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 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, Keq (T), [[TCNE]22- ]/[[TCNE].- ]2 , enthalpy, ΔH, and entropy, ΔS, of dimerization for [Mepy]2 [TCNE]2 (Mepy=N-methylpyridinium, H3 CNC5 H5+ ) 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 Keq (298 K) are 1.6 and 1.3 m-1 , respectively. The observed Keq (145 K) are 3 and 2 orders of magnitude greater for [NMe4 ]2 [TCNE]2 and [Mepy]2 [TCNE]2 , respectively. The Keq (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 Keq (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 Keq (T) are also discussed.