Magnetic clusters based on octacyanidometallates

Abstract

Octacyanidometallates make an important branch of cyanide-based molecular magnets that not only follow the trends in modern magnetochemistry and materials science but also have stimulated these fields from the very beginning and still blaze a trail by introducing new concepts such as heterotrimetallic systems, new functionalities and cross-effects like photo-switching of magneto-optical properties and setting new records in magnetic ordering temperatures or magnetic exchange interactions. The following paper focuses on a special class of molecular magnets which, to the best of our knowledge, have not been reviewed so far: octacyanide-based magnetic clusters. A complete list of all known molecules incorporating octacyanides with an extensive discussion of their structures, topologies and magnetic properties, with special attention paid to multifunctional systems, is provided. Several milestone-clusters are discussed thoroughly to emphasize their particular importance in the development of crystal engineering and molecular magnetism.

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Dawid Pinkowicz

Dawid Pinkowicz has been an Associate Professor at Jagiellonian University, Faculty of Chemistry since 2012. He received his PhD from Jagiellonian University with Prof. Barbara Sieklucka and then moved to Prof. Masahiro Yamashita's group for the research project ‘Photo-Switchable Single-Molecule Quantum Magnets’ within the Matsumae International Fellowship Program. Afterwards he joined Prof. Kim Dunbar for the research project ‘Multifunctional Molecular Materials through Cyanide Chemistry’ within the Marie Curie International Outgoing Fellowship funded by the European Commission within the 7^th Framework Programme. His recent research interests cover the design of tailor-made functional ligands for the construction of multifunctional molecular materials.

Robert Podgajny

Robert Podgajny received his PhD from Jagiellonian University in 2002. He developed his skills in molecular magnetism at P. & M. Curie University in Paris and in handling metallic nanoparticles in LCC CNRS in Toulouse (France). In 2013 he received his habilitation (post-doctoral) degree from Jagiellonian University. His area of expertise involves crystal engineering of magnetic coordination polynuclear assemblies. Currently he is developing methods for construction of molecular materials using polynuclear clusters of tuneable composition, structure and magnetic properties, and studying the role of non-covalent anion–π interactions.

Beata Nowicka

Beata Nowicka completed her PhD in coordination chemistry at Jagiellonian University in 1998. She gained experience from the synthetic chemistry laboratories at the University of Erlangen-Nürnberg in Germany and the Institute of Physical and Chemical Research (RIKEN) in Japan. In 2005 she joined the Inorganic Molecular Materials Group and she is now working on guest-responsive porous magnetic networks.

Szymon Chorazy

Szymon Chorazy received his PhD from Jagiellonian University, Faculty of Chemistry in 2014 for the investigation of multifunctional chiral, luminescent, and spin transition molecule-based magnets based on polycyanidometallates. During the PhD studies, he realized the International PhD Study Programme in the cooperation between Jagiellonian University (Inorganic Molecular Materials Group, Prof. Barbara Sieklucka) and The University of Tokyo (Solid State Physical Chemistry Group, Prof. Shin-ichi Ohkoshi). He is now continuing his research within this cooperation, investigating a variety of luminescence, non-linear optical and spin transition phenomena in the magnetically ordered state of novel cyanido-bridged coordination polymers.

Mateusz Reczyński

Mateusz Reczyński is a PhD student and a member of the Inorganic Molecular Materials Group. His area of interest includes the study of the stimulus–response relationship in dynamic molecular networks based on polycyanometallates and 3d-ion complexes. He is a grantee of the Diamond Grant Programme aimed at accelerating the scientific career of outstanding students in Poland.

Barbara Sieklucka

Barbara Sieklucka is a Full Professor of Inorganic Chemistry at Jagiellonian University in Krakow, Poland and the Head of the Inorganic Molecular Materials Group, a co-founder of the European Institute of Molecular Magnetism and a co-author of 106 research papers. Her research focuses on the crystal engineering of highly structured functional magnetic materials on the basis of polynuclear cyanido-bridged coordination compounds, which allow imposing specific functionalities (dynamics, sorption, magnetism, photomagnetism, porosity, luminescence, chirality, non-linear optics etc.) on the target material, with the ultimate goal of achieving multifunctionality and efficient engineering of the nanospace within the crystal network.

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1. Introduction

Molecular magnetic clusters^1–5 are obtained through chemical assembly of appropriate geometrically restricted spin-carriers, organic radicals (p-centres)^6,7 and/or paramagnetic metal ions (d- or f-centres),^8–10 connected in such a way that they form closed discrete structures with magnetic interactions transmitted through intervening overlapping magnetic orbitals of diamagnetic bridges (superexchange) or directly (exchange). In addition to paramagnetic building blocks other molecules bearing various additional functionalities, chirality,¹¹ redox activity,^12,13 photochromism¹⁴ or luminescence,^15,16 can be incorporated. Proper ‘chemical blend’ of functional units can lead to a multifunctional molecule/compound,¹⁷ which apart from interesting magnetism may also exhibit additional properties in response to external factors including light,^18–21 pressure,^22–25 electric field,^26–29 and guest molecules/solvent vapors.^21,30–34 Research on multifunctional magnetic molecules that incorporates basic concepts of chemistry, physics, and materials science has much to offer to the future of science and technology and the elaboration of new types of devices on the nanoscale. The evolution of the field has been described in detail in several reviews and books.^2,5,35–52

The modular approach⁵³ is the most efficient strategy that allows the construction of larger molecules ‘by design’ from specific molecular fragments with properties strongly influenced by individual molecular modules. This step-wise method enables the synthesis of families of structurally related multinuclear molecules with tailored magnetic, electronic, optical and redox properties. Cyanidometallates are perfect candidates for such a modular strategy because of a large variety of known stable homoleptic cyanide complexes, which can be used to link to other metal centres resulting in basically an infinite number of heterometallic^54,55 combinations. Moreover, the cyanide anion is well known for providing short and effective pathways for ferromagnetic or antiferromagnetic superexchange interactions⁵⁶ between metal ions depending on the symmetry of the overlapping magnetic orbitals in the linear or bent M–CN–M′ linkage.^50,57–67 The ligand field at each metal centre and therefore their electronic properties can be controlled by the choice of suitable blocking ligands. Indeed this strategy has led to well-defined and robust multinuclear molecules that mimic extended Prussian blue analogs and allowed the systematic evaluation and modeling of properties due to their discrete nature.^{60,66,68–70} Octacyanidometallates are a specific class of cyanidometallates which are able to form up to eight CN-bridges with other metal centres and, unlike other cyanide complexes, can easily adopt various geometries, which allows uncommon topologies of the resulting CN-bridged assemblies.^71,72 The topology of the heterometallic molecular assemblies is usually controlled by the auxiliary ligands at the metal centre (blocking and bridging).^73–75

In the following contribution we will focus on clusters based on all octacyanidometallates: [Nb^IV(CN)₈]⁴⁻, [Mo^IV/V(CN)₈]^4−/3−, [W^IV/V(CN)₈]^4−/3− and [Re^V(CN)₈]³⁻. They will be reviewed according to their topology-class with particular attention paid to their functionality. Throughout this review a multinuclear molecule being ‘a group of metal centres with direct or indirect interactions i.e. through bridging ligands’ will be referred to as a cluster.⁷⁶

2. Topology

Heterometallic cyanido-bridged clusters show a variety of topologies, which depend mostly on (i) the coordination number and geometry of the polycyanidometallate, (ii) the presence of blocking ligands decreasing the number of bridging CN⁻ groups, (iii) the number and spatial arrangement of labile ligands on the cationic building block, and (iv) relative charges of the building blocks. The most common cluster topologies are presented in Fig. 1. They comprise simple and decorated squares (for 4–8 metal centres), trigonal bipyramids (5), octahedra (6), cubes (8), face-centred cubes (14) and six-capped body-centred cubes (15).

