Pairing of Propellers: Dimerization of Octahedral Ruthenium(II) and Osmium(II) Complexes of Eilatin via π−π Stacking Featuring Heterochiral Recognition

Five octahedral eilatin complexes of the type [M(L−L)2(eilatin)]2+ (M = Ru, Os; L−L = bipyridyl-type ligands) were synthesized, and their dimerization via π−π stacking was studied by crystallography and 1H NMR techniques. The X-ray structures of these racemic complexes were solved and revealed that the eilatin complexes are organized as discrete dimers in which the eilatin residues of each complex are stacked in centrosymmetric packing. Chemical shift dependence on concentration in the 1H NMR spectra support fast dimer−monomer equilibrium, and the structures of the dimers in acetonitrile solution are proposed to be analogous to their solid-state structures. Dimerization constants in acetonitrile were measured for the five racemic eilatin complexes that exhibit different structural parameters, as well as for the two enantiomeric forms of one of these complexes. They were found to be independent of the metal (Ru vs Os), strongly dependent on the steric effects introduced by the L−L ligands (2,2‘-bipyridine, 1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline, and 2,2‘-biquinoline), and dependent on the optical purity of the complexes. A clear preference for heterochiral over homochiral dimer formation was demonstrated. This is the first report of chiral recognition in solution, exhibited by simple chemical systems held solely by π-stacking interactions.本研究合成了5种通式为[M(L−L)₂(eilatin)]²⁺的八面体依莱汀（eilatin）配合物，其中M为钌（Ru）、锇（Os），L−L为联吡啶类配体；并借助晶体学与核磁共振氢谱（¹H NMR）技术，对其通过π-π堆积发生的二聚化过程展开了研究。研究人员解析了该类外消旋配合物的X射线晶体结构，结果显示，依莱汀配合物以离散二聚体形式存在，每个配合物中的依莱汀残基以中心对称堆积方式发生堆叠。核磁共振氢谱中化学位移随浓度的变化规律，佐证了二聚体与单体间存在快速平衡；乙腈溶液中二聚体的结构被推测与其固态结构相似。本研究测定了5种具有不同结构参数的外消旋依莱汀配合物，以及其中一种配合物的两种对映异构体形式在乙腈中的二聚化常数。研究发现，二聚化常数与金属中心（钌与锇）无关，而与L−L配体引入的位阻效应密切相关（配体包括2,2'-联吡啶（2,2'-bipyridine）、1,10-邻菲啰啉（1,10-phenanthroline）、2,9-二甲基-1,10-邻菲啰啉（2,9-dimethyl-1,10-phenanthroline）以及2,2'-联喹啉（2,2'-biquinoline）），同时也受配合物的光学纯度影响。实验证实，该类配合物在二聚体形成时，明显优先选择异手性而非同手性二聚体。本研究首次报道了仅依靠π-堆积相互作用维系的简单化学体系在溶液中展现出手性识别现象。