Octacyanidometallates for multifunctional molecule-based materials.

Octacyanidometallates have been successfully employed in the design of heterometallic coordination systems offering a spectacular range of desired physical properties with great potential for technological applications. The [M(CN)8]n- ions comprise a series of complexes of heavy transition metals in high oxidation states, including NbIV, MoIV/V, WIV/V, and ReV. Since the discovery of the pioneering bimetallic {MnII4[MIV(CN)8]2} and {MnII9[MV(CN)8]6} (M = Mo, W) molecules in 2000, octacyanidometallates were fruitfully explored as precursors for the construction of diverse d-d or d-f coordination clusters and frameworks which could be obtained in the crystalline form under mild synthetic conditions. The primary interest in [M(CN)8]n--based networks was focused on their application as molecule-based magnets exhibiting long-range magnetic ordering resulting from the efficient intermetallic exchange coupling mediated by cyanido bridges. However, in the last few years, octacyanidometallate-based materials proved to offer varied and remarkable functionalities, becoming efficient building blocks for the construction of molecular nanomagnets, magnetic coolers, spin transition materials, photomagnets, solvato-magnetic materials, including molecular magnetic sponges, luminescent magnets, chiral magnets and photomagnets, SHG-active magnetic materials, pyro- and ferroelectrics, ionic conductors as well as electrochemical containers. Some of these materials can be processed into the nanoscale opening the route towards the development of magnetic, optical and electronic devices. In this review, we summarise all important achievements in the field of octacyanidometallate-based functional materials, with the particular attention to the most recent advances, and present a thorough discussion on non-trivial structural and electronic features of [M(CN)8]n- ions, which are purposefully explored to introduce desired physical properties and their combinations towards advanced multifunctional materials.

Chiral Photomagnets Based on Copper(II) complexes of 1,2-Diaminocyclohexane and Octacyanidomolybdate(IV) Ions.

Chiral photomagnets compose a class of multifunctional molecule-based materials with light-induced alteration of magnetization and chiral properties. The rational design and synthesis of such assemblies is a challenge, and only few such systems are known. Herein, the remarkable octacyanide-bridged enantiomeric pair of 1-D chains [Cu((R,R)-chxn)2]2[Mo(CN)8]·H2O (1R) and [Cu((S,S)-chxn)2]2[Mo(CN)8]·H2O (1S) exhibiting enantiopure structural helicity, which results in optical activity in the 350-800 nm range as confirmed by natural circular dichroism (NCD) spectra, is reported. The photomagnetic effects of 1R, 1S, and 1rac result from the blue light excitation (436 nm) of the photomagnetically active octacyanidomolybdate(IV) ions. In the excited state MoIVHS centers with S = 1 couple antiferromagnetically with the neighboring CuII centers with JCuMo values of -1.3, -1.0, and -1.1 cm-1 for 1R, 1S, and 1rac, respectively. The values of thermal relaxation energy barriers have been estimated as 142 and 356 K for 1R and 1S, being comparable with the energy range of the thermal bath. The value for 1rac reveals a significantly lower value of 75 K. On the basis of these results the value of gMoHS has been estimated to be in the range 4.8-5.8.

Bulky ligands shape the separation between the large spin carriers to condition field-induced slow magnetic relaxation.

Crystal engineering of magnetic relaxation in supramolecular networks based on almost isotropic cyanido-bridged {Mn9[W(CN)8]6L8(solv)8} clusters decorated by bulky 4,4'-di-tert-butyl-2,2'-bipyridine (But2bpy) and 4,7-diphenyl-1,10-phenanthroline (Ph2phen) ligands is presented. The three new compounds {MnII9[WV(CN)8]6(But2bpy)8(MeOH)8}·Pri2O·13MeOH (1), {MnII9[WV(CN)8]6(But2bpy)8(MeOH)6(H2O)2}·4Pri2O·2H2O (1a), and {MnII9[WV(CN)8]6(Ph2phen)8(MeOH)8}·29MeOH·6H2O (2) were characterized structurally and magnetically. Compound 1 exhibits unequivocal domination of repulsive intercluster contacts operating between the side But groups leading to intercluster distances exceeding 10 Å in all three dimensions. Compound 1a reveals a 1-dimensional (1D) supramolecular chain structure with very close intercluster distances of 6.7 Å realized through the direct W-CNHO-Mn hydrogen bonds along the chain, further isolated by the above repulsive ButBut synthons. Compound 2 shows significant separation in all three directions with the intercluster distances close to 10, 12 and 13.5 Å. However, in contrast to 1, these separations are accompanied by indirect hydrogen bond arrays and local π-π interactions of potential to assist in the transfer of weak magnetic interactions. The dc magnetic data show the signature of S = 39/2 in the ground state, which is typical in this group of compounds. The high-spin clusters are accompanied by different intercluster interactions, illustrated by the effective zJ' values of +0.010 cm-1 (1), +0.008 cm-1 (1a) and +0.001 cm-1 (2). The low temperature ac susceptibility measurements revealed a temperature- and field-dependent magnetic relaxation time for all 1, 1a and 2 compounds (τ1, τ1a-fast, and τ2-fast in the range 10-3-10-4 s). In contrast and only in the case of 1a and 2, an additional temperature independent slow process was detected (τ1a-slow and τ2-slow located between 0.1 s and 1 s). The magnetic relaxations were correlated with the obtained supramolecular networks, indicating the significant role of dipolar fields, weak non-covalent interactions, hydrogen bonds and π-π interactions.

