Control of chirality inversion kinetics of triple-helical metallocryptands

Dynamic helical molecules, which can undergo a reversible chirality inversion between the P and M forms, are useful as a platform for switchable chiral functional molecules. The chirality inversion of these molecules has been extensively studied. However, it has mostly been discussed from the viewpoint of the equilibrated P/M ratios before and after the inversion; control of the response speeds or kinetic profiles has rarely been explored. In order to construct helical structures with controllable kinetic profiles, triple-helical metallocryptands, LM3, have been designed and synthesized. These molecules can undergo a relatively slow dynamic P/M chirality inversion (helicity inversion) to produce an equilibrated mixture. The P/M equilibration was accelerated or decelerated based on the following two strategies. One is based on the guest binding in the cryptand cavity. The P/M racemization kinetics of LNi3 was significantly decelerated by recognition of guanidinium ion in the cavity. The other strategy is based on the ligand exchange reactions at the octahedral cobalt(III) centers in LCo3(amine)6. The P/M chirality inversion speeds were controlled by changing the initial and entering amine ligands. In addition, a unique transient chirality inversion behavior was observed when chiral amine ligands were removed from the metallocryptand by the ligand exchange reaction with piperidine.