Fig. 1 Typical topologies of cyanido-bridged clusters (cyanidometallate [M(CN)_x] centres – blue spheres, secondary transition metal centres [M′(L)_y(solv)_z] – red spheres).

The simplest M′₂M₂square motif composed of two cyanidometallate anions M and two metal cations M′ is very common. It can be encountered in clusters based on planar tetracyanidometallates,^77–79 as well as hexa-^80–86 and octacoordinated^87–93 cyanido complexes, both with or without blocking ligands at M′ centres. Due to the rigid geometry and 90° angle between cis-CN groups in square-planar and octahedral complexes, the clusters based on tetra- and hexacyanidometallates are usually regular squares.^77,80,81,94 In some cases distortions are caused by the presence of bulky blocking ligands.^79,95–98 For octacyanidometallate-based squares, out-of-plane distortion to the rhombic or rhomboid shape is a rule, as the angle between CN-bridges is lower than 90°. Square clusters based on divalent metal cations M′ and [M(CN)_6/8]^3−/4− ions are negatively charged and require the presence of additional cations in the structure. Neutral square clusters are formed if two CN groups are replaced by neutral ligands in hexacyanidometallates(II)⁹⁹ or octacyanidometallates(IV).¹⁰⁰ The square motif often appears in larger clusters composed of two¹⁰¹ or three⁸⁹vertex-sharing squares and/or containing one or two side-chains.¹⁰¹ The so-called decorated squares may have single additional ion bound to cyanidometallates or cationic building blocks,^101–106 but sometimes the side-chains consist of two¹⁰⁷ or three¹⁰⁸ elements.

Pentanuclear trigonal bipyramid clusters M′₃M₂ are typically composed of two axially arranged polycyanidometallates forming bridges with three equatorially located octahedral metal complexes with four coordination sites blocked by chelating ligands. In this arrangement the cyanidometallate bridges through three facial CN ligands, therefore this kind of cluster is easily formed by hexacyanidometallates, as well as hexa-coordinated tricyanidometallates. This topology is very common for [M(CN)₆]³⁻ ions with divalent 3d metals, as it results in neutral clusters. For octacyanidometallates there are only two known isomorphic trigonal bipyramids, both [Ni(tmphen)₂]₃[M(CN)₈]₂.¹⁰⁹ Among larger clusters this topological motif appears in the form of decorated and vertex-sharing trigonal bipyramids. Interestingly, both apical¹¹⁰ and equatorial¹¹¹ vertex-shared structures are known. In the first case the connecting metal centre is a hexacyanidometallate, in the second case it is the tetrahedral Co²⁺ ion.

Octahedral topology is unique for hexanuclear clusters based on octacyanidometallates. This topology requires a building block with four CN-bridges in facial arrangement, which cannot be achieved in hexacyanidometallates. The known structures are neutral clusters of the M′₄M₂ type that consist of two [M(CN)₈]⁴⁻ ions in axial positions bound to four divalent M′ cations in the equatorial plane.^112–116

Conversely, the topology of a cube is attainable only for octahedral hexa-coordinated cyanidometallates. In M′₄M₄ clusters metal ions are placed alternatingly in the corners of a cube, with each metal connected to three neighbours. This topology is most often realised by the use of octahedral tricyanidometallates.^117–124 Ligands blocking three facial positions at the cationic building blocks^{118,125–128} or pentadienyl tricyanido-stool-type complexes are also utilised.^98,129–132 Cube-shaped cluster structures are numerous. They are widely studied as models of Prussian-blue type structures. A variation of this topology is shown by vertex sharing cubes of the M′₇M₈ type.¹³³

Among larger clusters, comprising more than ten metal centres, the most common is the highly symmetrical topology of face-centred cubes. It is characteristic of M′₈M₆ type clusters, where eight metal cations are placed at the corners of a cube and each is connected to three cyanidometallates located at the six faces, thus forming an octahedron. This type of cluster is often formed by planar tetracyanidometallates.^{119,134–137} The same topology can be realised by reversed placement of building blocks in M′₆M₈ clusters. In this type of structure eight hexa-coordinated cyanidometallates containing a tridentate facial blocking ligand occupy the corners of the cube. The six cations at the centres of each face can have octahedral¹³⁸ or square-pyramid coordination geometry.^119,139 If the face-centred metal ions have no ligand pointing inside the cube (for square-planar or square-pyramid geometry), clusters with this topology can accommodate guest molecules inside.^119,134 Like in the case of trigonal bipyramid or cube topology, there is also an example of a larger cluster composed of vertex-sharing face-centred cubes.¹⁴⁰

The face-centred cube topology clusters are not known to form octacyanidometallates. However, [M(CN)₈]³⁻ ions combined with divalent cations in alcoholic media commonly form pentadecanuclear clusters M′₉M₆, which can be treated as an equivalent of face-centred cube topology appended by one additional cation inside. In this type of cluster M′ cations located at the corners of the cube are connected to three octacyanidometallates each. The [M(CN)₈]³⁻ anions, usually in distorted dodecahedral coordination geometry, are located above the faces of the cube, therefore the topology is more accurately described as a six-capped cube rather than a face-centred cube. Three CN groups of each octacyanidometallate point outside the cube and one inside the cube, binding the additional cation in the centre. The body-centred six-capped cubic topology is unique to octacyanidometallates. Above twenty characterised structures make M′₉M₆ type clusters the most abundant group among discrete octacyanidometallate-based assemblies. Additionally, the clusters, which are stable in solution, have been employed as secondary building units (SBU) for the construction of extended structures.^141–143

The second largest cluster type based on octacyanidometallates is octadecanuclear M′₁₂M₆ assembly, which was obtained by the use of a tetradentate macrocyclic Schiff base blocking ligand on Ni(II). Two such structures based on Nb(IV) and W(IV), respectively, are known.^144,145 They show the topology of the hexadecorated dodecanuclear ring.

3. Discussion of the topology and properties

3.1 Squares and square-motif based clusters

Molecular squares are the simplest examples of polycyanidometallate-based clusters. The M′₂M₂ core, where metal centres are connected with CN bridges, can be expanded to more complex discrete systems containing more than four metal centres, such as decorated¹⁴⁶ or vertex-sharing^89,114 squares, or more intricate metal-capped squares,^147,148 where additional metal ions are incorporated in compartmental ligands. The M′₂M₂ motive can also be recognised in almost every kind of cluster incorporating octacyanidometallate moieties. All square-based clusters are gathered in Table 1.

