Dynamic Aminal-Based Tpa Ligands

The use of dynamic covalent reactions (DCRs) is gaining popularity for the construction of self-assembling architectures. We have recently introduced DCRs that exchange alcohols and aldehydes to create hemiaminal ethers within tri(2-picolyl)amine (TPA) ligands, all of which are templated by Zn-II. To expand the scope of this assembly, aromatic imines derived from pyridine-2-carboxyaldehyde were explored as dynamic covalent receptors for di(2-picolyl)amine in the presence of Zn-II to create TPA ligands that contain aminal linkages. This represents another metal-templated in situ multicomponent assembly. The stability of the assembly was successfully modulated through substituent effects, and the equilibrium constants from imines to aminals were correlated by a linear free energy relationship (LFER) with sigma(+) values. Dynamic component exchange was investigated as a means of probing multiple equilibriums quantitatively in the system. Further, the mechanism was analyzed with a qualitative kinetics study. NMR spectra reveal the different extents of two competing pathways for assembly depending upon whether the aromatic amine has electron-withdrawing or electron-donating groups on the ring. Finally, mass spectral evidence supports the presence and differing extents of dominance of the two pathways as a function of the substituents.