Reversible Single-Crystal-to-Single-Crystal Transformation in Photomagnetic Cyanido-Bridged Cd4M2 Octahedral Molecules.

Two new hexanuclear octahedral cyanido-bridged clusters, {[CdII(bpy)2]4[WIV(CN)8]2}·10H2O (Cd4W2) and {[CdII(bpy)2]4[MoIV(CN)8]2}·10H2O (Cd4Mo2), have been obtained and characterized structurally and photomagnetically. Both compounds show a very rare and reversible single-crystal-to-single-crystal transformation upon dehydration accompanied by marked color changes in the case of Cd4W2. Moreover, irradiation of Cd4Mo2 using 436 nm light induces a reversible photomagnetic effect due to the LIESST-like singlet-triplet transition at the MoIV center. Analogous photomagnetic experiments for Cd4W2 did not lead to any significant change of its magnetic moment.

Photoswitchable CuMoIV and CuMoIV cyanido-bridged molecules.

The self-assembly of copper(ii) complexes with two Schiff base ligands: L1O = N3 and L2 = N4 and octacyanidomolybdate(iv) ions yields two discrete molecules of odd nuclearity, namely pentametallic [Cu(L1O)(py)]4[MoIV(CN)8]·14H2O (1), Cu4Mo and trimetallic [Cu(L2)]2[MoIV(CN)8]·9H2O (2), Cu2Mo. Both molecular systems have been characterised structurally and magnetically, revealing a photomagnetic effect. In the case of (1) a metal-to-metal charge transfer (MMCT) mechanism is proposed. The analysis of magnetic interactions in the photogenerated state of (1) assumes the formation of the metastable cluster CuCuIMoV where metal centres in MoV-CN-CuII linkages are ferromagnetically coupled with J2 = 104(3) cm-1. In (2) the increase in the magnetisation is due to the singlet-triplet transition on the MoIV centre leading to the formation of the metastable CuMo. The presence of the paramagnetic Mo (S = 1) centre linking the CuII (S = 1/2) centres allows for effective ferromagnetic interaction of 3 paramagnetic centres with coupling constant J2 = 20.2(7) cm-1.

X-ray absorption spectroscopy study of novel inorganic-organic hybrid ferromagnetic Cu-pyz-[M(CN)8]3- assemblies.

We present a unique interpretation of X-ray absorption spectroscopy (XAS) spectra at Cu:K, W:L(3), and Mo:K edges of structurally related magnetic Cu(II)-[M(V)(CN)(8)](3-) compounds. The approach results in description of the structure of novel three-dimensional (3-D) Cu(II)(3)(pyz)[M(V)(CN)(8)](2)·xH(2)O, M = W (1), Mo, (2) polymers. Assemblies 1 and 2 represent hybrid inorganic-organic compounds built of {Cu(II)[W(V)(CN)(8)](-)}(n) double-layers linked by cyanido-bridged {Cu(II)-(µ-pyz)(2+)}(n) chains. These Cu(II)-M(V) systems reveal long-range magnetic ordering with T(c) of 43 and 37 K for 1 and 2, respectively. The presence of the 3-D coordination networks and 8 cyanido-bridges at M(V) centers leads to the highest Curie temperatures and widest hysteresis loops among Cu(II)-[M(V)(CN)(8)](3-) systems.

The role of carboxylate ligands in two novel cyanido-bridged 2D coordination networks Cu(II)-W(V) and Mn(II)-Nb(IV).

