Synthesis, X‐ray Crystal Structure and Magnetic Properties of Oxalato‐Bridged Copper( II ) Complexes with 2,3‐Bis(2‐pyridyl)pyrazine, 2,3‐Bis(2‐pyridyl)quinoxaline and 2,2′‐Bipyrazine as Peripheral Ligands

Five oxalate‐containing copper( II ) complexes of formula [Cu(dpq)(H 2 O)(ox)]·5H 2 O ( 1 ), [Cu(dpp)(H 2 O)(ox)]·H 2 O ( 2 ), [Cu(bpz)(ox)] n ( 3 ), [Cu 2 (dpp) 2 (H 2 O) 2 (NO 3 ) 2 (ox)]·4H 2 O ( 4 ) and [Cu 2 Cl 2 (bpz) 2 (H 2 O) 2 (ox)][Cu(bpz)(H 2 O) 2 (ox)]·2H 2 O ( 5 ) [dpq = 2,3‐bis(2‐pyridyl)quinoxaline; dpp = 2,3‐bis(2‐pyridyl)pyrazine; bpz = 2,2′‐bipyrazine; ox = oxalate] were prepared and their structures were determined by X‐ray diffraction on single crystals. Complexes 1 and 2 are discrete mononuclear complexes with oxalate and dpq ( 1 )/dpp ( 2 ) acting as bidentate ligands. Complex 3 is a neutral oxalato‐bridged (2,2′‐bipyrazine)copper( II ) chain where the oxalate adopts a bidentate/monodentate coordination mode, whereas 4 is an oxalato‐bridged copper( II ) dinuclear complex with ox and dpp acting as bis‐bidentate and bidentate ligands, respectively. The structure of 5 contains neutral [Cu(bpz)(H 2 O) 2 (ox)] (mononuclear) and [Cu 2 (bpz) 2 (H 2 O) 2 Cl 2 (ox)] (dinuclear) units where the bpz acts as a bidentate ligand and the oxalate group adopts the bidentate (mononuclear) and bis‐bidentate (dinuclear) coordination modes. The copper atoms have distorted square pyramidal ( 1 and 2 ) and elongated octahedral ( 3 − 5 ) geometries: two nitrogen atoms from heterocyclic N‐donors and two oxalate‐oxygen atoms occupy the equatorial positions ( 1 − 5 ) and a water molecule ( 1 and 2 ), two oxalate‐oxygen atoms ( 3 ), a water molecule and a nitrate‐oxygen ( 4 ), and either two water molecules ( 5 ), mononuclear) or a chlorine atom and a water molecule ( 5 , dinuclear) fill the axial ones. Magnetic susceptibility measurements for 3 − 5 in the temperature range 1.9−290 K reveal the occurrence of weak ( J = −1.4 cm −1 , 3 ) and strong (−312 and −345 cm −1 for 4 and 5 , respectively; H ˆ = − J · S ˆ A · S ˆ B ) antiferromagnetic interactions between the copper( II ) atoms in agreement with the out‐of‐plane ( 3 ) and in‐plane ( 4 and 5 ) exchange pathways involved. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)