# Cyanide cubic cages: soluble molecular counterparts of prussian blue analogues

Jana Glatz, Volodymyr Malytskyi, Rodrigue Lescouëzec

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Jana Glatz, Volodymyr Malytskyi, Rodrigue Lescouëzec. Cyanide cubic cages: soluble molecular counterparts of prussian blue analogues. Coordination Chemistry Reviews, 2026, 549, 10.1016/j.ccr.2025.217363. hal-05393795

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Review

# Cyanide cubic cages: soluble molecular counterparts of prussian blue analogues

Jana Glatz*,<fnref n="1" /> ID, Volodymyr Malytskyi* ID, Rodrigue Lescouëzec ID

Institut Parisien de Chimie Moléculaire, CNRS UMR 8232, Sorbonne Université, 4 place Jussieu, Paris 75005, France

ARTICLE INFO

This review is dedicated to the memory of Professor Miguel Julve; the scientist, colleague, and friend.

Keywords:

Cyanide-bridged assembly
Molecular PBAs
Heterometallic complexes
Molecular cages
Ion encapsulation
Electrochromism
Molecular magnetic materials
Molecular switches
Surface deposition

A B S T R A C T

Herein, we present a comprehensive review focusing on cyanide-bridged cubic complexes that have emerged over the last years as soluble molecular models of the well-known three-dimensional Prussian Blue Analogues (PBAs). We first describe the synthetic strategies and highlight the peculiarities of molecular cubes in comparison to the inorganic polymeric PBAs. Then, we review solution studies carried out with these soluble counterparts of PBAs, considering in particular supramolecular aspects. We also summarize efforts to transfer the original properties of PBAs to cubic complexes by highlighting both their similarities and specificities. Furthermore, outstanding electronic properties such as the access of multiple electronic states resulting in multiple bands electrochromism, or the stimuli-induced switching of electronic states associated with a change in physical properties are surveyed. Finally, recent attempts at surface deposition of functional molecular cubes or their integration into devices are presented.

1. Introduction

1.1. *Structure and properties of PBAs: A brief overview*

Around 1703, a Berliner draper, J. Diesbach accidentally discovered a cheap, dark blue pigment, “Prussian blue”, [1,2] which soon became popular among painters and is still found in many works of art today. Prussian blue (PB) is actually a ferric ferrocyanide of general formula K₄xFe₄[FeII(CN)₆]₃+x·14–16 H₂O, (0 ≤ x ≤ 1) and is recognized as the first synthetic coordination polymer. The first insights into its structure became possible with the advent of X-ray diffraction methods, [3] but a first detailed structure of Prussian blue was only proposed in 1977 by Ludi and Güdel. [4,5] Consequently, one year later, the first electro-chemical study of a PB thin film was published, revealing its electro-chromic behaviour. [6]

Importantly, the iron ions can be exchanged with other d-block metal ions, leading to a huge variety of so-called Prussian Blue Analogues (PBAs) of general formula A_yM'₄[M(CN)₆]_{4-z}□z·nH₂O where M' and M

are transition metal ions. A is a monovalent cation compensating a charge of the coordination frame, and □ denotes a [M(CN)₆] vacancy. [7] From a crystallographic point of view, most of PBA's structures can be described using a face-centred unit cell where M' cation are located in its nodes, hexacyanide metal anions in the octahedral sites (with a variable number of vacancies, z per unit cell) and A⁺ cation in some of the tetrahedral sites (Fig. 1). The synthesis conditions strongly influence the number and distribution of vacancies in the structure and, consequently, the properties of the resulting material. [8]

Indeed, as shown by paramagnetic NMR [9–11] or X-ray diffraction studies, [12] the local structure can be complex, as several different M'(NC-M)₆-p(OH₂)p coordination spheres of the metal ions M' are detectable, associated with the existence of distinct vacancies, even in stoichiometric compounds such as Mₕ[FeIII(CN)₆]₂. [9,10,13–15] In contrast, the coordination sphere of M is always M(CN-M')₆, but the presence of a variable amount of A⁺ in their vicinity may also alter their local electronic properties.

The structural complexity of PBAs may render structure-property

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: jana.glatz@uv.es (J. Glatz), volodymyr.malytskyi@sorbonne-universite.fr (V. Malytskyi), rodrigue.lescouezec@sorbonne-universite.fr (R. Lescouëzec).</footnote>

<footnote n="1">¹ Present address: Universitat de Valencia - Institut de Ciència Molecular, Catedrático José Beltrán Martínez 2, 46980 Paterna, Spain</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Schematic representation of FeCo PBA 3D network. Fe ions are depicted in orange, Co in dark blue, C in grey and N in light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

correlations challenging; however, it can be controlled to some extent through synthesis conditions. The success of PBAs in materials science arises from their easy syntheses, and their chemical versatility: the nature and amount of M, M' and A can be adjusted to tune their properties. Moreover, two peculiar features turned out to be beneficial for various applications. The first feature, contributing to the majority of the properties, is the nature of the cyanide linker, which effectively promotes electronic and magnetic communication between neighbouring M and M', [16,17] enabling electron transfer as well as delocalization in some cases. For example, the deep blue colour of the original PB arises from the metal-metal charge transfer in the Fe<sup>II</sup>-CN-Fe<sup>III</sup> bimetallic pair. Furthermore, the linker transmits magnetic exchange interaction between the spins of two paramagnetic ions resulting in interesting magnetic properties (see below). Secondly, PBAs are nanoporous materials due to the presence of varying amounts of hexacyanide vacancies and unfilled tetrahedral interstitial sites. The latter, in fact, are interconnected, forming lattice channels approximately 3.2 Å in width. [18]

The adjustable structural and electronic features of PBAs have made them versatile platforms for a broad range of applications in materials science. Since the early sixties, they have been used as sequestering agents for radioactive caesium-137 [19–21] or toxic thallium. [22–24] Subsequently, PBAs have been explored for a wide range of applications, including (bio)sensing, [25–27] (hydrogen) gas storage, [28,29] molecular sieves, [30,31] heterogeneous electro- or photo-catalysis, [32–35] biomedical uses, [36,37] electrochromic [38,39] devices and nanoscale systems for information storage. [40–43] In recent years, research focusing on energy storage [44–46] application, battery materials [47–51] and supercapacitors [52,53] has become particularly active. Moreover, the physical and chemical properties of PBAs are strongly influenced by the nature and oxidation state of the metal ions M and M'. As previously mentioned, electronic communication between these metal centres underlies processes such as metal-metal charge transfer and light absorption. By combining specific metal ions, controlling their stoichiometry and tuning the amount and nature of A⁺ ions, the excited electronic state resulting from such electron transfer can be rendered metastable. This enables the design of responsive materials that exhibit a range of functionalities including thermochromism, [54] photomagnetism, [55–58] and electrochromism. [59,60].

## 1.2. From three-dimensional PBAs to discrete molecular cubes

In the last decades, research on PBAs has not only focused on the investigation of new properties, but also on the development of methods to obtain PBAs with original nanoscopic structures. [36] Interestingly, Catala et al. demonstrated that epitaxial growth could be controlled on surfactant-free PBA nanoparticles leading to core-shell and multi-shell nanoparticles. [61] Later, hollow particles from nanometre to micrometre sizes were obtained using emulsion [62] and selective etching in acidic conditions [63,64] or from treatments of core-shell nanoparticles. [65,66] Such nano-structures have been investigated for a variety of applications such as catalysis [67], electrochemical energy storage [68] and biomedical applications. [69,70] In these studies, the impact of downscaling on the properties of PBAs has been particularly scrutinised. [71] Meanwhile, molecular chemists have continuously focused on designing a wide variety of lower dimensional cyanide compounds. [72] The development of “molecular PBAs” is often motivated by the idea of designing molecular models that elucidate the origin of the distinctive electronic properties observed in PBAs, and above all, to assess whether they can be preserved at the molecular scale. Additionally, this research is driven by the pursuit of novel functionalities that may arise from lower dimensionality, for example the occurrence of single molecule magnet behaviour. At this stage, it is worth noting that cyanido-cages exhibit distinctive features compared to many other polymetallic cages. The specific electronic communication enabled by the cyanide linker gives rise to unique properties. For example, the access to reversible photo-induced electron transfer (associated with photomagnetism) is typical of the M-CN-M' unit, and not found with any other bridging ligands. The occurrence of remarkable electrochromic properties with a controllable access to multiple optical signatures, linked to distinct optical transitions, also arises from the electronic properties of the cubic cage. Here the electronic communication between first and second neighbours, gives rise to two distinct first-neighbour MMCT bands and two distinct second-neighbour MMCT bands, in addition to two LMCT and two MLCT bands that are associated with the metallic subunits. [73] It can also be noted here that reducing the dimensionality down to the single-cage level offers distinct advantages compared to recent advances in nanoparticle PBAs. While nanoparticle systems have demonstrated remarkable progress – with tunable size, morphology, and surface chemistry leading to enhanced catalytic, redox, and transport properties – their behaviour remains strongly dependent on extrinsic factors such as particle size, shape, and interparticle interactions. In contrast, molecular PBAs represent the ultimate dimensional reduction of Prussian Blue frameworks, isolating the fundamental electronic units of the lattice and enabling precise control over oxidation state, coordination geometry, and electronic coupling. This molecular-level control not only disentangles intrinsic electronic effects from collective lattice phenomena but also enables the rational design of discrete systems that exhibit new or enhanced functionalities, including reversible redox switching and finely tunable charge-transfer behaviour. Together, molecular and nanoparticle PBAs provide complementary pathways for exploring and harnessing the rich chemistry of cyanide-based coordination frameworks.

To obtain discrete molecules, the metal ions must be partially blocked by ancillary ligands to reduce the number of available coordination sites and to limit the growth of extended structures. There are several possibilities for doing this and the adopted shape of the cyanide cluster depends on the available coordination sites (which in turn depends on the denticity of the blocking ligands as well as their stoichiometry). A typical strategy involves the synthesis of partially blocked cyanide precursors of formula [M(L)(CN)x]y⁻ developed for example by Julve, [74] Dunbar, [75] Long, [76,77] and co-workers, which are then used to react with another metal ion. [78]

A wide variety of cyanide-bridged metal clusters featuring diverse topologies and properties has been reported. These range from large polymetallic polyhedra with sophisticated structures [76,79–88] to
smaller clusters such as molecular pairs [89], squares [90–93] and cubes [94–97] which can be considered as low-dimensional elementary building blocks of PBA polymer networks. Fig. 2 shows some selected examples.

In this review we focus on the octametallic complexes with cubic structure, which have emerged at the turn of century and are now part of the research interest of different groups. They consist of four metal centres linked to the carbon donor atom of the cyanide [M(L)(CN)₃] alternated with four other metal centres linked to the nitrogen donor atom of the cyanide, [M'(L')(NC)₃] (Fig. 2 right). All eight corners are surrounded by blocking ligands L and L' of the same or different nature. Just like the polymeric PBAs, these molecular cubes can include small guest ions or molecules, noted A. For the purpose of an easier reading, the cubic molecules in this review will hereafter be denoted by the formula A ⊂ {[M(L)(CN)₃]₄ [M'(L')]₄} where the first metal ion M is linked to the carbon atom of the cyanide and the second M' is connected to nitrogen. In the following, we will first discuss the different synthetic pathways that have been explored, leading to molecular cubic systems with various metals M and M' as well as ligands L and L'. Then, we will further describe the main properties that have been investigated over the years. The focus will mainly lie on soluble molecular systems, as this is one of the reasons why they are so interesting compared to their insoluble parent polymeric PBA materials but does include examples of cubic molecules that have only been characterised in the solid-state.

## 2. Synthetic strategies: Tips and tricks

### 2.1. Choice of the blocking ligands

The general synthetic strategy for molecular cubic models closely resembles that of their parent compounds: combining metal cations (acting as Lewis acids) coordinated by labile solvent molecules with preformed cyanidometallates (playing the role of Lewis bases). However, to limit the growth of 3D networks and to ensure the formation of the cubic structure, careful selection of precursors and blocking ligands is essential. Indeed, the topology of the molecular species resulting from the reaction of a capped cyanometallate with a (capped) solvated metal cation depends on both the number and spatial arrangement of the available binding sites of the cyanometallate and the capped solvated metal cation. To obtain a cubic structure, at least three binding sites, arranged approximately perpendicular to one another, are necessary on each octahedral synthon. At the same time, to prevent the formation of extended 3D networks, three coordination sites on at least one synthon should be blocked by a non-labile ligand. This requirement highlights the advantage of using tridentate ligands that coordinate in a facial (fac-) mode. Scheme 1 illustrates four different classes of ligands that have been employed in the synthesis of cubic complexes to date.

### 2.2. Synthetic routes.

#### 2.2.1. General considerations

One of the first cubic boxes synthesised, dating back to 1998, is the organometallic, diamagnetic complex {[Co(Cp)(CN)₃]₄ [Rh(Cp*)]₄}⁴⁺ (Cp = C₅H₅; Cp* = C₅Me₅). [97] Here, both the cyanometallate and the metal cation are capped with cyclopentadienyl-based blocking ligands, which coordinate in a pentahapto (η⁵) binding mode, formally occupying three facial coordination positions of the octahedral coordination polyhedron. In this case, the cube was obtained via a stepwise synthetic approach. First, a square intermediate {[Co(Cp)(CN)₃]₂[Rh(Cp*)Cl]₂}⁴⁺ complex was isolated, and the cage {[Co(Cp)(CN)₃]₄ [Rh(Cp*)]₄}⁴⁺ was formed through chloride abstraction using AgPF₆ (Scheme 2). The resulting charged cube is stable as a PF₆ salt in the polar solvent CH₃CN, under inert atmosphere. To our knowledge, this synthetic route, involving a preformed, inert square complex containing low-spin Co³⁺ and Rh³⁺ ions, is quite unique. It is certainly favoured by the location of the terminal cyanide ligands on the same face of the square complex. This topology contrasts with that observed in most of the cyanide square complexes that have been recently reported, which exhibit an inversion centre and terminal cyanides oriented in opposite directions. [72,90] The peculiar topology of the Cp-based square is probably due to steric hindrance between Cp and Cp* ligands, which forces them into trans- positions relative to each other.

In all other reported examples, the octametallic complexes have been obtained directly via one-pot synthesis from monometallic precursors. Three main synthetic strategies have proven successful (Scheme 3):

A. reacting fac-capped cyanidometallate [M(L)(CN)₃]ⁿ⁻ with fac-capped tris-solvated cation [M'(L')(S)₃]ⁿ⁺ yielding a {[M(L)]₄ [M'(L')]₄} cube directly;

B. reacting fac-capped cyanidometallate [M(L)(CN)₃]ⁿ⁻ with an hexasolvated cation [M'(S)₆]ⁿ⁺ initially forming a {[M(L)]₄ [M'(S)₃]₄} cube (sometimes called pro-cube) before adding the ligand L' in a second step;

C. reacting hexacyanidometallate [M(CN)₆]ⁿ⁻ with fac-capped tris-solvated cation [M'(L')(S)₃]ⁿ⁺ yielding a {[M(CN)₃]₄ [M'(L')]₄} cube.

Herein ligand L always denotes a tripodal ligand, ligand L', coordinated to M', is often also tridentate but in some cases may signify a combination of bidentate and monodentate ligands or even three monodentate ligands (vide infra).

Moreover, it should be emphasised that fulfilling the aforementioned topological requirements for the precursors does not, by itself, guarantee the assembly of cubic structures. Synthetic conditions are crucial. The cube may be regarded as a polymacrocyclic compound, with each of its

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Some molecular models of PBAs: {Fe-Co} pair [89] (left), {Fe₂-Co₂} square [92] (middle) and {Fe₄-Co₄} cube [96] (right). Fe-ion in orange and Co-ions in blue, N in light purple, B in pink and C in grey. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Structures, names and abbreviations of some of tridentate ligands used in the preparation of cubic octametallic complexes and cited in this article.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** The {[Co(Cp)(CN)₃]₄ [Rh(Cp*)]₄}⁴⁺ cube synthesis from two square complexes. Co ion in blue, Rh ion in turquoise, N in light purple and C in grey.</figcaption></figure>

precursors possessing multiple reactive centres, several M-CN donor atoms on the one hand, and several labile M'-solvent sites on the other. Such reactants can lead to the formation of polymeric species. [98] Typically, the macrocyclic product is kinetically favoured at lower concentrations whereas polymeric species dominate at higher concentrations. However, in the case of coordination-bond-based assemblies, excessively low concentrations also promote dissociation of polymetallic complexes. Therefore, the synthesis of cubic complexes requires a finely optimised concentration of reagents, high enough to shift the equilibrium toward complex formation and low enough to avoid polymerisation. The viable reaction window also depends on the nature of M', the solvent, and the temperature. The template effect is known to strongly favour macrocyclisation over polymerization by preorganising the synths. Since a cubic cage consists of negatively charged cyanides, a template must be cationic but not interfere with coordination. Typically, this role is fulfilled by alkali metal or ammonium cations. Finally, obtained complexes are often charged and the nature of the counterion can affect crystallisation, thereby influencing isolation and overall yields. [99,100] In the following, the synthesis of several cubes will be

discussed in further detail.

### 2.2.2. Syntheses of cubes from two fac-complexes precursors

In the late nineties and around 2000, most researchers reported the use of synthetic strategy (A). For instance, one of the first cubes, reported by Long et al. [95] in 1998, is obtained by mixing [Co(tacn)(CN)₃] with either [Cr(H₂O)₃(tacn)]³⁺ or [Co(H₂O)₃(tacn)]³⁺ precursors (tacn is a cyclic tris-amine ligand, Scheme 1) in water, followed by boiling to dryness. Yields of 46 % {[Co(tacn)(CN)₃]₄ [Cr(tacn)]₄}¹²⁺ and 86 % {[Co(tacn)(CN)₃]₄ [Co(tacn)]₄}¹²⁺ were reported. The use of such extreme conditions suggests that direct formation of a highly charged cube from all-inert precursors at room temperature is kinetically disfavoured. In this particular case, however, the combination of hydrophilic tacn ligands and inert metal centres renders water a viable medium – an exception rather than the rule in the synthesis of such highly charged cubic assemblies.

In 1999, Rauchfuss' group published a direct synthesis of the {[Rh(Cp*)(CN)₃]₄ [Mo(CO)₃]₄}⁴⁻ cubic box using mesitylene-based precursor [Mo(η⁶-C₆H₃Me₃)(CO)₃]. [101] Here, the presence of labile
<figure><img src="image_7.png" /><figcaption>**Scheme 3.** Three different approaches to the one-pot synthesis of cyanide bridged cubes. The blocking trident ligands are schematized by grey and cyan triangles while labile coordination sites are represented by red spheres. Orange and blue spheres represent two different metal centres.</figcaption></figure>

π-ligand on the Mo�0 metal centre significantly enhanced reaction kinetics, allowing complete formation of the cubic complex within minutes at room temperature in acetonitrile. Initially, the pure product was isolated in low yield only in the presence of a potassium ion, which had been inadvertently introduced as an impurity in the (Et₄N)[Rh(Cp*) (CN)₃] precursor. This unexpected encapsulation of K⁺ was later rationalized and addition of a K⁺ source led to an improved 64 % yield of the cubic complex. Notably, the use of Cs⁺ further increased the yield to 73 %, forming Cs⁺ ⊂ {[Rh(Cp*)](CN)₃}₄ [Mo(CO)₃]₄⁴⁻ cube. The authors highlighted two key points in this study: 1. The encapsulated alkali cation greatly stabilizes such anionic cages and 2. It plays a crucial role in the formation of the cubic box, as polymeric species form in its absence. [102] Note that all these syntheses of inert cubes featuring a moderate overall charge (3+) were carried out in polar aprotic solvent.

