Oxalato-bridged dinuclear complexes of Cr(III) and Fe(III):: Synthesis, structure, and magnetism of [(C2H5)4N]4[MM′(ox)(NCS)8] with MM′ = CrCr, FeFe, and CrFe

A new series of homo- and heterometallic oxalato-bridged dinuclear compounds of formulas [Et4N](4)[MM'(ox)(NCS)(8)] ([Et4N](+) = [(C2H5)(4)N](+); ox = C2O42-) with MM' = Cr(III)-Cr(III) (1), Fe(III)-Fe(III) (2), and Cr(III)-Fe(III) (3) is reported. They have been structurally characterized by infrared spectra and single-crystal X-ray diffraction. The three compounds are isostructural and crystallize in the orthorhombic space group Cmca with Z = 8, a = 16.561(8) Angstrom, b = 13.481(7) Angstrom, and c = 28.168(8) Angstrom for 1, a = 16.515(2) Angstrom, b = 13.531(1) Angstrom, and c = 28.289(4) Angstrom for 2, a = 16.664(7) Angstrom, b = 13.575(6) Angstrom, and c = 28.386(8) Angstrom for 3. The structure of 3 is made up of a discrete dinuclear anion [CrFe(ox)(NCS)(8)](4-) and four disordered [Et4N](+) cations, each of them located on special positions. The anion, in a crystallographically imposed C-2h symmetry, contains metal cations in distorted octahedral sites. The Cr(ox)Fe group, which is planar within 0.02 Angstrom, presents an intramolecular metal-metal distance of 5.43 Angstrom. Magnetic susceptibility measurements indicate antiferromagnetic pairwise interactions for 1 and 2 with J = -3.23 and -3.84 cm(-1), respectively, and ferromagnetic Cr-Fe coupling with J = 1.10 cm(-1) for 3 (J being the parameter of the exchange Hamiltonian H = -2JS(1)S(2)). The ESR spectra at different temperatures confirm the magnetic susceptibility data.