The binding affinity of monoalkyl phosphinic acid ligands towards nanocrystal surfaces

We recently introduced monoalkyl phosphinic acids as a ligand class for nanocrystal synthesis. Their metal salts have interesting reactivity differences with respect to metal carboxylates and phosphonates, and provide cleaner work-up compared to phosphonates. However, there is little known about the surface chemistry of nanocrystals with monoalkyl phosphinate ligands. Here, we probe the relative binding affinity of monoalkyl phosphinate ligands with respect to other X-type ligands. We perform competitive ligand exchange reactions with carboxylate and phosphonate ligands at the surface of HfO2, CdSe, and ZnS nanocrystals. We monitor the ligand shell composition by solution 1H and 31P NMR spectroscopy. Using a monoalkyl phosphinic acid with an ether functionality, we gain an additional NMR signature, apart from the typical alkene resonance in oleic acid and oleylphosphonic acid. We find that carboxylate ligands are easily exchanged upon exposure to monoalkyl phosphinic acids, whereas an equilibrium is reached between monoalkyl phosphinates and phosphonates, slightly in the favour of phosphonate (K = 2). Phosphinic acids have thus an intermediate binding affinity between carboxylic acids and phosphonic acids for all nanocrystals studied. These results enable the sophisticated use of monoalkyl phosphinic acids for nanocrystal synthesis and for post-synthetic surface engineering.