Subsequently, the same group expanded their investigation on the role of the inserted cation, demonstrating that it acts as a template, pre-organising reagents to form an octametallic cage and greatly improving reaction kinetics. [103] It is worth noting that besides K⁺ and Cs⁺, such templating effects can also be achieved by other alkali metal ions, Tl⁺ and organic ammonium cations, notably NH₄⁺, MeNH₃⁺, and EtNH₃⁺. While NH₄⁺, MeNH₃⁺ are quite small (the van der Waals volume of the MeNH₃⁺ is 43.3 Å³) and remain within the octametallic cage, EtNH₃⁺ (V = 60.2 Å³) leaves the cavity before completion, enabling the synthesis of the empty cube {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₄}, a structure otherwise inaccessible without the use of templating cations. The templating effect was further confirmed in our group on Tp-based cubes as well as by Martinez et al. who studied tacn-based cubic cages. [104,105]

Part of the Rauchfuss groups' research was particularly interested in the mechanism of the box formation, including the synthesis of “defect” cubic structures lacking one M' cation. These have been prepared by reacting four equivalents of [Co(Cp)(CN)₃]⁻ or [Rh(Cp*)](CN)₃]⁻ with three equivalents of [Ru(Cp*)(NCCH₃)₃]⁺ in presence of Cs⁺ in acetonitrile. [106] The neutral charge of the resulting species may enhance their stability in organic solvents (note that some NMR spectra were carried out in the less polar aprotic solvent, THF). Interestingly, with three terminal CN groups, such defect boxes can accommodate one additional transition metal cation and pave the way toward more elaborated heterotetrametallic Cs{M₄-M'₃M''} cubes, such as Cs⁺ ⊂

[[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₃[Rh(Cp*)]]₃⁺. It is obtained by reacting Cs⁺ ⊂ {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₃}⁻ with 1 equivalent of [Rh(Cp*)]₂⁺. In contrast, Rh(Cp*)-based “defect” cubes Cs⁺ ⊂ {[Rh(Cp*) (CN)₃]₄ [Ru(Cp*)]₃}⁻ and NH₄⁺ ⊂ {[Rh(Cp*)(CN)₃]₄ [Ru(Cp*)]₃}⁻ were found to be unreactive. The authors suggested that the increased steric hindrance of the Cp* ligand likely contributes to this inertness.

In further works, the scope of metals reacting with the “defect” cube was extended. By mixing Cs⁺ ⊂ {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₃}⁻ with [Cu(NCCH₃)₄]PF₆, AgPF₆, [Ni(NCCH₃)₆](PF₆)₂ or [Fe(NCCH₃)₆] (PF₆)₂, the following heterotetrametallic cubes were isolated: [107] Cs⁺ ⊂ {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₃[Cu(NCCH₃)₄]₁}, Cs⁺ ⊂ {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₃[Ag(NCCH₃)₄]₁}, Cs⁺ ⊂ {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₃[Ni(NCCH₃)₃]₁}⁺, and Cs⁺ ⊂ {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₃[Fe(NCCH₃)₃]₁}⁺. These complexes feature labile CH₃CN ligands that can be readily substituted by phosphine- or thiol-based ligands in an additional step as demonstrated by the authors. (Scheme 4).

<figure><img src="image_8.png" /><figcaption>**Scheme 4.** Synthesis of a Co-Ru-Ni-Cs heterotetrametallic cube through the preparation of a cube with a missing corner. Co ions are depicted in blue, Ru ions in turquoise, Ni ion in light green, N in light purple and C in grey.</figcaption></figure>
Among the first reported cubes, some exhibit slightly different topologies, with alternating octahedral and pseudo-tetrahedral sites at neighbouring metal vertices. [108,109] The cluster geometry is presumed to be governed by the steric effects of the capping ligands. Indeed, most of the described cubic molecules typically have C-M-C and N-M'-N angles close to 90° (with distortions not exceeding 2°-3°). However, the use of bulky ligands can change this general tendency. For instance, Dunbar et al. [110] reported the synthesis of neutral cubes (in acetonitrile) with [Re(triphos)(CN)₃]⁻ corners where C-Re-C angles are around 85° due to the bulky trisphosphine ligand. Smaller C-Re-C angles lead to an increase in the N-M'-N angles to approximately 100°, forcing the second metal cation (MnII, FeII, CoII, NiII or ZnII) to adopt a tetrahedral rather than octahedral coordination geometry.

The main drawback of the straightforward method (A) is that it requires the preparation of two pre-formed cube corners, each typically synthesised via multiple steps. In contrast, alternative approaches (B) or (C) require only one precursor to be synthesised while the second component is easily available, whether it be the hexacyanometallate or the solvated cation.

### 2.2.3. Syntheses of cubes from hexacyanoferrate complex

Regarding method (C), to our knowledge, only Martinez's group has recently published a few examples. [105,111,112] Reaction of the [CoCl₃(Me₃tacn)] complex [113] with a threefold excess of A₄[Fe(CN)₆] (A = Li⁺, Na⁺, K⁺) afforded {[Fe(CN)₆]₄ [Co(Me₃tacn)]₄}⁴⁻ cubes with the corresponding alkali cation encapsulated inside. In the case of Li⁺-salt, only a minor fraction of cubes contained encapsulated Li⁺-OH₂ adducts while the majority were empty. Moreover, the NMR signals (in D₂O) of this minor product {Li⁺-OH₂} ⊂ {[Fe(CN)₆]₄ [Co(Me₃tacn)]₄}⁴⁻ disappeared within a week. Li⁺ likely plays a templating role during the synthesis, kinetically favouring the cube's formation. However, due to its small size and weak interaction with the cage, it does not stabilise the final complex thermodynamically and eventually leaves the cage. Moderate yields of 30–40 % compared with the above-mentioned method are due to the formation of inorganic polymers as side-products, which can be explained by the presence of N-uncoordinated CN⁻ groups pointing outside the cube. The use of polyanionic hexacyanido precursors, ensures a significant anionic charge of the cube complex and influences its solubility, making this family of complexes the only water-soluble cubes reported to date. A Cs-analogue of the cubes is selectively obtained by adding CsCl to the aqueous reaction mixture. The caesium cation outcompetes sodium or potassium inclusion, probably because of a more efficient templating effect. [112]

### 2.2.4. Two step syntheses of cubes starting from one fac-complex precursor

Almost all recent cyanide-bridged cubes are formed by a one-pot, two-steps synthesis in polar solvent (e.g. DMF or water) combining capped tris-cyanidometallates, [M(L)(CN)₃]ⁿ, with [M'(S)₆]X₂ transition-metal salts (S represents solvent molecule, method B from Scheme 3) followed by the addition of the second capping ligand. In such cases, the formation of an intermediate species, so-called pro-cubes, that are only half-capped and bear labile ligands (often solvent molecules) on the second metal ion M', can occur. In several cases, those pro-cubes are even stable enough to be isolated.

This was first observed by Long *et coll.* Who synthesised cubes with cyclic tris-amine capping ligands from aqueous solution upon slow evaporation. [114] They reported that intramolecular hydrogen bonding involving the capping ligand on cyanometallate could stabilise pro-cubes. In fact, the tach ligand featuring primary amine donor groups (Scheme 1) enabled the isolation of crystals of three highly hygroscopic pro-cube complexes ({[Co(tach)(CN)₃]₄ [M'(H₂O)₃]₄}⁸⁺ (M' = NiII or CoII) and {[Fe(tach)(CN)₃]₄ [Ni(H₂O)₃]₄}⁸⁺). These are stabilized by hydrogen-bonding interactions between tach-ligands, surrounding halogen anions and water molecules coordinated to the M' cation. In contrast, the reaction of tacn-capped tris-cyanidometallates (with N-donor atoms being secondary and included in the macrocycle) with

hexasolvated cation did not allow the isolation of aqueous pro-cubes. Furthermore, even isolated tach-based complexes were found to be unstable in aqueous solution (dissociating into reactants) and insoluble in organic media (due to high cationic charge) limiting their study to a solid state only.

Recently, Glatz demonstrated that soluble and stable pro-cubes can be obtained using [Fe(RTp)(CN)₃]⁻ cyanidometallates as precursors (where RTp is a derivative of the tris(pyrazol-1-yl)borate ligand). [115,116] Such pro-cubes with M' = CoII, and NiII, were synthesised in DMF or acetonitrile and precipitated using diethyl ether. Although these species were not isolated as single crystals, the FT-IR spectra of obtained solids show only bridging type of cyanides (M-CN-M'), without any terminal ones (see below for typical IR analysis of cubes). NMR, UV-vis and cyclic voltammetry analyses in solution, further confirmed successful pro-cube formation. Even without blocking ligand on M' cation, these pro-cubes are stable at a high enough concentration (ca. 0.1 M), especially with encapsulated alkali ions (Cs⁺).

In most cases, the pro-cube is not isolated and its formation is directly followed by the addition of the second ligand to cap the M' transition metal cation. Holmes' group was the first to report the synthesis of a cubic complex capped with scorpionate ligands. [117] These ligands have been extensively used since then in other cubes' syntheses, as they are versatile and offer a wide range of possible chemical modifications, allowing the introduction of functional groups and thereby tuning the electronic properties of resulting complexes. [118] The first cube of this family was obtained by reacting the preformed [Fe(pzTp)(CN)₃]⁻ complex with Ni(OTf)₂ salt (ca. 1.1 equivalent) in DMF. [117] After 2 h at room temperature, a precipitate was isolated (Authors' note: the nature of this precipitate was not investigated, but based on our research, we conclude that it is not a pure pro-cube, as non-bridging cyanide vibrations are observed [94]). The precipitate was then redissolved in CH₂Cl₂ before the neutral Tpe ligand was added in a second step and stirred for another 2 h. The resulting solution was evacuated to dryness and redissolved in DMF. Crystallisation (by slow diffusion of Et₂O) afforded the desired cationic cubic complex {[Fe(pzTp)(CN)₃]₄ [Ni(Tpe)]₄}⁴⁺ in 50 % yield. According to our experience, the isolation of the precipitate can be skipped without decreasing the yield. [100] Holmes' protocol was further extended to synthesise a cube bearing a thio-acetate group. [119] So far, surface deposition has not been reported, likely due to limited stability. Soon after, Holmes *et coll.* also reported the preparation of a Fe—Co analogue {[Fe(pzTp)(CN)₃]₄ [Co(Tpe)]₄}⁴⁺ with remarkable switchable (optical, magnetic) properties. [96] It is important to note that when using the bulkier per-methylated Tp⁺ ligand to prepare the tris-cyanido-Fe precursor, the formation of the cubic coordination cluster is hindered, and the formation of lower-symmetry polymetallic compounds is favoured. [120]

Related Fe—Ni cubes were also obtained by the group of Oshio and Newton. [121] They employed functionalised Tpe-like ligands incorporating electro-active TTF units. These cubes were obtained via slow evaporation of alcoholic (or acetone) solution of [Fe(pzTp)(CN)₃]⁻ mixed with an acetone (or nitromethane) solution containing a NiII salt and the Tpe-functionalized ligand (L2). Yields were modest, reaching only ca. 8–12 %. However, when the pzTp scorpionate ligand on the Fe was changed to the smaller Tp and NaBPh₄ salt was added to a reaction mixture as a Na⁺ source, a Na⁺-encapsulating mixed-valence complex Na⁺ ⊂ {[FeII/III(Tp)(CN)₃]₄ [NiII(L2)]₄}²⁺ was isolated in a significantly higher 22 % yield.

The groups of Kim [122] and Oshio [123] were the first to explore the synthesis of cubes using only Tp-type anionic ligands on both metal centres. Such a combination reduces the overall charge of the cubes. In particular, Oshio *et coll.* Prepared a mixed valence neutral {[FeII(Tp)(CN)₃]₄ FeIII(Tp)]₄} empty cube in which all eight corners are capped by the anionic Tp ligand. For this, [[NBu₄][Fe(Tp)(CN)₃] was first mixed with an equimolar quantity of Fe(OTf)₂ in DMF, followed by the addition of the Tp ligand. Single crystals were obtained by slow diffusion of diethyl ether into the reaction solution. Interestingly, the resulting
neutral isolated cube is stable in organic solvent and its redox properties were investigated (vide infra).

More recently, our group started exploring a related series of cubes, which contain encapsulated cations. Similar to Rauchfuss' organometallic cubes, we observed that alkali cations can act as templating agents, stabilizing the cubic cage – particularly the $Cs^+$, which fits best the cubic cavity. The resulting cubes have a neutral or moderately positive overall charge, and an organic shell made of hetero-aromatic groups further enhances their stability in polar or weakly polar organic solvents. Indeed, all of them have been proven to be stable in various organic solvents – some even in boiling butyronitrile (bp = 117 °C). [104] This approach enabled the preparation of several complexes in yields close to 60 %, notably:

$$K^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-, [124,125].$$

$$Rb^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-, [125].$$

$$Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-, [104,125].$$

$$K^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{III}(pzTp)]_4 \cdot ClO_4^-\}, [125].$$

$$Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{III}(pzTp)]_4 \cdot PF_6^-\}, [99].$$

$$Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(Tp)]_4\}^-, [116].$$

$$Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{III}(Tp)]_4 \cdot PF_6^-\}, [116].$$

$$Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{III}(3TPhTp)]_4 \cdot PF_6^-\}, [126].$$

$$Ti^+ \subset \{[Fe^{II}(2TPhTp)(CN)_3]_4 [Co^{II/III}(Tp)]_4\}^-, [127].$$

$$Cs^+ \subset \{[Fe^{III}(Tp)(CN)_3]_4 [Ni^{II}(Me_2NTp)]_4 \cdot BF_4^-\}, [128].$$

$$Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Fe^{II/III}(pzTp)]_4\}^-, [129].$$

$$Cs^+ \subset \{[Fe^{II/III}(Tp)(CN)_3]_4 [Mn^{II/III}(pzTp)]_4\}^-, [130].$$

as well as several yet unpublished ones described in recent PhD theses. [100,131,132]

In general, synthetic method B seems to be the most versatile. Since the L' ligand is introduced in the final step, it can be extensively functionalised and tailored for specific applications. Moreover, because the pro-cube is already formed at the time of L' addition, there are minimal constraints on the nature of this ligand. For example, Zhang et al. recently demonstrated that two different tris-(pyrazol-1-yl)methane ligands could be coordinated to the M' metal centres, forming cubes with three different scorpionates ancillary ligands. [133] In this case the cube was directly prepared in acetonitrile and isolated as crystals by slow evaporation. Our group has also shown that the M' cation can be successfully capped with three mono-dentate imidazole molecules, the cube being crystallized again by diethyl ether diffusion into DMF solution. The resulting complex remains stable even in the absence of the chelate effect. [115]

As observed in various related PBAs and smaller cyanido-bridged complexes, metal-metal electron transfer often occurs during the synthesis, particularly in the case of Fe-Co derivatives. Typically, for their synthesis, the $Fe^{III}$ tris-cyanoferrate precursor is mixed with $Co^{II}$ salt forming a cube with $Fe^{II}$ and $Co^{III}$ corners. This oxidation state change can, to some extent, be tracked visually: during the synthesis of Tp-capped cubes, a colour change from red ($Fe^{III}$-CN-$Co^{II}$) to deep green ($Fe^{II}$-CN-$Co^{III}$) is usually observed. The exchange of the coordinated solvent ligands by Tp increases the ligand field strength, thereby lowering the redox potential of the $Co^{III/II}$ favouring the intramolecular electron transfer. The occurrence of this change depends on the ligand's induced ligand field and does not happen for example with neutral Tpm-based ligands that induce a weaker ligand field.

In many cases, the isolated species adopts a mixed-valent electronic configuration, such as $\{Fe_4^{II}-Co_1^{II}Co_3^{III}\}$, where the attainment of overall charge neutrality is proposed as the principal thermodynamic driving force stabilizing the system. In this state, when all capping ligands are anionic (i.e. of Tp-type), the overall charge of cubic cage is monoanionic and is balanced by the encapsulated cation. This observation supports the likelihood of “outer-sphere” electron transfer processes, wherein reduction of one or more metal centres arises from trace redox-active species, such as coordinating solvents or water molecules, rather than from internal redistribution alone. The reduction of $Fe^{III}$ centres has also been observed in polycationic $\{Fe_4-M^4\}$ cubes bearing neutral Tpm-based or imidazolyl ligands upon cation encapsulation. [100,115,132]

In some cases, the encapsulated $Cs^+$ ion can be expelled over time, further reducing the net positive charge and enhancing the cube's overall stability in solution. [100]

## 2.3. Routine characterisation methods

The cubic complexes discussed in this review are large polymetallic assemblies composed of eight metal centres. As described in the previous sections, these metal centres exhibit variable redox states, and some 3d-block cations may also adopt different spin configurations. Despite this electronic complexity, the cube architectures are generally robust and amenable to characterisation in both the solid state and in solution using common organic solvents. This section outlines standard analytical techniques routinely employed to monitor cube formation and to probe the redox and spin states of cubic of the metal centres. More advanced techniques for studying supramolecular organization, electrochemical behaviour, and optical or magnetic properties will be discussed in detail in subsequent sections.

### NMR spectroscopy

The solution stability of cubic complexes enables their routine characterisation by nuclear magnetic resonance (NMR) spectroscopy. In the case of organometallic diamagnetic complexes, ¹H NMR provides direct information about the identity and relative ratios of capping ligands. [103,134] However, cubes incorporating 3d transition metals often contain cations with unpaired electrons. As a result, protons in proximity to paramagnetic metal centres exhibit broadened signals and large hyperfine shifts. Although typical H-H coupling cannot be detected, paramagnetic NMR offers a significant advantage: even small differences in the local chemical environment of protons result in substantial chemical shifts dispersion, often spanning several (or even tens of) ppm. For instance, pyrazole resonance of non-coordinated borate scorpionate ligands (e.g. Tp) typically appear between $\delta = 6$ and 7.5 ppm. In contrast, in the $Cs^+ \subset \{[Fe^{III}(Tp)(CN)_3]_4 [Ni^{II}(Me_2NTp)]_4\}$ cube, the Tp ligand coordinated to Fe displays chemical shifts at $\delta = -12.8, -15.7$ and $-54.7$ ppm, while its dimethylamino-substituted analogue $Me_2NTp$ coordinated to the Ni cation shows signals at $\delta = +32.8$ ppm, $+57.8$ and $+49.4$ ppm. [128]

Owing to the expanded spectral window in paramagnetic ¹H NMR, any chemically distinct proton appears as a separate signal. In the case of the aforementioned mixed-valence $A^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II}(pzTp)]_1 [Co^{III}(pzTp)]_3\}^-$ cubes, the presence of three $Co^{III}$ cations and one $Co^{II}$ centres reduce the symmetry of complex to the $C_{3v}$ point group rather than $T_d$ geometry. The ¹H NMR spectra of $K^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-$ and $Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-$ exhibit exactly 24 signals corresponding to the pyrazole protons, with relative intensities matching the expected stoichiometry, thereby confirming the lowered molecular symmetry. [104,124] Such dissymmetry supports the conclusion that electron transfer between non-adjacent $Co^{II}$ and $Co^{III}$ centres does not occur, or occurs too slowly to be detected on the NMR timescale (acquisition time ~ 0.2 s). Finally, the observed chemical shift in paramagnetic ¹H NMR strongly governed by hyperfine interactions between the nucleus and the paramagnetic centre. These interactions are temperature-dependent and increase in magnitude at lower temperatures as demonstrated by variable-temperature ¹H NMR spectra of $K^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-$, which revealed progressively larger shifts upon cooling. [124]

In cases where a cube complex encapsulates a molecule containing hydrogen atoms, DOSY analysis can provide diffusion coefficients for the ¹H peaks corresponding to both the cage itself and the presumed guest compound. Using this approach, the group of Martinez confirmed that a $Li^+$ and $Na^+$ cations are encapsulated in a cubic cage together with one water molecule. [105,111]

¹³C NMR spectroscopy is less commonly employed in the characterisation of cubic complexes, primarily due to the lower receptivity and natural isotopic abundance of the ¹³C nucleus. Nevertheless, ¹³C
chemical shifts of cyanide carbon atoms can yield valuable structural insight into the coordination environment and encapsulation phenomena.