Table 1 Molecular squares and square-motive based clusters

Centres	Formula/metallic core	Shape	Functionality		Ref.
			Type	Description
a HL1 = pentadentate ligand – see ref. 73. b L2 = bis(2-aminoethyl)amine.
4	[Cu(phen)3]2{[Cu(phen)2]2[W(CN)8]2}ClO4·10H2O/Cu2W2	Square	—	—	88
	[Mn(tptz)(ac)(H2O)2]2{[Mn(tptz)(MeOH)1.58(H2O)0.48]2[W(CN)8]2}·5MeOH·9.85H2O/Mn2W2		HSM	S = 8/2	89
	[Co(tpm)2]{[Co(H2O)(tpm)]2[W(CN)8]2}·4H2O/Co2W2		HSM	S = 8/2	90
	[Mn(tpm)2]{[Mn(H2O)(tpm)]2[W(CN)8]2}·4H2O/Mn2W2		Spin-glass	τ 0 = 7.1 × 10^−16 s ΔE/kB = 63.1 K	90
	{[Ni(HL1^a)]2[W(CN)8]2}·5H2O/Ni2W2		HSM	S = 8/2	91
	{[Mn(hmp)2]2Cl2}{[Mn(hmp)2W(CN)8]2}·5H2O/Mn2W2		HSM	S = 8/2	149
	{[Mn(bpy)2]2(ox)}{[Mn(bpy)2W(CN)8]2}·4H2O/Mn2W2		Spin-glass	τ 0 = 6.3 × 10^−15 s ΔE/kB = 63.4 K	92
	[Mn(phen)3]2[Mn(phen)2(μ-NC)2W(CN)6]2(ClO4)2·9H2O/Mn2W2		—	—	93
	K2{[Co(tren)]2[W(CN)8]2}·9H2O/Co2W2		—	—	87
	{[Ni(bpy)2]2[W(CN)6(bpy)]2}·12H2O/Ni2W2		—	—	100
	{[Cu(dpa)]2[W(CN)6(bpy)]2}·8H2O/Cu2W2		—	—	100
	{[Fe(bik)2]2[Mo(CN)8]2}(H^MeIm)2·5H2O/Fe2Mo2		SCO/photomagnetism	Gradual	150
6	[Co(bik)3]{[W(CN)8]3[Co(bik)2]3}·2H2O·13MeCN/Co3W3	Decorated square	CTIST/photomagnetism	Gradual	146
	{[Mn(tptz)(MeOH)(NO3)]2[Mn(tptz)(MeOH)(DMF)]2 [W(CN)8]2}·6MeOH/Mn4W2	Double-decorated square	HSM	S = 18/2	89
	{Cu(L2^b)]2[Cu(tren)]2[Mo(CN)8]2}·12H2O/Cu4Mo2		—	—	151
	{[((valen)M′)Ln(OH2)2(μM(CN)8)]2·2[M′′(tpy)2]}(ClO4)2 (Ln = Gd^III, Tb^III; M′ = Cu^II, Ni^II; M = Mo^IV, W^IV; M′′ = Ni^II, Ru^II, Os^II)/M′2Ln2M2	Metal-capped square	SMM	—	147
9	[{Mn(phen)2}6{M(CN)8}3(H2O)2]·nH2O (M = W, Nb; n = 21, 24)/Mn6M3	Double decorated two vertex-sharing squares	HSM (M = Nb)	S = 27/2	114
10	{[Mn6(tptz)6(MeOH)4(DMF)2W4(CN)32]}·8.2H2O·2.3MeOH/Mn6W4	Three vertex-sharing squares	HSM	S = 26/2	89
	{[((valen)M)Tb(H2O)2]4[Mo(CN)8]2}[Mo(CN)8]·nH2O/(M = Ni^II, Cu^II; n = 21, 19)/Cu4Tb4Mo2	Double-decorated metal-capped square	SMM (M = Cu)	τ 0 = 2.12 × 10^−6 s ΔE/kB = 19.3 K	148

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Simplicity and finite sizes of square-core based systems make them excellent objects for theoretical studies and modelling of magnetic exchange between metal centres through the cyanido bridge and intermolecular magnetic interactions. Several models explaining experimental data were proposed and investigated with the spin Hamiltonian,^88,90,114 many-body electronic model Hamiltonian⁹¹ or DFT calculations⁸⁸etc. The comparison of calculation results with structural details gives better understanding of magneto-structural correlations which allows prediction of the character of magnetic interactions.

Most of the square-motive based clusters are high spin molecules. The highest value of the ground spin state S = 27/2 was observed for the nonanuclear {[Mn(phen)₂]₆[Nb(CN)₈]₃(H₂O)₂} cluster¹¹⁴ exhibiting unique topology of double-decorated two vertex-sharing squares. SMM behaviour was observed in a large group of hexanuclear M′₂Ln₂M₂ (M′ = Ni, Cu; M = Mo, W; Ln = Gd, Tb) assemblies with metal-capped square topology, where lanthanide centres from the Ln₂M₂ core are coordinated by a complex of the valen ligand and M′.¹⁴⁷ M′₂Ln₂M₂ species were applied to obtain a new wide class of isostructural hetero-tetrametallic {[((valen)M′)Ln(OH₂)₂(μ-M(CN)₈)]2·2[M′′(tpy)₂]}(ClO₄)₂ systems (M′′ = Ni, Ru, Os; Fig. 2) exhibiting field-induced SMM behaviour at 1.6 kOe, with τ₀ = 3.93 × 10⁻⁸ s and ΔE/k_B = 16.8 K for tungsten derivatives.

Fig. 2 Structural diagram showing the structure of a heterotetrametallic system {[((valen)M′)Ln(OH₂)₂(μ-M(CN)₈)]2·2[M′′(tpy)₂]}(ClO₄)₂ with four different metal centres.¹⁴⁷

The M′₂Ln₂M₂ unit was also observed in another SMM {[((valen)Cu)Tb(OH₂)₂]₄[Mo(CN)₈]₂}[Mo(CN)₈] system,¹⁴⁸ where the central metal-capped square is decorated with two [((valen)Cu)Tb(H₂O)₂] units connected by CN bridges. Its SMM character is described by the relaxation time τ₀ = 2.12 × 10⁻⁶ s and the magnetisation reversal barrier ΔE/k_B = 19.3 K. The frequency dependence of the AC magnetic susceptibility was also noted in two systems incorporating Mn^II₂W^V₂ molecular squares.^90,92 Its character could be rather described as a spin-glass behaviour rather than an SMM one.

There are two examples of photo-switchable square-motif-based compounds: [Co(bik)₃]{[W(CN)₈]₃[Co(bik)₂]₃}·2H₂O·13MeCN¹⁴⁶ and {[Fe(bik)₂]₂[Mo(CN)₈]₂}(HMeIm)2.5H₂O.¹⁵⁰ The structure of [Co(bik)₃]{[W(CN)₈]₃[Co(bik)₂]₃}·2H₂O·13MeCN contains a kite-like (square decorated with a bimetallic tail) mixed-valence {(Co^II/IIIW^IV/V)₂Co^IIIW^V} molecule (Fig. 3). This complex exhibits thermally induced electron transfer in Co^II/III–W^IV/V pairs accompanied by the HS ↔ LS transition on Co centres. Interestingly, the transition occurs only on metal centres within the square core. The low-temperature {(Co^III_LSW^IV)₂Co^III_LSW^V} form exhibits a significant photomagnetic effect upon irradiation at various wavelengths. The proposed mechanism suggests photo-induced CTIST from the Co^III_LS–W^IV to Co^II_HS–W^V state. {[Fe(bik)₂]₂[Mo(CN)₈]₂}(HMeIm)2.5H₂O, on the other hand, shows a gradual thermal SCO transition and photomagnetic effect based on LIESST occurring at the Fe^II centres.

Fig. 3 A kite-like molecule of {[W(CN)₈]₃[Co(bik)₂]₃}·2H₂O·13MeCN.¹⁴⁶

3.2 Trigonal bipyramidal clusters

Trigonal bipyramidal clusters of the M′₃M₂ type based on hexacyanidometallates^{53,68,109,118,152} exhibit numerous magnetic and optical functionalities, including high spin molecules, SMM behaviour, thermal- and photo-induced SCO and CTIST phenomena and linkage isomerism. Among octacyanidometallate-based clusters there are only two examples of trigonal bipyramid topology: {[Ni(tmphen)₂]₃[W(CN)₈]₂}·MeOH·2H₂O and its Mo(V) analogue.¹⁰⁹ In both clusters ferromagnetic interactions between Ni(II) and M(V) centres result in the ground spin state S = 4, however no SMM behaviour was detected. The [Ni(tmphen)₂(NC)₂] units are chirality centres. Similar to many hexacyanidometallate-based clusters with diimine blocking ligands they adopt one configuration within one cluster resulting in chiral ΔΔΔ or ΛΛΛ clusters. Unfortunately both enantiomers are present in the centrosymmetric lattice, which excludes the possibility to observe the optical activity of these compounds. However, trigonal bipyramidal clusters form a great molecular platform for the implementation of chirality in magnetic materials and possible observation of magneto-optical phenomena. Their potential in that field should be exploited.

