Monitoring Self-Sorting by Electrospray Ionization Mass Spectrometry: Formation Intermediates and Error-Correction during the Self-Assembly of Multiply Threaded Pseudorotaxanes

Three binary pseudorotaxanes, which are based on two different secondary ammonium/crown ether binding motifs, have been studied by (1)H NMR and (1)H,(1)H EXSY NMR experiments with respect to their thermodynamic stabilities and their axle exchange kinetics. The stability ranking does not follow the order of axle exchange rates, and the thermodynamically most stable axle-wheel combinations assemble only slowly. On the basis of these binding motifs, a series of self-sorting systems have been studied ranging from simple four-component mixtures through sequence-specific pseudorotaxanes to multiply threaded complexes. Because of the mismatch of kinetic and thermodynamic order, wrongly assembled structures are unavoidable, which require error-correction steps to yield the final thermodynamically controlled self-sorted products. These error-correction steps can easily be monitored by electrospray mass spectrometry, when a mixed-flow microreactor is coupled to the ion source to cover second time scales. Self-assembly intermediates, wrongly assembled structures, and the final thermodynamic products can be simultaneously identified. The determination of preferred assembly pathways as well as the formation of dead-end structures provides a clear picture of a rich kinetic behavior of the self-sorting systems under study.