Synthesis, crystal structures, and magnetostructural correlations in two new one-dimensional nickel(II) complexes with azido as bridging ligand. Effect of temperature

Two new nickel(II) trans-monodimensional end-to-end azide-bridged complexes were synthesized and characterized: [Ni(1,3-diamino-2,2-dimethylpropane)(2)(mu-N-3)](n)(PF6)(n)(1) and [Ni(1,3-diaminopropane)(2)(mu-N-3)](n)(PF6)(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) Angstrom, b = 8.7050(12) Angstrom, c = 6.139(2) Angstrom, and Z = 2. Complex 2 crystallizes in the orthorhombic system, space group Pnmn with a = 15.185(3) Angstrom, b = 8.808(2) Angstrom, c = 5.983(2) Angstrom, 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 chi(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 Sigma SiSj, 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 degrees at room temperature) and should thus decrease the antiferromagnetic coupling, as is experimentally observed.