Series of trinuclear Ni^IILn^IIINi^II complexes derived from 2,6-di(acetoacetyl)pyridine: Synthesis, structure, and magnetism

Eighteen trinuclear Ni^II₂Ln^III complexes of 2,6-di(acetoacetyl)pyridine (H₂L) (Ln = La-Lu except for Pm) were prepared by a "one-pot reaction" of H₂L, Ni(NO ₃)₂·6H₂O, and Ln(NO₃) ₃·nH₂O in methanol. X-ray crystallographic studies indicate that two L^2- ligands sandwich two Ni^II ions with the terminal 1,3-diketonate sites and one Ln^III ion with the central 2,6-diacylpyridine site, forming the trinuclear [Ni₂Ln(L) ₂] core of a linear NiLnNi structure. The terminal Ni assumes a six-coordinate geometry together with methanol or water molecules, and the central Ln assumes a 10-coordinate geometry together with two or three nitrate ions. The [Ni₂Ln(L)₂] core is essentially coplanar for large Ln ions (La, Ce, Pr, Nd) but shows a distortion with respect to the two L^2- ligands for smaller Ln ions. Magnetic studies for the Ni ₂Ln complexes of diamagnetic La^III and Lu^III indicate an antiferromagnetic interaction between the terminal Ni^II ions. A magnetic analysis of the Ni₂Gd complex based on the isotropic Heisenberg model indicates a ferromagnetic interaction between the adjacent Ni^II and Gd^III ions and an antiferromagnetic interaction between the terminal Ni^II ions. The magnetic properties of other Ni₂Ln complexes were studied on the basis of a numerical approach with the Ni₂La complex and analogous Zn₂Ln complexes, and they indicated that the Ni^IILn^III interaction is weakly antiferromagnetic for Ln = Ce, Pr, and Nd and ferromagnetic for Ln = Gd, Tb, Dy, Ho, and Er.