A new {Fe₄Co₄} soluble switchable nanomagnet encapsulating Cs⁺: enhancing the stability and redox flexibility and tuning the photomagnetic effect.

Abstract

We report a new cyanide-bridged Cs⊂{FeCo} box, a soluble model of photomagnetic Prussian blue analogues (PBAs). The Cs ion has a high affinity for the box and can replace the K ion in the preformed K-cube. This exchange is kinetically impeded at room temperature but is accelerated by heating and using the 18-crown-6 ether. The inserted Cs ion confers a high robustness to the cube, which withstands boiling, as shown by variable-temperature NMR studies. The stability of this model complex in solution allows the probing of the electronic interaction between the alkali ion and the cyanide cage by using various techniques. These interactions are known to play a role in the photomagnetic behaviour of PBAs. Firstly, the Cs NMR spectroscopy proves that there is an electronic communication between the encapsulated alkali ion and the cyanide cage. The measured up-field signal, observed at ca. -200 ppm at 300 K, reveals that a certain amount of spin density is transferred through the bonds from the paramagnetic Co(ii) ion to the encapsulated cation. Secondly, cyclovoltammetric studies show that the nature of the inserted ions affects the redox properties of the cage and influences the electronic communication between the metal ions. However, the differences in the electrochemical properties of the K-cube and the Cs-cube remain moderate. As the switching properties are influenced by the redox potential of the Fe and Co centers, similar photomagnetic behaviour is observed, with both of them being highly photomagnetic. This result contrasts strikingly with previous studies on the 3D polymeric PBAs, where the PBAs with a high amount of Cs show poor photomagnetic behaviour. In that case, cooperative behaviour likely influences the switching properties. Finally, EPR spectroscopy shows that the K-cube is more anisotropic than the Cs-cube. This difference is reflected in the changes occurring in the slow magnetic relaxation (single molecule magnet behaviour) observed in the two cubes.

Electron Transfer in the Cs⊂{Mn4Fe4} Cubic Switch: A Soluble Molecular Model of the MnFe Prussian‐Blue Analogues

“…The strong and negative paramagnetic chemical shift is due to the spin-density transfer from the paramagnetic cage to the intercalated Cs + ion, as already observed in related systems. [24] The addition of CsClO 4 salt to the solution allows confirming this assignment, as it leads to the occurrence of a new signal in the diamagnetic range at 32 ppm.…”

“…In particular, we demonstrated that the encapsulation of a Cs + ion within the anionic {Co 4 Fe 4 } cube causes the entire complex to be remarkably stable. [21] Similar A&{Ni 4 Fe 4 } cubes could also be obtained by following a similar synthetic strategy. [22] Our present work intends to extend these results toward other switchable molecules that could show interesting properties such as electro-, thermo-, or photo-induced switching of the electronic states.…”

“…[18,19] Recently, in search of original molecular switches, we focused our research on cyanide-bridged cubic molecules, A&{[Co-( pz Tp)] 4 [Fe(Tp)(CN) 3 ] 4 } (noted A&{Co 4 Fe 4 } herein, where Tp = hydrotris(pyrazol-1-yl)borate, pz Tp = tetrakis(pyrazol-1yl)borate, and A is an alkali ion). [20,21] Interestingly, the photomagnetic A&{Co 4 Fe 4 } complexes were shown to be stable in organic solvents. In particular, we demonstrated that the encapsulation of a Cs + ion within the anionic {Co 4 Fe 4 } cube causes the entire complex to be remarkably stable.…”

“…Cs&{Mn 4 Fe 4 } crystallizes in the RÀ3 space group and exhibits a heterocubane structure, which is similar to the one previously observed in the A&{Co 4 Fe 4 } derivatives. [20,21] The unit cell is composed of two crystallography non-equivalent cubes, six cubes a and one cube b. In cube a (Figure 1 a), the Fe and Mn ions are crystallographically independent and well localized, as are the Tp and pz Tp ligands.…”

“…The respective positions of the Mn II and Mn III sites in the cube are not well identified (or localized), as previously observed for the Co III /Co II sites in a related Cs&{Co III 3 Co II Fe II 4 } cube. [24] However, the position of the caesium ion, which is closer to some faces of the cube, allows to suggest the probable positions of the trivalent M III and divalent M II ions. The larger the distance between the caesium and the face the more trivalent M III ions are present.…”