Heterobimetallic Oxalato-Bridged Cu^IIRe^IV Complexes. Synthesis, Crystal Structure and Magnetic Properties

Three copper(II)−rhenium(IV) bimetallic complexes of formula [ReCl4(μ-ox)Cu(phen)2] (1), [ReCl4(μ-ox)Cu(phen)2]·CH3CN (2), and [ReCl4(μ-ox)Cu(terpy) (H2O)][ReCl4(μ-ox)Cu(terpy)(CH3CN)] (3) (ox = oxalate anion, phen = 1,10-phenanthroline, and terpy = 2,2‘:6,2"- terpyridine) have been synthesized and their crystal structures determined by single-crystal X-ray diffraction. Complex 1 crystallizes in the triclinic system, space group P(−1), with a = 9.776(2), b = 11.744(3), c = 14.183(3) Å, α =102.09(2)°, β = 109.42(2)°, γ = 107.11(2)°, and Z = 2, whereas 2 and 3 crystallize in the monoclinic system, space groups P21/n and P21/c, respectively, with a = 12.837(3), b = 17.761(4), c = 12.914(3) Å, β = 91.32(2)°, and Z = 4 for 2, and a = 8.930(2), b = 18.543(4), c = 27.503(6) Å, β = 94.67(2)°, and Z = 4 for 3. The structures of 1 and 2 are made up of neutral [ReCl4(μ-ox)Cu(phen)2] bimetallic units. Re(IV) and Cu(II) metal ions exhibit distorted octahedral coordination geometries, being bridged by a bis(bidentate) oxalato ligand. The presence of acetonitrile molecules of crystallization in 2 causes a somewhat greater separation between the bimetallic complexes and a different packing of these units in the crystal structure with respect to 1. The copper−rhenium separation across oxalato is 5.628(2) in 1 and 5.649(3) Å in 2. The structure of 3 is made up of two different and neutral bimetallic units, [ReCl4(μ-ox)Cu(terpy)(H2O)] and [ReCl4(μ-ox)Cu(terpy)(CH3CN)]. In the first one, the oxalate group behaves as a bis(bidentate) ligand occupying one equatorial and one axial position in the elongated octahedral environment of Cu(II). The water molecule is axially coordinated. In the second one, the oxalate group behaves as a bidentate/monodentate ligand occupying the axial position in the square pyramidal environment of Cu(II). The acetonitrile molecule occupies a basal coordination position around the copper atom. These units are arranged in such a way that a chlorine atom of the first unit (Cl(1)) points toward the copper atom (Cu(2))of the second one (3.077(2) Å for Cl(1)...Cu(2)), forming a tetranuclear species. The copper−rhenium separation across bis(didentate) oxalato is 5.504(3) Å, whereas that through bidentate/monodentate oxalato is 5.436(2) Å. The magnetic behavior of 2 and 3 has been investigated over the temperature range 1.8−300 K. A very weak and nearly identical antiferromagnetic coupling between Re(IV) and Cu(II) through bis(bidentate) oxalato occurs in 2 (J = −0.90 cm-1) and 3 (J = −0.83 cm-1); it is ferromagnetic in 3 through both the bidentate−monodentate oxalato (J = +5.60 cm-1) and the chloro (J = +0.70 cm-1) bridges.