New charge transfer salts based on bis(ethylenedithio)tetrathiafulvalene (ET) and ferro or antiferromagnetic oxalato-bridged dinuclear anions: Syntheses, structures and magnetism of ET5[MM′(C2O4)(NCS)8] with MM′ = CrIIIFeIII, CrIIICrIII

Abstract : Electrochemical combination of the magnetic dinuclear anion [MM′(C2O4)(NCS)8]4- (MM′ = CrIIICrIII, CrIIIFeIII) with the ET organic π-donor (ET = BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) gives rise to two new isostructural molecular hybrid salts ET5[MM′(C2O4)(NCS)8], with MM′ = CrCr (1), CrFe (2). The molecular structure of compound 1 has been determined by single crystal X-ray diffraction. The particular arrangement of the organic units consists of an unprecedented two-dimensional organic sublattice nearly similar to that observed in κ-phase structures. For both compounds, the magnetic susceptibility measurements indicate (i) the ET radicals do not contribute to the magnetic moment probably due to the presence of strong antiferromagnetic interaction between them, and (ii) in the anion, the magnetic coupling is antiferromagnetic for 1 (J = -3.65 cm-1) and ferromagnetic for 2 (J = 1.14 cm-1, J being the parameter of the exchange Hamiltonian H = -2JS1S2). The field dependence of the magnetization of compound 2 at 2.0 K gives further evidence of the S = 4 ground-state arising from the interaction between S = 3/2 Cr(III) and S = 5/2 Fe(III). EPR measurements confirm the nature of the magnetic interactions and the absence of any contribution from the organic part, as observed from the static magnetic measurement. Conductivity measurements and electronic band structure calculations show that both salts are semiconductors with low activation energies.