New Heterometallic (Cu II and Cr III ) Complexes – First Crystal Structure of an Oxalate‐Bridged Ferromagnetically Coupled [Cu II Cr III Cu II ] System

Two new heterotemetallic complexes [{Cu(μ‐C 2 O 4 )(bpy) 2 } 2 Cr(C 2 O 4 )]NO 3 · H 2 O ( 1 ) and [Cu(bpy) 3 ] 2 [Cr(C 2 O 4 ) 3 ]NO 3 · 9H 2 O ( 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(C 2 O 4 ) 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 N 3 O 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(C 2 O 4 ) 3 ] 3– anion, one NO 3 – 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(C 2 O 4 ) 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 Cu II and Cr III with J = +1.20 cm –1 [using H INT =– J ( S Cu1 S Cr + S Cu2 S Cr )].(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)