In the aforementioned study, Martínez et al. utilized $^{13}$C NMR to probe $Li^+$ and $Na^+$ encapsulation, [105] demonstrating its utility for detecting guest-host interactions within the cubic cavity. However, due to the long acquisition times required for sufficient signal-to-noise ratio, this technique is limited to the study of relatively slow dynamic processes.

While $^{13}$C NMR has proven effective for diamagnetic cubic assemblies, to the best of our knowledge, it has not been applied to paramagnetic analogues. This is primarily because paramagnetic broadening exacerbates the inherently low sensitivity, rendering the signals difficult to detect or interpret.

$^7$Li, $^{23}$Na and $^{133}$Cs NMR spectroscopies are powerful tools for probing alkali cation encapsulation. In diamagnetic cubes, the NMR spectra of encapsulated cations typically show only slight shifts compared to the spectra of their free, solvated counterparts. For example, $^7$Li NMR of $\{Li^+\cdot OH_2\} \subset \{[Fe^{II}(CN)_6]_4 [Co^{III}(Me_3-tacn)]_4\}^{4-}$ shows a + 0.1 ppm shift, $^{23}$Na NMR of $\{Na^+\cdot OH_2\} \subset \{[Fe^{II}(CN)_6]_4 [Co^{III}(Me_3-tacn)]_4\}^{4-}$ shows +5 ppm and $^{133}$Cs NMR of $Cs^+ \subset \{[Fe^{II}(CN)_6]_4 [Co^{III}(Me_3-tacn)]_4\}^{4-}$ shows +52 ppm (all reported by Martinez group [105,111,112], recorded in $D_2O$). In related works, the Rauchfuss group [101,134] reported $^{133}$Cs NMR shifts of $Cs^+ \subset \{[Rh^{III}(Cp\*)](CN)_3\}_4 [Mo^{0}(CO)_3\}_4\}^{4-}$ and $Cs^+ \subset \{[Co^{III}(Cp)(CN)_3\}_4 [Co^{III}(Cb\*)]_4\}$ shows encapsulated Cs to free Cs shift of -37 and -34 ppm, respectively (in $CD_3CN$) where $Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3\}_4 [Co^{III}(Tp)]_4\}$ demonstrates +15 ppm shift (our group, [116] in $CDCl_3$). Beyond chemical shift analysis, alkali metal NMR can provide direct evidence of encapsulation via heteronuclear spin-spin coupling. In $Cs^+$-containing cages with isotopically enriched $^{13}$CN linkers, encapsulation

was evidenced by a 1.6-fold broadening of a $^{133}$Cs NMR signal due to $^{133}$Cs-$^{13}$C coupling. [101] Similarly, in the case of $Li^+$ encapsulation, $^7$Li-$^1$H coupling was observed between the cation and a water molecule inside the cage. [105]

As with $^1$H NMR, the presence of unpaired electrons in the complex can drastically affect the chemical shifts of alkali cations. This phenomenon was evidenced by our group in $^{133}$Cs NMR studies of paramagnetic $Cs^+$-containing cubes. Two complexes with identical composition, differing only in the redox state of one single metal centre, exhibited drastically distinct spectral features: in $Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3\}_4 [Co^{III}(Tp)]_4\}$, the $^{133}$Cs NMR signal is observed at +48 ppm, whereas in $Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3\}_4 [Co^{III}(Tp)]_1 [Co^{III}(Tp)]_3\}^-$ it shifts to -227 ppm. [116]

$^{133}$Cs is highly sensitive to subtle changes in its local, chemical environment. While studying $Cs^+ \subset \{[Fe^{II/III}(Tp)(CN)_3\}_4 [Co^{II}(Tpms)]_4\}^{2-}$ and $Cs^+ \subset \{[Fe^{II/III}(Tp)(CN)_3\}_4 [Ni^{II}(Tpms)]_4\}^-$ cages (Tpms a tris(pyrazolyl)methane ligand functionalized with a sulfonate moiety [135]), our group observed multiple peaks (at least three clearly resolved) in the $^{133}$Cs NMR spectra within a narrow spectral window (between -410 and -440 ppm for the Fe—Co complex and -730 to -750 ppm for the Fe—Ni, Fig. 3b and 3c). [131] These observations support the existence of cube isomers with identical metal oxidation states but differing coordination modes of the Tpms ligands $\kappa^3$-N,N,N and $\kappa^3$-N,N,O; Fig. 3a). Such ligand isomerisation had been previously observed in the solid state, [136] but to our knowledge, has never been demonstrated in solution.

### Vibrational spectroscopy

The primary structural motif of the cubic complexes, as well as polymeric PBAs, is a bridging cyanide ligand. This linear, polar ligand possesses a C≡N triple bond, which is easily detectable in infrared (IR) and Raman spectroscopy. Importantly, the stretching frequency of the

<figure><img src="image_9.png" /><figcaption>Fig. 3. a) Different coordination modes of Tpms ligand and $^{133}$Cs NMR spectra of b) $Cs^+ \subset \{[Fe^{II/III}(Tp)(CN)_3\}_4 [Co^{II}(Tpms)]_4\}^{2-}$ and c) $Cs^+ \subset \{[Fe^{II/III}(Tp)(CN)_3\}_4 [Ni^{II}(Tpms)]_4\}^-$ complexes.</figcaption></figure>
cyanide ligand is highly sensitive to its chemical environment, including the nature and oxidation state of the coordinated metal cation, as well as whether the ligand is terminal or bridging. In this section, we briefly summarize key data on cyanide stretching vibrations $v(C≡N)$ that can serve as a reference for cubic polymetallic complexes.

The main trends in the vibrational spectra of cyanide complexes were initially established for homoleptic monomolecular complexes, primarily exhibiting octahedral or square-planar geometries. [137] Free cyanide in aqueous solution exhibits a C≡N stretching vibration at 2060 cm⁻¹. The highest occupied $σ_3$ molecular orbital of CN⁻ is relatively high in energy (−3.1 eV) [138] and has significant antibonding character. Consequently, σ-donation from the ligand's HOMO to the metal centre strengthens the C—N bond, resulting in an increase in the stretching frequency (and corresponding wavenumber). Simultaneously, π-backbonding from the metal into the antibonding π* LUMO of CN⁻ weakens the C—N bond and leads to a decrease in vibrational frequency. Therefore, among similar iron cyanidometallate complexes, Fe³¹-based species (strong σ-acceptor, weak π-donor) exhibit higher-energy C≡N stretching vibrations than Fe¹¹ complexes (weaker σ-acceptor, stronger π-donor). For instance, the vibrational spectra of K₃[Fe³¹(CN)₆] display bands at 2135 cm⁻¹, 2130 cm⁻¹ (Raman) and 2118 cm⁻¹ (IR) whereas K₄[Fe³¹(CN)₆]·3H₂O exhibits absorptions at 2098 cm⁻¹, 2062 cm⁻¹ (Raman) and 2044 cm⁻¹ (IR). [139] When comparing such hexacyanido to tris-cyanido complexes, the metal cation exhibits stronger π-donating capacity, resulting in a weaker C—N bond and slightly lower stretching frequencies. The IR spectrum of PPh₄[Fe³¹(Tp)(CN)₃] shows bands at 2123 cm⁻¹ and 2116 cm⁻¹ [74] while K₂[Fe³¹(Tp)(CN)₃] displays lower CN stretching frequencies at 2057 cm⁻¹, 2040 cm⁻¹, 2033 cm⁻¹, 2017 cm⁻¹. [100] Finally, replacing the anionic Tp ligand with the neutral C-analogue Tpm (increasing the charge of the complex from 2- to 1-) slightly enhances the σ-accepting character of the metal centre, resulting in a modest upshift in the CN stretching frequencies as observed in K[Fe³¹(Tpm)(CN)₃] (2064 cm⁻¹, 2055 cm⁻¹ and 2047 cm⁻¹). [94]

In the case of cyanide ligands bridging two metals centres, M and M', σ-bonding involves the interaction of the CN⁻ $σ_3$ HOMO with both metal cations, forming coordination bonds at both the carbon and nitrogen termini. In contrast, π-backbonding is exclusively occurs at the C-side of the cyanide ligand (where π*CN are localised), and is considered negligible on the N-side. Therefore, a reduction in electronic density in the antibonding $σ_3$ orbital (through M' coordination), without significant alteration of the π orbitals, consistently leads to an increase in the wavenumbers of $v(CN)$ stretching vibrations in bridging cyanide compared to the terminal one. This feature has been extensively demonstrated on polymeric PBAs based on iron hexacyanates. [140–142]

Hence, analysis of cyanide stretching vibrations serves as a crucial tool for characterizing cubic complexes, which are octanuclear heteroleptic species composed of cyanide-bridged metal centres in multiple oxidation states.

Hexacyanoferrate-based cubes prepared by Martinez's group contain two types of cyanide ligands – $μ_1$ and $μ_2$. The IR spectrum of $Cs^+ \subset \{[Fe^{II}(CN)_6]_4 [Co^{III}(Me_3-tacn)]_4\}^{4-}$ exhibits three CN stretching bands at 2155 cm⁻¹, 2124 cm⁻¹ and 2053 cm⁻¹. [112] The lowest wavenumber can be unambiguously attributed to the terminal cyanide, while the other two correspond to bridging cyanide vibrations, with values close to those observed in polymeric PBAs containing Fe¹¹ and Co³¹. The introduction of various guest alkali cations causes minimal changes in the cyanide stretching bands. Reported values are 2151 cm⁻¹, 2125 cm⁻¹, 2063 cm⁻¹ for $K^+ \subset \{[Fe^{II}(CN)_6]_4 [Co^{III}(Me_3-tacn)]_4\}^{4-}$ and 2148 cm⁻¹, 2123 cm⁻¹, 2055 cm⁻¹ for $\{Na^+·OH_2\} \subset \{[Fe^{II}(CN)_6]_4 [Co^{III}(Me_3-tacn)]_4\}^{4-}$. [105,111] The slight influence of the encapsulated cation may arise from either CN⁻ → A⁺ interaction or minor distortions in the cube's geometry that affect σ- and π-bonding between Fe, Co and CN.

In most cases, both metal cations are capped by fac-ligands and only

bridging cyanides are present in the cubes. For example, for the $Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{III}(Tp)]_4\}$ cube that contains only Fe¹¹ and Co³¹ cations, two stretching bands are observed at 2125 cm⁻¹ and 2107 cm⁻¹ [116] In contrast, mixed-valence cubes such as $K^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-$ and $Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-$, exhibit CN stretching bands with at least three components: 2121 cm⁻¹, 2103 cm⁻¹, 2090 cm⁻¹ for the $K^+$-containing cage and 2121 cm⁻¹, 2102 cm⁻¹, 2088 cm⁻¹ for the $Cs^+$ analogue. [104,124,125] Bands above 2100 cm⁻¹ are attributed to Fe¹¹-CN-Co³¹ bridges, whereas those below 2100 cm⁻¹ correspond to Fe¹¹-CN-Co²¹ bridges.

As discussed above, so called pro-cube, presumably of the formula $Cs^+ \subset \{[Fe(Tp)(CN)_3]_4 [Co(S)_3]_4\}^{4+}$ (where S represents H₂O, DMF or a mixture thereof), can be isolated during the synthesis. Although crystallisation has not been achieved, we were able to characterise the complex using spectroscopic methods, which revealed a single CN stretching band at 2162 cm⁻¹. [116] This high wavenumber corresponds to Fe³¹-CN-Co²¹ linkages, the sole linkage present in the pro-cube structure. Similarly, high vibration wavenumbers have been reported by the groups of Holmes and Zhang [96,133] in complexes featuring anionic pzTp ligands on Fe and neutral Tpm-type ligands on the Co centre. Here, the Co cation is in the +II oxidation state, stabilized by the “neutral” N₆ coordination environment, which imposes relatively low crystal field splitting while generating a high redox potential – thereby preventing oxidation by the neighbouring Fe³¹ metal centre.

In the Fe—Fe cubes, the iron metal bonded to the C-atom of the cyanide is always in +II oxidation state, while that linked to the N-atom can be either Fe¹¹ or Fe³¹, depending largely on the overall charge of the complex. The main trends in the IR spectra are similar to Fe—Co pairs. Examples include:

* Fe¹¹-CN-Fe¹¹ vibrations at 2070 cm⁻¹ and 2057 cm⁻¹ in $\{BF_4\} \subset \{[Fe^{II}(Tpm*)](CN)_3\}_4 [Fe^{II}(H_2O)_3]_4\}^{4+}$ with neutral capping ligands on both metals; [143]

* Fe¹¹-CN-Fe³¹ vibrations at 2098 cm⁻¹, 2079 cm⁻¹ and 2057 cm⁻¹ in the empty $\{[Fe^{II}(Tp)(CN)_3]_4 [Fe^{III}(Tp)]_4\}^0$ cube with anionic ligands on both metals; [123]

* both types in a mixed-valence cube with an encapsulated cation, e.g. $Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Fe^{II/III}(pzTp)]_4\}^-$ with CN stretching bands at 2109 cm⁻¹, 2094 cm⁻¹, 2067 cm⁻¹, 2051 cm⁻¹ and 2016 cm⁻¹. [129]

In the latter case, the low-frequency band is unusual for bridging cyanide but probably shifted to lower wavenumbers due to significant CN-GS⁴ interaction. Considering these interactions as well as the distorted C₃v geometry of this family of compounds, additional theoretical modelling is required to further support this hypothesis.

In Fe—Ni cubes, the Ni cation is always in the +II oxidation state whereas the Fe cation is often in the +III but sometime also in the +II oxidation state. Examples include the $\{[Fe^{III}(pzTp)(CN)_3]_4 [Ni^{II}(L')]_4\}^{4+}$ [117,119] and $\{[Fe^{III}(Tp)(CN)_3]_4 [Ni^{II}(L')]_4\}^{4+}$ [144] cubes (L' are Tpm-type ligands) with $v(CN)$ values of 2174 cm⁻¹ and 2170 cm⁻¹, respectively. Remarkably, using IR spectroscopy, Oshio et al. were able to monitor Fe³¹ to Fe¹¹ reduction in a gel formulation, even at low concentration (4 wt%) of the metallic complexes. [144].

When Fe—Ni cubes with neutral L' capping are assembled in the presence of templating $Cs^+$, resulting compound are highly positively charged, leading to partial reduction of Fe³¹ and the formation of mixed-valence complexes. IR spectra of $Cs^+ \subset \{[Fe^{II/III}(Tp)(CN)_3]_4 [Ni^{II}(Im)]_4\}^{n+}$ cubes (isolated in our group) exhibit vibrations at 2162 cm⁻¹ (Fe³¹-CN-Ni³¹), 2107 cm⁻¹ and 2086 cm⁻¹ (both Fe¹¹-CN-Ni¹¹) with relative intensities varying according to preparation conditions. [115]

To summarize, vibrational spectroscopy is an invaluable analytical technique for investigating cyanido-complexes and is routinely employed to determine the oxidation states of metal cations in cubic complexes. It should be noted, however, that cyanide stretching
vibrations often appear as multiple bands or shoulders, especially in mixed-valence species or complexes with distorted geometries. Therefore, accurate interpretation of these spectral features requires either theoretical modelling, which, to date, has not been published, or additional data from complementary techniques. While this section of the review has been limited to the narrow spectral region corresponding to cyanide stretching vibrations, it is important to note that vibrational modes associated with capping ligands and metal-cyanide bonds may likewise provide significant structural information in specific contexts.

### X-ray diffraction techniques.

Cubic molecular complexes readily crystallize, allowing for detailed single-crystal X-ray diffraction (XRD) analysis typically resulting in well-resolved structures. This feature clearly distinguishes these molecular models of PBAs from polymeric and nanomaterials. The arrangement of these cubic complexes in the crystal lattice is mainly influenced by interactions between capping ligands and, in some cases, by solvent molecules and counterions, which serve to isolate the cyano-metallic cages from one another.

When the M and M' cations differ and are coordinated by distinct ligands L and L', the resulting structures are well-resolved and unambiguous. However, cubes with identical ligands L and L' often present challenges in structural resolution. For instance, the XRD structure of the {[FeII(Tp)(CN)₃]₄ [FeIII(Tp)]₄} cube obtained by the Oshio group [123] is centrosymmetric with unresolved C and N atom positions; any Fe cation at a vertex may correspond to either Fe(II) bonded to C or Fe (III) bonded to N. Thus, while the Tp-ligand shells exhibit ordering, the Fe₈(CN)₁₂ cores may be relatively disordered. A similar observation was reported for the {[FeIII(Tp)(CN)₃]₄ [NiII(Tp)]₄} cube by Kim et al. [122]

Another challenge is posed by mixed-valence compounds. For example, the Cs⁺ ⊂ {[FeII(Tp)(CN)₃]₄ [CoII(pzTp)]₁ [CoIII(pzTp)]₃}⁻ cube [104] contains one CoII and three CoIII cations, as evidenced by IR spectroscopy, electrochemical and magnetic measurements. XRD data corroborate this redox state, confirming the cage's mono-anionic charge due to the encapsulated Cs⁺ and absence of counterions. However, the exact location of the CoII ion cannot be revealed, as only two different pairs of cobalt sites can be distinguished from the XRD data with Co-N bond lengths of 1.91 Å and 1.98 Å. Thus, obtained structures should often be considered as an average of several (two in this example) possible ones.

In certain cases, X-ray diffraction analysis can be extended to amorphous materials. For example, when a bis-scorpionate ligand was employed to link cubic complexes, the resulting network lacked regularity and did not exhibit long-range order. Nevertheless, atomic pair distribution function (PDF) analysis demonstrated the presence of short-range ordering in such polymers, [116] exhibiting the same interatomic distances as those observed in a model molecular material. These features were attributed to cubic cages and provided compelling evidence for the structural integrity of the cube complexes within the polymeric matrix.

In summary, driven by contributions of several research groups, the general synthetic strategy for preparing octametallic cubic complexes is now well established. Task-specific compounds can be obtained through rational design of capping ligands. While the earliest complexes were typically characterised only in the solid state, numerous examples of solution-stable cubes have since been reported. This advancement has enabled in-depth studies of the properties of such molecular structures, which will be discussed in more detail below. Table 1 summarizes published complexes.

