Two-dimensional molecular sheets are of prime interest in nanoscience and technology. A promising class of such materials is 2D metal–organic frameworks (MOFs), assembled by cross-linking precursors with metal ions. It was recently demonstrated that such MOFs can be synthesized from monomers confined at an air–water interface. In order to elucidate this process at the atomic scale, we study a large flat tris-terpyridine-derived molecule (TTPB) on a water surface using ab initio molecular dynamics. We investigate the properties of the molecule and examine its reaction with Zn ions from the liquid phase. The fluid substrate significantly stabilizes the adsorbate while maintaining sufficient conformational flexibility to allow dynamic rearrangement and chemical reactions. The successful uptake and binding of ions is the first step toward linking TTPB molecules to dimers and large 2D MOFs.