Molecular Approach to Alkali-Metal Encapsulation by a Prussian Blue Analogue Fe II /Co III Cube in Aqueous Solution: A Kineticomechanistic Exchange Study

• Published in: Inorganic chemistry 2021-12, Vol.60 (23), p.18407-18422
• Main Authors: Gonzálvez, Miguel A, Bernhardt, Paul V, Font-Bardia, Mercè, Gallen, Albert, Jover, Jesús, Ferrer, Montserrat, Martínez, Manuel
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.1c03001

The preparation of a series of alkali-metal inclusion complexes of the molecular cube [{Co (Me -tacn)} {Fe (CN) } ] (Me -tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), a mixed-valent Prussian Blue analogue bearing bridging cyanido ligands, has been achieved by following a redox-triggered self-assembly process. The molecular cubes are extremely robust and soluble in aqueous media ranging from 5 M [H ] to 2 M [OH ]. All the complexes have been characterized by the standard mass spectometry, UV-vis, inductively coupled plasma, multinuclear NMR spectroscopy, and electrochemistry. Furthermore, X-ray diffraction analysis of the sodium and lithium salts has also been achieved, and the inclusion of moieties of the form {M-OH } (M = Li, Na) is confirmed. These inclusion complexes in aqueous solution are rather inert to cation exchange and are characterized by a significant decrease in acidity of the confined water molecule due to hydrogen bonding inside the cubic cage. Exchange of the encapsulated cationic {M-OH } or M units by other alkali metals has also been studied from a kineticomechanistic perspective at different concentrations, temperatures, ionic strengths, and pressures. In all cases, the thermal and pressure activation parameters obtained agree with a process that is dominated by differences in hydration of the cations entering and exiting the cage, although the size of the portal enabling the exchange also plays a determinant role, thus not allowing the large Cs cation to enter. All the exchange substitutions studied follow a thermodynamic sequence that relates with the size and polarizing capability of the different alkali cations; even so, the process can be reversed, allowing the entry of {Li-OH } units upon adsorption of the cube on an anion exchange resin and subsequent washing with a Li solution.