4‑Bromopyridine-Induced Chirality in Magnetic M^II-[Nb^IV(CN)₈]^4– (M = Zn, Mn, Ni) Coordination Networks

The introduction of 4-bromopyridine (4-Brpy) to a self-assembled M^II-[Nb^IV(CN)₈] (M = 3d metal ion) coordination system results in the formation of three-dimensional cyanido-bridged networks, {[M^II(4-Brpy)₄]₂­[Nb^IV(CN)₈]}­·<i>n</i>H₂O (M = Zn, <i>n</i> = 1, <b>1</b>; M = Mn, <i>n</i> = 0.5, <b>2</b>; M = Ni, <i>n</i> = 2, <b>3</b>). All these compounds are coordination frameworks composed of octahedral [M^II(4-Brpy)₄­(μ-NC)₂] complexes bonded to square antiprismatic [Nb^IV(CN)₈]^4– ions bearing four bridging and four terminal cyanides. <b>1</b> and <b>2</b> crystallize in the chiral <i>I</i>4₁22 space group as the mixture of two enantiomorphic forms, named <b>1</b>(<b>+</b>)/<b>1</b>(<b>−</b>) and <b>2</b>(<b>+</b>)/<b>2</b>(<b>−</b>), respectively. The chirality is here induced by the spatial arrangement of nonchiral but sterically expanded 4-Brpy ligands positioned around M^II centers in the distorted square geometry, which gives two distinguishable types of coordination helices, A and B, along a 4-fold screw axis. The (+) forms contain left handed helices A, and right handed helices B, while the opposite helicity is presented in the (−) enantiomers. On the contrary, <b>3</b> crystallizes in the nonchiral <i>Fddd</i> space group and creates only one type of helix. Half of them are right handed, and the second half are left handed, which originates from the ideally symmetrical arrangement of 4-Brpy around Ni^II and results in the overall nonchiral character of the network. <b>1</b> is a paramagnet due to paramagnetic Nb^IV centers separated by diamagnetic Zn^II. <b>2</b> is a ferrimagnet below a critical temperature, <i>T</i>_c of 28 K, which is due to the CN^–-mediated antiferromagnetic coupling within Mn–NC–Nb linkages. <b>3</b> reveals a ferromagnetic type of Ni^II–Nb^IV interaction leading to a ferromagnetic ordering below <i>T</i>_c of 16 K, and a hysteresis loop with a coercive field of 1400 Oe at 2 K. Thus, <b>1</b> is a chiral paramagnet, <b>3</b> is a nonchiral ferromagnet, and <b>2</b> combines both of these functionalities, being a rare example of a chiral molecule-based magnet whose chirality is induced by the noninnocent 4-Brpy ligands

Green to Red Luminescence Switchable by Excitation Light in Cyanido-Bridged Tb^III–W^V Ferromagnet

Green to Red Luminescence Switchable by Excitation Light in Cyanido-Bridged Tb^III–W^V Ferromagne

Dehydration of Octacyanido-Bridged Ni^II-W^IV Framework toward Negative Thermal Expansion and Magneto-Colorimetric Switching

An inorganic three-dimensional [Ni^II­(H₂O)₂]₂­[W^IV­(CN)₈]·​4H₂O (<b>1</b>) framework undergoes a single-crystal-to-single-crystal transformation upon thermal dehydration, producing a fully anhydrous phase Ni^II₂­[W^IV­(CN)₈] (<b>1d</b>). The dehydration process induces changes in optical, magnetic, and thermal expansion properties. While <b>1</b> reveals typical positive thermal expansion of the crystal lattice, greenish-yellow color, and paramagnetic behavior, <b>1d</b> is the first ever reported octacyanido-based solid revealing negative thermal expansion, also exhibiting a deep red color and diamagnetism. Such drastic shift in the physical properties is explained by the removal of water molecules, leaving the exclusively cyanido-bridged bimetallic network, which is accompanied by the transformation of the octahedral paramagnetic [Ni^II­(H₂O)₂­(NC)₄]^2– to the square-planar diamagnetic [Ni^II­(NC)₄]^2– moieties

