[MeNC5H5]2[TCNE]2 (TCNE = tetracyanoethylene). Single crystal X-ray and neutron diffraction characterization of an exceptionally long 2.8 Å C–C bond

Abstract

The reaction of N-methylpyridinium iodide, Mepy⁺I⁻, and tetracyanoethylene (TCNE) forms [Mepy]₂[TCNE]₂, which possesses [TCNE]₂²⁻ with an intradimer C–C bond distance of 2.806(1) Å at 50 K from X-ray diffraction, and 2.801(4) Å at 50 K from neutron diffraction. In the IR it exhibits ν_C≡N absorptions at 2191, 2174, 2169, 2163 and a ν_CC absorption at 1366 cm⁻¹, with UV/Vis absorption bands at 26,880 and 18,520 cm⁻¹. Analysis of the cation-hydrogen to [TCNE]₂²⁻ interactions do not provide evidence that the cation stabilizes formation of the [TCNE]₂²⁻ dimer, which is stabilized by the intradimer 2e⁻-4 center C–C bonding interaction.