Metal string complexes:: Synthesis, crystal structures and magnetic properties of heptanuclear nickel(II) complexes, [Ni7(μ7-teptra)4X2] (teptra equals tetrapyridyltriamido, X = Cl-, NCS-)

The synthesis, crystal structures and magnetic properties of linear heptanuclear nickel(II) complexes [Ni-7 (mu(7)-teptra)(4)X-2], (teptra = tetrapyridyltriamido), with the axial ligand X = Cl- (1), NCS- (2), are reported. The hepta-nuclear metal chain is helically wrapped by four syn-syn-syn-syn-syn-syn teptra(3.) ligands. Both of the [Ni-7(mu(7)-teptra)(4)](2+) moiety are isostructural involving a Ni, linear chain unit with all of the angle Ni-Ni-Ni being 180 degrees, terminated by two axial ligands. Three types of Ni-Ni distances are found in these complexes. The longest ones bonded with the axial ligands are 2.383(1), 2.374(2) and 2.375(2), 2.354(2), and the intermediate ones are 2.310(1), 2.304(1) and 2.300(2), 2.303(2) Angstrom for (I) and (2), respectively. The innermost Ni-Ni distances are the shortest ones with the distances of 2.226(2), 2.214(2) and 2.194(2), 2.206(2) Angstrom for (1) and (2), respectively. Two terminal Ni(II) ions bonded with the axial ligands are in a square-pyramidal (NiN4X) environment and exhibit long Ni-N bonds (-2.10 Angstrom) which are consistent with a high-spin Ni(II) configuration. The inner five Ni(II) ions displayed short Ni-N (similar to 1.90 Angstrom) bond distances which are consistent with a square-planar (NiN4,), diamagnetic arrangement of a low-spin Ni(II) configuration. The magnetic measurement of (1) shows an antiferromagnetic interaction of two terminal high-spin Ni(II) ions with the coupling constant J = -3.8 cm(-1). The Ni-Ni, Ni-N distances and magnetic behavior among tri-, penta-, and hepta-nickel(II) complexes are compared and discussed.