Synthesis, Ion Recognition Ability, and Metal-Assisted Aggregation Behavior of Dinuclear Metallohosts Having a Bis(Saloph) Macrocyclic Ligand

Macrocyclic molecule 1 that has two saloph coordination sites was designed and synthesized. The macrocycle 1 was easily converted into the corresponding metallohosts 2 and 3 by the reaction with nickel­(II) and palladium­(II), respectively. As expected from the molecular structure of these metallohosts having an 18-crown-6-like cavity, the nickel­(II) metallohost 2 showed excellent binding affinity toward Na+, Ca2+, and Sr2+ to give 1:1 host–guest complexes. Preorganization effect due to the extremely rigid metal-containing macrocycle was suggested to be a major factor for the strong binding. Larger cations such as K+, Rb+, Cs+, and Ba2+ gave higher aggregated host–guest complexes such as 22M, 23M2, and 24M3. Density functional theory calculations revealed that smaller metal ions do not occupy the center of each macrocycle in the sandwich structures 22M, while larger Cs+ simultaneously interacts with all the 12 oxygen donor atoms. On the basis of the interaction energy calculations, the preference for 2·Na over 22Na can be explained by destabilization of 22Na due to the elongated Na–O bonds and repulsive three-body interactions. When the ionic radius of the guest ion increases (K+, Rb+, Cs+), this destabilization becomes less significant and the formation of sandwich complexes 22M is favored. Such aggregation would significantly affect the physical and chemical properties of the metal complexes due to the interplane interactions between the metal centers.

设计并合成了带有两个saloph配位位点（saloph coordination sites）的大环分子1。该大环分子1可分别与镍(II)、钯(II)发生反应，便捷地转化为相应的金属主体2与3。正如这类具有类18-冠-6空腔的金属主体的分子结构所预期的那样，镍(II)金属主体2对Na+、Ca2+及Sr2+展现出优异的结合亲和力，可生成1:1型主客体配合物。研究表明，由极具刚性的含金属大环所带来的预组织效应，是实现强结合作用的核心因素。对于K+、Rb+、Cs+及Ba2+等体积更大的阳离子，则会形成更高阶的聚集态主客体配合物，如22M、23M2与24M3。密度泛函理论（Density Functional Theory）计算结果显示，在三明治型结构22M中，较小的金属离子并未占据每个大环的中心位置；而体积更大的Cs+则可同时与全部12个氧供体原子产生相互作用。基于相互作用能的计算分析，2·Na相较于22Na的结合偏好性，可通过22Na因Na-O键伸长以及三体排斥相互作用导致的不稳定来解释。当客体离子的离子半径依次增大（K+、Rb+、Cs+）时，这类不稳定效应会逐渐减弱，三明治型配合物22M的形成则更为有利。此类聚集作用会因金属中心之间的平面间相互作用，显著影响该金属配合物的物理与化学性质。