Supramolecular Chains and Coordination Nanowires Constructed of High-Spin Co^II₉W^V₆ Clusters and 4,4′-bpdo Linkers

Cyanido-bridged high-spin {Co^II[Co^II­(MeOH)₃]₈­[W^V­(CN)₈]₆} (<b>Co</b>_<b>9</b><b>W</b>_<b>6</b>) clusters revealing single-molecule magnet behavior were combined with 4,4′-bipyridine-<i>N</i>,<i>N</i>′-dioxide (4,4′-bpdo) linkers, giving unique H-bonded supramolecular {Co^II₉­(MeOH)₂₄­[W^V­(CN)₈]₆}·4,4′-bpdo·MeOH·2H₂O (<b>1</b>) chains and one-dimensional coordination {Co^II­[Co^II­(4,4′-bpdo)_1.5­(MeOH)]₈­[W^V­(CN)₈]₆}·2H₂O (<b>2</b>) nanowires. The hydrogen-bonded chains of <b>1</b> are embedded within the three-dimensional supramolecular network stabilized by the series of noncovalent interactions between <b>Co</b>_<b>9</b><b>W</b>_<b>6</b> clusters, 4,4′-bpdo, and solvent molecules. The coordination nanowires <b>2</b>, revealing an average core diameter of about 11 Å, are arranged parallel with the significant separation in the crystal structure, leading to a microporous supramolecular network with broad channels (12 × 12 Å) filled by methanol and water. Both <b>1</b> and <b>2</b> are stable only in a mother solution or an organic protectant, whereas they undergo the fast exchange of methanol ligands to water molecules during drying in the air. Synthesized materials preserve the magnetic characteristics of <b>Co</b>_<b>9</b><b>W</b>_<b>6</b> clusters with an effective ferromagnetic coupling, giving a ground-state spin of 15/2. For <b>2</b>, the additional antiferromagnetic intercluster interactions are observed. Below 3 K, the frequency-dependent χ_M″(<i>T</i>) signals of <b>1</b> and <b>2</b> indicate the onset of slow magnetic relaxation. For <b>1</b>, the relaxation time follows the Arrhenius law with an energy gap of Δ/<i>k</i>_B = 10.3(5) K and τ₀ = 4(1) × 10^–9 s, which is consistent with single-molecule magnet behavior

Cesium Cyano-Bridged Co^II–M^V (M = Mo and W) Layered Frameworks Exhibiting High Thermal Durability and Metamagnetism

Two-dimensional cesium bimetal cyano-bridged assemblies Cs^I₄Co^II[Mo^V(CN)₈]­Cl₃ (<b>CsCoMo</b>) and Cs^I₄Co^II[W^V(CN)₈]­Cl₃ (<b>CsCoW</b>) were synthesized. The negatively charged and solvent-free {Co^II[M^V(CN)₈]­Cl₃}^4– (M = Mo, W) coordination layers are separated by Cs⁺ ions. Themogravimetric measurements show that these compounds reveal high thermal durability up to 523 K (250 °C), which is due to the absence of solvent molecules in their crystal structures. The magnetic measurements show that <b>CsCoMo</b> and <b>CsCoW</b> are metamagnets showing the field-induced transition from an antiferromagnetic phase with Néel temperature of 25 K to a ferromagnetic phase, which is observed at high critical magnetic field of 24 kOe at 1.8 K. These originate from antiferromagnetic interactions between ferromagnetically coupled cyano-bridged Co^II–M^V layers, and the contribution from single-ion anisotropy of Co^II

Conjunction of Chirality and Slow Magnetic Relaxation in the Supramolecular Network Constructed of Crossed Cyano-Bridged Co^II–W^V Molecular Chains

