Magnetostructural Correlations in Two New One-Dimensional Nickel(II) Complexes with Azido as Bridging Ligand. Effect of Temperature

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Two new nickel(II) trans -monodimensional end-to-end azide-bridged complexes were synthesized and characterized: [Ni(1,3-diamino-2,2-dimethylpropane) 2 (μ-N 3 )] n (PF 6 ) n ( 1 ) and [Ni(1,3-diaminopropane) 2 (μ-N 3 )] n (PF 6 ) n ( 2 ). The crystal structures of 1 and 2 were solved. Complex 1 crystallizes in the orthorhombic system, space group Pnmn with a = 18.046(6) Å, b = 8.7050(12) Å, c = 6.139(2) Å, and Z = 2. Complex 2 crystallizes in the orthorhombic system, space group Pnmn with a = 15.186(3) Å, b =8.808(2) Å, c = 5.983(2) Å, and Z = 2. In the two complexes, the chains run along the c axis and the nickel atoms are situated in similar distorted octahedral environments. The two complexes are very similar and may be described as one-dimensional systems with the azido bridging ligands in the trans position. The magnetic properties of the two compounds were studied by susceptibility measurements vs temperature. 1 and 2 show a discontinuity at 235 and 230 K, respectively, which may be attributed to a phase transition at these relatively high temperatures. The χ M vs T plot for 1 and 2 show the typical shape for antiferromagnetically coupled nickel(II) one-dimensional complexes with a maximum near 35 ( 1 ) and 100 K ( 2 ), respectively. From the spin Hamiltonian − J Σ S i S j, J values for 1 and 2 were −41.1 and −70.6 cm - 1 in the high-temperature zone and −19.4 and −55.8 cm - 1 for the low-temperature range. This difference in magnetic behavior may be explained in terms of the phase transition, which changes the crystal parameters a and b, as has been shown from powder diffraction data. This transition changes the dihedral Ni−NNN−Ni angles (0° at room temperature) and should thus decrease the antiferromagnetic coupling, as is experimentally observed.