Polyoxometalates (POMs) are molecular metal‐oxide anions applied in energy conversion and storage, manipulation of biomolecules, catalysis, as well as materials design and assembly. Although often overlooked, the interplay of intrinsically anionic POMs with organic and inorganic cations is crucial to control POM self‐assembly, stabilization, solubility, and function. Beyond simple alkali metals and ammonium, chemically diverse cations including dendrimers, polyvalent metals, metal complexes, amphiphiles, and alkaloids allow tailoring properties for known applications, and those yet to be discovered. This review provides an overview of fundamental POM–cation interactions in solution, the resulting solid‐state compounds, and behavior and properties that emerge from these POM–cation interactions. We will explore how application‐inspired research has exploited cation‐controlled design to discover new POM materials, which in turn has led to the quest for fundamental understanding of POM–cation interactions.

The selective hydrolysis of proteins by non‐enzymatic catalysis is difficult to achieve, yet it is crucial for applications in biotechnology and proteomics. Herein, we report that discrete hafnium metal‐oxo cluster [Hf18O10(OH)26(SO4)13⋅(H2O)33] (Hf18), which is centred by the same hexamer motif found in many MOFs, acts as a heterogeneous catalyst for the efficient hydrolysis of horse heart myoglobin (HHM) in low buffer concentrations. Among 154 amino acids present in the sequence of HHM, strictly selective cleavage at only 6 solvent accessible aspartate residues was observed. Mechanistic experiments suggest that the hydrolytic activity is likely derived from the actuation of HfIV Lewis acidic sites and the Brønsted acidic surface of Hf18. X‐ray scattering and ESI‐MS revealed that Hf18 is completely insoluble in these conditions, confirming the HHM hydrolysis is caused by a heterogeneous reaction of the solid Hf18 cluster, and not from smaller, soluble Hf species that could leach into solution.

Zr/Hf aqueous-acid clusters are relevant to inorganic nanolithography, metal−organic frameworks (MOFs), catalysis, and nuclear fuel reprocessing, but only two topologies have been identified. The (Zr 4 ) polyoxocation is the ubiquitous square aqueous Zr/Hfoxysalt of all halides (except fluoride), and prior-debated for perchlorate. Simply adding peroxide to a Zr oxyperchlorate solution leads to a striking modification of Zr 4 , yielding two structures identified by single-crystal X-ray diffraction. Zr 25 , isolated from a reaction solution of 1:1 peroxide/Zr, is fully formulatedZr 25 is a pentagonal assembly of 25 Zr-oxy/peroxo/ hydroxyl polyhedra and is the largest Zr/Hf cluster topology identified to date. Yet it is completely soluble in common organic solvents. ZrT d , an oxo-centered tetrahedron fully formulated [Zr 4 (OH) is isolated from a 10:1 peroxide/Zr reaction solution. The formation pathways of ZrT d and Zr 25 in water were described by small-angle X-ray scattering (SAXS), pair distribution function (PDF), and electrospray ionization mass spectrometry (ESI-MS). Zr 4 undergoes disassembly by 1 equiv of peroxide (per Zr) to yield small oligomers of Zr 25 that assemble predominantly in the solid state, an unusual crystal growth mechanism. The self-buffering acidity of the Zr-center prevents Zr 25 from remaining intact in water. Identical species distribution and cluster fragments are observed in the assembly of Zr 25 and upon redissolution of Zr 25 . On the other hand, the 10:1 peroxide/Zr ratio of the ZrT d reaction solution yields larger prenucleation clusters before undergoing peroxide-promote disassembly into smaller fragments. Neither these larger cluster intermediates of ZrT d nor the smaller intermediates of Zr 25 have yet been isolated and structurally characterized, and they represent an opportunity to expand this new class of group IV polycations, obtained by peroxide reactivity and ligation.

