Graham Adora G, Fedin Matvey V, Miller Joel S
Department of Chemistry, University of Utah, Salt Lake City, UT, 84112-0850, USA.
International Tomography Center SB RAS and, Novosibirsk State University, Novosibirsk, 630090, Russia.
Chemistry. 2017 Sep 12;23(51):12620-12629. doi: 10.1002/chem.201702492. Epub 2017 Aug 15.
[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.
[四氰基乙烯](TCNE = 四氰基乙烯)已被分离为具有2.9 Å长的多中心2电子 - 4中心(2e/4c)C - C键的Dπ-[TCNE]，以及具有更长的3.04 Å多中心2e/6c(4个C + 2个N原子)键的Cπ-[TCNE]。在2 - 甲基四氢呋喃(MeTHF)中进行的温度依赖紫外/可见光谱测量，已确定了固态下具有Dπ-[TCNE]构象的[甲基吡啶][TCNE](甲基吡啶 = N - 甲基吡啶鎓，HCNCH)和具有Cπ-[TCNE]构象的[四甲基铵][TCNE]的二聚化平衡常数K(T)、[π-[TCNE]]/[[TCNE]]、焓变ΔH和熵变ΔS；然而，两者在MeTHF溶液中均形成Dπ-[TCNE]。基于MeTHF中[四甲基铵][TCNE]和[甲基吡啶][TCNE]的ΔH分别为 - 3.6±0.1 kcal·mol⁻¹(-15.2 kJ·mol⁻¹)和ΔS为 - 11±1 eu(-47 J·mol⁻¹·K⁻¹)，以及ΔH为 - 2.4±0.2 kcal·mol⁻¹(-10.2 kJ·mol⁻¹)和ΔS为 - 8±1 eu(-32 J·mol⁻¹·K⁻¹)，计算得到的K(298 K)分别为1.6和1.3 m。观察到的[四甲基铵][TCNE]和[甲基吡啶][TCNE]在145 K时的K值分别比上述计算值大3和2个数量级。[四甲基铵][TCNE]、[甲基吡啶][TCNE]、[三乙铵][TCNE]和[正丁基铵][TCNE]在130 K时的K值分别为4470、257、≈0.8和≪0.1 m，随阳离子尺寸增大而减小。在标准条件及低于环境温度下，对于NMe和Mepy阳离子，平衡有利于二聚体的形成。从NMe>Mepy时焓变的降低，以及随着NMe>Mepy>NEt>N(nBu)二聚体形成K(T)的降低来看，在MeTHF中，二聚体键能随阳离子尺寸增大而降低。这归因于随着阳离子尺寸增大，[A]⋅⋅⋅[TCNE]吸引相互作用的减弱。报道了[四甲基铵][TCNE]的固态紫外/可见光谱测定结果，并与Dπ-[TCNE]构象体进行了比较。还讨论了温度依赖电子顺磁共振(EPR)测量用于确定K(T)的可行性和局限性。
