Binding Affinity of Monoalkyl Phosphinic Acid Ligands toward Nanocrystal Surfaces

Abstract

We recently introduced monoalkyl phosphinic acids as a ligand class for nanocrystal (NC) synthesis. Their metal salts have interesting reactivity differences compared to metal carboxylates and phosphonates and provide a cleaner work-up compared to phosphonates. However, little is known about the surface chemistry of NCs 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 hafnia, cadmium selenide, and zinc sulfide NCs. We monitor the ligand shell composition by solution 1 H and 31 P 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 favor of phosphonate (K = 2). Phosphinic acids have thus an intermediate binding affinity between carboxylic acids and phosphonic acids for all the NCs studied. These results enable the sophisticated use of monoalkyl phosphinic acids for NC synthesis and for post-synthetic surface engineering.

The Surface Chemistry of Colloidal Nanocrystals Capped by Organic Ligands

“…Anionic ligands bound to a cation-rich nanocrystal core cannot desorb in nonpolar solvents and are thus strongly bound. Comparing these X-type ligands in nonpolar solvents, we found across many nanocrystal types the following order (see also Figure ): In water, nitrocatechol ligands bind most strongly to oxide nanocrystals. , For cadmium and zinc based nanocrystals, thiols seem much stronger X-type ligands than carboxylic acids, but it is unclear how thiols compare to phosphonic acids. Also for noble metals, soft thiol and phosphine ligands are preferred. ,,,, In the case of PbS nanocrystals, thiols do not show a clean X-type exchange but also behave as Lewis bases .…”

“…Anionic ligands bound to a cationrich nanocrystal core cannot desorb in nonpolar solvents and are thus strongly bound. Comparing these X-type ligands in nonpolar solvents, we found across many nanocrystal types the following order (see also Figure 9): 86 < carboxylic acid monoalkyl phosphinic acid phosphonic acid…”

“…111−113 Furthermore, the monoalkyl phosphinic acids can be used for convenient surface functionalization. 86 Their binding affinity is almost equal to the one of phosphonic acids while their binding mode (LXtype) is similar to that of carboxylic acids. Circling back to our NMR methods in the first section, it is often useful to have an isolated resonance in the NMR spectrum that can be monitored to assess ligand exchange.…”

“…(C) The synthesis of monoalkyl phosphinic acids with steric hindrance or special functionalities. The synthesis is described in detail in references , , and .…”

“…Interestingly, the reactivity of cadmium phosphinate was higher than the reactivity of cadmium phosphonate, but lower than the one of cadmium oleate. This provides the nanochemist with an additional handle on the reaction, which is currently mostly limited to varying the chalcogen precursor. − Furthermore, the monoalkyl phosphinic acids can be used for convenient surface functionalization . Their binding affinity is almost equal to the one of phosphonic acids while their binding mode (LX-type) is similar to that of carboxylic acids.…”