Williams Avril R, Northrop Brian H, Houk Kendall N, Stoddart J Fraser, Williams David J
California NanoSystems Institute and Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1569, USA.
Chemistry. 2004 Oct 25;10(21):5406-21. doi: 10.1002/chem.200400221.
Three constitutionally isomeric bis(naphthylmethyl)ammonium ions, in which the two naphthyl groups are substituted 1) both at their 1-positions, 2) one at its 1-position and the other at its 2-position, and 3) both at their 2-positions, have been investigated separately in solution for their propensities to undergo spontaneous self-assembly with three different [24]crown-8 derivatives, namely, pyrido[24]crown-8 (P24C8), dipyrido[24]crown-8 (DP24C8) and dibenzo[24]crown-8 (DB24C8), in turn to form [2]pseudorotaxanes. The strengths of the 1:1 complexes depend on the composition of the secondary dialkylammonium ions and on the nature of the crown ether hosts; generally, as far as the guest cation is concerned, the 1/1- and 2/2-isomers form stronger complexes, as indicated by stability constant measurements, than the 1/2-isomer and, as far as the crown ethers are concerned, the more flexible P24C8 is a much more efficient host than either DP24C8 or DB24C8. The rates of formation of the [2]pseudorotaxanes are fast (i.e., taking no more than a few minutes) in solution with the exception of one case, that is, in which the crown ether host is DB24C8 and the guest cation is the 1/1-isomer, when it can take upwards of one month for the complexation-decomplexation equilibrium to be established at room temperature. In all cases, the equilibrium between complexed and uncomplexed species is slow on the NMR timescale, allowing the determination of stability constants to be made readily using the single-point method. X-ray crystallography and molecular modeling have been used to gain insight into ground and transition state interactions, respectively, in some of the [2]pseudorotaxanes. The relative stabilities of the three [2]pseudorotaxanes formed by each guest cation in the presence of the three crown ether hosts were also evaluated in solution by competition experiments that were monitored by (1)H NMR spectroscopy. By and large the results of the competition experiments could be predicted on the basis of the derived stability constants for the individual [2]pseudorotaxanes.
三种结构异构的双（萘甲基）铵离子，其中两个萘基分别在以下位置被取代：1）均在其1位；2）一个在其1位，另一个在其2位；3）均在其2位。分别在溶液中研究了它们与三种不同的[24]冠-8衍生物，即吡啶并[24]冠-8（P24C8）、二吡啶并[24]冠-8（DP24C8）和二苯并[24]冠-8（DB24C8）自发进行自组装形成[2]准轮烷的倾向。1:1配合物的强度取决于仲二烷基铵离子的组成和冠醚主体的性质；一般来说，就客体阳离子而言，如稳定性常数测量所示，1/1-和2/2-异构体形成的配合物比1/2-异构体更强，而就冠醚而言，更灵活的P24C8是比DP24C8或DB24C8更有效的主体。除了一种情况外，[2]准轮烷在溶液中的形成速率很快（即不超过几分钟），在这种情况下，冠醚主体是DB24C8，客体阳离子是1/1-异构体，在室温下络合-解络合平衡的建立可能需要一个多月的时间。在所有情况下，络合和未络合物种之间的平衡在核磁共振时间尺度上很慢，这使得使用单点法很容易测定稳定性常数。X射线晶体学和分子建模已分别用于深入了解一些[2]准轮烷中的基态和过渡态相互作用。通过（1）H核磁共振光谱监测的竞争实验，还在溶液中评估了每种客体阳离子在三种冠醚主体存在下形成的三种[2]准轮烷的相对稳定性。总的来说，竞争实验的结果可以根据推导的各个[2]准轮烷的稳定性常数来预测。
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