Crystal structure, magnetic properties, and single-crystal EPR spectra of a copper-nickel [CuNi]2 bis heterobinuclear compound: complementarity of the magnetic and EPR techniques

The compound C~(salen)Ni(hfa)w~h ere salen is N,N'-ethylenebis(oxosalicy1diiminato) and hfa is hexafluoroacetylacetonato has been synthesized and its crystal structure has been solved at -123 K. It crystallizes in the monoclinic system, space group P2,/c: a = 9.287 (4) A, b = 21.533 (2) A, c = 14.733 (3) A, fl = 94.36 (3)' with Z = 4 CuNi units. The copper is in a planar environment and the nickel in a distorted octahedral environment. The CuOzNi bridging angle is bent with a dihedral angle of 141.4'. Two CuNi units of this kind are related through a symmetry center, giving rise to a remarkable [C~(salen)Ni(hfa)e~n]t~it y with a relatively short copper-copper separation, 3.432 (1) A. The magnetic properties of this compound have been studied in the 1.27-300 K temperature range. They correspond to what is expected for an antiferromagnetically coupled CuNi pair with Scu = and SNi = 1 local spin. The interaction gives rise to a doublet and a quartet pair state. The doublet state is the lowest and the doubletquartet splitting is 35.4 cm-'. In contrast, the EPR properties are entirely associated with the bis heterobimetallic entity. The EPR spectrum is that of a triplet state arising from the interaction between two doublet pair states. A single-crystal EPR investigation has been carried out at X-band frequency, at 4.2 K. The principal values and the orientations of the g and D tensors associated with the triplet state have been determined. A model is presented, allowing the calculation of the dipolar contribution, which is found to be much larger than the experimental D tensor. The difference is ascribed to both the specific nature of the interacting doublet states and the anisotropic exchange. Finally, the complementarity of the magnetic and EPR techniques is emphasized. The magnetic properties lead to the interaction between copper(I1) and nickel(I1) within the CuNi unit, and the EPR properties specify the interaction between two symmetry- related units within the bis heterobinuclear entity.