Hybrid Molecular Materials Based upon Magnetic Polyoxometalates and Organic π-Electron Donors:  Syntheses, Structures, and Properties of Bis(ethylenedithio)tetrathiafulvalene Radical Salts with Monosubstituted Keggin Polyoxoanions

The syntheses, crystal structures, and physical properties of the series of radical salts made with bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) and monosubstituted α-Keggin polyoxoanions of formula [XZ(H 2 O)M 11 O 39 ] 5 - (XZM 11 = Si IV Fe III Mo 11, Si IV Cr III W 11, P V Co II W 11, P V Ni II W 11, P V Cu II W 11 P V Zn II W 11, P V Mn II W 11, and P V Mn II Mo 11 ) containing a magnetic metal ion Z on a peripheral octahedral site of the Keggin anion are reported. They all crystallize in two related series called α 2 and α 3 . The general structure consists of alternating layers of the organic donor and the Keggin polyoxometalates. While the stoichiometry and α-packing arrangement within the organic ET layers are the same in the two phases, significant differences are observed in the anion layers. Thus, in the α 3 phase the anions polymerize to form an unprecedented linear chain of Keggin anions along the c axis, while in the remaining compounds the anion layers are formed by discrete Keggin units. A salient feature of these structures is that in both phases the magnetic metal ion Z appears to be localized on two of the 12 possible octahedral sites of the monosubstituted Keggin anion, even if the Keggin anions are not bonded, as in the α 2 phase. The organic layers are formed by two different kinds of stacks: an eclipsed chain with almost totally ionized ET molecules and a dimerized one with partially charged ET molecules that accounts for the semiconducting character of these salts. Magnetic measurements of the radical salt with the diamagnetic Zn-containing Keggin anion indicate the presence of antiferromagnetic interactions in the organic sublattice (/ J / ≈ 70 cm - 1 ), together with a Curie tail at low temperatures, which has been attributed to a paramagnetic contribution coming from the progressive electron localization in the mixed-valence dimerized chain when cooling. All the salts having discrete magnetic anions (α 2 phase) show magnetic behaviors which correspond to the sum of the magnetic contributions of the two sublattices. This observation indicates that the interactions between the two sublattices are quite negligible, a result in agreement with the EPR measurements. However, with the two radical salts having chains of Keggin anions (α 3 phase: PMnW 11 and PMnMo 11 ) small differences between the ET and the tetrabutylammonium salts have been detected at low temperatures. In the [PMnW 11 ] derivative these differences have been attributed to single-ion effects. In the [PMnMo 11 ] derivative, these differences have been attributed to a weak antiferromagnetic Mn−Mn interaction promoted by the presence of a delocalized electron on the mixed-valence polyanion.