Transient chirality inversion during racemization of a helical cobalt(III) complex

While most responsive molecules show a trivial first-order exponential relaxation upon stimulation, increasing interest has recently focused on unusual nontrivial time responses such as chemical clocks and chemical oscillators. These unique time profiles principally originate from complicated multistep reactions involving an autocatalytic redox process of small inorganic species such as BrO(3)(–) or from supramolecular assembling of organic molecules. In this study, we have found that a very unusual time response was observed in a unimolecular platform just by integrating multiple reaction centers in a molecule. A triple-helical metallocryptand, [LCo(3)A(6)](3+) (A = (S)-1-phenylethylamine), underwent a transient chirality inversion from P-rich (right-handed) to M-rich (left-handed) then racemic during the racemization driven by the multistep ligand exchange with piperidine. In contrast, the reverse reaction showed a simple monotonic change with an induction time without showing the transient chirality inversion. Consequently, the M helicity appeared only in the forward reaction. Detailed mass spectrometric analysis suggested that the forward and reverse reactions proceeded in different pathways to make a unique hysteretic cycle.