### 3. Supramolecular chemistry of cubic octametallic cages

The cavity of a cyanide-bridged molecular box is about 125–135 Å³, depending on the internuclear distances between the metal ions of the cube. This cavity is sufficiently large to accommodate small guest molecules. Similar to polymeric Prussian blue analogues (PBAs), molecular

cubic counterparts exhibit guest-exchange behaviour. However, it is should be noted that the porosity of PBAs originates not only from the intrinsic cavities of the cubic units (due to the partially occupied tetrahedral interstitial sites mentioned above) but also from hexacyanometallate vacancies and the structuration of PBAs in hollow nanoparticles. [152] Molecular cubes replicate only the nanoporosity associated with the tetrahedral voids. To date, a variety of cationic guests have been successfully encapsulated, including alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺), the p-block metal ion Tl⁺, and small cationic molecules such as N₂H₅⁺, MeNH₃⁺, NH₄⁺. [99,103,111,134] In contrast to PBAs, the small anion BF₄⁻ has also been intercalated into these cubic cavities. [143]

Among these, Cs⁺-encapsulated cubes consistently display enhanced stability compared to analogues containing other guests or the empty cubic frameworks.

This finding aligns with the well-known affinity of PBAs for Cs⁺, which underlies their application in radioactive caesium decontamination. Several studies from our group have explored the origin of this enhanced stability. [104] In particular, solution-phase studies on {Fe₄-Co₄} and {Fe₄-Ni₄} pro-cubes confirm that Cs⁺ not only templates cube formation but also stabilizes the uncapped charged species. [100,115] In fact, in the absence of Cs⁺, gradual degradation into precursors is observed.

As discussed in greater detail below, the selectivity and stability of host–guest assemblies depend on the nature of interactions between the guest and the cubic framework. [103,104] Electrostatic (ionic and/or dipolar) interactions may be complemented by orbital overlap. In some complexes, short A⁺...cyanide distances are observed, indicating weak Lewis acid–base interactions between the encapsulated alkali ion and the π-electronic cloud of the cyanide bridges. [125] Consequently, the charge, size, and electronic structure of the guest species are of primary importance. As is typical in supramolecular chemistry, void space is energetically unfavourable; the cube cavity therefore preferentially accommodates a guest of optimal size.

The most comprehensive studies on host–guest properties of cyanide cubes have been conducted by Rauchfuss and co-workers. They were the first to report and study the templating effect during the formation of {Co₄-Co₄}, [134] {Rh₄-Co₄} [134] and {Rh₄-Mo₄} [101] boxes. These cubes display a clear affinity for Cs⁺. For example, when K⁺ ⊂ {[Rh(Cp*)](CN)₃]₄ [Mo(CO)₃]₄}⁻ was treated with 1 eq. of CsOTf in presence of 9 eq. of KB(C₆H₄–4-Cl)₄, only the Cs⁺ ⊂ {[Rh(Cp*)](CN)₃]₄ [Mo(CO)₃]₄}⁻ cube was detected by NMR, with an estimated exchange equilibrium constant exceeding 3780. To further explore encapsulation, Rauchfuss and co-workers prepared the “empty” neutral complex {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₄} mentioned above, and used it to encapsulate a series of guests. [103] They observed that K⁺, Rb⁺, Cs⁺, Tl⁺, NH₄⁺, MeNH₃⁺, and N₂H₅⁻ cations were successfully encapsulated, whereas Na⁺ (too small), EtNH₃⁺ (too large), and divalent cations (too highly charged and/or possibly solvated) were not. The selectivity and kinetics of encapsulation were also investigated. Competitive experiments using a 1:2:2 mixture of the {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₄} cube, KPF₆ and CsOTf revealed that K⁺-containing cages formed rapidly (major kinetic product within 10 min) but were gradually replaced by Cs⁺-containing cages (thermodynamic product) over several days, as confirmed by ESI-MS analysis (Figs. 4a and 4b). A similar kinetic-to-thermodynamic profile was observed for the MeNH₃⁺/Cs⁺ pair (Figs. 4c and 4d).

Two general mechanisms are proposed for guest entry or exchange: (1) guest insertion through a {M₂(CN)₄M'₂} "window" or (2) temporary dissociation of an M' corner via rupture of a weaker M'-NC bond. The first mechanism is expected to depend more strongly on the size of the guest ion or molecule, whereas the second is governed by the lability of the M' cation. In case of the {[Co(Cp)(CN)₃]₄ [Ru(Cp*)]₄} cube, the second mechanism has been excluded by an encapsulation test using a mixture of this cube and its close analogue {[Co(Cp)(CN)₃]₄ [Ru(CpMe₄Et)]₄}. The ESI-MS analysis showed that both cages successfully
<table><caption>**Table 1**</caption><caption>Summary of cubic complexes published to date and discussed in this review together with selected information concerning their composition, synthetic route, characterization and magnetic properties.</caption><thead><tr><th>M-CN</th><th>L</th><th>M'-NC</th><th>L'</th><th>A (encapsulated guest ion)</th><th>Synthetic method</th><th>Cube charge</th><th>ℓ (CN) in cm<sup>-1</sup></th><th>Solution Studies</th><th>Magnetic Properties</th><th>References</th></tr></thead><tbody><tr><td>Co<sup>III</sup></td><td>tacn</td><td>Cr<sup>III</sup><br>Co<sup>III</sup></td><td>tacn</td><td>-</td><td>A</td><td>12+</td><td>2177<br>2200</td><td>UV-vis, Mass</td><td>-</td><td>[95]</td></tr><tr><td>Co<sup>III</sup></td><td>Cp</td><td>Ru<sup>II</sup></td><td>Cp*</td><td>-</td><td>-</td><td>4+</td><td>2180</td><td>NMR</td><td>-</td><td>[97]</td></tr><tr><td>Co<sup>III</sup></td><td>Cp</td><td>Ru<sup>II</sup></td><td>Cp*</td><td>K<sup>+</sup>, Rb<sup>+</sup> Cs<sup>+</sup>, Tl<sup>+</sup>, N<sub>2</sub>H<sub>5</sub><sup>+</sup>, NH<sub>4</sub><sup>+</sup>,<br>MeNH<sub>3</sub><sup>+</sup></td><td>A</td><td>0<br>1+</td><td>2174<br>2158</td><td>NMR, Mass</td><td>-</td><td>[103,106,145]</td></tr><tr><td>Co<sup>III</sup></td><td>tach</td><td>Ni<sup>II</sup><br>Co<sup>II</sup></td><td>3 × H<sub>2</sub>O</td><td>-</td><td>B</td><td>8+</td><td>2175<br>2174</td><td>-</td><td>-</td><td>[114]</td></tr><tr><td>Co<sup>III</sup> or<br>Rh<sup>III</sup></td><td>Cp</td><td>Co<sup>l</sup></td><td>Cb*</td><td>-</td><td>A</td><td>0<br>1+</td><td>2159<br>2147</td><td>CV, Mass</td><td>-</td><td>[134]</td></tr><tr><td>Rh<sup>III</sup></td><td>Cp*</td><td>Mo<sup>0</sup><br>Mn<sup>II</sup><br>Fe<sup>II</sup></td><td>3 × CO</td><td>K<sup>+</sup>, Cs<sup>+</sup></td><td>A</td><td>3-</td><td>2147<br>2096, 2081<br>1992, 2011</td><td>NMR, Mass</td><td>-</td><td>[101]</td></tr><tr><td>Re<sup>II</sup></td><td>triphos</td><td>Co<sup>II</sup><br>Ni<sup>II</sup><br>Zn<sup>II</sup></td><td>Cl<sup>-</sup>, I<sup>-</sup> or<br>OMe</td><td>-</td><td>B</td><td>0</td><td>2111, 2096<br>2122, 2109<br>2115, 2102, 2074</td><td>CV, UV-vis, Mass</td><td>SMM</td><td>[108-110]</td></tr><tr><td>Fe<sup>III</sup></td><td>tach</td><td>Ni<sup>II</sup></td><td>3 × H<sub>2</sub>O</td><td>-</td><td>B</td><td>8+</td><td>2175</td><td>-</td><td>SMM</td><td>[114]</td></tr><tr><td>Fe<sup>III</sup></td><td>pzTp</td><td>Ni<sup>II</sup></td><td>Tpe or RTpm</td><td>-</td><td>B</td><td>4+</td><td>2174</td><td>-</td><td>SMM</td><td>[117,119,146]</td></tr><tr><td>Fe<sup>III</sup></td><td>pzTp</td><td>Ni<sup>II</sup></td><td>Phen/<br>CH<sub>3</sub>OH</td><td>-</td><td>B</td><td>4+</td><td>2169</td><td>-</td><td>SMM</td><td>[147]</td></tr><tr><td>Fe<sup>III</sup></td><td>Tp</td><td>Ni<sup>II</sup></td><td>Tp</td><td>-</td><td>B</td><td>0</td><td>2163</td><td>-</td><td>SMM</td><td>[122]</td></tr><tr><td>Fe<sup>III</sup> Fe<sup>II</sup>/<br>Fe<sup>III</sup></td><td>Tp or<br>pzTp</td><td>Ni<sup>II</sup></td><td>RTpm</td><td>Na<sup>+</sup></td><td>B</td><td>4+</td><td>2170<br>2168, 2110, 2086</td><td>CV, UV-vis, Mass,<br>electrochromic</td><td>SMM</td><td>[121,144]</td></tr><tr><td>Fe<sup>III</sup></td><td>Tp</td><td>Ni<sup>II</sup></td><td>Me<sub>2</sub>NTp</td><td>Cs<sup>+</sup></td><td>B</td><td>3+</td><td>2160</td><td>NMR, UV-vis, CV, Mass</td><td>SMM</td><td>[128]</td></tr><tr><td>Fe<sup>II</sup>/Fe<sup>III</sup></td><td>Tp</td><td>Ni<sup>II</sup></td><td>3 × Im</td><td>Cs<sup>+</sup></td><td>B</td><td>4+</td><td>2086, 2107, 2162</td><td>NMR, CV, UV-vis</td><td>HS ground state</td><td>[115]</td></tr><tr><td>Fe<sup>II</sup></td><td>Tpm*</td><td>Fe<sup>II</sup></td><td>H<sub>2</sub>O</td><td>BF<sub>4</sub><sup>-</sup></td><td>B</td><td>3+</td><td>2070, 2057</td><td>-</td><td>-</td><td>[143]</td></tr><tr><td>Fe<sup>III</sup></td><td>Tp</td><td>Fe<sup>II</sup></td><td>Tp</td><td>-</td><td>B</td><td>0</td><td>2098, 2079, 2057</td><td>CV, UV-vis, electrochromic</td><td>-</td><td>[123]</td></tr><tr><td>Fe<sup>II</sup></td><td>Tp</td><td>Fe<sup>III</sup>/Fe<sup>II</sup></td><td>pzTp<sup>[a]</sup></td><td>Cs<sup>+</sup></td><td>B</td><td>0</td><td>2109, 2094, 2051,<br>2016</td><td>NMR, CV, UV-vis, Mass,<br>electrochromic</td><td>ST</td><td>[129]</td></tr><tr><td>Fe<sup>II</sup></td><td>Tp or<br>pzTp</td><td>Fe<sup>III</sup>/Fe<sup>II</sup></td><td>Tpm</td><td>-</td><td>B</td><td>4+</td><td>2156, 2073</td><td>CV</td><td>ET</td><td>[148]</td></tr><tr><td>Fe<sup>III</sup></td><td>pzTp</td><td>Co<sup>II</sup></td><td>Tpe/<br>TpEtOP</td><td>-</td><td>B</td><td>4+</td><td>2169, 2159</td><td>-</td><td>ETCST</td><td>[133]</td></tr><tr><td>Fe<sup>III</sup></td><td>pzTp</td><td>Co<sup>II</sup></td><td>TpEtOAn</td><td>-</td><td>B</td><td>4+</td><td>2166, 2156</td><td>UV-vis</td><td>ETCST</td><td>[149]</td></tr><tr><td>Fe<sup>III</sup></td><td>pzTp</td><td>Co<sup>II</sup></td><td>Tpe</td><td>-</td><td>B</td><td>4+</td><td>2168</td><td>UV-vis</td><td>ETCST</td><td>[96,150]</td></tr><tr><td>Fe<sup>II</sup></td><td>Tp</td><td>Co<sup>II</sup>/<br>Co<sup>III</sup></td><td>pzTp or<br>Tp, or TPhTp</td><td>K<sup>+</sup>, Rb<sup>+</sup>, Cs<sup>+</sup>, Tl<sup>+</sup></td><td>B</td><td>0/1+</td><td>2103, 2085</td><td>NMR, CV, UV-vis, Mass</td><td>ETCST</td><td>[104,116,124-127,151]</td></tr><tr><td>Fe<sup>II</sup></td><td>3 × CN<sup>-</sup></td><td>Co<sup>III</sup></td><td>Me<sub>3</sub>tacn</td><td>-</td><td>C</td><td>4-<br>3-</td><td>2161<sup>Li</sup>, 2148<sup>Na</sup>,<br>2151<sup>K</sup></td><td>NMR, CV, UV-vis, ICP</td><td>-</td><td>[105,111,112]</td></tr><tr><td>Fe<sup>II</sup>/Fe<sup>III</sup></td><td>Tp</td><td>Mn<sup>II</sup>/<br>Mn<sup>III</sup></td><td>pzTp</td><td>Cs<sup>+</sup></td><td>B</td><td>0</td><td>2150, 2095, 2071</td><td>NMR, CV, UV-vis, Mass,<br>electrochromic</td><td>ET</td><td>[130]</td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>**Fig. 4.** The competitive formation $K^+$ / $Cs^+$ (a and b) and $MeNH_3^+$ / $Cs^+$ (c and d) host-guest complexes with $\{[Co(Cp)(CN)_3]_4 [Ru(Cp*)]_4\}$ cage, studied by mass spectrometry. Reprinted with permission from [103]. Copyright 2003, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.</figcaption></figure>

encapsulated $Cs^+$ but no evidence of Ru-corner exchange found after the reaction. [103]

Interestingly, due to their encapsulating nature and slow exchange kinetics, these cages can “protect” their guest ions. For example, $NH_4^+$ $\subset$ $\{[Co(Cp)(CN)_3]_4 [Ru(Cp*)]_4\}$ does not undergo H—D exchange in $D_2O$ at neutral pH, unlike free $NH_4^+$. [103] Considering that H—D

exchange in free $NH_4^+$ typically proceeds rapidly, this finding suggests that $NH_4^+$ is unable to leave and re-enter the cage. This was further confirmed by the lack of $ND_4^+$ adduct formation upon addition of $ND_4BF_4$ salt. However, $NDH_3^+ \subset \{[Co(Cp)(CN)_3]_4 [Ru(Cp*)]_4\}$ was observed over time in the $^1H$ NMR spectra. The addition of $ND_4BF_4$ presumably lowered the pH, enabling slow $D^+$ transfer and H—D exchange in the ammonium cation inside the box.

Ion-exchange properties were also demonstrated for cubes of the tris (pyrazolyl)borate family, $A^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-$ ($A^+ = Cs^+, K^+$) by Jiménez et al. [104] These studies also confirmed higher affinity for $Cs^+$ than for $K^+$. The high stability and solubility of those complexes enabled monitoring of the K—Cs exchange reaction by paramagnetic $^1H$ and $^{133}Cs$ NMR. Upon addition of 2 equivalents of caesium salt to a solution containing the $K^+$-cube, exchange began immediately (Fig. 5). Although slow at room temperature, this exchange significantly accelerated at 65 °C, reaching 40 % conversion within 32 hours. Six days are required to complete the reaction under these conditions. In contrast, addition of 12-crown-6-ether, which strongly binds $K^+$, markedly accelerated the reaction. Compared to the aforementioned examples of ion-exchanges in compounds from Rauchfuss' group, this reaction exhibits significantly slower kinetics. A possible explanation lies in the size disparity between these octametallic cages: when using 4d-cations (e.g. $Rh^{III}$, $Mo^0$ or $Ru^{II}$) the M-CN-M' distances forming the cube edges are relatively long ($d(Rh-CN-Mo) = 5.36 Å$) [101]. In contrast, with 3d metal ions, these distances are substantially shorter ($d(Fe-CN-Co) = 4.9 Å$) resulting in narrower cube faces. This structural difference likely hinders the passage of the guest ions, thereby reducing the efficiency of the exchange process.

The third family of cubes in which guest exchange has been demonstrated are the anionic cages developed by Martinez' group. [105,111] As discussed above, water-soluble $A^+ \subset \{[Fe^{II}(CN)_6]_4 [Co^{III}(Me_3tacn)]_4\}^{4-}$ ($A^+ = Li^+, Na^+, K^+$) cubes were prepared from corresponding hexacyanoferrates (Fig. 6). It was found that in this series the smallest cations, $Na^+$ and $Li^+$, are encapsulated as $A^+·OH_2$ adducts, as confirmed by DOSY NMR, as well as $^1H$, $^7Li$ and $^{23}Na$ NMR. While $\{Na^+·OH_2\} \subset \{[Fe^{II}(CN)_6]_4 [Co^{III}(Me_3tacn)]_4\}^{4-}$ is stable, $\{Li^+·OH_2\} \subset \{[Fe^{II}(CN)_6]_4 [Co^{III}(Me_3tacn)]_4\}^{4-}$ is only detected in

<figure><img src="image_11.png" /><figcaption>**Fig. 5.** $K^+$ to $Cs^+$ exchange in $A^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{II/III}(pzTp)]_4\}^-$ cubes followed by $^1H$ NMR-spectroscopy. Fe ions in orange, Co ions in blue, $Cs^+$ in dark purple and $K^+$ in purple.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 6.** Synthesis and the structure of {Na⁺·OH₂} = {[FeII(CN)₆]₄ [CoIII(Me₃tacn)]₄}⁴⁻ cube with inset showing ¹H NMR signal of encapsulated H₂O molecule different from that of solution. Fe ions in orange, Co ions in blue, N in light purple and C atoms in grey.</figcaption></figure>

freshly-prepared samples and gradually transforms into the “void” cube, presumably containing a water cluster, {[FeII(CN)₆]₄ [CoIII(Me₃tacn)]₄}⁴⁻. In contrast, the larger K⁺ ion occupies the cube’s cavity without a water molecule, forming the stable K⁺ = {[FeII(CN)₆]₄ [CoIII(Me₃tacn)]₄}⁴⁻ complex. Hence, the small {Li⁺·OH₂} guest can be easily exchanged for the better fitting {Na⁺·OH₂}, which in turn can be replaced by K⁺. However, K⁺ could not be exchanged for Rb⁺. The Rb⁺ = {[FeII(CN)₆]₄ [CoIII(Me₃tacn)]₄}⁴⁻ cube could only be obtained from the more labile {Na⁺·OH₂}-complex. In contrast, the Cs⁺-containing cube could not be obtained even after prolonged treatment (two-week) of {Na⁺·OH₂} = {[FeII(CN)₆]₄ [CoIII(Me₃tacn)]₄}⁴⁻ with a large excess (50–100-fold) of Cs⁺ salt. Given that the internal volume of {[FeII(CN)₆]₄ [CoIII(Me₃tacn)]₄}⁴⁻ cubes is similar to that of the previously discussed {[FeII(Tp)(CN)₃]₄ [CoII/III(pzTp)]₄}⁻, the lack of Cs⁺ exchange is likely due to kinetic limitations. Both metal centres (FeII and CoIII) possess a t₆g₂ configuration and are inert, making Fe—C or N—Co bonds dissociation unlikely. Thus, the most plausible guest exchange mechanism involves diffusion through one of the {Fe₂(CN)₄Co₂} “windows”, which are large enough to permit the passage of Na⁺ and K⁺ but too narrow for Cs⁺. However, this hypothesis does not explain the Rb⁺ exchange, as Rb⁺ should also be sterically excluded. We suggest that solvation effects in water play critical role (these exchanges were all performed in aqueous media, in contrast to the use of acetonitrile for other cubes). The hydration shells of larger alkali may hinder them from accessing the cavity, limiting successful exchange. In a recent study by Martinez’s group, the Cs⁺-cube was successfully isolated by using Cs⁺ as a template during synthesis. The obtained complex demonstrated high robustness and resistance to guest leaching, consistent with the preferential trapping of Cs⁺ inside the cube. Furthermore, the selective encapsulation of Cs⁺ over Na⁺, the counter cation of the starting hexacyanidoferrate complex, was clearly evidenced. [112]

