Chiral Metallocycles: Rational Synthesis and Novel Applications

Abstract

Chiralsupramolecular systems have attracted a great deal of interest from synthetic chemists over the past two decades because of their ability to mimic complex biological processes and their potential applications in enantioselective events such as asymmetric catalysis and chiral sensing. Chiral metallocycles, among the simplest forms of chiral supramolecular systems, are of particular interest because of their relative ease of synthesis. In this Account, we survey recent developments in the rational design and synthesis of chiral metallocyclic systems based on metal-ligand coordination and their potential applications in enantioselective recognition and catalysis. General design principles for metallocycles are first introduced with particular focus on thermodynamic and kinetic considerations. The symmetry requirements for the linear and angular building units, the influence of stoichiometries and reaction concentrations, and the roles of solvents are discussed. Optimum synthetic conditions for the self-assembly and directed-assembly of metallocycles are also compared. Three synthetic strategies for chiral metallocycles are broadly categorized based on the source of chirality, namely, (1) introduction of metallocorners containing chiral capping groups, (2) use of metal-based chirality owing to specific coordination arrangements, and (3) introduction of chiral bridging ligands. The bulk of this Account focuses on the third synthetic strategy with examples of chiral metallocycles built from atropisomeric bridging ligands based on the 1,1'-binaphthalene framework. The influences of ligand geometries and metallocorner configurations on the metallocycle structures are demonstrated. The synthetic utility of directed-assembly processes is illustrated with numerous examples of cyclic polygons ranging from nanoscopic dimers to a mesoscopic 47mer. Moreover, the directed-assembly processes offer exquisite control on structure, chirality, and functionality of the metallocycles. A number of interesting applications have been demonstrated with chiral metallocycles with diverse sizes and functionalities. For example, metallocycles with the Pt(diimine) metallocorners show interesting behaviors as luminophores in prototype light-emitting devices, chiral molecular squares based on 1,1'-binaphthyl-derived bipyridyl bridging ligands and fac-ReCO3Cl corners exhibit enantioselective luminescence in the presence of the 2-amino-1-propanol analyte, and chiral metallocycles based on 1,1'-binaphthyl-derived bialkynyl bridging ligands and cis-PtPEt2 corners activate Ti(IV) centers to catalyze highly enantioselective diethylzinc additions to aromatic aldehydes to afford chiral secondary alcohols. Additionally, chiral metallocycles synthesized via the weak-link approach (WLA) are shown to exhibit allosteric regulation. They experience significant changes in the cavity sizes and shapes upon the introduction of other ligands, with the resulting open structures serving as a catalyst for acyl transfer reaction or as an enantioselective recognition pocket. In summary, chiral metallocycles with much enhanced stability, favorable solubility characteristics, unprecedentedly large sizes, well-positioned functional groups, and desired chirality have been synthesized using a combination of self- and directed-assembly strategies. The applications of these chiral metallocycles in light-emitting devices, allosteric regulation, chiral sensing, and asymmetric catalysis have been demonstrated. The examples illustrated in this Account give testimony to chemists' ability, through chemical manipulations, to create large and complex chiral metallocycles that can potentially serve as mimics of natural enzyme systems.