Disruption of a tautomeric equilibrium by coordination self-assembly depends upon a delicate interplay between supramolecular interactions but has not been studied for supramolecular polymers in two dimensions at the solid–liquid interface. Presented herein is the description of a functionalized molecular module which in DMSO solution generates a highly compact dynamic aggregate in its lactam (amide) form that undergoes reaction with Zn(II) ion to give a ML₂-type complex in which one lactam unit on each ligand has been converted to its iminol form, enabling bidentate binding also involving pyrimidine-N. These solution measurements, as well as a study by scanning tunneling microscopy at the liquid/graphite interface, corroborated by density functional studies, have revealed how the tautomeric equilibrium can be modulated by the addition of a metal salt.