Overall, the thermodynamic stability of all alkali “host-guest” complexes tends to increase with the ionic radius, following the trend from Li⁺ to Cs⁺. This pattern mirrors observations in polymeric PBAs, where encapsulation of Li⁺ cations is difficult, while Na⁺ and K⁺ are more readily included, and the most stable compound are formed with Cs⁺. Indeed, Cs⁺ readily exchanges other cations from PBAs, which lead to their application as Cs-capturing reagents. [153,154]

Detailed structural analysis of guest position in cubic complexes reveals that the guest is not always located at the geometric centre of a cage, instead, its position depends on the nature of the A, M and M' cations. For example, in {A⁺·OH₂} = {[FeII(CN)₆]₄ [CoIII(Me₃tacn)]₄}⁴⁻ (A⁺ = Li⁺ or Na⁺), Li⁺ is located close to one corner of a cube, interacting with three CN nitrogen atoms and one water molecule in a distorted tetrahedral geometry, while Na⁺ is situated closer to a cube face, forming four Na-CN contacts and one water, adopting a

square-pyramidal geometry. [105] Rauchfuss et al. pointed out that the only significant difference between A⁺ ⊂ {[Rh(Cp*)(CN)₃]₄ [Mo(CO)₃]₄}⁴⁻ (A⁺ = K⁺, Cs⁺) is the location of the alkali metal ion inside the box and its interaction with the surrounding cyanide bridges: the K⁺ is equally distributed over two slightly off-centred positions interacting with eight cyanide bridges, whereas the Cs⁺ is centred in the cavity, interacting with all twelve cyanide edges. [101] A similar finding was reported by our group in a series of A⁺ ⊂ {[FeII(Tp)(CN)₃]₄ [M'(pzTp)]₄}⁻ (A⁺ = K⁺, Rb⁺, Cs⁺; M' = CoII/III or FeII/III) cubes, where one M' metal centre is in reduced to +II to balance the inserted alkali ion's charge. Herein, the Cs⁺ is located at the centre of the cube, interacting with all twelve surrounding cyanide linkers. In contrast, the smaller K⁺ is displaced toward the M'(+II) corner, interacting with only three cyanide edges, while the Rb⁺ adopts an intermediate position and interacts with five cyanide bridges. [100, 104, 124] Upon oxidation, crystal structures of the resulting cubes A⁺ ⊂ {[FeII(Tp)(CN)₃]₄ [CoIII(pzTp)]₄} (A⁺ = K⁺, Cs⁺) reveal that both cations are now located at the centre of their respective cages. This implies that the y smaller K⁺ is sensitive to electrostatic gradients within the cavity, preferring the less positive corner when possible. However, spatial confinement limits the extent of displacement, a principle recently exploited in far-infrared spectroscopy studies on polymeric PBAs. [155] No such studies have yet been reported for molecular analogues.

Guest de-encapsulation has been less thoroughly explored, but can occur via drastic alterations of the cage’s electronic structure. For instance, Boyer et al. [134] reported two Co-only cubes A⁺ ⊂ {[CoIII(Cp)(CN)₃]₄ [CoI(Cb*)]₄} (A⁺ = K⁺ or Cs⁺), where the neutral octametallic framework readily accommodates alkali cations. Upon oxidation with ferricenium to a {CoIII₄-CoI₄} state, the cage becomes 4⁺ charged, which disfavours the retention of positively charged guests, and expulsion occurs. This process is reversible, reduction with 4 eq. of [CoCp₂] in the presence of 1 eq. of CsOTf regenerates Cs⁺ ⊂ {[CoIII(Cp)(CN)₃]₄ [CoI(Cb*)]₄}.

Our group observed a similar phenomenon in Cs⁺ ⊂ {[FeII/III(Tp)(CN)₃]₄ [NiII(Tpe)]₄}ⁿ⁺ cubes. Depending on synthetic conditions, the oxidation state of the iron metal ions and the cube’s cage charge (n+) can be tuned, ranging from 4⁺ (with four FeIII) up to 1⁺ (three FeII and one FeIII). At higher charged states (3⁺ or 4⁺), Cs⁺ is gradually expelled over several months at room temperature. This process could be accelerated by heating the solution to 80 °C as confirmed by ¹H and ¹³³Cs NMR. [100] (Fig. 7).

Unlike polymeric PBAs, the overall charge of molecular cyanide bridged boxes can be modulated by an appropriate choice of the transition metal cations and ligands. A positively charged cage can even encapsulate anions – a feature that has not been observed in polymeric PBAs. In 2008 Shi et coll. Reported the first and to date, only anion-
<figure><img src="image_13.png" /><figcaption>**Fig. 7.** Expulsion of $Cs^+$ from $Cs^+ \subset \{[Fe^{II}(Tp)]_4 [Ni^{II}(Tpe)]_4\}^{4+}$ cube in CD₃CN solution at 80 °C followed by ¹H (left) and ¹³³Cs (right) RMN. Fe ions in orange, Ni ions in light green and $Cs^+$ in purple.</figcaption></figure>

encapsulating cubic cyanidometallate cage $BF_4^- \subset \{[Fe^{II}(Tpm^*)(CN)_3]_4 [Fe^{II}(H_2O)_3]_4\}^{4+}$ (where $Tpm^* = \text{tris}(3,5\text{-dimethyl-1-pyrazolyl})$ methane). [143]

To conclude, octa-metallic cyanide-bridged cages are remarkable receptor structure capable of encapsulating small cations and molecules. The careful selection of corner metal centres and capping ligands allow control over charge, size, and lability of the cages, consequently influencing the thermodynamic and kinetic stability of resulting inclusion complexes. This allows for fine-tuned selectivity in guest inclusion and offers potential for applications in selective sensing or targeted ion trapping. It is noteworthy that the electronic flexibility of many of these cages opens the door to the development of stimuli-responsive host-guest systems. In the next section, we will continue by discussing electrochemical properties of these complexes.

## 4. Redox properties of cubes

The redox properties of PBA materials are critical for many of their application fields. For example, they play a pivotal role in their use as intercalation materials for batteries, [44] as active materials in electrochemical (bio)sensor, [36] or as electrochromic materials. [38] In this section, we explore the redox properties of their molecular cubic counterparts, emphasizing their distinct features in comparison to the three-dimensional PBA materials.

### 4.1. Multiple accessible redox states

In contrast to the insoluble, polymeric PBAs, that are generally non-stoichiometric compounds and display a distribution of metal coordination environments due to vacancies, hydration, and variable counterion interactions, [9–12] their molecular counterparts feature well-defined metal sites and uniform coordination environments. These structural differences typically lead to more resolved and reversible redox processes in solution-phase cyclic voltammetry. Although the broader and less defined redox features of extended PBAs partly arise from the solid-state measurement conditions and diffusion limitations, they also reflect the intrinsic heterogeneity of the framework, where local structural disorder and mixed-valence delocalization contribute to a distribution of redox potentials. The octametallic nature of the cube and the presence of two distinct types of metal ion corner, $[M(L)(CN)_3]$ and $[M'(L')(NC)_3]$, can give rise to two sets of redox events. Depending on the stability of the oxidized/reduced cube species in a given solvent, the exact number of redox events may vary.

Several valuable insights can be obtained from cyclic voltammetry experiments:

(1) The open circuit potential (OCP) gives a direct indication of the dominant redox state of the cubic cage in solution. When the redox wave

of each cation is resolved, the position of the OCP relatively to the redox waves provides insight into the electronic states of M and M'.

(2) The reversibility of the CV waves indicates the electrochemical stability of the cube complexes in general, and of specific redox states. Some cubes studied in our group exhibit remarkable stability over a wide potential range, spanning the entire electrochemical window of $CH_3CN$. [104]

(3) Coordination of a second metal ion M' on the nitrogen donor atom of cyanide significantly affects the redox potentials of M, compared to the tris-cyanide precursor $[M(L)(CN)_3]^{x-}$. The Lewis acidity of M' reduces electronic density on the CN bridge, thus increasing the redox potential of the M metal ions. For instance, in the $Fe^{III/II}$ couple of the $[Fe(Tp)(CN)_3]^{-}$ complex, binding three metal ions M' to the three N donor sites of CN ligands may results in shifts exceeding +1 V. [124] Conversely, replacement of a coordinating solvent molecule S in the M' coordination sphere by the donor NC-M may lead to a decrease in the redox potential of M'. When these shifts become sufficiently large, redox potentials can cross, enabling electron transfer between M and M' upon formation of the M-CN-M' bridge

Fig. 8, together with Table 2, summarizes the redox potentials of the $[Fe(Tp)(CN)_3]^{-}$ precursor as well as those of $Mn^{II}$, $Fe^{II}$, $Co^{II}$, and $Ni^{II}$ in their hexaaqua complexes and their Tp- and pztP-based bis-scorpionate

<figure><img src="image_14.png" /><figcaption>**Fig. 8.** Schematic illustration of the $M^{III/II}$ redox potential shift upon coordination with the pztP scorpionate ligand, compared to the reduction potential of the $[Fe^{III}(Tp)(CN)_3]^{-}$ complex. Fe ions in orange, Co ion in blue, N in light purple, B in pink and C atoms in grey.</figcaption></figure>
<caption>**Table 2**</caption>

Redox potential the metal hexaaqua complexes and metal bis-scorpionate complexes in dichloroethane.

<table><thead><tr><th rowspan="2"></th><th colspan="4">Potential (V vs SCE)</th></tr><tr><th>Mn</th><th>Fe</th><th>Co</th><th>Ni</th></tr></thead><tbody><tr><td>[M(H<sub>2</sub>O)<sub>6</sub>]<sup>2+/-3+</sup></td><td>1.29</td><td>0.52</td><td>1.59</td><td>&gt;1.75</td></tr><tr><td>[M(Tp)<sub>2</sub>]<sup>0+/-</sup></td><td>0.47</td><td>0.13</td><td>-0.13</td><td>1.21</td></tr><tr><td>[M(pzTp)<sub>2</sub>]<sup>0+/-</sup></td><td>0.46</td><td>0.23</td><td>-0.10</td><td>1.29</td></tr><tr><td>[Fe(Tp)(CN)<sub>3</sub>]<sup>-/2</sup></td><td colspan="4">-0.43</td></tr></tbody></table>

complexes, measured in dichloromethane. The most pronounced shift in redox potential upon coordination with N-donor ligands is observed for CoII. Furthermore, the sandwich complexes of Co and Fe – which exhibit the lowest redox potentials (and are therefore closest to that of the [Fe(Tp)(CN)<sub>3</sub>]<sup>-</sup> precursor) – are the metal ions that tend to undergo electron transfer upon coordination with the precursor, forming {[FeII(Tp)(CN)<sub>3</sub>]<sub>4</sub> [CoIII(pzTp)<sub>4</sub>] and {[FeII(Tp)(CN)<sub>3</sub>]<sub>4</sub> [FeIII(pzTp)<sub>4</sub>] cubes in the absence of alkali ions. [100] In contrast, upon coordination of NiII – whose bis-scorpionate complexes exhibit significantly higher redox potential, with differences exceeding 1 V compared to the [Fe(Tp)(CN)<sub>3</sub>]<sup>-</sup> precursor – no electron transfer is observed, and the resulting cubes are isolated in FeIII-NiII state.

We propose that comparing the redox potential of the precursor with that of the target ion in its bis-scorpionate complex form (using the same

ligand) may provide a useful indication of the likelihood of electron transfer during cube formation. Such predictive capability would be valuable in the development of PBA-based battery materials.

(4) Beyond measuring the redox activity of individual ions, CV is also valuable for probing the electronic interaction between metal centres mediated by the cyanide bridges. The splitting between the redox waves corresponding to M or M' metal ions is indicative of the degree of electronic communication between these metal centres. Studies by Endicott et al. [16] on square complexes and by Nihei [123] and Glatz et al. [129] on cubic systems, demonstrated that such interactions may occur between second-nearest neighbours – i.e., via M-CN-M'-NC-M pathways – as further supported by spectroscopic measurements. [156] Electrostatic interactions also contribute to the redox wave splitting. However, to our knowledge, the relative contributions of pure electrostatic effects versus through-bond electronic interactions have not yet been clearly distinguished in the case of cubic cages. Table 3 summarizes the redox properties of all published cubes normalized versus NHE.

## 4.2. Representative CV studies

The first CV study on cyanide cubic cages was conducted by Dunbar et al. on the family of {[ReII(triphos)(CN)<sub>3</sub>]<sub>4</sub> [M'Cl]<sub>4</sub>} complexes (M' = MnII, FeII, CoII, NiII, ZnII). [110] The [Re(triphos)(CN)<sub>3</sub>] precursor exhibits three distinct redox processes at -0.74 V (ReII/1), +0.19 V (ReIII/II) and +1.12 V (ReIV/III), versus an Ag/AgCl reference electrode. In the cube complex, these waves are split and shifted to higher potentials,

<caption>**Table 3**</caption>

Reported redox potentials of octametallic cube complexes. All values are given vs the Normal Hydrogen Electrode (NHE) for consistency, using reported conversion factors where necessary. Potentials obtained under different solvent or supporting-electrolyte conditions are noted as such.

<table><thead><tr><th rowspan="2"></th><th colspan="4">Redox potentials associated with</th><th rowspan="2">solvent</th><th rowspan="2">ref</th></tr><tr><th colspan="2">M cations</th><th colspan="2">M' cations</th></tr><tr><th></th><th colspan="4">in V vs. NHE</th><th></th><th></th></tr></thead><tbody><tr><td>K<sup>+</sup> &lt; {[Co(Cp)(CN)<sub>3</sub>]<sub>4</sub> [Co(Cb*)]<sub>4</sub>}</td><td></td><td></td><td></td><td>0.14 (4 e<sup>-</sup>)</td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Co(Cp)(CN)<sub>3</sub>]<sub>4</sub> [Co(Cb*)]<sub>4</sub>}</td><td></td><td></td><td></td><td>0.22 (4 e<sup>-</sup>)</td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td>K<sup>+</sup> &lt; {[Rh(Cp*)<sub>4</sub>(CN)<sub>3</sub>]<sub>4</sub> [Co(Cb*)]<sub>4</sub>}</td><td></td><td></td><td></td><td>0.13 (4 e<sup>-</sup>)</td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Ru(Cp*)<sub>4</sub>(CN)<sub>3</sub>]<sub>4</sub> [Co(Cb*)]<sub>4</sub>}</td><td></td><td></td><td></td><td>0.18 (4 e<sup>-</sup>)</td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td rowspan="2">{[Re(triphos)(CN)<sub>3</sub>]<sub>4</sub> [MnI]<sub>4</sub>}</td><td>0.17</td><td></td><td>1.21</td><td>1.48</td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td>0.91</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">{[Re(triphos)(CN)<sub>3</sub>]<sub>4</sub> [FeCl]<sub>4</sub>}</td><td>0.43</td><td>0.78</td><td>1.20</td><td>1.46</td><td>0.17<br>(2 e<sup>-</sup>)</td><td>CH<sub>3</sub>CN</td></tr><tr><td>-0.15</td><td>-0.28</td><td>-0.35</td><td>-0.51</td><td>-0.20</td><td>-0.55</td></tr><tr><td rowspan="2">{[Re(triphos)(CN)<sub>3</sub>]<sub>4</sub> [CoCl]<sub>4</sub>}</td><td>0.91</td><td>1.00</td><td>1.09</td><td></td><td></td><td></td></tr><tr><td>0.29</td><td>0.11</td><td>0.02</td><td>-0.19</td><td></td><td></td></tr><tr><td rowspan="2">{[Re(triphos)(CN)<sub>3</sub>]<sub>4</sub> [NiCl]<sub>4</sub>}</td><td>0.91</td><td>1.01</td><td>1.18</td><td>1.4</td><td></td><td></td></tr><tr><td>0.01</td><td>-0.16</td><td>-0.33</td><td>-0.48</td><td></td><td></td></tr><tr><td rowspan="2">{[Re(triphos)(CN)<sub>3</sub>]<sub>4</sub> [ZnCl]<sub>4</sub>}</td><td>0.90</td><td>0.98</td><td>1.20</td><td>1.4</td><td></td><td></td></tr><tr><td>0.23</td><td>0.43</td><td>0.70</td><td>0.90</td><td></td><td></td></tr><tr><td>Na<sup>+</sup> &lt; {[Fe(pzTp)(CN)<sub>3</sub>]<sub>4</sub> [Ni(RTp)m]<sub>4</sub>]<sup>2+</sup></td><td></td><td></td><td></td><td></td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td> {[Fe(pzTp)(CN)<sub>3</sub>]<sub>4</sub> [Ni(RTp)m]<sub>4</sub>]<sup>4+</sup></td><td></td><td></td><td>&lt;0.4 irreversible</td><td></td><td></td><td>CH<sub>2</sub>Cl<sub>2</sub></td></tr><tr><td> {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Ni(S)<sub>3</sub>]<sub>4</sub>]<sup>4+</sup></td><td></td><td></td><td>0.45 (4 e<sup>-</sup>)</td><td></td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Ni(S)<sub>3</sub>]<sub>4</sub>]<sup>4+</sup></td><td>0.39</td><td>0.51</td><td>0.64</td><td>0.77</td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Ni(Im)<sub>3</sub>]<sub>4</sub>]<sup>2+</sup></td><td>0.37</td><td>0.51</td><td>0.65</td><td>0.78</td><td></td><td>DMF</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Ni(Im)<sub>3</sub>]<sub>4</sub>]<sup>2+</sup></td><td>0.29</td><td>0.42</td><td>0.54</td><td>0.64</td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Ni(RTp)m]<sub>4</sub>}</td><td>-0.22</td><td>0</td><td>0.17</td><td>0.28</td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Mn(pzTp)<sub>4</sub>]<sup>-</sup></td><td>0.23</td><td>0.38</td><td>0.56</td><td>0.76</td><td></td><td>CH<sub>3</sub>CN</td></tr><tr><td> {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Fe(Tp)]<sub>4</sub>}</td><td>0.48</td><td>0.68</td><td>0.84</td><td>0.98</td><td></td><td>CH<sub>2</sub>Cl<sub>2</sub></td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Fe(pzTp)]<sub>4</sub>}-</td><td>1.38</td><td>1.55</td><td>1.70</td><td>1.79</td><td>0.64</td><td>0.39</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Fe(pzTp)]<sub>4</sub>}-</td><td>1.05</td><td>1.18</td><td>1.32</td><td>1.49</td><td>0.33</td><td>-0.03</td></tr><tr><td> {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Fe(Tpm)]<sub>4</sub>}-</td><td></td><td>-0.25</td><td>-0.05</td><td>0.13</td><td>1.25</td><td>1.14</td></tr><tr><td> {[Fe(pzTp)(CN)<sub>3</sub>]<sub>4</sub> [Fe(Tpm)]<sub>4</sub>}-</td><td></td><td>-0.34</td><td>-0.09</td><td>0.12</td><td>1.15</td><td>0.04</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Co(pzTp)]<sub>4</sub>}-</td><td>1.28</td><td>1.42</td><td>1.52</td><td>1.62</td><td>0.45</td><td>0.15</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Co(pzTp)]<sub>4</sub>}-</td><td>1.00</td><td>1.13</td><td>1.28</td><td>1.45</td><td>0.24</td><td>-0.05</td></tr><tr><td>Rb<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Co(pzTp)]<sub>4</sub>}-</td><td>1.27</td><td>1.41</td><td>1.51</td><td>1.63</td><td>0.46</td><td>0.15</td></tr><tr><td>K<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Co(pzTp)]<sub>4</sub>}-</td><td>1.22</td><td>1.37</td><td>1.48</td><td>1.64</td><td>0.50</td><td>0.20</td></tr><tr><td>K<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Co(pzTp)]<sub>4</sub>}-</td><td>0.96</td><td>1.11</td><td>1.29</td><td>1.50</td><td>0.29</td><td>0.00</td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Co(3TPhTp)]<sub>4</sub>}-</td><td>1.15</td><td>1.44</td><td>1.66</td><td>1.84</td><td>0.22</td><td>-0.01</td></tr><tr><td> {[Fe(CN)<sub>6</sub>]<sub>4</sub> [Co(Me<sub>3</sub>-tacn)]<sub>4</sub>}-</td><td>0.83</td><td>0.98</td><td>1.1</td><td>1.32</td><td></td><td></td></tr><tr><td> Na<sup>+</sup> &lt; {[Fe(CN)<sub>6</sub>]<sub>4</sub> [Co(Me<sub>3</sub>-tacn)]<sub>4</sub>}-</td><td>0.50</td><td>0.70</td><td>0.87</td><td>1.12</td><td></td><td></td></tr><tr><td>K<sup>+</sup> &lt; {[Fe(CN)<sub>6</sub>]<sub>4</sub> [Co(Me<sub>3</sub>-tacn)]<sub>4</sub>}-</td><td>0.82</td><td>0.97</td><td>1.05</td><td>1.31</td><td></td><td></td></tr><tr><td>Rb<sup>+</sup> &lt; {[Fe(CN)<sub>6</sub>]<sub>4</sub> [Co(Me<sub>3</sub>-tacn)]<sub>4</sub>}-</td><td>0.83</td><td>0.97</td><td>1.18</td><td>1.35</td><td></td><td></td></tr><tr><td>Cs<sup>+</sup> &lt; {[Fe(CN)<sub>6</sub>]<sub>4</sub> [Co(Me<sub>3</sub>-tacn)]<sub>4</sub>}-</td><td>0.82</td><td>0.97</td><td>1.05</td><td>1.31</td><td>-0.4</td><td></td></tr></tbody></table>
reflecting both electronic communication and the influence of CoII co-
ordination at the nitrogen termini of the CN ligand. For example, in
{[ReII(triphos)(CN)₃]₄ [CoIICl]₄}, two sets of quasi-reversible redox
waves are observed (Fig. 9a): One set between –0.71 V and – 0.35 V,
corresponding to ReII/ReI reduction, and a second set between +0.71 V
to 0.89 V, assigned to the ReII/ReIII oxidation. These assignments were
made by comparison with the redox behaviour of the [Re(triphos)
(CN)₃]⁻ precursor. However, this approach should be applied with
caution, as it does not hold for all M' metal ions.

