Cation Dependence of the Dimerization Enthalpy for A₂[tetracyanoethylene]₂ (A=NMe₄, Mepy, NEt₄) Possessing a Long, Multicenter Bond

Abstract

[TCNE] (TCNE=tetracyanoethylene) has been isolated as D π-[TCNE] possessing a long, 2.9 Å multicenter 2-electron-4-center (2e /4c) C-C bond, and as C π-[TCNE] 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] , equilibrium constants, K (T), [[TCNE] ]/[[TCNE] ] , enthalpy, ΔH, and entropy, ΔS, of dimerization for [Mepy] [TCNE] (Mepy=N-methylpyridinium, H CNC H ) possessing D π-[TCNE] and [NMe ] [TCNE] possessing C π-[TCNE] conformations in the solid state; however, both form D π-[TCNE] in MeTHF solution. Based on ΔH=-3.6±0.1 kcal mol (-15.2 kJ mol ), and ΔS=-11±1 eu (-47 J mol K ) and ΔH=-2.4±0.2 kcal mol (-10.2 kJ mol ), and ΔS=-8±1 eu (-32 J mol K ) in MeTHF for [NMe ] [TCNE] and [Mepy] [TCNE] , respectively, the calculated K (298 K) are 1.6 and 1.3 m , respectively. The observed K (145 K) are 3 and 2 orders of magnitude greater for [NMe ] [TCNE] and [Mepy] [TCNE] , respectively. The K (130 K) is 4470, 257, ≈0.8, and ≪0.1 m for [NMe ] [TCNE] , [Mepy] [TCNE] , [NEt ] [TCNE] , and [N(nBu) ] [TCNE] , respectively, decreasing with increasing cation size. At standard conditions and below ambient temperature the equilibrium favors the dimer for the NMe and Mepy cations. From the decreasing enthalpy, NMe >Mepy , along with the decrease in dimer formation K (T) as NMe >Mepy >NEt >N(nBu) , 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 [NMe ] [TCNE] are reported and compared to D π-[TCNE] conformers. The feasibility and limitations of temperature-dependent electron paramagnetic resonance (EPR) measurements for the determination of K (T) are also discussed.

Reversible Solution π‐Dimerization and Long Multicenter Bonding in a Stable Phenoxyl Radical

“…We note that a significant variation in Δ S appears depending on the level of approximation (see Table S15 and Figure S11, Supporting Information). Recent work has demonstrated that quantitative determination of thermodynamic parameters for radical dimerization from EPR data may be inconclusive . We therefore attempted to determine these same parameters from variable‐temperature UV/Vis spectroscopy and obtained Δ H and Δ S values of 6.8±0.3 kcal mol −1 and 13.7±1.6 cal mol −1 K −1 (Table S5).…”

“…Recent work has demonstratedt hat quantitative determination of thermodynamicp arameters for radicald imerizationf rom EPR data may be inconclusive. [31] We therefore attempted to determine these same parameters from variable-temperature UV/Vis spectroscopy and obtained DH and DS values of 6.8 AE 0.3 kcal mol À1 and 13.7 AE 1.6 cal mol À1 K À1 (Table S5).…”