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. 2021 Dec 6;60(23):18407-18422.
doi: 10.1021/acs.inorgchem.1c03001. Epub 2021 Nov 12.

Molecular Approach to Alkali-Metal Encapsulation by a Prussian Blue Analogue FeII/CoIII Cube in Aqueous Solution: A Kineticomechanistic Exchange Study

Miguel A Gonzálvez  1   2 , Paul V Bernhardt  1 , Mercè Font-Bardia  3 , Albert Gallen  2 , Jesús Jover  2   4 , Montserrat Ferrer  2   5 , Manuel Martínez  2   5
Affiliations

Molecular Approach to Alkali-Metal Encapsulation by a Prussian Blue Analogue FeII/CoIII Cube in Aqueous Solution: A Kineticomechanistic Exchange Study

Miguel A Gonzálvez et al. Inorg Chem. .

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.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Chart 1
Chart 1
Figure 1
Figure 1
(a) 1H NMR spectrum of the Sephadex G-25 eluate obtained from the crude mixture of the preparation of the lithium salt of the [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– cubic cage. (b) 7Li NMR spectrum of the same sample.
Figure 2
Figure 2
(a) DOSY NMR spectrum of the sample obtained from the crude mixture of the preparation of the lithium salt of the [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– structure. (b) 13C NMR spectrum of the same sample.
Figure 3
Figure 3
(a) Electronic spectra of the major component (i.e., void) of the lithium (black), sodium (red), and potassium (blue) salts of the [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– species in water. (b) Cyclic voltammograms of the FeIII/FeII region of the major component (i.e., void) of the lithium salt (black; the minor signal at ca. 500 mV is associated with the residual sodium salt of the species) and potassium salt (blue) of the [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– cubic cage.
Figure 4
Figure 4
Ball-and-stick representation of the cubic {{Li–OH2}⊂[{CoIII(Me3-tacn)}4{FeII(CN)6}4]} structure of the Li8{{Li–OH2}⊂[{CoIII(Me3-tacn)}4{FeII(CN)6}4]}·(ClO4)5·12H2O compound showing the encapsulated {Li–OH2}.
Figure 5
Figure 5
Relevant portal (a) and cavity (b) dimensions of the cubic cage of the Li8{{LiOH2}⊂[{CoIII(Me3-tacn)}4{FeII(CN)6}4]}·(ClO4)5·12H2O compound.
Figure 6
Figure 6
(a) Cyclic voltammograms of the FeIII/FeII signals of the void (black) and {Li–OH2}+-containing (blue) lithium salts of the [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– cubic cages. (b) Electronic spectra of the same samples.
Figure 7
Figure 7
Ball-and-stick representation of the cubic {{Na–OH2}⊂[{CoIII(Me3-tacn)}4{FeII(CN)6}4} structure of the Na3{{Na–OH2}⊂[{CoIII(Me3-tacn)}4{FeII(CN)6}4]}·22H2O compound showing the encapsulated {Na–OH2}.
Figure 8
Figure 8
Relevant portal (a) and cavity (b) dimensions of the cubic cage of the Na3{{Na–OH2}⊂[{CoIII(Me3-tacn)}4{FeII(CN)6}4]}·22H2O compound.
Figure 9
Figure 9
1H (top) and 23Na (bottom) NMR spectra of the Na3{{Na–OH2}⊂[{CoIII (Me3-tacn)}4{FeII(CN)6}4]}·22H2O crystals dissolved in D2O.
Figure 10
Figure 10
(a) Changes in the cyclic voltammogram of a 1 × 10–3 M solution of the sodium salt of the [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– cube (red) in a 0.20 M RbCl solution after 24 h (black). (b) Changes in the 1H (top) and 23Na (bottom) NMR spectra of the same species at 1 × 10–2 M in 1.0 M RbCl.
Figure 11
Figure 11
(a) [K+] dependence at 35 °C in aqueous solution of the values of kobs for exchange from the sodium salt of the [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– cubic cages. (b) Standard Eyring (top) or ln k versus P (bottom) plots for changes of the value of limk for the sodium-to-potassium cation-exchange process with temperature and pressure.
Figure 12
Figure 12
(a) UV–vis spectral changes observed upon reaction of a 2 × 10–4 M aqueous solution of the sodium salt of the [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– cubic cage with RbCl (0.15 M) at 25 °C. (b) Effect of [Na+] added on the value of the observed rate constants for the confined cation exchange at [[{CoIII(Me3-tacn)}4{FeII(CN)6}4]4–] = 1 × 10–4 M: (top) {NaOH2}+ to Rb+ at 0.15 M RbCl (33 °C); (bottom) {NaOH2}+ to K+ at 0.005 M KCl (35 °C). Note the 10-fold difference in the [Na+] concentration scale.
Figure 13
Figure 13
(a) Time-resolved 1H NMR changes observed upon solution of a sample of {{Li–OH2}⊂[{CoIII(Me3-tacn)}4{FeII(CN)6}4]} species in 0.10 M NaCl. (b) UV–vis spectral changes observed for a 5 × 10–5 M sample of the same compound with 0.10 M NaCl and 0.05 M LiCl at 35 °C.
Figure 14
Figure 14
(a) 1H NMR spectral changes observed after four and eight repetitions of Sephadex DEAE A-25 LiClO4-eluted chromatography of a sample of the sodium salt of the [{CoIII(Me3-tacn)}4{FeII(CN)6}4]4– cubic cage (see the text). (b) 13C NMR spectrum of the intermediate sample (Sephadex × 4).
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