Synthesis and Properties of a Mixed-Valence Compound with Single-Step Tunneling and Multiple-Step Hopping Behavior

The organic precursor bis(trimethylsilylethynyl)TTFMe2 (3, TTF = tetrathiafulvalene) was prepared as a 1:1 mixture of the cis and trans isomers. Pure samples of 3‐cis and 3‐trans were obtained by crystallization and identified by XRD analysis. The treatment of pure 3‐trans and a 1:1 mixture of 3‐cis/trans with (i) potassium carbonate, (ii) the iron complex Cp*(dppe)FeCl [5, Cp* = η5‐C5Me5, dppe = 1,2‐bis(diphenylphosphanyl)ethane] in the presence of KPF6, and (iii) tBuOK provided Cp*(dppe)Fe–C≡C–TTFMe2–C≡C–Fe(dppe)Cp* as the pure geometric isomer 6‐trans (85 %) and as the 60:40 mixture 6‐cis/trans (63 %), respectively. The oxidation of 6‐trans with [(C5H5)2Fe]PF6 gave [6‐trans][PF6]n (n = 1–3). Visible, IR, near‐IR (NIR), and electron paramagnetic resonance (EPR) spectroscopy together with DFT data show that [6‐trans][PF6] is a class II mixed‐valence complex (Hab = 85 cm–1) in which the spin distribution depends on the conformation of the molecule. Intramolecular electron transfer occurs through single‐step tunneling and a multistep hoping mechanism. The triplet state is thermally accessible for [6‐trans][PF6]2. The complex trans‐[Cp*(dppe)Fe–C≡C–TTFMe2–C≡C–Fe(dppe)Cp*][PF6] [Cp* = η5‐C5Me5, dppe = 1,2‐bis(diphenylphosphanyl)ethane, TTF = tetrathiafulvalene] is a class II mixed‐valence compound in which the spin density varies with the respective orientation of the metal termini. Intramolecular electron transfer occurs through single‐step tunneling and multiple‐step hopping mechanisms.