3.3 Octahedral clusters

Octahedral clusters (also referred to as hexanuclear cages; Fig. 4) are specific for non-rigid octacyanidometallates and are not known for other polycyanidometallates due to the geometrical restrictions. Their design and synthesis relies on the same principles as in the case of trigonal bipyramidal clusters discussed in the previous section. Combination of the divalent 3d transition metal complexes M′ with two chelating blocking groups and two ‘open coordination sites’ in cis-geometry [M′(L)(solv)₂]²⁺ with the flexible octacyanidometallate(IV) [M(CN)₈]⁴⁻ allows the formation of 90°-bent M′–CN–M linkages and consecutively closed CN-bridged architectures – octahedrons, where apical positions are occupied by octacyanidometallates and the equatorial ones – by 3d transition metals.

Fig. 4 Fragment of the crystal structure of {[Mn^II(bpy)₂]₄[Mo^IV(CN)₈]₂} showing the overall geometry and topology typical of the hexanuclear octahedral cluster family.¹¹²

There are only three isostructural octahedral clusters known so far featuring [Mn^II(bpy)₂] and [M(CN)₈] (M = Mo,^112,113 W,^112,113 Nb¹¹⁴) moieties as building blocks. Note that {[Mn^II(bpy)₂]₄[Mo^IV(CN)₈]₂} has been reported twice after its discovery by Sieklucka et al. in 2000¹¹² with different crystallization solvent contents.^115,116

Chronologically, {[Mn^II(bpy)₂]₄[Mo^IV(CN)₈]₂} and {[Mn^II(bpy)₂]₄[W^IV(CN)₈]₂} are the two octahedral clusters that were reported first. Their magnetic properties, however, are rather trivial due to the diamagnetism of octacyanidomolybdate(IV) and octacyanidotungstate(IV). Both show paramagnetic behaviour typical of Mn^II (S = 5/2) centres with very weak inter/intramolecular antiferromagnetic interactions between them (through space or through the diamagnetic –NC–Mo–CN– linkage). Both compounds have been subjected to detailed photomagnetic studies which revealed a strong photomagnetic effect upon UV light irradiation (Fig. 5).¹¹³ The observed photo-induced magnetization was rationalized on the basis of inter-valence charge transfer from the Mn^II–NC–M^IV to the Mn^I–NC–M^V motif involving an intra-ligand π*←π transition within the [Mn^II(bpy)₂(μ-NC)₂] moiety.

Fig. 5 ‘Photomagnetic effect’ for {[Mn^II(bpy)₂]₄[Mo^IV(CN)₈]₂} in the form of magnetic susceptibility vs. UV light irradiation time dependence showing the characteristic increase when the light is switched off after ca. 16 h of constant irradiation at T = 10 K and H_DC = 5 kOe.

The third octahedral cluster {[Mn^II(bpy)₂]₄[Nb^IV(CN)₈]₂}¹¹⁴ is a high-spin molecule with S = 18/2 ground state. Such high total spin is due to significant antiferromagnetic interactions between the two apical Nb^IV and four equatorial Mn^II centres estimated to be −9.1 cm⁻¹ (with the exchange coupling Hamiltonian defined as H = −2J₁₂S₁S₂). Molecules with such a high spin ground state can be used for magnetic cooling at very low temperatures.³

3.4 Pentadecanuclear clusters

3.4.1 Molecular structures. A family of pentadecanuclear six-capped-body-centred cube clusters are easily formed without the polydentate chelating ligands required for other cluster types. The general exact formula of the solvated clusters is {M′[M′(solv)₃]₈[M(CN)₈]₆}, or {M′₉M₆} in short, where M′ represents divalent 3d metal ions and M represents pentavalent 4d or 5d ions forming octacyanidometallate complexes (M = W^V Mo^V or Re^V; Fig. 6 and 1). The central M′ ions coordinate six [M′(μ-CN)₅(CN)₃]³⁻ units with M centres located in the corners of an octahedron. Each face of the octahedron is capped by a fac-[M′(μ-NC)₃(solv)₃] moiety with M′ centres located in the corners of a concentric cube. Clusters with the following combination of metal centres have been reported: M′ = Mn^II, Fe^II, Co^II or Ni^II and M = W^V, Mo^V or Re^V (Table 2).^153–157 The use of FeCl₂·4H₂O and CoCl₂·6H₂O in the same synthesis has been demonstrated to result in a mixed-metal trimetallic {Fe₆Co₃W₆}¹⁵⁸ analogue. The diamagnetic [Re^V(CN)₈]³⁻ has also been used by Long and co-workers for substitution of paramagnetic [M(CN)₈]³⁻ to give bimetallic {Mn₉Re₆} and mixed-metal trimetallic {M′₉M_6−xRe_x} clusters.¹⁵⁹

Fig. 6 The molecular structure of the solvated pentadecanuclear six-capped-body-centred cubes {M′[M′(ROH)₃]₈[M(CN)₈]₆}.

Table 2 Molecular pentadecanuclear six-capped-body-centred cubes and higher nuclearity clusters

