In 1987, Pedersen, Cram, and Lehn were awarded the Nobel Prize in Chemistry to honor their achievements in, among other things, the selective recognition of alkali metal ions by synthetic hosts. Almost three decades later, the 2016 Nobel Prize went to Stoddart, Sauvage, and Feringa for the development of artificial molecular machines, in which interlocked molecules play a significant role. Surprisingly, although many rotaxane- and catenane-based molecular machines have been constructed using various templating approaches, alkali metal ions, which are good templates for crown ether synthesis, have only rarely been applied as templates for the assembly of these interlocked molecules. This paucity of examples is probably due to the less well defined coordination numbers and geometries in the complexation of alkali metal ions to common oxygen-containing ligands, resulting in much weaker metal–ligand interactions and less predictable structures for their complexes compared with those formed between transition metal ions and common pyridine-containing ligands. Nevertheless, the ease of removing alkali metal ions from interlocked compounds and their much lower toxicity compared with that of transition metal ions are attractive features that have inspired their use as templates in the synthesis of interlocked molecules. About a decade ago, we began investigating the feasibility of using alkali metal ions to template the formation of catenanes and rotaxanes, with the hope of developing facile, broadly applicable, green, and efficient methods for their construction. We noticed that the interactions between oxygen-containing ligands and alkali metal ions can be strengthened by minimizing the effects of competing interactions from solvent molecules and counteranions. Thus, to increase the solubility of the metal ion salts in less polar solvents (e.g., CH₂Cl₂, CHCl₃) and minimize ion pairing, we chose tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), a weakly coordinating anion, as the counteranion for the alkali metal ions applied as templates. Our strategy has been based on the association of simple and general recognition units: (i) the orthogonal arrangement of two oligo(ethylene glycol) chains around an alkali metal ion and (ii) the encircling of a single urea/amide unit by an oligo(ethylene glycol)-containing macrocycle in the presence of a templating alkali metal ion. The former recognition system has allowed the facile construction of many interesting interlocked structures, including cyclic [2]catenane trimers and tetramers; the latter has provided several rotaxanes, including some incorporating monomers of practically important (macro)molecules (e.g., peptides, polymers) and some that behave as switches with unique functions (e.g., catalysis, gelation). The components in these recognition systems possess high flexibility in terms of their structures and the choice of suitable alkali metal ion templates.

中文翻译：

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使用碱金属离子模板化互锁分子的合成

1987年，Pedersen，Cram和Lehn被授予诺贝尔化学奖，以表彰他们在合成主体对碱金属离子的选择性识别等方面的成就。大约三十年后，由于人造分子机器的发展，2016年诺贝尔奖授予了Stoddart，Sauvage和Feringa，其中互锁的分子起着重要的作用。出人意料的是，尽管已经使用各种模板方法构建了许多基于轮烷和链烷的分子机器，但是碱金属离子是冠状醚合成的良好模板，但很少被用作组装这些互锁分子的模板。这种稀少的例子可能是由于碱金属离子与常见的含氧配体的络合中配位数和几何形状的定义不明确，与它们之间形成的配体相比，金属-配体之间的相互作用弱得多，其配合物的可预测性也较差过渡金属离子和常见的含吡啶配体。然而，与过渡金属离子相比，从互锁化合物中除去碱金属离子的难易程度和低得多的毒性是吸引人的特征，这些特征启发了它们在互锁分子合成中用作模板。大约十年前，我们开始研究使用碱金属离子模板化链烷烃和轮烷的可行性，以期开发出简便易行，广泛适用的绿色，高效的施工方法。我们注意到，含氧配体与碱金属离子之间的相互作用可以通过最大程度地减少溶剂分子和抗衡离子竞争性相互作用的影响来增强。因此，为了增加金属离子盐在极性较小的溶剂（例如CH₂氯₂，氯仿₃）并最大程度地减少了离子对，我们选择了弱配位阴离子四[3,5-双（三氟甲基）苯基]硼酸酯（TFPB）作为用作模板的碱金属离子的抗衡阴离子。我们的策略基于简单识别单元和通用识别单元的关联：（i）围绕碱金属离子的两个低聚（乙二醇）链的正交排列，以及（ii）一个低聚体环绕单个尿素/酰胺单元在模板碱金属离子的存在下，含有（乙二醇）的大环。以前的识别系统允许轻松构建许多有趣的互锁结构，包括环状[2]环烷三聚体和四聚体；后者提供了几种轮烷，包括一些掺入了实际上重要的（大分子）分子的单体（例如，肽，聚合物）和一些具有独特功能（例如催化，胶凝）的开关。这些识别系统中的组件在其结构和合适的碱金属离子模板的选择方面具有高度的灵活性。