Professor Joel Miller recently published his paper “Cation Dependence of the Dimerization Enthalpy for A₂[tetracyanoethylene]₂ (A=NMe₄, Mepy, NEt₄) Possessing a Long, Multicenter Bond” in Chemistry: A European Journal. It was written in collaboration with Adora G. Graham and Matvey V. Fedin and “Dedicated to Edel Wasserman on the occasion of his 85th birthday, and to the memory of the DuPont Central Research Department—the birthplace of cyanocarbon chemistry (1957–2016).”

Abstract: [TCNE]^.− (TCNE=tetracyanoethylene) has been isolated as D₂h π–[TCNE]₂²⁻ possessing a long, 2.9 Å multicenter 2-electron-4-center (2e⁻/4c) C−C bond, and as C₂ π–[TCNE]₂²⁻ possessing a longer, 3.04 Å multicenter 2e⁻/6c (4 C+2 N atoms) bond. Temperature-dependent UV/Vis spectroscopic measurements in 2-methyltetrahydrofuran (MeTHF) has led to the determination of the dimerization, 2[TCNE]^.−⇌π–[TCNE]₂²⁻, equilibrium constants, K_eq(T), [[TCNE]₂²⁻]/[[TCNE]^.−]², enthalpy, ΔH, and entropy, ΔS, of dimerization for [Mepy]₂[TCNE]₂ (Mepy=N-methylpyridinium, H₃CNC₅H₅⁺) possessing D₂h π–[TCNE]₂²⁻ and [NMe₄]₂[TCNE]₂ possessing C₂ π–[TCNE]₂²⁻conformations in the solid state; however, both form D₂h π–[TCNE]₂²⁻ in MeTHF solution. Based on ΔH=−3.6±0.1 kcal mol⁻¹ (−15.2 kJ mol⁻¹), and ΔS=−11±1 eu (−47 J mol⁻¹ K⁻¹) and ΔH=−2.4±0.2 kcal mol⁻¹ (−10.2 kJ mol⁻¹), and ΔS=−8±1 eu (−32 J mol⁻¹ K⁻¹) in MeTHF for [NMe₄]₂[TCNE]₂ and [Mepy]₂[TCNE]₂, respectively, the calculated K_eq(298 K) are 1.6 and 1.3 m⁻¹, respectively. The observed K_eq (145 K) are 3 and 2 orders of magnitude greater for [NMe₄]₂[TCNE]₂ and [Mepy]₂[TCNE]₂, respectively. The K_eq(130 K) is 4470, 257, ≈0.8, and ≪0.1 m⁻¹ for [NMe₄]₂[TCNE]₂, [Mepy]₂[TCNE]₂, [NEt₄]₂[TCNE]₂, and [N(nBu)₄]₂[TCNE]₂, respectively, decreasing with increasing cation size. At standard conditions and below ambient temperature the equilibrium favors the dimer for the NMe₄⁺ and Mepy⁺ cations. From the decreasing enthalpy, NMe₄⁺>Mepy⁺, along with the decrease in dimer formation K_eq(T) as NMe₄⁺>Mepy⁺>NEt₄⁺>N(nBu)₄⁺, the dimer bond energy decreases with increasing cation size in MeTHF. This is attributed to a decrease in the [A]⁺⋅⋅⋅[TCNE]⁻ attractive interactions with increasing cation size. Solid state UV/Vis spectroscopic determinations of [NMe₄]₂[TCNE]₂ are reported and compared to D₂h π–[TCNE]₂²⁻ conformers. The feasibility and limitations of temperature-dependent electron paramagnetic resonance (EPR) measurements for the determination of K_eq(T) are also discussed.

_{Structure (50 % electron density ORTEP) showing the intradimer bonding components and HOMO for D2h π–[TCNE]2²⁻ possessing a 2.9 Å, 2e⁻/4c C−C bond (a),
and C2 π–[TCNE]2²⁻ possessing a 3.04 Å 2e⁻/6c C−C (4 C+2 N atoms) bond (b) (adapted from Ref. [9]).}