We report on the synthesis, molecular structure and magnetic properties of two novel coordination polymers: [{Cu(II)(4)(pic)(2)(H(2)O)(2)(MeOH)}{W(V)(CN)(8)}(2)]·MeOH·4H(2)O (1) and [{Mn(II)(3)(HCOO)(2)(H(2)O)(4)}{Mn(II)(H(2)O)(3)(HCONH(2))}(2){Nb(IV)(CN)(8)}(2)]·4HCONH(2)·2H(2)O (2). The single-crystal X-ray diffraction analysis of 1 shows that its molecular structure can be interpreted as a cyanido bridged (3,4,7)-connected 2D bilayer built of two different subnets sharing the tungsten centers. The magnetic measurements suggest that the system reveals long-range ferromagnetic ordering between Cu(II) and W(V) centers below 13.4 K. The molecular structure of (2) reveals a 2D topology of layers built of cyanido and formato bridging ligands. The system reveals ferrimagnetic behavior with a critical temperature at 17.8 K.

Microwave-assisted construction of ferromagnetic coordination polymers of [W(V)(CN)8]3- with Cu(II)-pyrazole synthons.

The microwave-mediated self-assembly of [W(V)(CN)(8)](3-) with Cu(II) in the presence of pyrazole ligand resulted in the formation of three novel assemblies: Cu(II)(2)(Hpyr)(5)(H(2)O)[W(V)(CN)(8)](NO(3))·H(2)O (1), {Cu(II)(5)(Hpyr)(18)[W(V)(CN)(8)](4)}·[Cu(II)(Hpyr)(4)(H(2)O)(2)]·9H(2)O (2), and Cu(II)(4)(Hpyr)(10)(H(2)O)[W(V)(CN)(8)](2)(HCOO)(2)·4.5H(2)O (3) (Hpyr =1H-pyrazole). Single-crystal X-ray structure of 1 consists of cyanido-bridged 1-D chains of vertex-sharing squares topology. The structure of 2 reveals 2-D hybrid inorganic layer topology with large coordination spaces occupied by {Cu(Hpyr)(2)(H(2)O)(4)}(2+) ions. Compound 3 contains two types of cyanido-bridged 1-D chains of vertex-sharing squares linked together by formate ions in two directions forming hybrid inorganic-organic 3-D framework (I(1)O(2)). The magnetic measurements for 1-3 reveal a weak ferromagnetic coupling through Cu(II)-NC-W(V) bridges.

W-knotted chain {[Cu(II)(dien)]4[W(V)(CN)8]}(5+)∞: synthesis, crystal structure, magnetism, and theory.

The self-assembly of [Cu(II)(dien)(H(2)O)(2)](2+) and [W(V)(CN)(8)](3-) in aqueous solution leads to the formation (H(3)O){[Cu(II)(dien)](4)[W(V)(CN)(8)]}[W(V)(CN)(8)](2)·6.5H(2)O (1). The crystal structure of 1 consists of an unprecedented {[Cu(II)(dien)](4)[W(V)(CN)(8)]}(5+)(∞) chain of (2,8) topology, nonbridging [W(CN)(8)](3-) anions, and crystallization water molecules. The analysis of magnetic behavior of 1 was performed by the density functional theory (DFT) method and magnetic susceptibility measurements. The DFT broken symmetry approach gave two J(CuW) coupling constants: J(ax) = +2.9 cm(-1) assigned to long and strongly bent W-CN-Cu linkage, and the J(eq) = +1.5 cm(-1) assigned to short and less bent W-CN-Cu linkage, located at the axial and the equatorial positions of square pyramidal Cu(II) centers, respectively, in the hexanuclear {W(2)Cu(4)} chain subunit. The dominance of weak-to-moderate ferromagnetic coupling within the chain was confirmed by magnetic calculations. Zero-field susceptibility of the full chain segment {WCu(4)}(n) was calculated by a semiclassical analytical approach assuming that only one W(V) out of five ½ spins of the chain unit WCu(4) is treated as a classical commuting variable. The calculation of the field dependence of the magnetization was performed separately by replacing the same spin with the Ising variable and applying the standard transfer matrix technique. The intermolecular coupling between the chain segments and off-chain [W(CN)(8)](3-) entities was resolved using the mean-field approximation set to be of antiferromagnetic character. The magnetic coupling parameters are compared with those of other low dimensional {Cu(II)-[M(V)(CN)(8)]} systems.