Ion pairs and solubility related to ion-pairing in water influence many processes in nature and in synthesis including efficient drug delivery, contaminant transport in the environment, and self-assembly of materials in water. Ion pairs are difficult to observe spectroscopically because they generally do not persist unless extreme solution conditions are applied. Here we demonstrate two advanced techniques coupled with computational studies that quantify the persistence of ion-pairs in simple solutions and offer explanations for observed solubility trends. The system of study, (TMA,Cs)

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Inorganic aqueous metal-oxo clusters are both functional "molecular metal oxides" and intermediates to understand metal oxide growth from water. There has been a recent surge in discovery of aqueous Ti-oxo clusters but without extensive solution characterization. We use small-angle and total X-ray scattering, dynamic light scattering, transmission electron microscopy, and a single-crystal X-ray structure to show that heterometals such as bismuth stabilize labile Ti-oxo sulfate clusters in aqueous solution.[Ti Bi O (OH)(OH ) (SO ) ] features edge-sharing between the Ti and Bi polyhedra, in contrast to the dominant corner-linking of Ti-oxo clusters. Bi stabilizes the Ti-polyhedra, which are synergistically stabilized by the bidentate sulfates. Gained stability and potential functionality from heterometals is an incentive to develop more broadly the landscape of heterometallic Ti-oxo clusters.

The selective hydrolysis of proteins by non‐enzymatic catalysis is difficult to achieve, yet it is crucial for applications in biotechnology and proteomics. Herein, we report that discrete hafnium metal‐oxo cluster [Hf18O10(OH)26(SO4)13⋅(H2O)33] (Hf18), which is centred by the same hexamer motif found in many MOFs, acts as a heterogeneous catalyst for the efficient hydrolysis of horse heart myoglobin (HHM) in low buffer concentrations. Among 154 amino acids present in the sequence of HHM, strictly selective cleavage at only 6 solvent accessible aspartate residues was observed. Mechanistic experiments suggest that the hydrolytic activity is likely derived from the actuation of HfIV Lewis acidic sites and the Brønsted acidic surface of Hf18. X‐ray scattering and ESI‐MS revealed that Hf18 is completely insoluble in these conditions, confirming the HHM hydrolysis is caused by a heterogeneous reaction of the solid Hf18 cluster, and not from smaller, soluble Hf species that could leach into solution.

Understanding fundamental differences between zirconium and hafnium chemistry contributes to our fundamental understanding of the periodic table and leads to devising necessary separations for high-precision nuclear and microelectronics applications, developing water-based nanolithographic processes, and creating new robust metal−organic frameworks for catalysis and separations. Here we crystallize a rich matrix of polynuclear Zr and Hf species differentiating in complexation with peroxide and oxalate in mild acid, where the countercations influence polymerization. Hf only complexes oxalate, yielding polymericNa 6 [Hf 2 (OH) 2 (C 2 O 4 ) 6 ], and Li 2 K 4 [Hf 2 (OH) 2 (C 2 O 4 ) 6 ] and mononuclear K 4 Hf(C 2 O 4 ) 4 , Rb 4 Hf(C 2 O 4 ) 4 , and Cs 4 Hf-(C 2 O 4 ) 4 . Zr complexes both peroxide and oxalate to yield the ring structures (N-( C H 3 ) 4 ) 6 [ Z r 6 ( O 2 ) 6 ( O H ) 6 ( C 2 O 4 ) 6 ] , L i 1 2 [ Z r 8 ( O 2 ) 1 2 ( O H ) 4 ( C 2 O 4 ) 8 ] , K 18 [Zr 12 (O 2 ) 18 (OH) 6 (C 2 O 4 ) 12 ], and Rb 24 [Zr 16 (O 2 ) 24 (OH) 8 (C 2 O 4 ) 16]. The Zr ring nuclearity increases with countercation size, while Hf polymerization decreases with increasing countercation size. The Zr rings feature nine-coordinate face-sharing polyhedra in both solution and the solid state, unprecedented in Zr coordination complexes. These studies describe differentiating the coordination chemistry of Zr/Hf, exploiting simple aqueous reagents that could be further developed for aqueous synthesis of materials as well as challenging chemical separations.