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} (2Fe, M = Fe; 2Ru,
M = Ru) and {Cp*­(dppe)­Fe}­{CCC­[C­(CN)2]­C­[C­(CN)2]­CC}­{Ru­(dppe)­Cp*} (2FeRu) were obtained
by treatment of the binuclear precursors
Cp*­(dppe)­M-CCCCCC-M′(dppe)­Cp* (M =
M′ = Fe, 1Fe; M = M′ = Ru, 1Ru; M = Fe, M′ = Ru, 1FeRu) with TCNE in CH2Cl2 at 20 °C. Complexes 2Fe and 2FeRu were isolated as deep purple powders (82% and 87% yields,
respectively), and 2Ru was isolated as a brown-yellow
solid (55%). The paramagnetic salt [2FeRu]­[C3(CN)5] 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 [2FeRu]­[C3(CN)5] demonstrated that the coupling between the iron and
ruthenium centers is not completely removed by addition of TCNE to [1FeRu]+, but has produced a very strong attenuation
of the electronic coupling from Hab =
0.50 eV to Hab = 0.03 eV.