Magnetostructural correlations in Cu(II)-NC-W(V) linkage: the case of [Cu(II)(diimine)]2+-[W(V)(CN)8]3- 0D assemblies.

We report on the syntheses, crystal structures, and magnetic properties of two cyano-bridged molecular assemblies: [Cu(II)(phen)(3)](2){[Cu(II)(phen)(2)](2)[W(V)(CN)(8)](2)}(ClO(4))(2) x 10 H(2)O (phen = 1,10-phenanthroline) (1) and {[Cu(II)(bpy)(2)](2)[W(V)(CN)(8)]} {[Cu(II)(bpy)(2)][W(V)(CN)(8)]} x 4 H(2)O (bpy = 2,2'-bipyridyl) (2). Compound 1 consists of cyano-bridged [Cu(II)(2)W(V)(2)](2-) molecular rectangles and isolated [Cu(II)(phen)(3)](2+) complexes. The molecular structure of 2 reveals cyano-bridged trinuclear [Cu(II)(2)W(V)](+) and dinuclear [Cu(II)W(V)](-) ions. Magnetic interactions in 1 are interpreted in terms of the model of a tetranuclear moiety consisting of two ferromagnetic Cu(II)-NC-W(V) units (J(1) = +39(4) cm(-1)) interacting antiferromagnetically (J(2) = -1.6(4) cm(-1)). The analysis of the magnetic response of 2 reveals ferromagnetic interaction within the [Cu(II)-NC-W(V)-CN-Cu(II)](+) and [Cu(II)-NC-W(V)](-) isolated units, characterized by J = +35(7) cm(-1) and antiferromagnetic coupling between them (4J' = -0.30(8) cm(-1)). The discussion of the magnetic behavior and the correlation of the J(CuW) parameters with the geometry of cyano bridges at copper(II) centers of 1 and 2 is based on the DFT calculations, which yield J(av) = +13.2 cm(-1) for 1 and J = +31 cm(-1) for 2. The domination of the ferromagnetic Cu(II)-NC-W(V) interaction in 1 and 2 originates from the mutual orthogonality of natural magnetic orbitals in the case of coordination of cyano bridges at the equatorial sites of Cu(II) moieties.

Pressure-induced magnetic switching and linkage isomerism in K0.4Fe4[Cr(CN)6]2.8 x 16 H2O: X-ray absorption and magnetic circular dichroism studies.

The effect of applied pressure on the magnetic properties of the Prussian blue analogue K0.4Fe4[Cr(CN)6]2.8 x 16 H2O (1) has been analyzed by dc and ac magnetic susceptibility measurements. Under ambient conditions, 1 orders ferromagnetically at a critical temperature (T(C)) of 18.5 K. Under application of pressure in the 0-1200 MPa range, the magnetization of the material decreases and its critical temperature shifts to lower temperatures, reaching T(C) = 7.5 K at 1200 MPa. Pressure-dependent Raman and Mossbauer spectroscopy measurements show that this striking behavior is due to the isomerization of some Cr(III)-C[triple bond]N-Fe(II) linkages to the Cr(III)-N[triple bond]C-Fe(II) form. As a result, the ligand field around the iron(II) centers increases, and the diamagnetic low-spin state is populated. As the number of diamagnetic centers in the cubic lattice increases, the net magnetization and critical temperature of the material decrease considerably. The phenomenon is reversible: releasing the pressure restores the magnetic properties of the original material. However, we have found that under more severe pressure conditions, a metastable sample containing 22% Cr(III)-N[triple bond]C-Fe(II) linkages can be obtained. X-ray absorption spectroscopy and magnetic circular dichroism of this metastable sample confirm the linkage isomerization process.

High Tc ferrimagnetic organic-inorganic hybrid materials with MnII-L-MnII and MnII-NC-NbIV linkages (L=pyrazine, pyrazine-N,N'-dioxide, bipyrimidine).

