From molecular wires to molecular resistors: TCNE, a class-III/class-II mixed-valence chemical switch

Abstract

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 H ab = 0.03 eV.