The addition of chiral 2,2′-(2,6-pyridinediyl)­bis­(4-isopropyl-2-oxazoline) (<i>i</i>Pr-Pybox) to a self-assembled Co^II–[W^V(CN)₈] magnetic system gives two enantiomorphic cyano-bridged chains, {[Co^II((<i>S</i>,<i>S</i>)-<i>i</i>Pr-Pybox)­(MeOH)]₃[W^V(CN)₈]₂·​5.5MeOH·​0.5H₂O}_<i>n</i> (<b>1</b>-<i>SS</i>) and {[Co^II((<i>R</i>,<i>R</i>)-<i>i</i>Pr-Pybox) (MeOH)]₃[W^V(CN)₈]₂·​5.5MeOH·​0.5H₂O}_<i>n</i> (<b>1</b>-<i>RR</i>). Both compounds crystallize with a structure containing a unique crossed arrangement of one-dimensional chains that form a microporous supramolecular network with large channels (14.9 Å × 15.1 Å × 15.3 Å) filled with methanol. The investigated materials exhibited optical chirality, as confirmed by natural circular dichroism and UV–vis absorption spectra. <b>1</b>-(<i>SS</i>) and <b>1</b>-(<i>RR</i>) are paramagnets with cyano-mediated Co^II–W^V magnetic couplings that lead to a specific spin arrangement with half of the W^V ions coupled ferromagnetically with their Co^II neighbors and the other half coupled antiferromagnetically. Significant magnetic anisotropy with the easy axis along the [101] direction was confirmed by single-crystal magnetic studies and can be explained by the single-ion anisotropy of elongated octahedral Co^II sites. Below 3 K, the frequency-dependent χ_M^″(<i>T</i>) signal indicated slow magnetic relaxation characteristic of single-chain magnets

Conjunction of Chirality and Slow Magnetic Relaxation in the Supramolecular Network Constructed of Crossed Cyano-Bridged Co^II–W^V Molecular Chains

The addition of chiral 2,2′-(2,6-pyridinediyl)­bis­(4-isopropyl-2-oxazoline) (<i>i</i>Pr-Pybox) to a self-assembled Co^II–[W^V(CN)₈] magnetic system gives two enantiomorphic cyano-bridged chains, {[Co^II((<i>S</i>,<i>S</i>)-<i>i</i>Pr-Pybox)­(MeOH)]₃[W^V(CN)₈]₂·​5.5MeOH·​0.5H₂O}_<i>n</i> (<b>1</b>-<i>SS</i>) and {[Co^II((<i>R</i>,<i>R</i>)-<i>i</i>Pr-Pybox) (MeOH)]₃[W^V(CN)₈]₂·​5.5MeOH·​0.5H₂O}_<i>n</i> (<b>1</b>-<i>RR</i>). Both compounds crystallize with a structure containing a unique crossed arrangement of one-dimensional chains that form a microporous supramolecular network with large channels (14.9 Å × 15.1 Å × 15.3 Å) filled with methanol. The investigated materials exhibited optical chirality, as confirmed by natural circular dichroism and UV–vis absorption spectra. <b>1</b>-(<i>SS</i>) and <b>1</b>-(<i>RR</i>) are paramagnets with cyano-mediated Co^II–W^V magnetic couplings that lead to a specific spin arrangement with half of the W^V ions coupled ferromagnetically with their Co^II neighbors and the other half coupled antiferromagnetically. Significant magnetic anisotropy with the easy axis along the [101] direction was confirmed by single-crystal magnetic studies and can be explained by the single-ion anisotropy of elongated octahedral Co^II sites. Below 3 K, the frequency-dependent χ_M^″(<i>T</i>) signal indicated slow magnetic relaxation characteristic of single-chain magnets

Cyanido-Bridged Clusters with Remote N‑Oxide Groups for Branched Multimetallic Systems

