From Molecular Wires to Molecular Resistors: TCNE, a Class-III/Class-II Mixed-Valence Chemical Switch

The binuclear complexes {Cp*­(dppe)­M}₂{μ-CCC­[C­(CN)₂]­C­[C­(CN)₂]­CC} (<b>2Fe</b>, M = Fe; <b>2Ru</b>, M = Ru) and {Cp*­(dppe)­Fe}­{CCC­[C­(CN)₂]­C­[C­(CN)₂]­CC}­{Ru­(dppe)­Cp*} (<b>2FeRu</b>) were obtained by treatment of the binuclear precursors Cp*­(dppe)­M-CCCCCC-M′(dppe)­Cp* (M = M′ = Fe, <b>1Fe</b>; M = M′ = Ru, <b>1Ru</b>; M = Fe, M′ = Ru, <b>1FeRu</b>) with TCNE in CH₂Cl₂ at 20 °C. Complexes <b>2Fe</b> and <b>2FeRu</b> were isolated as deep purple powders (82% and 87% yields, respectively), and <b>2Ru</b> was isolated as a brown-yellow solid (55%). The paramagnetic salt <b>[2FeRu]­[C</b>_<b>3</b><b>(CN)</b>_<b>5</b><b>]</b> was also isolated in 51% yield. The structural and electronic properties of the new compounds were investigated by ¹H, ¹³C, and ³¹P NMR, XRD analysis, cyclic voltammetry, IR and UV–vis, EPR, and NIR spectroscopies. The experimental data clearly show that the insertion of the central −CC– triple bond of [M]–C₆–[M] into tetracyanoethene (TCNE) dramatically decreases the electronic interaction between the metal termini. NIR spectroscopy of the salt <b>[2FeRu]­[C</b>_<b>3</b><b>(CN)</b>_<b>5</b><b>]</b> demonstrated that the coupling between the iron and ruthenium centers is not completely removed by addition of TCNE to <b>[1FeRu]⁺</b>, but has produced a very strong attenuation of the electronic coupling from <i>H</i>_ab = 0.50 eV to <i>H</i>_ab = 0.03 eV