Static and Dynamic Adsorptions of Water Vapor by Cyclic [Zr 36 ] Clusters: Implications for Atmospheric Water Capture Using Molecular Solids

Atomically precise surface chemistry of zirconium and hafnium metal oxo clusters beyond carboxylate ligands

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 insight into the thermodynamics of ligand binding. Through a combination of theoretical calculations and experimental spectroscopic techniques, we determine the interaction between the M6O8 8+ (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 remains unaffected by the surrounding adsorbates. For example, dialkylphosphinic acids are too sterically hindered to yield complete ligand exchange, even though a single dialkylphosphinate has a high binding affinity. Monoalkyl or monoaryl phosphinic acids do replace carboxylates quantitatively and we obtained the crystal structure of M6O8H4(O2P(H)Ph)12 (M = Zr, Hf), giving insight into the binding mode of monosubstituted phosphinates. Phosphonic acids cause a partial structural reorganization of the metal oxo cluster into amorphous metal phosphonate as indicated by pair distribution function analysis. These results rationalize the absence of phosphonate-capped M6O8 clusters and the challenge in preparing Zr phosphonate metal-organic frameworks. We thus further reinforce the notion that monoalkylphosphinates are carboxylate mimics with superior binding affinity.

Molecular Insights into Sequence-Specific Protein Hydrolysis by a Soluble Zirconium-Oxo Cluster Catalyst

The development of catalysts for controlled fragmentation of proteins is a critical undertaking in modern proteomics and biotechnology. {Zr6O8}-based metal-organic frameworks (MOFs) have emerged as promising candidates for catalysis of peptide bond hydrolysis due to their high reactivity, stability, and recyclability. However, emerging evidence suggests that protein hydrolysis mainly occurs on the MOF surface, thereby questioning the need for their highly porous 3D nature. In this work, we show that the discrete and water-soluble [Zr6O4(OH)4(CH3CO2)8(H2O)2Cl3]+ (Zr6) metal-oxo cluster (MOC), which is based on the same hexamer motif found in various {Zr6O8}-based MOFs, shows excellent activity toward selective hydrolysis of equine skeletal muscle myoglobin. Compared to related Zr-MOFs, Zr6 exhibits superior reactivity, with near-complete protein hydrolysis after 24 h of incubation at 60 °C, producing seven selective fragments with a molecular weight in the range of 3-15 kDa, which are of ideal size for middle-down proteomics. The high solubility and molecular nature of Zr6 allow detailed solution-based mechanistic/interaction studies, which revealed that cluster-induced protein unfolding is a key step that facilitates hydrolysis. A combination of multinuclear nuclear magnetic resonance spectroscopy and pair distribution function analysis provided insight into the speciation of Zr6 and the ligand exchange processes occurring on the surface of the cluster, which results in the dimerization of two Zr6 clusters via bridging oxygen atoms. Considering the relevance of discrete Zr-oxo clusters as building blocks of MOFs, the molecular-level understanding reported in this work contributes to the further development of novel catalysts based on Zr-MOFs.

Expansion of Zirconium Oxide Clusters by 3d/4f Ions

Two series of charge-neutral coordination clusters featuring quasi-isostructural metal oxide cores, isolated as [Zr₆Fe₂Ln₂O₈(ib)₁₄(bda)₂(NO₃)₂]·xMeCN (Ln = La (1), Ce (2), Pr (3), and Nd (4); ib^- = isobutyrate; H₂bda = N-butyldiethanolamine) and [Zr₆Fe₂Ln₂O₈(ib)₁₄(mda)₂(NO₃)₂]·xMeCN (Ln = La (5), Ce (6), Pr (7), and Nd (8); H₂mda = N-methyldiethanolamine), were obtained via one-pot reactions of [Fe₃O(ib)₆(H₂O)₃]NO₃ as a critical precursor, Ln(NO₃)₃·6H₂O (Ln = La, Ce, Pr, and Nd), the respective aminoalcohol, and [Zr₆O₄(OH)₄(ib)₁₂(H₂O)]·3Hib in an acetonitrile solution. The coordination clusters in 1-8 feature {Zr₆O₈} cores that are structurally expanded by two 4f (Ln³⁺) and two 3d (Fe³⁺) metal ions, each individually coordinated to one of the eight oxide centers of {Zr₆O₈}, producing a metal skeleton where the 3d/4f positions cap four of the triangular faces of the central Zr₆ octahedron. The coordination clusters differ in the chosen aminoalcohol coligands, N-butyldiethanolamine or N-methyldiethanolamine, which lead to a different isobutyrate coordination pattern in the two series, while the {Fe₂Ln₂Zr₆O₈} core structure remains virtually unaffected. All eight coordination clusters are obtained in moderate to good yields of 29-66% after only several days. Complexes 1-8 are stable against air and moisture; they are also surprisingly thermally stable up to 280 °C in air and in nitrogen atmosphere, and they represent the first reported examples of 3d/4f-functionalized zirconium oxide clusters.