Dynamic control of topological asymmetry:
Chiral coordination complexes are frequently used in asymmetric
synthesis and chiral discrimination technologies. Recently, C3-symmetric chiral ligands have shown great potential for enantioselective reactions and enantioselective recognition, yet few such compounds are available. Usually such complexes involve a pre-organized rigid ligand with a C3 symmetry "built-in"; i.e., the geometry of the complex is determined by the bond connectivity in the ligand itself.
On the other hand, the coordination chemistry of the ligand tris((2-pyridyl)methyl)amine (TPA) with metal ions such as Zn(II) and Cu(II) has been extensively studied. Crystallographic data of these complexes shows that the metal ion usually displays a trigonal bipyramidal geometry. The ligand occupies four coordination sites, leaving one apical site available for anion or solvent coordination (e.g. [Zn(TPA)Cl]ClO4). In these complexes, enantiomeric conformations are adopted in which the pyridine rings occupy equatorial positions and are tilted with respect to the central axis of the molecule such that they display a propeller-like, C3-symmetrical arrangement. These conformations would be expected to interconvert rapidly at room temperature in solution.
If the direction of the twist could be controlled, it would provide a route to coordination complexes with highly asymmetric environments around the electrophilic coordination site of the metal ion. Here we discuss the idea that the control of the topology of the complex can be done dynamically using robust and conformationally flexible chiral ligands that wrap around Zn(II) and Cu(II) ions to form pseudo-C3-symmetric complexes with a helical asymmetry dictated by the configuration at a single carbon atom. This type of dynamic control could be applied to the design of a series of ligands that would be relatively easy to make and derivatize.
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