A series of heterobimetallic, cyano-bridged 3D inorganic-organic hybrid networks with MnII-L-MnII and MnII-NC-NbIV linkages are reported. Reaction of [Mn(H2O)6]2+ with [Nb(CN)8]4- in the presence of organic linker L (pyrazine (pyz), pyrazine-N,N'-dioxide (pzdo), and 2,2'-bipyrimidine (bpym)) in H2O affords {MnII2(pz)2(H2O)4[NbIV(CN)8]}.pz.3H2O (1), {MnII2(pzdo)(H2O)4[NbIV(CN)8]}.5H2O (2), and {MnII2(bpym)(H2O)2[NbIV(CN)8]} (3), respectively. 1-3 were examined by X-ray crystallography and vibrational and magnetochemical studies. 1 is characterized by the coexistence of 3D inorganic cyano-bridged and 1D organic [Mn-(micro-pyz)]n2n+ sublattices along with the presence of monocoordinated and crystallization molecules of pyrazine. Assemblies 2 and 3 exhibit dimeric {MnII2-(micro-L)}4+ coordination motifs. The magnetic behavior of heterobimetallic 1-3 complexes is dominated by antiferromagnetic coupling between MnII and NbIV centers mediated by cyano bridges, resulting in long-range ferrimagnetic ordering with a high TC of 27 (1), 37 (2), and 50 K (3). The magneto-structural correlation leads to the conclusion that the magnitude of TC is related to the type of coordination polyhedra of [Nb(CN)8] moieties (SAPR (1), intermediate between SAPR and DD (2), and DD (3)), the relative number of cyano bridges per Mn2Nb unit, and coexistence of inorganic and organic connectivity. FC/ZFC responses appear to be sensitive to the degree of organic connectivity. The discussion of magneto-structural correlation is based on the spin-density properties of adequate heterobimetallic systems containing octacyanometalates.

An unprecedented copper(I,II)-octacyanotungstate(V) 2-D network: crystal structure and magnetism of [CuII(tren)]{CuI[W(V)(CN)8]} . 1.5H2O.

A novel two-dimensional cyanide-bridged polymer [CuII(tren)]{CuI[W(V)(CN)8]} . 1.5H2O (tren = tris(2-aminoethyl)amine) formed via the simultaneous in situ metal-ligand redox reaction of [Cu(tren)(OH2)]2+ and self-assembly with [W(V)(CN)8]3- consists of a {CuI[W(V)(CN)8]} square grid built of CuI centres of tetrahedral geometry coordinatively saturated by CN bridges and [W(V)(CN)8]3- capped by [CuII(tren)]2+ moieties; it exhibits ferromagnetic coupling J1 = +5.8(1) cm(-1) within the CuII-W(V) dinuclear subunits and weak antiferromagnetic coupling J2 = -0.03(1) cm(-1) between them through diamagnetic CuI spacers.

Grid-type two-dimensional magnetic multinuclear metal complex: strands of ([CuII(mu-4,4'-bpy)]2+)n cross-linked by octacyanotungstate(V) ions.

Reaction of the preorganized strands of ([Cu(II)(mu-4,4'-bpy)](2+))n (4,4'-bpy = 4,4'-bipyridine) with [W(V)(CN)(8)](3)(-) leads to a novel cyano-bridged Cu(II)(3)W(V)(2) complex [Cu(mu-4,4'-bpy)(DMF)(2)][Cu(mu-4,4'-bpy)(DMF)](2)[W(V)(CN)(8)](2).2DMF. 2H(2)O 1. The structure of 1 consists of the expected 2-dimensional grid-type network which is built of infinite ([Cu(II)(mu-4,4'-bpy)](2+))n chains cross-linked by octacyanotungstate units. The Cu(II)-NC-W(V)-CN-Cu(II) linkage exhibits the topology of a 3,2-chain. The skeleton of the layer is additionally stabilized by a hydrogen bond network formed by terminal cyano ligands of the [W(CN)(8)](3-) moiety and water molecules. The distance between the adjacent Cu(3)(II)W(2)(V) chains within the layer is 11.12 A along the a axis. The layers are connected by H-bonds of NCN-NDMF-NCN linkages into 3-D supramolecular architecture. The magnetic properties correspond to a dominant ferromagnetic coupling within the Cu(II)(3)W(V)(2) pentamer units (J = +35(4) cm(-1)) and much weaker effective AF interunit coupling which include both intra- and inter-3,2-chain interactions between pentamers (J' = -0.05(1) cm(-1)).

2-D soft ferromagnet based on [Wv(CN)8]3- and CuII with a Tc of 34 K.

Self-assembly of [Cu(tetren)]2+ (tetren = tetraethylenepentamine) and [W(CN)8]3- in acidic aqueous solution yields the double-layered square grid cyanide-bridged polymer of ((tetrenH5)0.8CuII4[Wv(CN)8](4).7.2H2O)n with Cu(II) centres of square pyramidal geometry coordinatively saturated solely by CN bridges supplied by five [W(CN)8]3- ions; it exhibits soft ferromagnetic behaviour with an ordering temperature Tc of 34 K.