New heterometallic (Cu-II and Cr-III) complexes - First crystal structure of an oxalate-bridged ferromagnetically coupled [(CuCrCuII)-Cr-II-Cu-III] system

摘要

Two new heterotemetallic complexes [{Cu(mu-C2O4)(bpY)(2)}(2)-Cr(C2O4]NO3 center dot H2O (1) and [Cu(bpy)(3)](2)[Cr(C2O4)(3)]NO3 center dot 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(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-O-2 = 2.6439(18) angstrom] and one nitrogen atom from one of the bpy ligands [Cu-N1 = 2.1079(18) angstrom). The N3O 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(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) angstrom]. Analysis of the magnetic properties (1.8-290 K) of I showed a ferromagnetic exchange interaction between Cu-II and Cr-III with J = +1.20 cm(-1) [using H-INT = -J(SCu1SCr + SCu2SCr)].