Passing Two Strings through the Same Ring Using an Octahedral Metal Center as Template: A New Synthesis of [3]Rotaxanes

Octahedral transition metal centers such as Fe(II), Co(II), and Co(III) have been used as templates in the construction of [3]pseudorotaxanes and [3]rotaxanes from various acyclic and macrocyclic fragments. The species obtained consist of a ring threaded by two string-like compounds. Such systems are relatively uncommon in the [3]rotaxane family, the most usual form being made up of a single axis threaded through two rings. The key structural feature of the present systems is the coordinating unit incorporated in the various organic fragments and used in conjunction with the metal to gather and thread the two filaments through the ring. This bidentate chelate is derived from 8,8′-diphenyl-3,3′-bi-isoquinoline, a very rare example of an endotopic but nonsterically hindering ligand. The stoppered [3]rotaxanes were obtained by using an open-chain fragment bearing azide groups as end functions, followed by click chemistry using a propargyl ether attached to a very bulky group. A particularly attractive X-ray structure was obtained for a cobalt(III)-complexed [3]pseudorotaxane consisting of a 41-membered ring and two crescent-shaped threaded components. The Fe(II) and Co(III) complexes were characterized by 1H NMR and ES-MS. By taking advantage of the markedly different kinetic properties of the two oxidation states, Co(II) and Co(III), it was possible to proceed to fast coordination or decoordination reactions (for the divalent state) or, when needed, to “freeze” the complexes due to the kinetic inertness of the trivalent state and to study them by 1H NMR. Finally, demetalation of the two stoppered compounds prepared was performed. This demetalation reaction was fast for the Co(II)-complexed [3]rotaxane, whereas decomplexation of the Fe(II) equivalent required harsh conditions which were not compatible with the stability of the metal-free rotaxane. Interestingly, the thermal stability of the free [3]rotaxane toward unthreading and formation of its constitutive elements was only limited. 1H NMR measurements showed that the half-life of the rotaxane is about one week at room temperature in dichloromethane. A variable-temperature study revealed that the unthreading reaction leading to dissociation of the [3]rotaxane has a remarkably high entropy of activation, in agreement with the intuitive view that the unthreading process involves a highly ordered transition state.

八面体过渡金属中心（octahedral transition metal centers）包括Fe(II)、Co(II)与Co(III)，已被用作模板，从多种非环状及大环片段构筑[3]假轮烷（[3]pseudorotaxane）与[3]轮烷（[3]rotaxane）。所得产物为一个环被两条线状化合物穿入的结构。这类结构在[3]轮烷家族中相对罕见，最常见的[3]轮烷形式是单根轴穿过两个环的结构。本研究体系的核心结构特征在于：各类有机片段中引入了配位单元，可与金属中心协同作用，将两条线状组分收拢并穿入环中。该双齿螯合配体（bidentate chelate）衍生自8,8'-二苯基-3,3'-联异喹啉，是一类极为罕见的内位型但无空间位阻的配体。封端型[3]轮烷的合成路线为：首先使用末端带有叠氮基的开链片段，随后通过点击化学（click chemistry），与连接了超大位阻基团的炔丙基醚发生反应。研究中获得了一个极具研究价值的钴(III)配位[3]假轮烷的X射线单晶结构（X-ray structure），该结构包含一个41元环与两条呈月牙形的穿入组分。所制备的Fe(II)与Co(III)配合物均通过氢核磁共振（1H NMR）与电喷雾质谱（ES-MS）进行了表征。研究利用Co(II)与Co(III)两种氧化态间显著差异的动力学特性：对于二价钴配合物，可实现快速的配位或去配位反应；而三价钴配合物因动力学惰性可被“冻结”，从而能够通过氢核磁共振对其进行表征。最后，对所合成的两种封端型产物进行了脱金属处理。其中，钴(II)配位[3]轮烷的脱金属反应速率较快；而对应铁(II)配位体系的脱配合物反应则需要严苛的条件，且该条件与无金属轮烷的稳定性不兼容。值得注意的是，游离态[3]轮烷在发生解穿环并生成其组成单元过程中的热稳定性仅处于较低水平。氢核磁共振测试结果显示，该轮烷在二氯甲烷溶剂、室温条件下的半衰期约为一周。变温实验研究表明，引发[3]轮烷解离的解穿环反应具有极高的活化熵，这与“解穿环过程涉及高度有序过渡态”的直观认知相符。