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New Heterometallic (Cu^II and Cr^III) Complexes - First Crystal Structure of an Oxalate-Bridged Ferromagnetically Coupled [Cu^IICr^IIICu^II] System

Two new heterometallic complexes [[Cu(mu-C_2O_4)(bpy)_2]_2Cr(C_2O_4)]NO_3 center dot H2O (1) and [Cu(bpy)_3]_2[Cr(C_2O_4)_3]NO_3 center dot 9H_2O (2) (bpy = 2, 2'-bipyridine) have been synthesized. Their crystal structures were determined and magnetic and spectroscopic (IR, UV/Vis) characterization accomplished. In 1, the central [Cr(C_2O_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) Angstrom] and one nitrogen atom from one of the bpy ligands [Cu-N1 = 2.1079(18) Angstrom]. The N_3O equatorial environment consists of three bpy-nitrogen atoms [Cu-N = 1.995 Angstrom (mean value)] and the second oxygen atom from the bridging oxalate anion [Cu-O1 = 2.1168(15) Angstrom]. The chromium(III) atom is in a slightly distorted octahedral environment of four oxygen atoms from two bridging oxalate groups [Cr-O = 1.988 Angstrom (mean value)] and two oxygen atoms from the terminally bonded bidentate oxalate group [Cr-O = 1.9454(16) Angstrom]. Complex 2 consists of two mononuclear [Cu(bpy)_3]^2+ cations, one mononuclear [Cr(C_2O_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_2O_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) Angstrom]. Analysis of the magnetic properties (1.8-290 K) for 1 showed a ferromagnetic exchange interaction between Cu^II and Cr^III with J = +1.20 cm^-1 [using H_(INT) = -J(S_(Cu1)S_Cr + S_(Cu2)S_Cr)].

crystal structure; copper complexes; chromium complexes; oxalate bridge; ferromagnetic interaction

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