The {[Re(triphos)(CN)₃]₄[FeCl]₄} cluster shows peculiar redox
properties, as itis isolated with Re¹ and Fe³⁺ redox states, indicating an
intramolecular electron transfer during the formation of the poly-
metallic complex. As shown in Fig. 9b, seven quasi-reversible redox
events are observed. The four redox processes above 0.28 V are attrib-
uted to the oxidation of Re¹ to Re¹, while the three below +0.28 V
correspond to the reduction of three Fe³⁺ to Fe¹. However, for most of the
{[Re(triphos)(CN)₃]₄ [M'Cl]₄} complexes, small additional redox
waves appear, suggesting partial fragmentation in solution during
cycling.

More redox-stable complexes are systematically obtained using [Fe
(RTp)(CN)₃]⁻ building blocks. Selected examples of scorpionate-capped
{Fe₄-Mn₄}, {Fe₄-Fe₄}, {Fe₄-Co₄}, and {Fe₄-Ni₄} complexes are shown
in Fig. 10. Each of these cubes displays four reversible FeII/III redox
waves with no sign of fragmentation over cycling.

A related case is $K^+ = \{[Fe^{II}(CN)_6]_4[Co^{III}(Me_3tacn)]_4\}^{4-}$ cube, [111] which shows four well-defined successive redox events, assigned to the step-wise Fe$^{II/III}$ redox processes (Fig. 11a). The one-electron events are well-separated, spanning from 0.50 V and 1.40 V versus NHE.

By contrast, the Na⁺-encapsulating analogue shows a less resolved
cyclovoltammetry profile (Fig. 11b). Actually, the absence of the elec-
trochemical resolution of Fe-centres was also observed in our group in
the case of cubes without encapsulated cation. [100]

The observed splitting between the FeII/III redox events indicates electronic communication between second neighbour metal atoms. These potential differences allow estimation of comproportionation (or disproportionation) constants, which reflect the thermodynamic stability of the mixed-valence states relative to one another. [123] Splittings above 0.23 V suggests thermodynamically stable mixed valence states, smaller values typically indicate the presence of several oxidation states in solution where one is dominant. However, such constants should be interpreted cautiously, as they are sensitive to solvent and electrolyte effects. [157]

Moreover, our group recently reported several examples of com-
plexes demonstrating redox activity not only at the M-site (i.e. Fe-
cyanide) but also at the M'-site (M' = Fe [129] or Co [104,124]). A
representative example is the Cs⁺ = {[FeII(Tp)(CN)₃]₄ [CoII/
III(pzTp)]₄}⁻ cube, whose CV (Fig. 12) shows two well-separated sets of
four redox waves: the lower-potential set with three one-electron redox
processes corresponds to CoIII/II, and the high-potential set with four
well resolved one-electron waves to FeIII/II centres. [104]

The cobalt metal ion redox processes are slow and electrochemically
irreversible but chemically reversible. Such behaviour is characteristic
of sluggish redox systems, in which the electron transfer is coupled to a
spin transition on the metal complex. [158] In this system, oxidation of
the high-spin CoII centre (t⁵g₂g²e²ₙ) to CoIII leads to a low-spin configuration
(t⁵g₂g), accompanied by a pronounced structural reorganization of the
cobalt coordination sphere (a substantial shortening of the Co—N bonds,
Δd<sub>Co—N</sub> ≈ 0.18 Å), and thus a significant shift in the redox potential.

The sodium encapsulating $Na^+ = \{[Fe^{II/III}(pzTp)(CN)_3]_4[Ni^{II}(L')_4]^{2+}$ cube, bearing the Tpe-based capping ligand L' with TTF functionality was studied by Oshio *et coll.* [121] Its CV (Fig. 13) shows two reversible four-electron redox waves at E₁/₂ = 0.46 and 0.74 V (versus SCE) that are ascribed to the TTF moiety of the ligand. The two less intensive, monoelectronic reversible redox waves at 0.03 and 0.23 V are attributed to the Fe centres.

Multiple redox cycling experiments confirmed the stability of the
cage, even when all Fe-cations and TTF ligands are fully oxidized.

Our group recently developed similar cube complexes featuring
electroactive peripheral groups and studied their electrochemical
oxidation behaviour. [127] These results will be addressed in the final
section focused on cube-based materials and devices.

Overall, the cyanide cubes show rich electrochemical behaviour and
remarkable solution stability, reminiscent of polymetallic clusters such
as poly-oxometallates. At the same time, they may inherit the broad
applicability of polymeric PBAs as electroactive components in energy
storage systems (e.g. supercapacitors and ion batteries), electrocatalysis,
and electrochemical sensing. Their tunability via chemical modification
makes them appealing platforms for redox-active materials and devices.

4.3. *Electrochromism*

As mentioned at the beginning, the initial interest in Prussian Blue stemmed from its optical properties. The intense blue colouration of this material is due to a metal-to-metal charge transfer (MMCT) upon photon absorption. Molecular analogues of PBAs can also be intensely coloured compounds. In most cases, ligand-centred excitations are high-energy and lie in the ultraviolet region (e.g. 1H-pyrazole $\lambda_{max} = 211$ nm) while metal-centred processes (d-d transitions) are of low intensity. However, in addition to MMCT, the colour of cubes is strongly influenced by metal-to-ligand and ligand-to-metal charge transfers (MLCT and LMCT respectively). The ability of a metal cation to donate or accept an electron is defined by its redox potential, which directly affects the energy of charge transfer (CT) processes. Therefore, the optical properties of the cubes are inherently coupled to their electrochemistry.

The high stability of the redox-active Tp-based cubes across a wide electrochemical range has led to the observation of remarkable electrochromic properties in one {Fe₄-Mn₄} [130], several {Fe₄-Fe₄}, [123,129] one {Fe₄-Co₄} [124] and one {Fe₄-Ni₄} [121] cubic complexes.

Oshio *et coll.* Reported potential-controlled UV-vis spectra of

<figure><img src="image_15.png" /><figcaption>**Fig. 9.** Electrochemical response (top: DPV, bottom CV) of {[Re(triphos)(CN)₃]₄ [M'Cl]₄} complexes: a) M' = Co; b) M' = Fe in CH₃CN with 0.2 M TBAPF₆ vs Ag/AgCl. Reprinted with permission from [110]. Copyright 2007, American Chemical Society.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 10.** Cyclic voltammograms of selected cubes bearing scorpionate ligands: a) $Cs^+ = \{[Fe^{II/III}(Tp)(CN)_3]_4[Mn^{II/III}(pzTp)]_4\}^-$ (Reprinted with permission from [130]. Copyright 2020, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim), b) $\{[Fe^{II}(Tp)(CN)_3]_4[Fe^{III}(Tp)]_4\}$ (Reprinted with permission from [123]. Copyright 2008, American Chemical Society), c) $K^+ = \{[Fe^{II}(Tp)(CN)_3]_4[Co^{II/III}(pzTp)]_4\}^-$ (Reprinted under the terms of the Creative Commons CC BY license from [124]), and d) $Cs^+ = \{[Fe^{III}(Tp)(CN)_3]_4[Ni^{II}(Me_2NTp)]_4\}$ (Reprinted from [99]).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 11.** Cyclic voltammetry profiles of a) $K^+ = \{[Fe^{II}(CN)_6]_4[Co^{III}(Me_3tacn)]_4\}^{4-}$ and b) $Na^+ = \{[Fe^{II}(CN)_6]_4[Co^{III}(Me_3tacn)]_4\}^{4-}$ cubes in aqueous solution with 0.1 M electrolyte (KCl and NaClO$_4$ respectively). Reprinted with permission from [111]. Copyright 2020, American Chemical Society.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 12.** Cyclic voltammetry profile of $Cs^+ = \{[Fe^{II}(Tp)(CN)_3]_4[Co^{II/III}(pzTp)]_4\}^-$ cube in CH$_2$Cl$_2$ with 0.1 M TBAPF$_6$ electrolyte. Reprinted with permission from [104]. Copyright 2017, Royal Society of Chemistry.</figcaption></figure>

$\{[Fe^{II}(Tp)(CN)_3]_4[Fe^{III}(Tp)]_4\}$ and $Na^+ = \{[Fe^{II/III}(pzTp)(CN)_3]_4[Ni^{II}(L')]_4^{2+}$ complexes and investigated their electrochromic properties. [121,123] Electrochromic responses are associated with the redox activity of the $\{[Fe^{II/III}(RTp)(CN)_3]^-$ subunits such that up to five distinct redox states are accessible across a potential range limited by the

<caption>**Fig. 13.** Cyclic voltammetry profile of $Na^+ = \{[Fe^{II/III}(pzTp)(CN)_3]_4[Ni^{II}(L')]_4^{2+}$ cube in CH$_3$CN with 0.1 M TBAPF$_6$ electrolyte. Reprinted with permission from [121]. Copyright 2012, Royal Society of Chemistry.</caption>

stability of the molecular species. For example, UV-vis-NIR absorption spectra of $\{[Fe^{II}(Tp)(CN)_3]_4[Fe^{III}(Tp)]_4\}$ in CH$_2$Cl$_2$ were recorded between 0 and 0.85 V (vs Ag/Ag$^+$, Fig. 14). [123]

At 0 V (before oxidation), the spectrum is dominated by bands centred at 370 nm and 816 nm (red line in Fig. 14). Upon oxidation (Fig. 12b shows CV profile of the same complex) the band at 370 nm shifts to lower energy and its intensity increases. This band was attributed to a combination of the Fe$^{II}$-Tp metal-to-ligand (MLCT) and Fe$^{III}$-Tp ligand-to-metal charge transfer (LMCT) absorptions. Over the course of oxidation, the MLCT disappears while LMCT becomes stronger due to the disappearance of Fe$^{II}$ and the corresponding increase in Fe$^{III}$. For the same reason, the metal-to-metal CT band between neighbouring Fe$^{II}$.
<figure><img src="image_19.png" /><figcaption>**Fig. 14.** UV–vis spectra of {[**Fe**<sup>II</sup>(Tp)(CN)<sub>3</sub>]<sub>4</sub> [**Fe**<sup>III</sup>(Tp)]<sub>4</sub>} cube upon electrochemical oxidation (potential vs Ag/Ag<sup>+</sup>, 0.6 mM, 0.1 M TBAPF<sub>6</sub> in CH<sub>2</sub>Cl<sub>2</sub>). Arrows indicate absorbance changes upon electrochemical oxidation. Reprinted with permission from [123]. Copyright 2008, American Chemical Society.</figcaption></figure>

CN-Fe<sup>III</sup> at 816 nm steadily decreases as the potential increases. Meanwhile, a new broad band appears around 1000 nm during the oxidation but disappears once the process is complete. This band is attributed to second-neighbour MMCT between remote Fe<sup>III</sup>-CN-Fe<sup>III</sup>-NC-Fe<sup>II</sup> metal centres.

Later, we extended this study by investigating a related Cs⁺ c {[Fe<sup>II</sup>(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Fe<sup>II</sup>(pzTp)]<sub>1</sub> [Fe<sup>III</sup>(pzTp)]<sub>3</sub>⁻ cube [129] that shows up to nine accessible redox states over a range of ca. 2 V. The incorporation of a Cs⁺ ion confers greater stability to the cube, enabling the exploration of its electrochromic properties over a broader potential range. Indeed, as shown in Fig. 15, varying the potential allows switching on and off multiple CT bands: (1) LMCT, MLCT (of both [Fe(Tp)(CN)<sub>3</sub>] and [Fe(pzTp)(NC)<sub>3</sub>] corners), (2) the first neighbour MMCT (Fe<sup>II</sup>-CN-Fe<sup>III</sup>) and (3) the second neighbour MMCTs (Fe<sup>II</sup>-CN-Fe<sup>III</sup>-NC-Fe<sup>III</sup> and Fe<sup>II</sup>-NC-Fe<sup>II</sup>-CN-Fe<sup>III</sup>) bands. The result is a variety of optical signatures. Due to the accessible redox states at low potential, this study

also revealed two different second-neighbour MMCT bands, centred around 1100 and 1300 nm. Upon oxidation, (Fig. 15d) remote electron transfer occurs between pairs of [Fe<sup>II</sup>(Tp)(CN)<sub>3</sub>]/[Fe<sup>III</sup>(Tp)(CN)<sub>3</sub>] through Fe<sup>II</sup>-CN-Fe<sup>III</sup>-NC-Fe<sup>III</sup> linkages. Upon reduction, (Fig. 15b) the electron transfer takes place between the [Fe<sup>II</sup>(pzTp)(NC)<sub>3</sub>]/[Fe<sup>III</sup>(pzTp)(NC)<sub>3</sub>] moieties via Fe<sup>II</sup>-NC-Fe<sup>II</sup>-CN-Fe<sup>III</sup> linkages. Both bands are broad, low in energy but distinct.

Redox controlled absorption spectra of Na⁺ c {[Fe<sup>II</sup>/III](pzTp)(CN)<sub>3</sub>]<sub>4</sub> [Ni<sup>II</sup>(L')]<sub>4</sub>²⁺ cube [121] were recorded in CH<sub>3</sub>CN by applying potentials of 0.36 to -0.40 V and 0.36–1.00 V versus SCE. The spectrum shows similar variations in LMCT and MLCT bands corresponding to the [Fe<sup>II</sup>/III(pzTp)] corner. Meanwhile, the presence of redox active TTF moieties gives rise to specific electrochromic signatures, namely of the cation-radical TTF*⁺, its dimer and bication TTF²⁺. However, no intervalence charge transfer between TTF units and metal cations are observed. Thus, at in this compound at least, the cube and TTF redox systems appear to be largely independent.

To summarise, unlike polymeric PBAs, the cube complexes exhibit good solubility and solution stability over a wide electrochemical window, as well as a higher number of distinguishable redox states, each associated with several CT transitions. Therefore, they are promising candidates for the development of poly-electrochromic materials, offering the possibility of tuning the optical output through modulation of the chemical nature of the metals and ligands.

## 5. Magnetic properties

Prussian Blue and its analogues have attracted significant interest due to their magnetic properties. The M and M' paramagnetic centres, linearly connected through the cyanide linker, can be ferro- or antiferromagnetically coupled; [7] this interaction is well-rationalized by Kahn's model. [159,160] Several PBAs are emblematic magnetic materials, with the V<sup>II</sup>/III[Cr<sup>III</sup>(CN)<sub>6</sub>]<sub>γ</sub> PBA derivatives (of various stoichiometry) being among the few molecule-based systems exhibiting Curie temperatures above room temperature. [161–163] Some Fe—Co PBAs were also the first reported photomagnetic systems based on metal-metal electron transfer. In 1996 Hashimoto et al. demonstrated that diamagnetic Fe<sub>L</sub><sup>II</sup>Co<sub>L</sub><sup>III</sup> pairs could be converted into paramagnetic

<figure><img src="image_20.png" /><figcaption>**Fig. 15.** Electrophromism of Cs⁺ c {[Fe<sup>II</sup>(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Fe<sup>II</sup>/III(pzTp)]<sub>4</sub>⁻ cube in CH<sub>3</sub>CN: a) 9 accessible redox states with potentials needed to access fully reduced {Fe<sub>4</sub>-Fe<sub>4</sub>}⁴⁻, fully oxidized {Fe<sub>4</sub>-Fe<sub>4</sub>}⁴⁺ and meridional {Fe<sub>4</sub>-Fe<sub>4</sub>} states; c) CV of this cube; UV–vis spectra upon b) a reduction from {Fe<sub>4</sub>-Fe<sub>4</sub>} to {Fe<sub>4</sub>-Fe<sub>4</sub>}⁴⁻ and d) an oxidation from {Fe<sub>4</sub>-Fe<sub>4</sub>} to {Fe<sub>4</sub>-Fe<sub>4</sub>}⁴⁺. Reprinted with permission from [129]. Copyright 2020, Royal Society of Chemistry.</figcaption></figure>
$Fe_{LS}^{III}Co_{HS}^{II}$ pairs by light irradiation at low temperatures in the $K_{0.2}Co_{1.4}[Fe(CN)_6]·6.9H_2O$. [55] Since then, significant developments have occurred. It was shown that temperature, pressure or electric field could also trigger the reversible Electron Transfer-Coupled Spin Transition (ETCST). [164–170] Recently, this mechanism has been investigated using time-resolved spectroscopic techniques to gain deeper insight. [171,172] To date, the Fe—Co model remains the most studied system, but other M-CN-M' pairs have also been investigated, [169] including, for example, Fe—Mn PBAs. [173]

Significant research efforts are focused on the design of low-dimensional cyanide bridged complexes that could mimic the switchable properties of the PBAs and eventually showing additional physical functionalities. [72,78,174–177]

Low dimensional cyanide paramagnetic complexes, including some cubic complexes, have also found interest as slow magnetic relaxation systems (Single Molecule Magnets, “SMMs”) which can retain their magnetization in the absence of a magnetic field at low temperatures. They have gained strong attention in the last decades both for fundamental reasons and for potential applications in data storage at the molecular scale. [159,178,179] In the following section, we briefly describe some examples of SMMs based on cyanide bridged cubes, followed by a more detailed discussion of switchable cubic systems exhibiting either spin transition (ST) or ETCST properties.

