Designed Self-Assembly of Molecular Necklaces

This paper reports an efficient strategy to synthesize molecular necklaces, in which a number of
small rings are threaded onto a large ring, utilizing the principles of self-assembly and coordination chemistry.
Our strategy involves (1) threading a molecular “bead” with a short “string” to make a pseudorotaxane and
then (2) linking the pseudorotaxanes with a metal complex with two cis labile ligands acting as an “angle
connector” to form a cyclic product (molecular necklace). A 4- or 3-pyridylmethyl group is attached to each
end of 1,4-diaminobutane or 1,5-diaminopentane to produce the short “strings” (C4N42+, C4N32+, C5N42+,
and C5N32+), which then react with a cucurbituril (CB) “bead” to form stable pseudorotaxanes (PR442+,
PR432+, PR542+, and PR532+, respectively). The reaction of the pseudorotaxanes with Pt(en)(NO3)2 (en =
ethylenediamine) produces a molecular necklace [4]MN, in which three molecular “beads” are threaded on
a triangular framework, and/or a molecular necklace [5]MN, in which four molecular “beads” are threaded
on a square framework. Under refluxing conditions, the reaction with PR442+ or PR542+ yields exclusively
[4]MN (MN44T or MN54T, respectively), whereas that with PR432+ or PR532+ produces exclusively [5]MN
(MN43S or MN53S, respectively). The products have been characterized by various methods including
X-ray crystallography. At lower temperatures, on the other hand, the reaction with PR442+ or PR542+ affords
both [4]MN and [5]MN. The supermolecules reported here are the first series of molecular necklaces obtained
as thermodynamic products. The overall structures of the molecular necklaces are strongly influenced by
the structures of pseudorotaxane building blocks, which is discussed in detail on the basis of the X-ray
crystal structures. The temperature dependence of the product distribution observed in this self-assembly
process is also discussed.

本文报道了一种高效的分子项链（molecular necklace）合成策略：该策略借助自组装与配位化学原理，将多个小分子环穿入一个大环结构中。
我们的策略分为两步：(1) 将短“绳”穿入分子“珠”内，制备准轮烷（pseudorotaxane）；(2) 以带有两个顺位可解离配体的金属配合物作为“角连接器”，将准轮烷连接为环状产物，即分子项链。
将4-吡啶甲基或3-吡啶甲基接于1,4-丁二胺或1,5-戊二胺的两端，可得到四种短“绳”（C4N4²+、C4N3²+、C5N4²+与C5N3²+）；这些短绳随后与葫芦脲（cucurbituril, CB）“珠”反应，生成对应的稳定准轮烷：PR44²+、PR43²+、PR54²+与PR53²+。
准轮烷与Pt(en)(NO3)2（en = 乙二胺（ethylenediamine））反应，可得到两种分子项链：[4]MN（三角形骨架上穿有3个分子“珠”）和/或[5]MN（正方形骨架上穿有4个分子“珠”）。在回流条件下，与PR44²+或PR54²+反应仅能得到[4]MN（分别对应MN44T与MN54T）；而与PR43²+或PR53²+反应则仅能得到[5]MN（分别对应MN43S与MN53S）。上述产物已通过包括X射线晶体学（X-ray crystallography）在内的多种表征手段得以确认。
在较低温度下，与PR44²+或PR54²+反应则可同时生成[4]MN与[5]MN。本文报道的超分子体系是首批作为热力学产物分离得到的分子项链系列。分子项链的整体结构受准轮烷构筑基元的结构显著影响，本文将基于X射线晶体结构对此展开详细讨论。此外，本研究还探讨了该自组装过程中产物分布的温度依赖性。