Frish Limor, Vysotsky Myroslav O, Böhmer Volker, Cohen Yoram
School of Chemistry, The Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv 69978, Tel Aviv, Israel.
Org Biomol Chem. 2003 Jun 7;1(11):2011-4. doi: 10.1039/b302398b.
The affinities of ferrocene (2) and the cobaltocenium cation (3+), which have roughly the same size and differ in their charge, towards the inner cavity of the dimeric capsule formed by tetraurea calix[4]arene (1) were studied in C2D4Cl2 solutions. While 3+, which occupies more than 75% of the internal volume of the dimer, is readily encapsulated this is not the case for 2. This is probably due to cation-pi interactions, which operate only between 3+ and the aromatic rings of the calix[4]arene dimer. We found that the affinity of the cobaltocenium cation is higher than that of the tropylium cation (4+) and is only 2-3 times less than that of the tetraethylammoniun cation (5+). From the variable temperature 1H NMR spectra of this capsule, the free energy of activation at 298 K (deltaGdouble dagger(298K)) for the reorientation of the hydrogen bonded belt between the two parts of the dimer could be determined by total line shape analysis for the aromatic protons of the calixarene. The value of 14.3 +/- 0.2 kcal mol(-1) for the dimeric capsules of 3+ PF6- is very similar to the free activation energy found for dimeric capsules of 1 with 4+ PF6- and 5+ PF6- in C2D4Cl2. It becomes significantly lower, if PF6- is replaced by BF4-. We also found that ten times more DMSO is needed to disrupt the capsule 1 x 3+ x 1 than the corresponding 1 x 1 dimer containing benzene as guest. This demonstrates again the importance of the cation-pi interactions for the stability of such hydrogen-bonded dimeric capsules.
在C2D4Cl2溶液中研究了二茂铁（2）和钴鎓阳离子（3+）对由四脲杯[4]芳烃（1）形成的二聚体胶囊内腔的亲和力，它们大小大致相同但电荷不同。虽然占据二聚体内部体积超过75%的3+很容易被包封，但2并非如此。这可能是由于阳离子-π相互作用，其仅在3+与杯[4]芳烃二聚体的芳环之间起作用。我们发现钴鎓阳离子的亲和力高于环庚三烯酚酮阳离子（4+），并且仅比四乙铵阳离子（5+）的亲和力低2 - 3倍。从该胶囊的变温1H NMR光谱中，通过对杯芳烃芳质子的全谱线形状分析，可以确定二聚体两部分之间氢键带重排的298K活化自由能（ΔG‡(298K)）。对于3+ PF6-的二聚体胶囊，14.3±0.2 kcal mol(-1)的值与在C2D4Cl2中1与4+ PF6-和5+ PF6-的二聚体胶囊发现的自由活化能非常相似。如果用BF4-代替PF6-，该值会显著降低。我们还发现，破坏1×3+×1的胶囊所需的DMSO比含有苯作为客体的相应1×1二聚体多十倍。这再次证明了阳离子-π相互作用对于此类氢键二聚体胶囊稳定性的重要性。
