Synthesis, crystal structure and magnetism of centrosymmetric linear trinuclear copper(II) complex of pyridine nitronyl nitroxide derivative

Novel stable dark-green solid adducts (2) were isolated from the reaction of 2-(4,4,5,5-tetramethyl-3-oxylimidazoline-1-oxide)-5-bromopyridine (1) with copper(II)-hexafluoroacetylacetonate hydrate (Cu(hfac)2 · xH2O), namely [12 • {Cu(hfac)2}3]. The X-ray structure revealed that, the complex is linear and centrosymmetric in the triclinic space group. The striking feature of the complex is, the presence of two types of copper coordination geometries with the radical 1, a distorted octahedron at the center and two terminal octahedron structures, where the copper and the nitronyl nitroxide oxygen binding is stronger in the latter case due to “chelation effect”. The central copper is surrounded by O6 donor sets and the terminal coppers are surrounded by N,O5 donor sets. The three copper ions in the complex are collinear with shortest intramolecular metal–metal distance ca. 7.75 Å. The angles between Cu–O–N also differ considerably due to the different overlapping modes of nitronyl nitroxide oxygen with copper, for chelating type it is 111.25° and for the non-chelating one it is 142.85°. Two exchange couplings are possible for this kind of system. The magnetic behavior shows that the coupling is antiferromagnetic between the radical and the terminal coppers, with J1=−440 cm−1 and weakly ferromagnetic between the central copper and the neighboring radicals with J2=10 cm−1.