New Heterometallic (CuII and CrIII) Complexes - First Crystal Structure of an Oxalate-Bridged Ferromagnetically Coupled [CuIICrIIICuII] System

Two new heterotemetallic complexes [{Cu(μ‐C2O4)(bpy)2}2Cr(C2O4)]NO3·H2O (1) and [Cu(bpy)3]2[Cr(C2O4)3]NO3·9H2O (2) (bpy = 2,2′‐bipyridine) have been synthesized. The crystal structures were determined and magnetic and spectroscopic (IR, UV/Vis) characterization accomplished. In 1, the central [Cr(C2O4)3]3– unit acts as a bridge between two copper(II) ions, each of them having a distorted octahedral geometry. The apical positions are occupied by one oxygen atom belonging to the bridging oxalate anion [Cu–O2 = 2.6439(18) Å] and one nitrogen atom from one of the bpy ligands [Cu–N1 = 2.1079(18) Å]. The N3O equatorial environment consists of three bpy nitrogen atoms [Cu–N = 1.995 Å (mean value)] and the second oxygen atom from the bridging oxalate anion [Cu–O1 = 2.1168(15) Å]. The chromium(III) atom is in a slightly distorted octahedral environment of four oxygen atoms from two bridging oxalate groups [Cr–O = 1.988 Å (mean value)] and two oxygen atoms from the terminally bonded bidentate oxalate group [Cr–O = 1.9454(16) Å]. Complex 2 consists of two mononuclear [Cu(bpy)3]2+ cations, one mononuclear [Cr(C2O4)3]3– anion, one NO3– anion and nine molecules of water of crystallisation. The copper(II) ions in two crystallographically independent [Cu(bpy)3]2+ cations are in a tetragonally distorted octahedral coordination environment of six N atoms from the bpy ligands. The chromium(III) ion in [Cr(C2O4)3]3– has a slightly distorted octahedral environment of oxygen atoms from three oxalate groups [with the Cr–O bond lengths ranging from 1.964(3) to 1.982(3) Å]. Analysis of the magnetic properties (1.8–290 K) of 1 showed a ferromagnetic exchange interactionbetween CuII and CrIII with J = +1.20 cm–1 [using HINT =–J(SCu1SCr + SCu2SCr)].(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)