Formula/metallic core	Topology	Functionality		Ref.
		Type	Description
a It was later proved that intracluster Mn^II–Mo^V coupling is rather of AF nature leading to SGS = 39/2 as shown for analogous Mn9W6.^159,164 b It was later proved that intracluster Co^II–W^V and Co^II–Mo^V coupling is rather of F nature leading to SGS = 15/2 assuming the effective Co^II spin of 1/2 with an isotropic average g-value of 13/3 operating at low temperatures.^143 c It was later shown that intercluster Co^II–W^V and Co^II–Mo^V couplings are rather of F nature with the effective Co^II spin of 1/2 with an isotropic average g-value of 13/3 operated for Co^II at low temperatures.^143
15-centred clusters with solvent blocking ligands, 15 compounds
{Mn[Mn(EtOH)3]8[W(CN)8]6}·12EtOH/Mn9W6	Six-capped body centred cube	HSM	Mn^II–W^V AF coupling leading to SGS = 39/2	153
{Mn[Mn(MeOH)3]8[Mo(CN)8]6}·5MeOH·2H2O/Mn9Mo6	Six-capped body centred cube	HSM	Mn^II–Mo^V F leading to SGS = 51/2^a	154
{Mn[Mn(EtOH)3]8[Mo(CN)8]6}·6EtOH·3iPrOH·3H2O/Mn9Mo6	Six-capped body centred cube	HSM	Mn^II–Mo^V AF coupling leading to SGS = 39/2	164
{Mn[Mn(MeOH)3]8[Re(CN)8]6}·MeOH·7.8H2O/Mn9Re6	Six-capped body centred cube	Paramagnet	—	159
{Mn[Mn(MeOH)3]8[W(CN)8]5[Re(CN)8]}·nMeOH·mH2O/Mn9W5Re1	Six-capped body centred cube	HSM	Mn^II–W^V AF coupling leading to SGS = 38/2	159
{Mn[Mn(MeOH)3]8[Mo(CN)8]5[Re(CN)8]}·5.5MeOH·4.5H2O/Mn9Mo5Re1	Six-capped body centred cube	HSM	Mn^II–Mo^V AF coupling leading to SGS = 38/2	159
{Mn[Mn(MeOH)3]8[Mo(CN)8]3[Re(CN)8]3}·0.5MeOH·6H2O/Mn9Mo3Re3	Six-capped body centred cube	HSM	Mn^II–Mo^V AF coupling leading to SGS = 36/2	159
{Co[Co(MeOH)3]8[W(CN)8]6}·19H2O/Co9W6	Six-capped body centred cube	HSM, SMM behaviour	Co^II–W^V AF coupling leading to SGS = 21/2^b/τ0 = 7.39 × 10^−11 s, ΔE/kB = 27.8 K	156
{Co[Co(MeOH)3]8[W(CN)8]6}·4,4′-bpdo·MeOH·2H2O/Co9W6	Six-capped body centred cube	HSM, SMM behaviour	Co^II–W^V F coupling leading to SGS = 15/2; τ0 = 4(1) × 10^−9 s, ΔE/kB = 10.3(5) K	143
{Co[Co(MeOH)3]8[Mo(CN)8]6}·4MeOH·16H2O/Co9Mo6	Six-capped body centred cube	HSM	Co^II–Mo^V AF coupling leading to SGS = 21/2^b	156
{Co[Co(MeOH)3]8[W(CN)8]5[Re^V(CN)8]}·nMeOH·mH2O/Co9W5Re1	Six-capped body centred cube	HSM	Co^II–W^V AF coupling leading to SGS = 20/2^c	159
{Co[Co(MeOH)3]8[Mo(CN)8]5[Re(CN)8]}·3MeOH·8H2O/Co9Mo5Re1	Six-capped body centred cube	HSM	Co^II–Mo^V AF coupling leading to SGS = 20/2^c	159
{Ni[Ni(MeOH)3]8[W(CN)8]6}·15MeOH/Ni9W6	Six-capped body centred cube	HSM	Ni^II–W^V F coupling leading to SGS = 24/2	155
{Fe[Fe(MeOH)3]8[W(CN)8]6}·nMeOH/Fe9W6	Six-capped body centred cube	CT phase transition	Thermal CT: Fe^II-HS–W^V (HT) to Fe^III-HS–W^IV (LT); thermal hysteresis loop of 8 K (Tc↓ = 206 K, Tc↑ = 214 K)	157
{Co[Co(MeOH)3]2[Fe(MeOH)3]6[W(CN)8]6}·nMeOH/Co3Fe6W6	Six-capped body centred cube	CT/CTIST phase transition	Thermal CT: Fe^II-HS–W^V (HT) to Fe^III-HS–W (LT) concomitant with thermal CTIST: Co^II-HS–W^V (HT) to Co^III-LS–W^IV (LT); thermal hysteresis loop of 14 K (Tc↓ = 193 K, Tc↑ = 207 K)	158
15-centred clusters with bidentate blocking ligands, 9 compounds
{Ni[Ni(btz)(MeOH)]8[W(CN)8]6}·6MeOH·18H2O/Ni9W6	Six-capped body centred cube	HSM	Ni^II–W^V F coupling leading to SGS = 24/2	162
{Ni[Ni(2,2′-bpy)(H2O)]8[W(CN)8]6}·41H2O/Ni9W6	Six-capped body centred cube	HSM, SMM behaviour	Ni^II–W^V F coupling leading to SGS = 24/2;/τ0 = 1.5 × 10^−13 s, ΔE/kB = 47 K	160
{Ni[Ni(2,2′-bpy)(H2O)]8[W(CN)8]6}·16H2O/Ni9W6	Six-capped body centred cube	HSM	Ni^II–W^V F coupling leading to SGS = 24/2	163
{Ni[Ni(tmphen)(MeOH)]6[Ni(H2O)3]2[W^V(CN)8]6}·6DMF/Ni9W6	Six-capped body centred cube	HSM, SMM behaviour	Ni^II–W^V F coupling leading to SGS = 24/2; hysteresis loop below TB = 0.7 K	109
{Ni[Ni(4,4′-dmbpy)(H2O)]8[W(CN)8]6}·18H2O/Ni9W6	Six-capped body centred cube	HSM	Ni^II–W^V F coupling leading to SGS = 24/2	163
{Ni[Ni(5,5′-dmbpy)(H2O)]8[W(CN)8]6}·8H2O/Ni9W6	Six-capped body centred cube	HSM	Ni^II–W^V F coupling leading to SGS = 24/2	163
{Ni[Ni(4,4′-dtbpy)(H2O)]8[W(CN)8]6}·17H2O/Ni9W6	Six-capped body centred cube	HSM	Ni^II–W^V F coupling leading to SGS = 24/2	163
{Ni[Ni(2,2′-bpy)(H2O)]8[Mo(CN)8]6}·12H2O/Ni9Mo6	Six-capped body centred cube	HSM	Ni^II–Mo^V F coupling leading to SGS = 24/2	161
{Ni[Ni(tmphen)(MeOH)]6[Ni(H2O)3]2[Mo(CN)8]6}·6DMF/Ni9Mo6	Six-capped body centred cube	HSM	Ni^II–Mo^V F coupling leading to SGS = 24/2	109
15-centred clusters in the coordination network, 3 compounds
{Mn[Mn(4,4′-bpy)(EtOH)]2[Mn(4,4′-bpy)0.5(EtOH)2]2[Mn(4,4′-bpy)0.5(EtOH)(H2O)]2[Mn(EtOH)2(H2O)]2[W(CN)8]6}·10EtOH/Mn9W6	Six-capped body centred cube in I^0O^2 network	HSM embedded in I^0O^2 network	Mn^II–W^V AF coupling leading to SGS = 39/2	141
{Mn[Mn(dpe)]4[Mn(dpe)0.5(MeOH)2]2[Mn(MeOH)3]2[W(CN)8]6}·14MeOH/Mn9W6	Six-capped body centred cube in I^1O^2 network	Superpara-magnetic chain of Mn9W6 clusters in I^1O^2 network	Mn^II–W^V AF coupling along the chain built of Mn9W6 clusters, weak AF interchain interactions	142
{Co[Co(4,4′-bpdo)1.5(MeOH)]8[W(CN)8]6}·2H2O/Co9W6	Six-capped body centred cube in I^0O^1 network	HSM, embedded in I^0O^1 network	Co^II–W^V F coupling leading to SGS = 15/2	143
18-centred clusters, 2 compounds
{[Ni(L1)]6[Ni(L1)(H2O)]6[Nb(CN)8]6}·100H2O/Ni12Nb6	12-Metallic Ni6Nb6 ring decorated by 6Ni^II centres	Paramagnet	Overall AF interactions	144
{[Ni(L1)]6[Ni(L1)(H2O)]6[W(CN)8]6}·36H2O/Ni12W6	12-Metallic Ni6W6 ring decorated by 6Ni^II centres	Paramagnet	—	145
20-centred clusters, 1 compound
{[Cu(Me3tacn)]12[Cu(H2O)][W(CN)8]2[W(CN)8]5}·24H2O (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane)/Cu13W7	20-Metallic open winged cage	Paramagnet	Overall F interactions	165

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M′ centres can be coordinated by solvent molecules (solv = H₂O, MeOH, EtOH) or bidentate chelating N-donor or N,O-donor ligands L forming [M′(μ-NC)₃(L)(solv)] units (Fig. 7a).^160–163 However, in {Ni^II[Ni^II(tmphen)(MeOH)]₆[Ni(H₂O)₃]₂[μ-CN]₃₀[W^V(CN)₃]₆} two Ni^II centres located at the opposite external M′ sites do not coordinate 3,4,7,8-tmphen, preserving the original composition: [M′(μ-NC)₃(solv)₃]. The remaining six Ni^II centres adopt [Ni(μ-NC)₃(tmphen)(solv)] coordination, with all six ligands located on the perimeter and oriented almost perfectly parallel to each other (pancake bonds). This results in a disc-like shape with the molecular symmetry lowered from O_h to approximate C_3v (Fig. 7b).¹⁰⁹

Fig. 7 Molecular structures of {Ni₉W₆(4,4′-dmbpy)₈} (a) and {Ni₉W₆(tmphen)₆} (b) presented perpendicular (left) and along (right) the C₃ axis. Colour code: {Ni(solv)_x} – red, {W(CN)₃} – blue, CN⁻ bridges – grey, organic blocking ligands – green.

