New hybrid semiconductor materials based on viologen salts of bimetallic Fe-Pt and Fe-Au carbonyl clusters:: First structural characterization of the diradical π-dimer of the diethylviologen monocation and EPR evidence of its triplet state

The synthesis, structure, spectroscopic characterization and electrical resistivity of the [EtV](2)[Fe4Pt(CO)(16)], [EtV][Fe3Pt3(CO)(15)]center dot THF, [EtV][Fe(4)AU(CO)(16)](2)center dot 2THF (EtV = 1,1'-diethyl-4,4'-bipyridilium cation) and [NEt4](2)[Fe4Au(CO)(16)] is reported. The crystal structure of [EtV](2)[Fe4Pt(CO)(16)] is based on infinite stacks of [(EtV)(.+)](2) pi-dimers rotated by 90 degrees and isolated [Fe4Pt(CO)(16)](2-) ions. Within each pi-dimer, the [EtV](.+) radical ions are perfectly eclipsed and the distance between their mean planes is 3.275 angstrom. The EPR spectrum of the solid material at room temperature clearly indicates the presence of a significantly populated triplet state of the pi-dimer, by showing signals both at Delta m=1 and Delta m=2. The solid-state structure of [EtV][Fe3Pt3(CO)(15)]center dot THF is based on the ionic packing of [EtV](2+) and [Fe3Pt3(CO)(15)](2-) ions, inferred from a comparison of their molecular parameters with literature data. Significant electron transfer could have been expected to give [EtV](.+) [Fe3Pt3(CO)(15)](.-) on the basis of their formal redox potentials. In spite of their different stoichiometries, the structures of [EtV][Fe4Au(CO)(16)](2). 2THF and [NEt4](2)[Fe4Au(CO)(16)] both contain an isomer of the monoanion [Fe4Au(CO)(16)](-) in solution (previously characterized in the solid state as a [NMe3CH2Ph](2)[Fe4Au(CO)(16)]Cl mixed salt). Resistivity measurements on pellets of powdered samples of the above compounds indicate that their tetrasubstituted ammonium salts, and complex [EtV] [Fe4Au(CO)(16)](2).2 THF, largely behave as insulators. However, the [EtV](2)[Fe4Pt(CO)(16)] and [EtV][Fe3Pt3(CO)(15)].THF samples respectively display resistivities 3 and 4 orders of magnitude less than those of their corresponding ammonium salts and could be classified as semiconductor materials.