Group IV metal-organic frameworks (MOFs) with phosphonate linkers offer enhanced stability and potential applicability compared to their carboxylate-based counterparts. However, synthetic strategies for accessing such frameworks remain underdeveloped. The inorganic nodes of MOFs - group IV metal oxo clusters, are considered the smallest conceivable nanocrystal prototypes. Their structurally similar inorganic core, capped with an organic ligand shell, allows them to be treated as model systems for colloidally stable nanocrystals of the same composition.[2]

The effectiveness of nanocrystals in many applications depends on their surface chemistry. Here, we leverage the atomically precise nature of zirconium and hafnium oxo clusters to gain fundamental insights into the thermodynamics of ligand binding. Using a combination of theoretical calculations and experimental spectroscopic techniques, we investigate the interactions between the M₆O₈⁸⁺ (M = Zr, Hf) cluster surface and various ligands: carboxylates, phosphonates, dialkylphosphinates, and monosubstituted phosphinates. We refute the common assumption that the adsorption energy of an adsorbate is unaffected by the surrounding adsorbates. Through ligand exchange from the carboxylate-capped clusters, we find that dialkylphosphinic acids possess too much sterical hindrance, preventing complete exchange. Monoalkyl or monoaryl phosphinic acids drive off carboxylates quantitatively and we obtained the crystal structure of M₆O₄(OH)₄(O₂P(H)Ph)₁₂ (M = Zr, Hf), first fully phosphinate-capped clusters. Phosphonic acids, however, cause a structural reorganization into amorphous metal phosphonate as indicated by Pair Distribution Function analysis.

These findings rationalize the absence of phosphonate-capped M₆O₈ clusters and underscore the challenges in preparing group IV phosphonate MOFs. Our results further reinforce the notion that monoalkylphosphinates, as carboxylate analogs with superior binding affinities, are promising alternative linkers for MOFs. We infer that while metal oxo clusters serve as minimalistic prototypes for oxide nanocrystals, their surface chemistries exhibit significant diversity due to variations in surface curvature. The development of a versatile toolkit for precise manipulation of cluster surfaces could unlock new opportunities in designing advanced cluster-based materials.