The combination of [W^V(CN)₈]^3– anions with 3d metal cations M^II in MeOH leads to the formation of pentadecanuclear spherical cyanido-bridged clusters {M­[M­(solv)₃]₈[M′(CN)₈]₆}, <b>M</b>_<b>9</b><b>M′</b>_<b>6</b>. By decorating their surface with organic ligands or/and by installation of different ions in their coordination skeleton, one could tune high spin in the ground state, slow relaxation of magnetization, or structural/spin phase transition. In this work we present the extended molecular high spin (<i>S</i>_GS = ¹⁵/₂, <i>g</i>_eff = 3.4) clusters or chains of clusters {Co₉W₆(<i>N</i>,<i>O</i>-L)_<i>x</i>} (<i>N</i>,<i>O</i>-L – pyrazine mono-N-oxide, <i>pzmo</i>; 4,4-bipyridine mono-N-oxide – 4,4′-<i>bpmo</i>) equipped with the structurally ordered remote (2–2.5 nm) N-oxide functions, as a result of deliberate combination of solvated Co₉W₆ supercomplexes with asymmetric <i>N</i>,<i>O</i>-donor linkers L. The systematic occurrence of such motifs in the series <b>1</b>–<b>3</b> is a result of preference for the Co–N_L coordination over the Co–O_L coordination, controlled also by strongly competing supramolecular interactions including simple hydrogen bonding {_LNO···H-donor} as well as cooperative π-costacked hydrogen bonding in double cyclic synthons {Co–O–H_MeOH···O–N_bpmoN−}₂. The observed coordination backbones are discussed in terms of the potential to bind the specific external molecular units and create the new type of branched molecular organization. The magnetic properties are confronted with structural differences along <b>1</b>–<b>3</b>, considering coordination polyhedra, Co–N bond lengths, Co–N–C angles, and hydrogen bonds. The diversity of slow magnetic relaxation images for the known Co₉W₆ based phases are discussed in terms of local deformation of Co coordination polyhedra and global deformation of cyanide bridged backbones

Double Magnetic Relaxation and Magnetocaloric Effect in the {Mn₉[W(CN)₈]₆(4,4′-dpds)₄} Cluster-Based Network

Cyanide-bridged {Mn^II₉[W^V(CN)₈]₆} clusters with the ground state spin <i>S</i>_SG = 39/2 were connected by a 4,4′-dipyridyl disulfide (4,4′-dpds) linker into 2-D double-connected coordination layers of the I⁰O² type, {Mn^II₉­(4,4′-dpds)₄­(MeOH)₁₆­[W^V(CN)₈]₆}·12MeOH (<b>1</b>). The intercluster contacts are controlled by the bridging Mn^II–(4,4′-dpds)–Mn^II coordination modes and direct hydrogen bonds W–CN···HO_MeOH–Mn in three crystallographic directions, with the <i>vertex-to-vertex</i> contact unprecedented in {Mn₉W₆}-based networks dominating over the typical <i>edge-to-edge</i> contacts. The resulting 3D supramolecular network of high-spin clusters was subjected to a thorough magnetic characterization in context of two critical issues. First, the intracluster W^V–CN–Mn^II exchange coupling and intercluster interaction were successfully modeled through the combination of dc measurements, Quantum Monte Carlo simulations, and mean-field calculations, yielding a reasonable <i>J</i>_ap = −8.0 cm^–1, <i>J</i>_eq = −19.2 cm^–1 (related to apical and equatorial CN bridges, depending on the angle they form with the <i>S</i>₄ axis of dodecahedral [W­(CN)₈]^3– units, respectively), and <i>zJ</i>′ = 0.014 cm^–1 with the average <i>g</i>_W <i>= g</i>_Mn = 2.0 parameter set. Continuing this approach, we simulated the magnetocaloric effect (MCE) and compared it to the experimental result of Δ<i>S</i>^max = 7.31 J kg^–1 K^–1 for fields >5.0 T. Second, two relaxation processes were induced by a relatively weak magnetic field, <i>H</i>_dc = 500 Oe, at an <i>H</i>_ac field frequency range of up to 10 kHz, which are related to dipole–dipole interactions between high-spin (39/2) moieties. The observed relaxation times significantly differ from each other, the slow process with τ_slow at tenths of a second being temperature independent and the faster process being 3–5 orders of magnitude faster with the effective energy barrier Δ_eff = 17.6 K. These dynamic properties are surprising, since the compound is made up of isotropic high-spin molecules