3.4.2 Isomerism. The consequence of the presence of [M′(μ-NC)₃(L)(solv)] moieties is stereoisomerism as was presented by Nowicka et al.¹⁶³ A total number of possible spatial arrangements of external ligands on eight independent [M′(μ-NC)₃(L)(solv)] moieties per cluster can be calculated as 3⁷ = 2187 (variations with repetition, three combinations per one M′ site). In the case of four symmetrically independent external Ni cations observed for centrosymmetric clusters the number of possible combinations is reduced to 3³ = 27, with the independent crystallographic unit being “half” of the cluster. As several combinations are identical (clusters can be superimposed) six different isomers were identified and their point symmetry was ascribed as D_4h (one), C_4v (one), C_2h (two) and C_i (two combinations; Fig. 8). [M′(μ-NC)₃(L)(solv)]-based isomerism was identified in the case of Ni₉W₆ clusters with bidentate ligands: 2,2′-bpy and its alkyl-substituted derivatives (Table 3).¹⁶³

Fig. 8 Geometrical isomers of centrosymmetric {M′[M′X₂Y]₈[M(CN)₈]₆} type clusters; X – blue sticks = identical monodentate or symmetrical bidentate ligands (bpy-derivatives), Y – red balls = ligand (solvent). For clarity only one half of each cluster is depicted.¹⁶³ Reproduced from ref. 163 with permission from The Royal Society of Chemistry.

Table 3 Comparison of structural data and isomeric forms for known pentadecanuclear clusters with eight symmetrical bidentate ligands

Cluster formula	Cell volume, space group P1̄	No. of symmetry non-equivalent clusters	Effective cell volume per cluster and solvent molecules	Isomers	Ref.
{Ni[Ni(bpy)(solv)]8[W(CN)8]6}	8536	2	4268	III and IV	163
	9392	2	4696	III and IV	163
	9593	2	4797	IV	160
{Ni[Ni(bpy)(H2O)]8[Mo(CN)8]6}	9666	2	4833	IV	161
{Ni[Ni(btz)(H2O)]8[W(CN)8]6}	5036	1	5036	IV	162
{Ni[Ni(4,4′-dmbpy)(solv)]8[W(CN)8]6}	6273	1	6273	VI	163
	6026	1	6026	II and VI	163
{Ni[Ni(5,5′-dmbpy)(solv)]8[W(CN)8]6}	5828	1	5828	IV	163
	12 054	2	6027	IV	163
{Ni[Ni(4,4′-dtbpy)(solv)]8[W(CN)8]6}	7615	1	7615	II	163

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3.4.3 Secondary building blocks. M′₉M₆ clusters can be used as secondary building blocks (SBB) in the construction of more complex supramolecular systems (Fig. 9).^{124,166–168} The fac-[M′(μ-NC)₃(HOR)₃] groups of a M′₉M₆ cluster located in the corners of a cube can be considered as open coordination sites and [M(CN)₈] groups located in the corners of an octahedron with their terminal CN⁻ ligands can act as molecular bridges (Fig. 9a). The use of 4,4′-bpy and its derivatives like dpe led to the formation of extended hybrid coordination polymers unprecedented in cyanide chemistry like the 2-D I⁰O² (I – inorganic connectivity with its dimensionality in the superscript, O – organic connectivity with its dimensionality in the superscript)¹⁶⁹ {Mn₉W₆-(4,4′-bpy)}_∞ polymer revealing double 4,4′-bpy intercluster bridging (Fig. 9b)¹⁴¹ or the 3-D I¹O² {Mn₉W₆-(dpe)}_∞ showing unique quadruple intercluster cyanide-bridging towards a coordination nanowire (Fig. 9c). The observed hybrid organic–inorganic coordination resulted in the hexagonal arrangement of the nanowires.¹⁴² The use of 4,4′-bpdo and Co^II centres leads to two different products depending on the bpdo : Co molar ratio. The 1 : 1 ratio gives 1-D supramolecular zig-zag chains {Co₉W₆⋯(bpdo)}_∞ with hydrogen bonding (MeOH)₂⋯bpdo⋯(HOMe)₂ patterns (Fig. 9d). The excess of bpdo gave the 1-D coordination nanochain I⁰O¹ {Co₉W₆-(bpdo)}_∞ with quadruple linear connectivity and additional monodentate side coordination of bpdo (Fig. 9e).¹⁴³

Fig. 9 Observed reactivity of {M′[M′(ROH)₃]₈[W(CN)₈]₆} (M′ = Mn or Co) molecules in ROH media. {M′(solv)_x} – red, {W(CN)₃} – blue, CN⁻ bridges – grey, organic linkers – green. The green-to-red arrows and blue-to-red arrows indicate the propagation directions of the M′₉M₆ coordination skeleton; (a) M′₉M₆ cluster; (b) {Mn₉W₆-(4,4′-bpy)}_∞ polymer; (c) {Mn₉W₆-(dpe)}_∞ coordination polymer; (d) {Co₉W₆⋯(bpdo)}_∞ H-bonded chain; (e) {Co₉W₆-(bpdo)}_∞ quadruple chain. For details see the text.

3.4.4 Magnetic properties and functionalities. The most important magnetic properties of pentadecanuclear clusters and the relevant parameters are presented in Table 2. The high spin ground states S = 39/2 for Mn₉M₆,^153,154S = 12 for Ni₉M₆,^{155,160,161,163}S = 15/2 for Co₉M₆ (M = Mo, W)^143,156S = 18 for Mn₉W₅Re and Mn₉Mo₅Re and S = 17 for Mn₉Mo₃Re₃ ¹⁵⁹ have been recognized due to the strong magnetic interactions between the CN-bridged metal ions. The magnetic coupling constants J_M′M were calculated from direct fitting or simulation of magnetic data as well as using DFT methods for cyanide-bridged fragments of Mn₉M₆ and Ni₉M₆ clusters. These J_M′M values are within the 0 to −18 cm⁻¹ range for Mn analogues^170,171 and 0 to +16 cm⁻¹ for Ni analogues (for the following exchange Hamiltonian: Ĥ = −2J_M′MŜ₁Ŝ₂).^109,155,171 The antiferromagnetic interactions in Mn₉M₆ clusters were also confirmed by polarized neutron diffraction.¹⁷⁰ The slow magnetic relaxation was reported for Co₉W₆,¹⁵⁶ Co₉W₅Re,¹⁵⁹ Co₉W₆⋯(bpdo)¹⁴³ and Ni₉M₆(bpy)₈ (M = W, Mo).^160,161 The SMM character has been confirmed only for Ni₉W₆(tmphen)₆ by measuring a magnetic hysteresis loop below T = 0.7 K.¹⁰⁹ In the hybrid Mn₉W₆-(bpy) material the magnetic properties related to the high spin ground state of S = 39/2 are preserved,¹⁴¹ while in the Mn₉W₆-(dpe) long range antiferromagnetic ordering below T_c = 3.5 K and double metamagnetic transitions at H = 100 Oe and 480 Oe are observed.¹⁴² The onset of slow magnetic relaxation for the supramolecular Co₉W₆-(bpdo) network below 4 K allowed us to calculate the relaxation time τ₀ = 2 × 10⁻⁷ s and the effective energy barrier ΔE/k_B = 19 K (S = 15/2) being very similar to those found for the original solvated Co₉M₆ clusters. The onset of magnetic relaxation for the coordination polymer Co₉W₆-(bpdo) was shifted below 3 K disabling its reliable analysis. This difference was correlated with the differences in local structural symmetry of the external Co^II centres, structural distortion of the cyanido-bridged skeleton, possible anisotropy of magnetic W^V–CN–Co^II interactions and intercluster magnetic interactions.¹⁴³ The influence of spatial orientation of three terminal MeOH ligands coordinated to the external M′ sites (M′ = Co, Ni) on the magnetic anisotropy was tested theoretically using DFT methods.¹⁷²

