Abstract
The preparation of a series of alkali-metal inclusion complexes of the molecular cube [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– (Me3-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–OH2}+ (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–OH2}+ 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–OH2}+ units upon adsorption of the cube on an anion exchange resin and subsequent washing with a Li+ solution.
Highlights
Colored transition-metal mixed-valence complexes such as Prussian Blue have attracted the attention of both the chemical and general public communities for centuries
As mixed-valence compounds, Prussian Blue analogues (PBAs) have been used academically for the establishment of fundamental mixed-valent classifications related to the electronic coupling between the metal centers and the symmetry-allowed inner-sphere optical electron transfer occurring between them.[1−8] In reactivity aspects, these species have been utilized as cheap, metal-abundant catalysts in water oxidation processes[9,10] and electrochemical applications.[11]
The use of these types of complexes in the development of photomagnetic switching materials has been developed by several groups.[12−16] both anionic Prussian Blue and PBAs are known to act as hosts for cationic guests in general, which has led to various applications.[14,17−24] Their use as sequestering agents has been explored, their ion-exchange properties in aqueous solution are difficult to measure
Summary
Colored transition-metal mixed-valence complexes such as Prussian Blue (or ferric ferrocyanide) have attracted the attention of both the chemical and general public communities for centuries. As mixed-valence compounds, Prussian Blue analogues (PBAs) have been used academically for the establishment of fundamental mixed-valent classifications related to the electronic coupling between the metal centers and the symmetry-allowed inner-sphere optical electron transfer occurring between them.[1−8] In reactivity aspects, these species have been utilized as cheap, metal-abundant catalysts in water oxidation processes[9,10] and electrochemical applications.[11] Recently, the use of these types of complexes in the development of photomagnetic switching materials has been developed by several groups.[12−16] both anionic Prussian Blue and PBAs are known to act as hosts for cationic guests in general, which has led to various applications.[14,17−24] Their use as sequestering agents has been explored, their ion-exchange properties in aqueous solution are difficult to measure. The use and applicability of PBAs in medicinal chemistry have been recently reviewed.[27]
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