Aqueous Stability of Zirconium Clusters, Including the Zr(IV) Hexanuclear Hydrolysis Complex [Zr₆O₄(OH)₄(H₂O)₂₄]¹²⁺, from Density Functional Theory

Abstract

Framework materials constitute a broad family of solids that range from zeolites and metal–organic frameworks (MOFs) to coordination polymers. The synthesis of such network structures typically rely on precursor molecular building blocks. As an example, the UiO-66 MOF series is constructed of hexanuclear [Zr6O4(OH)4(CO2)12] cluster nodes and linear carboxylate linkers. Unfortunately, these Zr MOF cluster nodes cannot currently be manufactured in a sustainable way, motivating a search for “green” alternative synthesis methods. Stabilizing the hexanuclear Zr­(IV) cluster (i.e., the hexamer, {Zr6 12+}) without the use of organic ligation would enable the use of environmentally friendly solvents such as water. The Zr­(IV) tetranuclear cluster (i.e., the tetramer, {Zr4 8+}) can be stabilized in solution with or without organic ligands, yet the hexamer has yet to be synthesized without supporting ligands. The reasons why certain zirconium clusters are favored in aqueous solution over others are not well understood. This study reports the relative thermodynamic instability of the hypothetical hexamer {Zr6 12+} compared to the ubiquitous {Zr4 8+} tetramer. Density functional theory calculations were performed to obtain the hydrolysis Gibbs free energy of these species and used to construct Zr Pourbaix diagrams that illustrate the effects of electrochemical potential, pH, and Zr­(IV) concentration. It was found that the aqueous {Zr6 12+} hexamer is ∼17.8 kcal/mol less stable than the aqueous {Zr4 8+} tetramer at pH = 0, V = 0, and [Zr­(IV)] = 1 M, which is an energy difference on the order of counterion interactions. Electronic structure analyses were used to explore trends in the highest occupied molecular orbital–lowest unoccupied molecular orbital gap, frontier molecular orbitals, and electrostatic potential distribution of these clusters. The evidence suggests that the aqueous {Zr6 12+} hexamer may be promoted with more strategic syntheses incorporating minimal ligands and counterions.

Oxo-Cluster-Based Zr/HfIV Separation: Shedding Light on a 70-Year-Old Process

“…Group IV Zr 4+ and Hf 4+ are strong Lewis acids that promote hydrolysis, even at pH < 2, giving rise to a variety of oxo-cluster-based materials . Both tetravalent metals have similar ionic radii and prefer coordination of seven to nine bonds .…”

“…Group IV Zr 4+ and Hf 4+ are strong Lewis acids that promote hydrolysis, even at pH < 2, giving rise to a variety of oxocluster-based materials. 1 Both tetravalent metals have similar ionic radii and prefer coordination of seven to nine bonds. 2 A few percent of hafnium is substituted in all earth materials containing zirconium, a testament to the proclamation that these are the two most similar elements on the periodic table.…”

The Chemistry of Zirconium/Carboxylate Clustering Process: Acidic Conditions to Promote Carboxylate-Unsaturated Octahedral Hexamers and Pentanuclear Species

“…In this sense, a precise control of the amount of water is crucial for the first steps of the formation of these entities. 25 On the other hand, the acidity of the reaction media exerts a strong influence on the deprotonation of the coordinated water molecules to afford bridging hydroxide and oxide anions but also on the readiness of the carboxylic ligands to coordinate to the metal centers. 26 Herein, we report on several discrete zirconium entities ranging from the ubiquitous hexanuclear [Zr 1).…”

“…A crucial stage of the setup of the Zr–O/OH polynuclear entities resides on the oxygen source from which these species emerge. In this sense, a precise control of the amount of water is crucial for the first steps of the formation of these entities . On the other hand, the acidity of the reaction media exerts a strong influence on the deprotonation of the coordinated water molecules to afford bridging hydroxide and oxide anions but also on the readiness of the carboxylic ligands to coordinate to the metal centers …”

Shedding light on intrinsic characteristics and optical properties of novel selenite and tellurite crystals ZrSe2O6, HfSe2O6 and HfTe3O8

“…Moreover, there are 3 kinds of ternary zirconium selenite compounds, namely, ZrSe 2 O 6 , ZrSe 2 O 7 , Zr 4 (SeO 3 )(SeO 4 ) 7 , 17 and 4 kinds of hydrates of zirconium selenite reported in the Zr–Se–O system. 18 Wildner, M. et al reported the preparation and single crystal structure analysis of only 10 μm ZrSe 2 O 6 microcrystals. 19 Performance and application studies of Zr/Hf selenites or tellurites are still limited.…”