Introduction of Fe^II ions as M′ sites in the M′₉M₆ clusters did not lead to a high spin ground state because the obtained compounds Fe₉W₆ and Fe₆Co₃W₆ undergo a reversible charge-transfer assisted solid state phase transition based on Fe^IIW^V (HT)⇄Fe^IIIW^IV (LT) equilibrium. The transitions occur with a thermal hysteresis loop with T_c↓ = 208 K and T_c↑ = 214 K for Fe₉W₆,¹⁵⁷ and with T_c↓ = 194 K and T_c↑ = 207 K for Fe₆Co₃W₆,¹⁵⁸ which was consequently shown by monocrystalline and powder X-ray diffraction, DSC and magnetic measurements. It was also shown that in Fe₆Co₃W₆ electron transfer occurs simultaneously along two different channels: (NC)₇W^V–CN–^HSFe^II⇄(NC)₇W^IV–CN–^HSFe^III (pure electron transfer) and (NC)₇W^V–CN–^HSCo^II⇄(NC)₇W^IV–CN–^LSCo^III (electron transfer combined with the ^HSCo^III⇄^LSCo^III spin transition), the first process being most probably a driving force for the second one (Fig. 10a). The related structural solid state transformation consisted of (i) a local isotropic change in Co–N bond-lengths of ca. 0.16 Å at the central Co site and in M′–O and M′–N bond-lengths of 0.08 Å at external Co or Fe sites (Fig. 10b), (ii) moderate and nearly isotropic cluster compression/expansion of ca. 0.2–0.3 Å (up to 2%, measured between the most remote atoms of terminal ligands) (Fig. 10c) and (iii) shortening/elongation of intercluster distances through hydrogen bonds (up to 4%). It was suggested that the spin transition ^HSCo^III⇄^LSCo^III took place only at the central Co site, while the ^HSFe^II⇄^HSFe^III processes together with a simple shortening of bond lengths in [Co^II(μ-NC)₃(MeOH)₃] moieties occurred in the external M′ corners. Interestingly, the identical change in the crystallographic cell volume between ca. 16 000 and ca. 17 000 ų was observed for both Fe₉W₆ and Fe₆Co₃W₆, parallel to different local changes at M′ sites.

Fig. 10 Structural and spin transition in {Fe₆Co₃W₆}: (a) χ_MT(T) dependence at H = 1 kOe and electronic configurations of involved d metal ions (tungsten – blue, Co/Fe – red), (b) shortening/elongation of bond-lengths at Co/Fe sites (pink) and (c) changes in the molecular structure (red – HT phase, blue – LT phase).

3.5 Octadecanuclear decorated molecular wheels

The efficient approach towards octacyanidometallate-based coordination clusters with the nuclearity even higher than the pentadecanuclear molecules was presented first by Sutter and co-workers in 2003.¹⁴⁴ They exploited the potential of 3d metal complexes with pyridine-based macrocyclic Schiff bases occupying the equatorial coordination sites, leaving only two axial positions to grow the coordination skeleton. The spontaneous assembly of such an equatorially blocked {Ni^IIL} unit with octacyanidoniobate(IV) gave the very unique octadecanuclear {[Ni^II(L)]₆[Ni^II(L)(H₂O)]₆[Nb^IV(CN)₈]₆}·100H₂O, composed of cyclic {Ni₁₂Nb₆} molecules. A 12-centred cyclic core {Ni₆Nb₆} resembles a giant cyclohexane in the chair conformation with six Nb^IV centres located on the vertexes, and six CN–Ni–NC linkages forming the sides of this spatial structure. The cyclic core is further decorated by six additional {Ni^IIL(H₂O)} modules through the cyanide-bridges (Fig. 11a). The longest Ni–Ni and Nb–Nb distances within the Ni₆Nb₆ core are around 15 Å, and the largest size of the whole molecule is defined by the distance of 30 Å between O atoms of the opposite {Ni^IIL(H₂O)} modules.

Fig. 11 Topology and dynamic properties of 18-metallic {[Ni^II(L)]₆[Ni^II(L)(H₂O)]₆[M^IV(CN)₈]₆}·nH₂O, {Ni₁₂M₆} (L – pyridine-based macrocyclic Schiff base shown in the top right corner, M = Nb, n = 100 or M = W, n = 36) cyclic motifs: (a) the structural views along c (left) and a (right) directions (Ni – dark green, W – gold, CN – green sticks, N, O – red, C – yellow), and (b) the scheme of water-controlled syntheses and single-crystal transformations exploring dynamic properties of {Ni₁₂M₆} clusters.^144,145

The presence of both paramagnetic Ni^II and Nb^IV centres connected by cyanide bridges in the 18-metallic cluster suggested the possible observation of a high spin ground state S = 15 or S = 9 in the case of ferromagnetic or antiferromagnetic coupling, respectively. Surprisingly, the magnetic measurements did not prove the high-spin nature, showing only weak overall antiferromagnetic interactions operating in the crystal lattice.

The special structural feature of {Ni₁₂Nb₆} molecules is the presence of two types of Ni^II complexes: the bridging {Ni^IIL(μ-NC)₂} and the terminal {Ni^IIL(H₂O)(μ-NC)} for which the coordinated water prevents the further growth of the cyanido-bridged skeleton. This suggests the existence of a very subtle equilibrium between cyanides and water coordinated at the axial positions of Ni^II complexes. In fact, it was shown for the W^IV-containing analogues that depending on the amount of water used in the synthesis it is possible to obtain repeatedly {[Ni^II(L)]₆[Ni^II(L)(H₂O)]₆[W^IV(CN)₈]₆}·36H₂O, {Ni₁₂W₆} molecules, or two types of three-dimensional cyanido-bridged {[Ni^II(L)]₂[W^IV(CN)₈]}_n networks of Cc or Ia3̄d symmetry, both built of {Ni₁₂W₆} molecular subunits (Fig. 11b).¹⁴⁵ Moreover, these 3D networks can be post-synthetically transformed to 0D {Ni₁₂W₆} decorated cyclic molecules by the addition of water to the mother solution containing single crystals. The observed solvent-mediated transformations are accompanied by the unprecedentedly significant change of the crystal shape from a flower with diamond-like leaves for the Ia3̄d network to block crystals for the Cc network, and needles for {Ni₁₂W₆} molecules. This observation points to the possible dynamic properties of 18-metallic decorated {Ni₁₂M₆} cyclic molecules, which can be further explored in the reversible synthetic switching between different cyanido-bridged magnetic frameworks of various dimensionalities.