## 5.1. High spin molecule and single molecule magnet

Several groups have demonstrated the utility of metal-cyanide chemistry in the preparation of high-spin coordination clusters that can exhibit SMM behaviour. Among these, some cyanide cubes were investigated in the early 2000s.

In 2003 Long *et coll.* Reported the first cube, {[**Fe**<sup>III</sup>(tach)(CN)<sub>3</sub>] <sub>4</sub> [Ni<sup>II</sup>(H<sub>2</sub>O)<sub>3</sub>] <sub>4</sub>}<sup>8+</sup> in which the metals at both sites are paramagnetic. [114] Magnetic measurements indicated the presence of weak ferromagnetic exchange interactions, with a coupling constant $J = +7.9$ K, resulting in an “S = 6” ground state. This ferromagnetic coupling arises from the interaction between four $Fe_{LS}^{III}(t_{2g}^5)$ ($S = 1/2$) and four $Ni^{II}(t_{2g}^6e^{2g})$ ($S = 1$) centres, with unpaired electrons occupying orthogonal orbitals. However, no SMM behaviour was observed in this system.

Subsequent studies of {Fe<sub>4</sub><sup>III</sup>-Ni<sub>4</sub><sup>II</sup>} systems identified several cubic complexes exhibiting SMM behaviour, including:

(1) {[**Fe**<sup>III</sup>(Tp)(CN)<sub>3</sub>] <sub>4</sub> [Ni<sup>II</sup>(Tp)] <sub>4</sub>} [122],

(2) {[**Fe**<sup>III</sup>(pzTp)(CN)<sub>3</sub>] <sub>4</sub> [Ni<sup>II</sup>(Tpe)] <sub>4</sub>}<sup>4+</sup> [117]

(3) and {[**Fe**<sup>III</sup>(pzTp)(CN)<sub>3</sub>] <sub>4</sub> [Ni<sup>II</sup>(phen)(CH<sub>3</sub>OH)] <sub>4</sub>}<sup>4+</sup>. [147]

The $\chi T$ vs T plots for these cubes indicate ferromagnetically coupled $Fe^{III}$ and $Ni^{II}$ centres (Fig. 16a demonstrate that of compound (2)) and simulations of the experimental magnetic data yielded J values of approximately 9.6 K [117] and 8.6 K [147]. Least square fits of the M vs

H/T data (inset Fig. 16a) suggest effective spin reversal energy barriers $U_{eff}$, of approximately 14.0 K, [122] 11.9 K [117] and 20.2 K [147] for complexes (1)–(3) respectively. These compounds behave as SMM at temperatures below 1 K.

The inherent magnetic anisotropy of the metal ions is a crucial factor in achieving SMM behaviour. For this reason, the use of anisotropic 4d and 5d transition metal ions has been explored. In 2004, Dunbar et al. reported the first example of a cubic SMM cluster {[**Re**(triphos)(CN)<sub>3</sub>] <sub>4</sub> [MnCl]<sub>4</sub>} [108,110] which, at that time, was also the only known SMM containing a 5d transition metal. The $\chi_M T$ vs T profile suggests a “S = 8” ground-state, resulting from antiferromagnetic coupling between four $Re^{II}$ ($S = 1/2$) and four $Mn^{II}$ ($S = 5/2$) ions (Fig. 17a). AC magnetic susceptibility and µ-SQUID measurements (Fig. 17b) confirm the SMM behaviour of this cube, and the effective energy barrier was estimated to be close to $U_{eff} = 12.7$ K.

Finally, field-induced SMM behaviour was also recently reported in {Fe<sub>4</sub>-Co<sub>4</sub>} cubes containing a unique paramagnetic high-spin Co<sup>II</sup> centre. [104,124] In contrast to the previously described Fe—Ni and Re—Mn complexes, which feature every quartet of cations in uniform oxidation states and an overall $T_d$ geometry, the $A^+ = \{[Fe^{II}(Tp)(CN)<sub>3</sub>] <_4 [Co^{II}(pzTp)] <_1 [Co^{III}(pzTp)] <_3\}^-$ ($A^+ = K^+$ or $Cs^+$) cube adopts C<sub>3v</sub> point group symmetry and is notably less symmetric. It was found that the magnetic relaxation time and the effective energy barrier are related to the distortion in the Co<sup>II</sup> coordination environment, which is itself influenced by the nature of the encapsulated alkali ion. The K-cube which exhibits greater structural distortion, shows a higher energy barrier compared to the Cs-cube (130.6 K vs 71.6 K, respectively).

Overall, although some cubic complexes show slow magnetic relaxation, these effects are typically observed only at very low temperatures and do not compete with the performance of more extensively studied SMMs, such as {Mn<sub>12</sub>} oxoclusters or lanthanide-based systems. The high symmetry of the molecule is generally unfavourable for inducing strong magnetic anisotropy, and thus limits their SMM behaviour. Instead, these systems have garnered more attention for their switchable magnetic properties, which are associated with reversible electron transfer and/or spin-crossover phenomena.

## 5.2. Magnetic molecular switches

Molecular switches have significant potential for application in the fabrication of multifunctional materials and nanodevices. [180–182] In particular, magnetic switches, such as spin-crossover (SCO) [183] or charge-transfer complexes, [175] may find numerous applications as sensors, actuators, memory device, or in molecular electronics. [184,185]

### Spin-Crossover Systems (SCO)

Numerous examples of compounds with cyanide coordinated SCO

<figure><img src="image_21.png" /><figcaption>**Fig. 16.** Magnetic data for {[**Fe**<sup>III</sup>(pzTp)(CN)<sub>3</sub>] <sub>4</sub> [Ni<sup>II</sup>(Tpe)] <sub>4</sub>}<sup>4+</sup>: a) $\chi T$ vs T plot with M vs H/T inset and µ-SQUID magnetization vs H measurements b) from 0.04 to 7 K at 0.14 Ts<sup>-1</sup>, and c) at different scan rates. Reprinted with permission from [117]. Copyright 2006, American Chemical Society.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. 17.** a) χT vs T plot and b) µ-SQUID magnetization vs H measurements at 0.14 T·s⁻¹ showing hysteretic behaviour at T = 0.04 K of {[Re(triphos)(CN)₃]₄[MnCl]₄}. Reprinted with permission from [110]. Copyright 2007, American Chemical Society.</figcaption></figure>

metal centres have been reported. Among these are cyanide-bridged squares, such as the first example, $[Fe_4^{II}(CN)_4(bpy)_4(tpa)_2](PF_6)_4$ (tpa = tris(2-pyridylmethyl)amine), presented by Oshio et al., which shows a two-step spin transition. [186] However, to date, only one cubic SCO molecule namely the $Cs^+ = \{[Fe^{II}(Tp)(CN)_3]_4 [Fe^{II}(pzTp)]_1 [Fe^{III}(pzTp)]_3\}^-$ cube has been reported by our group in 2020. [129] A gradual rise of $\chi_M T$ values with $T_{1/2} = 184$ K corresponds to that expected for a complete Fe$^{II}$ SCO (ca. 3.5 cm³ mol⁻¹ K). It is noteworthy that the very similar empty cube– $\{[Fe^{II}(Tp)(CN)_3]_4 [Fe^{III}(Tp)]_4\}$, earlier prepared by Oshio et al., [123] does not exhibit any spin transition. In this system, Fe$^{II}$ centres in $C_3N_3$ coordination environment are in low spin state while Fe$^{III}$ centres in the $N_6$ environment are in high-spin state and these electronic configurations remain unchanged under temperature variations as revealed by Mössbauer spectroscopy at 20, 200, and 293 K. However, the encapsulation of a cation stabilises an anionic cube and leads to the reduction of one Fe$^{III}$ corner to Fe$^{II}$. The Fe$^{II}$ centre in a $N_6$ coordination environment with a tris(pyrazolyl) donor ligand is a possible candidate for SCO. In fact, nearly all $A^+ = \{Fe_4^{II}-Fe_4^{II/III}\}^-$ cubes studied by our group exhibit a gradual spin transition, attributed to the SCO occurring at this single Fe$^{II}$(N₆) site. [100] However, this transition is highly sensitive to lattice solvent molecules and disappears upon drying the sample, which locks this Fe$^{II}$ in its high-spin state. [129] The nature of the alkali cation does not significantly influence SCO behaviour. In contrast, changing the blocking ligand (from Tp to pzTp) allows for fine-tuning of the transition temperature. Due to the stronger ligand field of pzTp, the LS state is stabilized, resulting in higher SCO temperatures.

### Electron-Transfer (ET) and Electron-Transfer-Coupled-Spin Transition (ETCST)

In contrast to SCO compounds, where electronic reorganisation occurs at a single metal centre, electron-transfer (ET) switches involve charge transfer between two metal centres, resulting in the formation of two distinct valence isomers. While SCO is typically observed in complexes of 3d transition metal ions complexes, reversible metal-metal ET is not restricted to 3d metals and can also involve heavier 4d or 5d metal ions. The accompanying changes in physical properties (e.g., optical and magnetic) strongly depend on the identity of the two metal ions involved. Numerous studies have demonstrated that reversible charge transfer can occur in molecular systems, including cyanide-bridged cubic molecules $\{Fe_4-M'4\}$ with M' = Mn, Fe or Co.

Notably, such switchable properties have never been observed in the original Fe–Fe Prussian blue compound. This is because reversible ET requires precise tuning of the redox potentials of both metal ions – an intrinsic limitation in PBAs, but one that is achievable in discrete molecular systems where ligand field and redox potential can be adjusted by tuning the nature of ancillary ligands. For instance, Wen et al. published a $\{Fe_{13}\}$ cluster [187] exhibiting thermally induced SCO

alongside an irreversible ET at high temperatures. Meanwhile Sato et al. described several chain-like Fe$^{II/III}$ coordination polymers [188,189] which undergo reversible ET from the $Fe_{LS}^{III}-CN-Fe_{HS}^{II}$ state to $Fe_{LS}^{II}-CN-Fe_{HS}^{III}$ upon cooling. All these compounds were synthesised from Fe$^{III}$ triscyanido precursors capped by borate scorpionate ligands.

Evidence of thermally induced ET from an $Fe_{LS}^{II}-Fe_{HS}^{III}$ to an $Fe_{LS}^{III}-Fe_{HS}^{II}$ state was also reported by our group in Glatz's doctoral thesis, using the aforementioned family of $A^+ = \{Fe_4-Fe_4\}$ cubes. [100] In the $K^+ = \{[Fe^{II}(Tp)(CN)_3]_4 [Fe^{III}(Tp)]_4\} \cdot ClO_4$, $Cs^+ = \{[Fe^{II}(pzTp)(CN)_3]_4 [Fe^{II/III}(Tp)]_4\}^-$, $Cs^+ = \{[Fe^{II}(Tp)(CN)_3]_4 [Fe^{II/III}(pzTp)]_4\}^-$ and $Cs^+ = \{[Fe^{II}(pzTp)(CN)_3]_4 [Fe^{II/III}(pzTp)]_4\}^-$ cubes, an increase in $\chi_M T$ values by approximately 0.3–0.5 cm³mol⁻¹K was observed above 300 K, suggesting partial ET conversion. More definitive evidence for the Fe–Fe ET in cubic complexes was recently provided by Zhang et al., who reported two mixed-valence, cyanide-bridged cubes $\{[Fe^{II}(RTp)(CN)_3]_4 [Fe^{III}(Tpm)]_4\}^{4+} \cdot 4OTf^-$ (where RTp = Tp or pzTp). [148] In these cases, thermally reversible ET transitions, between the $Fe_{LS}^{II}-CN-Fe_{HS}^{III}$ (low temperature) and $Fe_{LS}^{III}-CN-Fe_{HS}^{II}$ (high temperature) states, were observed at lower temperatures ($T_{1/2} = 230$ and 274 K, respectively), allowing full characterisation by magnetic measurements, single-crystal XRD, variable-temperature FT-IR and Mössbauer spectroscopies.

Although the $\{Fe_4-Fe_4\}$ cubes from our group and those by Zhang et al. are not structurally identical, electrochemical factors may explain differences in transition temperatures. In both systems, the M-site (Fe bonded to the C-terminal of CN) is in the $Fe_{LS}^{II}$ state at low temperature, while the M'-site (Fe bound to the N-end) is $Fe_{HS}^{III}$. When Fe$^{III}$ is coordinated by the neutral Tpm ligand, the cube corner can be considered “more cationic” compared to when coordinated by Tp or pzTp. This facilitates reduction, leading to ET at a lower temperature in Tpm-based cubes (room temperature) than in Tp-based cubes (onset above 300 K). Moreover, the higher $T_{1/2}$ observed for $\{[Fe^{II}(pzTp)(CN)_3]_4 [Fe^{III}(Tpm)]_4\}^{4+} \cdot 4OTf^-$ compared to its Tp analogue likely results from the stronger ligand field splitting of pzTp, which stabilises Fe$^{II}$ and makes it harder to oxidise, rendering ET more energetically demanding. This hypothesis is supported by cyclic voltammetry measurements of iron-triscyanido precursors: $[Fe^{III}(pzTp)(CN)_3]^-$ is easier to reduce than $[Fe^{III}(Tp)(CN)_3]^-$ (reduction potentials: –0.27 V and –0.33 V, respectively, vs Ag/AgCl reference). [148]

In 2020, our group reported a mixed-valence manganese-based cube, $Cs^+ = \{[Fe^{II/III}(Tp)(CN)_3]_4 [Mn^{II/III}(pzTp)]_4\}^-$. [130] Its magnetic properties were studied between 2 and 370 K. At room temperature, the $\chi_M T$ value is approximately 17.3 cm³ mol⁻¹ K, which is consistent with two $Fe_{LS}^{III}$, three $Mn_{HS}^{II}$ and one $Mn_{HS}^{III}$. Around 300 K, the $\chi_M T$ vs T curve rises to approximately 19.0 cm³mol⁻¹K indicating the onset of a transition attributable to electron transfer between the $\{Fe_{LS}^{II}-CN-Mn_{HS}^{III}\}$ pair generating $\{Fe_{LS}^{III}-CN-Mn_{HS}^{II}\}$ one. The difference of ca. 1.7 cm³mol⁻¹K
aligns with the expected value for the conversion of one {FeII-CN-MnIII} pair. Mössbauer spectroscopy confirmed the intramolecular ET, albeit with incomplete conversion possibly due to variable solvation states. At 80 K, the Fe<sub>LS</sub><sup>II</sup>/Fe<sub>HS</sub><sup>II</sup> ratio was about 50:50, shifting to 36:64 once heated to 400 K. To date this complex remains the unique example of molecular analogue of the FeMn switchable PBAs.

Among all studied systems, Fe-CN-Co based switchable systems are the most extensively investigated. At low temperature, the switchable pairs are in a diamagnetic Fe<sup>II</sup>Co<sup>III</sup> state and can be converted upon increasing temperature to a paramagnetic Fe<sup>II</sup>Co<sup>III</sup> state. This phenomenon is entropically driven since the FeCo paramagnetic state show higher entropy. Importantly, the electron transfer is accompanied by a spin state change on the cobalt from Co<sup>II</sup><sub>LS</sub> to Co<sup>II</sup><sub>HS</sub> on cobalt cation. This coupled phenomenon is referred to as *Electron Transfer Coupled Spin Transition* (ETCST). (Fig. 18) While detailed discussion of the exact mechanisms of ETCST (e.g. ET before or after ST) is beyond the scope of this article, [171,172,190] it is important to note here that both processes occur on ultrafast timescales – a few hundred femtoseconds – making them unresolvable by conventional magnetic or spectroscopic characterization methods.

Control over Fe and Co redox potentials via ligand tuning has been shown to be crucial for the occurrence of ETCST phenomenon. [191,192] The most systematic work published on this correlation was conducted by Nihei et al. on a family of {Fe<sub>2</sub>-Co<sub>2</sub>} square complexes, where they established a quantitative link between the transition temperature and the redox potentials of Fe and Co precursors [193] To our knowledge, no equivalent study has been conducted for the {Fe<sub>4</sub>-Co<sub>4</sub>} cube family.

However, a comparison is possible between the cubes reported by Li et al, [96] and by our group [124]. The {[Fe(pzTp)(CN)₃]₄ [Co(Tpe)]₄}⁴⁺·cube, [96] which features a neutral pyrazolyl-based blocking ligand, undergoes a complete thermally induced ET around 250 K (Fig. 19a). This is evidenced by the χ<sub>M</sub>T value of ca. 12.7 cm³ K mol⁻¹ that is in agreement with four Fe<sub>LS</sub><sup>III</sup> ions and four Co<sub>HS</sub><sup>II</sup> ions. The electronic states before and after the transition were corroborated using variable-temperature UV-vis and FTIR spectroscopy (Fig. 19b) in the solid state, as well as by single-crystal X-ray diffraction analysis. In contrast, the related K⁺ = {[Fe<sup>II</sup>(Tp)(CN)₃]₄ [Co<sup>II</sup>(pzTp)]₁ [Co<sup>III</sup>(pzTp)]₃}⁻ cube, [124] bearing anionic tris(pyrazolyl)borate ligands, exhibits ET above 300 K. The stronger donor ability and the stronger ligand field of the anionic ligand on the Co centre stabilise the Co<sup>III</sup> state over Co<sup>II</sup>, and therefore favouring the diamagnetic Fe<sub>LS</sub><sup>II</sup>-Co<sub>LS</sub><sup>III</sup>

<figure><img src="image_23.png" /><figcaption>**Fig. 18.** Simplified scheme showing *d*-orbitals occupation before and after ETCST between cyanide-bridged Fe and Co cations. Fe ions in orange, Co in blue, N in light purple and C atoms in grey. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

state and raising the transition temperature. These trends are consistent with observations in the aforementioned {Fe<sub>4</sub>-Fe<sub>4</sub>} cubes.

