Probing the Delicate Balance between Pauli Repulsion and London Dispersion with Triphenylmethyl Derivatives
Probing the Delicate Balance between Pauli Repulsion and London Dispersion with Triphenylmethyl Derivatives
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DOI：
10.1021/jacs.8b09145
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发表时间：
2018-10-31
影响因子：
15
通讯作者：
Schreiner, Peter R.
中科院分区：
文献类型：
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作者：
Roesel, Soeren;Becker, Jonathan;Schreiner, Peter R.
用六苯乙烷（HPE）衍生物探测了长期已知的、普遍存在的且总是有吸引力的伦敦色散（LD）相互作用。合成了一系列全间位烃基[Me，Pr-i，Bu-t，Cy，Ph，1-金刚烷基（Ad）]取代的三苯甲基（TPM）衍生物[TPM-H，TPM-OH，（TPM-O）（2），TPM中心点]，并对几种衍生物进行了单晶X射线衍射（SC-XRD）表征。多个二聚体头对头SC-XRD结构的特征在于间位取代基之间的优异几何拟合;这对于空间上要求最高的Bu-t和Ad取代基尤其如此。获得Pr-1-、Bu-t-和Cy-derived三苯甲基自由基的NMR光谱，并与EPR和UV-Vis光谱数据一起揭示，所有间位烷基取代对三苯甲基支架的电子性质的影响是微不足道的。因此，我们得出结论，HPE稳定性的最重要因素来自LD相互作用。除了全meta Bu-t-HPE之外，我们还鉴定了迄今未报道的全meta Ad-HPE。对温度依赖性解离常数的复杂数学分析允许我们从全间位Bu-t-HPE的NMR实验中提取Δ G（d）（298）（exptl）= 0.3（5）kcal mol（-1），与先前的实验值和B3 LYP-D3（BJ）/def 2-TZVPP（C-PCM）计算非常一致。这些计算显示了与全间位Ad-HPE一致的取代基大小的稳定趋势（Δ G（d）（298）（exptl）= 2.1（6）kcal mol（-1））比其Bu-t同源物更稳定。也就是说，大的、刚性的和对称的烃部分充当优异的分散能供体。提供良好的几何拟合，它们能够通过强的分子内LD相互作用稳定不稳定的分子，如HPE，即使在溶液中。
The long-known, ubiquitously present, and always attractive London dispersion (LD) interaction was probed with hexaphenylethane (HPE) derivatives. A series of all-meta hydrocarbyl [Me, Pr-i, Bu-t, Cy, Ph, 1-adamantyl (Ad)]-substituted triphenylmethyl (TPM) derivatives [TPM-H, TPM-OH, (TPM-O)(2), TPM center dot] was synthesized en route, and several derivatives were characterized by single-crystal X-ray diffraction (SC-XRD). Multiple dimeric head-to-head SC-XRD structures feature an excellent geometric fit between the meta-substituents; this is particularly true for the sterically most demanding Bu-t and Ad substituents. NMR spectra of the Pr-i-, Bu-t-, and Cy-derived trityl radicals were obtained and reveal, together with EPR and UV-Vis spectroscopic data, that the effects of all-meta alkyl substitution on the electronic properties of the trityl scaffold are marginal. Therefore, we concluded that the most important factor for HPE stability arises from LD interactions. Beyond all-meta Bu-t-HPE we also identified the hitherto unreported all-meta Ad-HPE. An intricate mathematical analysis of the temperature-dependent dissociation constants allowed us to extract Delta G(d)(298)(exptl) = 0.3(5) kcal mol(-1) from NMR experiments for all-meta Bu-t-HPE, in good agreement with previous experimental values and B3LYP-D3(BJ)/def2-TZVPP(C-PCM) computations. These computations show a stabilizing trend with substituent size in line with all-meta Ad-HPE (Delta G(d)(298)(exptl) = 2.1(6) kcal mol(-1)) being more stable than its Bu-t congener. That is, large, rigid, and symmetric hydrocarbon moieties act as excellent dispersion energy donors. Provided a good geometric fit, they are able to stabilize labile molecules such as HPE via strong intramolecular LD interactions, even in solution.