3.6 Eicosanuclear open winged cages

The divalent 3d cations of the octahedral geometry combined with octacyanidometallates (Mo, W, Re) strongly prefer the construction of 15-centred M′₉[M(CN)₈]₆ clusters of a six-capped body-centred cube topology, especially in alcoholic media. However, such molecules are not known for Cu^II ions probably due to the strong Jahn–Teller effect leading to the preferable D_4h or C_4v symmetry which strongly hampers the growth of Cu₉[M(CN)₈]₆ clusters. To prevent the formation of 3-D coordination polymers and to impose the C_3v symmetry obtained typically at fac-[M′(solv)₃(μ-NC)₃] moieties the self-assembly between Cu^II and [W^V(CN)₈]³⁻ was carried out with the preprogrammed tridentate Me₃tacn ligand (Me₃tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) in a methanolic solution. Instead of 15-centred molecules much larger 20-metallic {[Cu^II(Me₃tacn)]₁₂[Cu^II(H₂O)][W^V(CN)₈]₂[W^IV(CN)₈]₅}·24H₂O, {Cu₁₃W₇} clusters of a very specific open winged molecular cage topology were obtained (Fig. 12).¹⁶⁵ The opening cage part of this molecule consists of nine Cu^II ions constructing a body-centred cube which is essentially the same as in M′₉[M(CN)₈]₆ clusters but only five [M(CN)₈] units cover the cube which is due to the strong axial elongation of the central Cu^II ion. The resulting {Cu₉W₅} units adopt a five-capped body-centred cube topology which is an open-cage structure different from spatially closed M′₉[M(CN)₈]₆ clusters. In addition, four trinuclear {Cu₂W} wings with half-occupancies are bonded to four W and four Cu centres of {Cu₉W₅} cages giving the unique eicosanuclear {Cu₁₃W₇} cluster core, which is the largest among [M(CN)₈]-based polynuclear molecules. Such a sophisticated spatial structure was primarily caused by the geometrical preferences of Cu^II complexes but the reported partial reduction of W^V to W^IV plays also a crucial role in the successful formation of a neutral 20-metallic cluster. This, however, strongly limits the cyanido-mediated intracluster magnetic interactions, as five diamagnetic [W^IV(CN)₈]⁴⁻ are embedded in each {Cu₁₃W₇} molecule. For the remaining paramagnetic Cu^II and W^V centres, an expected ferromagnetic coupling was detected but this does not lead to the high ground state spin of the whole molecule due to the partial isolation of the spin carriers. Despite this, it is clearly proved that the application of geometrically flexible Cu^II complexes together with flexible and redox active [W^IV/V(CN)₈]^4−/3− ions can be a successful synthetic pathway towards very high nuclearity clusters.¹⁶⁵

Fig. 12 Topology of the largest [M(CN)₈]-based cluster – 20-metallic {[Cu^II(Me₃tacn)]₁₂[Cu^II(H₂O)][W^V(CN)₈]₂[W^IV(CN)₈]₅}·24H₂O, {Cu₁₃W₇} (Me₃tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) open winged molecular cage in the views along a (top) and c (bottom) axes (left panel). The 15-centred {M′[M′(L)₃]₈[M(CN)₈]₆}·nsolv, {M′₉M₆} (M′ = Mn–Ni; M = Mo, W, Re) cluster was shown as the comparison (right panel) in the corresponding structural views. The blue balls and sticks present the identical parts for both types of clusters, while the orange part represents the differences between these two topologies. Note that all four trinuclear Cu₂W wings of {Cu₁₃W₇} clusters reveal only half crystallographic occupancies.^153,165

4. Conclusions

Octacyanidometallates are unique building blocks that are extremely useful in the construction of various low dimensional magnetic clusters ranging from tetranuclear squares, pentanuclear trigonal bipyramids, hexanuclear octahedrons, to pentadecanuclear six-capped-body-centred cubes and even larger molecules. Some of these cluster topologies are unique in the sense that they are not known for other cyanidometallates ([M(CN)₆]ⁿ⁻ or [M(CN)₇]ⁿ⁻) as it is in the case of hexanuclear and pentadecanuclear cluster families. Owing to the intrinsic geometrical flexibility, octacyanidometallates can easily adopt different coordination modes resulting in all possible topologies which are relatively easy to control by the use of appropriate ‘protecting groups’ – blocking ligands at the 3d metal centre.

Combination of various metal centres in octacyanidometallate-based clusters enables observation of many functionalities: thermal charge transfer,¹⁵⁸ spin cross-over¹⁵⁰ and photomagnetic effects¹⁸ (due to the availability of at least two oxidation states at the metal centres), magnetic bistability and SMM-type slow magnetic relaxation¹⁰⁹ (thanks to significant magnetic anisotropy originating from strong spin–orbit coupling of the building blocks) or high spin ground states leading to the observation of the magnetocaloric effect (due to strong exchange coupling within the M–CN–M′ linkages of a multinuclear system and weak intermolecular interactions).¹⁷³ Therefore, octacyanidometallate-based clusters can be considered as perfect molecular platforms for the development of molecular devices for future applications in spintronics and molecular electronics, nanoscale information storage, molecular sensors or light-triggered molecular switching devices.

A systematic study of octacyanidometallate-based clusters led to the discovery of new physical and chemical phenomena like simultaneous electron transfer through two different charge transfer pathways in Fe₆Co₃W₆,¹⁵⁸ or the Fe^II–W^V⇄Fe^III–W^IV charge transfer equilibrium in Fe₉W₆ ¹⁵⁷ clusters or the strong photomagnetic effect in Mn₄Mo₂ and Mn₄W₂ hexanuclear octahedral molecules that is not based on charge transfer within the Cu^II–Mo^IV or Co^II–W^V structural motifs.¹¹³ It is expected that further in-depth research on similar heterometallic systems will lead to the discovery of other unknown phenomena and functionalities important in the development of chemistry, physics and materials science.

Abbreviations

1-D	One dimensional
2,2′-bpy	2,2′-Bipyridine
2-D	Two dimensional
3-D	Three dimensional
4,4′-bpdo	4,4′-Bipyridine-N,N′-dioxide
4,4′-dmbpy	4,4′-Dimethyl-2,2′-bipyridine
4,4′-dtbpy	4,4′-Ditertbutyl-2,2′-bipyridine
5,5′-dmbpy	5,5′-Dimethyl-2,2′-bipyridine
ac	Acetate
AC	Alternating current
AF	Antiferromagnetic
bik	Bis(1-methylimidazole-2-yl)ketone
bpy	2,2′-Bipyridine
btz	2,2′-Bis(4,5-dihydrothiazine)
CT	Charge transfer
CTIST	Charge transfer induced spin transition
DFT	Density functional theory
DMF	N,N′-Dimethylformamide
DMSO	Dimethylsulfoxide
dpa	Di-(2-picolyl)amine
dpe	1,2-Di(4-pyridyl)ethylene
DSC	Differential scanning calorimetry
EtOH	Ethanol
F	Ferromagnetic
H^MeIm	N-Methyl imidazole
hmp	2-Hydroxymethylpyridine
HS	High spin
HSM	High spin molecule
HT	High temperature
iPrOH	Isopropanol
LS	Low spin
LT	Low temperature
Me3tacn	1,4,7-Trimethyl-1,4,7-triazacyclononane
MeCN	Acetonitrile
MeOH	Methanol
NOA	Natural optical activity
ox	Oxalate
phen	1,10-Phenanthroline
PrOH	Propanol
SCO	Spin crossover
SMM	Single molecule magnet
solv	Solvent
T c	Critical temperature
tmphen	3,4,7,8-Tetramethyl-1,10-phenanthroline
tpm	Tri(1H-pyrazol-1-yl)methane
tptz	2,4,6-Tris(2-pyridyl)-1,3,5-triazine
tpy	2,2′:6′,2′′-Terpyridine
tren	Tris(2-aminoethyl)amine
valen	N,N′-Bis(3-methoxy-salicylidene)-ethylenediamine

Acknowledgements

This work was partially financed by the Polish National Science Centre within the Research Project DEC-2011/01/B/ST5/00716. DP gratefully acknowledges the financial support of the EC REA within the Marie Curie International Outgoing Fellowship, project MultiCyChem (grant agreement no. PIOF-GA-2011-298569) and the financial support of the Polish Ministry of Science and Higher Education within the Fellowship for Outstanding Young Scientists programme (2014 edition). SC gratefully acknowledges the financial support of the Foundation for Polish Science within the START fellowship. MR gratefully acknowledges the financial support of the Polish Ministry of Science and Higher Education within the Diamond Grant (grant agreement no. 0195/DIA/2013/42).

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