Structure, stacking interactions and magnetism of compounds with oxalate-bridged dinuclear Cu IICu II and Cu IINb V units

Abstract

Two novel compounds of the formulae [{Cu(phen) 2} 2(μ-C 2O 4)][Cu(phen) 2(μ-C 2O 4)NbO(C 2O 4) 2] 2 · 8H 2O ( 1) and [{Cu(bpy) 2} 2(μ-C 2O 4)][Cu(bpy) 2(μ-C 2O 4)NbO(C 2O 4) 2] 2 · 0.5bpy · 7H 2O ( 2) (phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine) have been prepared and characterized by single crystal X-ray diffraction, IR spectroscopy and magnetic susceptibility measurements. The molecular structure of both 1 and 2 consists of a discrete Cu IICu II oxalate-bridged dinuclear [{Cu(L) 2} 2(μ-C 2O 4)] 2+ cation ( A unit) and two Cu IINb V oxalate-bridged dinuclear [Cu(L) 2(μ-C 2O 4)NbO(C 2O 4) 2] − anions ( B units) (L = phen, bpy). In 1 a crystallographic inversion centre is located in the middle of the oxalate bridge of the A unit, whereas in 2 an analogous inversion centre is missing. In the A units the copper(II) atoms adopt a tetragonally elongated octahedral coordination with the equatorial planes being perpendicular to the mean planes of the oxalate bridge and parallel to each other. In both structures, similar one-dimensional motifs are generated through the ligand stacking interactions, with a difference that one free bipyridine molecule, present in 2, intercalates into one of the motifs. It is shown that the phenanthroline ligand, due to its ability of stacking through the central aromatic ring, causes longer intermolecular Cu⋯Cu distances than the bipyridine ligand. The magnetic susceptibility measurements (1.8–290 K) show the ferromagnetic exchange interaction between the copper(II) atoms in the A units of both compounds, with J = +5.9 cm −1 and +7.9 cm −1 for 1 and 2, respectively ( J – the exchange parameter in the isotropic spin Hamiltonian H INT = − J S 1 · S 2).