While redox potential is critical, it is not the sole determinant of ETCST. As in SCO complexes, solid-state effects significantly influence the transition. Recently, Zhang et al. [133] showed that crystal packing modulates ETCST by comparing two cube variants: {[Fe(pzTp)(CN)₃]₄ [Co(TpEtOP)]₄}⁴⁺·4ClO₄⁻, and {[Fe(pzTp)(CN)₃]₄ [Co(Tpe)]₂[Co(TpEtOP)]₂}⁴⁺·4ClO₄⁻ (where Tpe is a tris(pyrazolyl)ethanol, Scheme 1, and TpEtOP is its derivative with propargyl function). They demonstrated that the complex with four TpEtOP functionalized ligands upon cooling exhibit partial ETCST while the second one, with a mix of two different ligands on Co corners had a complete and sharp transition. Such difference was attributed to mechano-elastic properties of molecular crystals. The compound with four alkyne functions per cube feature intermolecular π-π and CH-π interactions that lead to competitive elastic interactions in the crystal lattice. Such elastic frustration was shown to limit structural transition and favour in some case the occurrence of half transition. [194] This conclusion is corroborating with results obtained by the same group with tetra-anthryl-functionalized {[Fe(pzTp)(CN)₃]₄ [Co(TpEtOAn)]₄}⁴⁺ cubes. [149]

The encapsulated guest ion can also affect the ETCST. Our group studied a series of A⁺ = {[Fe<sup>II</sup>(Tp)(CN)₃]₄ [Co<sup>II/III</sup>(pzTp)]₄}⁻ cubes with A⁺ = K⁺, Rb⁺, or Cs⁺. [125] Thermal ETCST in the solid state gradually sets in between 300 and 400 K for K⁺ ⊂ {Fe<sub>4</sub>-Co<sub>4</sub>} and Rb⁺ ⊂ {Fe<sub>4</sub>-Co<sub>4</sub>} but is not yet observed for the Cs⁺-containing cube, whose switching temperature is above 400 K. This trend in increasing thermal transition temperature is also observed in PBA bulk materials.

The metal-metal electron transfer can also be triggered by light irradiation, typically at low temperatures. It is worth emphasizing that the direct photo-conversion represents a spin-forbidden process. Therefore, it proceeds through intermediate excited states. [171,172,190] However, the final electronic state resulting from ETCST is metastable and can be trapped at low temperatures, allowing it to be characterised by magnetic and spectroscopic measurements.

A comparable phenomenon is well-known in the context of spin-crossover complexes and is referred to as the LIESST effect (Light-Induced Excited Spin-State Trapping), which was first observed several decades ago in the [Fe<sup>II</sup>(propyl-tetrazole)₆](BF₄)₂ compound. [195] The first observation of light-induced ETCST in a molecular cubic complex was described in 2008 for the already mentioned {[Fe<sup>III</sup>(pzTp)(CN)₃]₄[Co<sup>II</sup>(Tpe)]₄}⁴⁺·4ClO₄⁻ compound. [96] Fig. 19a (red dots) depicts the photo-induced switching of this cube. At low temperatures, the χ<sub>M</sub>T product is close to zero, indicating a diamagnetic ground state corresponding to {Fe<sub>4</sub>-Co<sub>4</sub>}³⁺.

This complex, when exposed to white light irradiation at 30 K, demonstrates a progressive increase in the χ<sub>M</sub>T product, reaching almost ca. 10 cm³ K mol⁻¹ after 20 h. When the temperature rises, the photo-induced paramagnetic state then relaxes back into the diamagnetic one (T<sub>relax</sub> = 180 K).

An important point should be noted here. The ground state of this cube (or any similar compound) at temperatures below T₁/₂ is the diamagnetic {Fe<sub>LS</sub><sup>II</sup>·4-Co<sub>LS</sub><sup>III</sup>}⁴⁺·and above T₁/₂ the paramagnetic {Fe<sub>LS</sub><sup>III</sup>·4-Co<sub>HS</sub><sup>II</sup>}⁴⁺ state. Light excitation at low temperature enables the generation of the {Fe<sub>LS</sub><sup>III</sup>·4-Co<sub>HS</sub><sup>II</sup>}⁴⁺ metastable state, stabilized through an energy barrier E<sub>a</sub>, which is partly related to the structural reorganization that accompanied the ETCST (Fig. 21). Thus, the photoinduced ETCST state is a kinetically trapped state, in contrast to the thermally induced state, which is thermodynamically stable.

Three further examples of photomagnetic cubes are the already mentioned cubes of formula A⁺ = {[Fe<sup>II</sup>(Tp)(CN)₃]₄ [Co<sup>II/III</sup>(pzTp)]₄}⁻ (A⁺ = Cs⁺, Rb⁺ K⁺) prepared by our group. [104,124,125,151] In this case, the relaxation temperatures are considerably lower than for the above-mentioned cube, typically below 70 K (Fig. 20). This difference could be correlated with their higher thermal transition temperature. Actually, as for SCO complexes, when comparing similar molecules, lower T<sub>relax</sub> temperature can be expected
<figure><img src="image_24.png" /><figcaption>**Fig. 19.** χT vs T plots (a) before irradiation (black curve), after irradiation (red) and after thermal quenching (blue), as well temperature dependent FT-IR spectra of cyanide stretching vibrations (b) for the cube {[Fe<sup>III</sup>(pzTp)(CN)<sub>3</sub>]<sub>4</sub> [Co<sup>II</sup>(Tpe)]<sub>4</sub>}<sup>4+</sup>·4ClO<sub>4</sub><sup>-</sup>. Reprinted with permission from [96]. Copyright 2008, American Chemical Society.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Fig. 20.** XRD structure (a) and χT vs T plots (b) of K<sup>+</sup> = {[Fe<sup>II</sup>(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Co<sup>II/III</sup>(pzTp)]<sub>4</sub>}<sup>-</sup>. After irradiation (red dots) and without irradiation (black dots). Reprinted under the terms of the Creative Commons CC BY license from [124]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

when higher T<sub>1/2</sub> are observed. [196] The lower relaxation temperature may also be influenced by the distinct electronic structure of the neutral alkali-containing cubes, which feature a Fe<sub>4</sub>Co<sub>3</sub>Co<sup>II</sup> ground state in contrast to the Fe<sub>4</sub>Co<sub>4</sub><sup>III</sup> ground state of Li's cube. Despite their lower relaxation temperature, the remarkable solution stability of the

A<sup>+</sup> = {[Fe<sup>II</sup>(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Co<sup>II/III</sup>(pzTp)]<sub>4</sub>}<sup>-</sup> molecular cubic complexes has allowed to access accurate electronic information which has helped to enlighten the role of the inserted alkali ions on the photomagnetic process. First, photomagnetic studies were carried out in frozen solution to remove the crystal lattice solid state effects that are

<figure><img src="image_26.png" /><figcaption>**Fig. 21.** a) Schematic representation of the influence of structural reorganization on the activation energy barrier of the relaxation and b) photo-induced ETSCT in A<sup>+</sup> = {[Fe<sup>II</sup>(Tp)(CN)<sub>3</sub>]<sub>4</sub> [Co<sup>II/III</sup>(pzTp)]<sub>4</sub>}<sup>-</sup> complexes with different guest cations: Cs<sup>+</sup> (green), Rb<sup>+</sup> (red) or K<sup>+</sup> (blue). Reprinted with permission from [125]. Copyright 2022 American Chemical Society.</figcaption></figure>
known to influence the switchable properties. Then variable tempera-
ture cyclic voltammetry allowed probing the activation energy associ-
ated to the structural reorganization accompanying the Co<sup>III</sup><sub>LS</sub> ↔ Co<sup>II</sup><sub>HS</sub>
electronic change. It was shown to be higher for smaller alkali ion, K<sup>+</sup>.
Interestingly the CV measurements also point to a better stabilisation of
the Co<sup>II</sup> redox state in Cs-cube than in the K-cube, which notably differs
from previous hypotheses that have been drawn to justify the better
stabilization of the Fe<sup>III</sup>Co<sup>II</sup> metastable state (and higher relaxation
temperature) in PBA with smaller cations. In brief, these results ascribe
the origin of the higher relaxation temperature observed with small
cations to larger structural reorganization (horizontal displacement on
the Fig. 21a) rather than to a stabilisation of the Co<sup>II</sup> state (vertical
displacement on the Fig. 21a). Single-crystal X-ray analyses support the
higher structural reorganization. Distinct interactions between the alkali
cations and the cage are observed. In contrast to the large Cs<sup>+</sup> ion, which
fits well within the cubic cavity and interacts uniformly with all 12
cyanide edges, the smaller K<sup>+</sup> ion is mobile in the cavity. For example, it
is displaced toward one the Co corners of reduced charge {Co<sup>II</sup>(Tp)
(NC)<sub>3</sub>} in the Fe<sub>4</sub>Co<sub>3</sub><sup>III</sup>Co<sup>II</sup> state. [125] This leads to larger geometrical
distortions on the Co<sub>HS</sub><sup>II</sup> ion as compared with the Cs-cube, but also larger
distortion on the cubic cage framework.

Thus, as the electronic configuration of the cage changes, more sig-
nificant structural change could be expected in the more distorted and
"flexible" K⁺-containing cage than in the symmetrical and "rigid" Cs⁺
one. Consequently, the activation barrier between the metastable and
ground state would be higher for smaller guest cations.

In conclusion, cubic octametallic complexes have proven to be ver-
satile systems within the field of molecular magnetism, functioning as
single-molecule magnets, spin-crossover complexes, or mixed-valence
switches. Their combined magnetic and electrochemical properties
make them strong candidates for the design of next-generation

functional materials. Furthermore, their stability in solution enables the
use of deposition techniques that are not applicable to polymeric PBAs,
broadening their potential for integration into device architectures.

6. Toward molecular devices

This section provides an overview of research efforts aimed at inte-
grating cubic complexes into functional devices. To date, only a few such
examples have been reported in the literature. Ultimately, the primary
objective is to develop strategies for depositing these complexes onto
surfaces while ensuring their stability and preserving their switchable
properties. A further step involves the exploitation of their electro-
chemical and magnetic behaviours for device applications.

Recently, Benchohra et al. [126] reported the successful deposition of a molecular cubic switch $Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{III}(3TPhTp)]_4\} \cdot ClO_4^-$ (where 3TPhTp = [4-(3-thienyl)phenyl]tris (pyrazol-1-yl)borate ligand) onto a (111)-oriented gold substrate. The diamagnetic $Cs^+ \subset \{Fe_4-Co_4\}$ cage was deposited using electrospray ionization (ESI), and its stability and structural integrity in the monolayer were confirmed by IR spectroscopy and X-ray photoelectron spectroscopy (XPS). The IR spectrum, recorded by PM-IRRAS after ESI deposition, displayed the characteristic cyanide stretching bands of the bulk molecule and the XPS spectra of both the deposited layer and the bulk $Cs^+ \subset \{Fe_4-Co_4\}$ revealed identical characteristic signatures for $Co^{III}$, $Fe^{II}$ and $Cs^+$ cations (Fig. 22). The deposited layer was found to be sub-monolayer in thickness, as evidenced by the attenuation of the surface Au XPS signal.

A further advancement toward device fabrication was recently ach-
ieved with the preparation of ultrathin films (approximately 20 nm
thick) of the cubic $Ti^{+} \subset \{[Fe^{II}(2TPhTp)(CN)_3]_4 [Co^{II/III}(Tp)]_4\}^{-}$
switch on a conductive surface ($2TPhTp = [4-(2-thienyl)phenyl]tris$

<figure><img src="image_27.png" /><figcaption>**Fig. 22.** XPS spectra of $Cs^+ \subset \{[Fe^{II}(Tp)(CN)_3]_4 [Co^{III}(3TPhTp)]_4\}$ cube powder (a and b) and of the surface after electrospray deposition (c and d). Reprinted with permission from [126]. Copyright 2020 Royal Society of Chemistry.</figcaption></figure>
(pyrazol-1-yl)borate). [127] This was achieved via electrochemical polymerization of the peripheral thiophene groups of the TPhTp ligands. The Tl⁺ cation was selected for encapsulation due to the photomagnetic properties of related complexes. [99] The resulting film was amorphous and homogeneous, with low surface roughness, as revealed by AFM imaging. XPS analysis revealed a Tl/Fe/Co atomic ratio of approximately 0.24:1:0.9, confirming that the structural integrity of the cubes was preserved during deposition. Finally, low-temperature XAS measurements under light irradiation confirmed the occurrence of photo-induced ETCST within the film, with a reversible conversion of FeII to FeIII and CoIII to CoII in similar amounts. Ongoing research in our group has now led to the fabrication of the first two-electrode devices incorporating such electropolymerized films, paving the way for future applications in molecular electronics and spintronics. [132]

The first microdevice incorporating molecular ETCST switchable complexes was reported by Clerac et al. [150] They described a micro-electromechanical system (MEMS) incorporating a molecular switchable material that alters the device’s mechanical properties. To fabricate the MEMS, a piezoelectric cantilever was coated with a layer of the switchable material via drop-casting from solution. The temperature-dependent resonance frequency of the cantilever was investigated. The {[Fe<sup>III</sup>(pzTp)(CN)<sub>3</sub>]₄ [Co<sup>II</sup>(Tpe)]₄}<sup>4+</sup>·4ClO<sub>4</sub><sup>-</sup> cube served as the thermosensitive component. At room temperature, the deposited complex is in a paramagnetic state. Upon cooling, an ETCST occurs, converting the cube into its diamagnetic Fe<sup>II</sup>-Co<sup>III</sup> state (T<sub>1/2</sub> close to 230 K). This transition is accompanied by a reduction in molecular volume and changes in the mechanical properties of the deposited layer. A decrease in the cantilever’s resonance frequency (relative to the uncovered cantilever at the same temperature) was observed. Remarkably, the pristine resonance frequency at 80 K could be restored via white-light irradiation, which induces the reverse ETCST from the diamagnetic Fe<sup>II</sup>-Co<sup>III</sup> state back to the paramagnetic Fe<sup>III</sup>-Co<sup>II</sup> configuration. This metastable photoinduced state persists up to 150 K (Fig. 23). Hence, the MEMS device features can be addressed both thermally and through light irradiation at low temperature. However, the switching behaviour in the film differs from that of the bulk molecular material. The transition is more gradual, with shifts in T<sub>1/2</sub> and in T<sub>relax</sub>, which could be correlated with a partial loss of crystallinity.

The last example of an electronic device based on octametallic complexes is the magnetic tunnel junction (MTJ) reported by Friebe et al. [146] These consist of two ferromagnetic electrodes separated by a nanometre-thick insulating layer. A highly paramagnetic cubic complex,

<figure><img src="image_28.png" /><figcaption>**Fig. 23.** High spin fraction of {[Fe<sup>III</sup>(pzTp)(CN)<sub>3</sub>]₄ [Co<sup>II</sup>(Tpe)]₄}<sup>4+</sup> in a drop-casted film as a function of the temperature. The blue and red dots are deduced from the experimental response of MEMS in cooling (in the dark) and heating (in the dark after light irradiation at 80 K) modes, respectively. Reprinted under the terms of the Creative Commons CC BY license from [150].</figcaption></figure>

{{[Fe<sup>III</sup>(pzTp)(CN)<sub>3</sub>]₄ [Ni<sup>II</sup>(L')]₄}<sup>4+</sup>·4OTf<sup>-</sup> (L' = thiol-functionalized ligand, 1-S(acetyl)tris(pyrazolyl)decane), with a spin state S = 6, was used to bridge the two ferromagnetic electrodes across an insulating layer. (Fig. 24). The presence of the paramagnetic cube strongly altered the spin density of states and significantly reduced the tunnelling current between the electrodes. This effect was not stable over time and varied under thermal treatment, but was definitely attributed to molecular linkage of the electrodes by the paramagnetic complex. Importantly, no such effect was observed when (1) the insulating layer was significantly thicker than the complex itself (thus preventing molecular bridging), or (2) when an organic linker with S = 0 was used.

## 7. Conclusion

Over the past fifteen years, cyanide-bridged octametallic cubic complexes have emerged as molecular counterparts of the well-known Prussian Blue Analogues (PBAs). They were proven to retain essential features of extended PBAs, including metal-metal charge transfer, cooperative redox behaviour, and reversible ion accommodation, all within a single, well-defined entity. More specifically, it is worth underlying that the cubic cyanido-based nanocages show remarkable electronic features. First, they can exhibit a stimuli-induced metal-metal electron transfer through the cyanide bridge, a unique property not found with any other bridging ligand. Secondly, these discrete nanocages can undergo sequential redox switching of all their metal centres, allowing access to as many as nine distinct redox states within a single molecule. This exceptional redox richness translates into an unprecedented tunability of their optical, magnetic, and electronic properties, making them one of the most versatile families of polymetallic nanocages known to date.

Among various synthetic approaches, the use of preformed *fac*-[M(L)(CN)<sub>3</sub>]n⁻ precursors as cube corners – sometime in the presence of templating alkali cations – is often encountered and has proven particularly effective. Unlike the non-stoichiometric, insoluble bulk PBAs which often contain variable amounts of vacancies and inserted alkali cations, and show structural disorder, the molecular models can be isolated as single crystals with uniform coordination environments. This structural precision enables a fine-tuning of the electronic properties through an adequate choice of metal ions and ancillary ligands, offering insights into intrinsic electronic processes that are sometimes obscured in extended PBA frameworks. While intermolecular interactions and matrix effects still remain critical to the properties of the crystal phases of cubic complexes, their solubility and crystallinity facilitate accurate spectroscopic and theoretical investigations.

From a fundamental perspective, ultrafast spectroscopies and electrochemical studies should now be directed at these molecular systems to capture the primary electron- and spin-transfer events that govern photo- and electro-switching. Combined theoretical, and spectroscopic studies, similar to those recently applied to solid PBAs or related discrete molecules, [171,172] could reveal transient states and cooperative pathways hardly accessible to bulk analyses.

Beyond reproducing known PBA behaviour, the design of molecular cubes, in particular those that are stable in solution, open new avenues for the design of advanced materials and molecule-based devices. The ability to access in a controlled manner multiple electronic states at the single-molecule level open routes for designing electronically active materials and molecular spintronic elements. Furthermore, the ability to encapsulate specific cations introduces opportunities for responsive molecular recognition, or coupling between guest identity and cube redox state – features that could lead to adaptive or stimuli-programmed materials. These features highlight the potential of molecular PBAs as adaptive or stimuli-responsive components in functional materials.

The challenge lies in connecting individual molecular cubes into electronically or magnetically communicating arrays without erasing their intrinsic bistability. Thin-film deposition, supramolecular polymerization, and framework assembly of pre-synthesised cubes represent
<figure><img src="image_29.png" /><figcaption>**Fig. 24.** MTJ with {[**Fe**<sup>III</sup>(pzTp)(CN)<sub>3</sub>]<sub>4</sub> [**Ni**<sup>II</sup>(L)]<sub>4</sub>}<sup>4+</sup> molecular cube as channel between NiFe top and bottom electrode (FM2 and FM1). Reprinted with permission from [146]. Copyright 2018, Elsevier.</figcaption></figure>

promising pathways as shown by recent works. [127] The anchoring of such molecular building blocks to conductive substrates or 2D materials is still to be achieved, and could lead to active charge and spin transfer interfaces.

In summary, cyanido-based cubic complexes are not merely models of the polymeric PBAs but represent modular, electronically versatile molecules that can be assembled into hierarchically organized materials. We expect the appearance of new cubic based molecular materials and cubic-based electronic devices.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

No data was used for the research described in the article.

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