Single-Chain Magnets Based on Octacyanotungstate with the Highest Energy Barriers for Cyanide Compounds

3 nm-wide Cyanometallate Fe-Co Tape Exhibiting Single-Chain Magnet Behavior

Treatment of Co(OTf)2·6H2O, Li[(pzTp)FeIII(CN)3], and H3PMo12O40·nH2O in protic solvents afforded two structurally related Fe-Co cyanometallate complexes: [{(pzTp)Fe(CN)3}3Co3(MeOH)10][PMo12O40]·H2O·11MeOH (1, pzTp- = tetra(pyrazolyl)borate) and {[(pzTp)Fe(CN)3]4Co3(MeOH)5(H2O)3}n[HPMo12O40]n·3 nMeOH·6.5nH2O (2). Complex 1 consists of a cyanide-bridged hexanuclear [Fe3Co3] cage, characterized by the fused conjunction of two mutually perpendicular trigonal bipyramids (TBPs, [Fe2Co3] and [Co2Fe3]), while complex 2 showcases an intricate cyanide-bridged Fe-Co tape comprising a central chain backbone of vertex-sharing [Fe2Co3] TBPs alongside peripheral [Fe2Co2] squares. Complex 2 is among the widest one-dimensional coordination assemblies characterized by the single-crystal X-ray diffraction technique. Magnetic studies revealed that complex 2 behaved as a single chain magnet with an effective energy barrier (Ueff/kB) of 46.8 K. Our findings highlight the possibilities in the development of cyanometallate-POM hybrid materials with captivating magnetic properties.

Modulation of single-chain magnet behaviour in a heterometallic Fe2Co cyanide-bridged 2D sheet

A cyanide-bridged Fe2Co 2D sheet exhibiting electron transfer coupled spin transition (ETCST) with co-existence of magnetic ordering below 50 K is reported. The complex exhibits single-chain magnet behaviour where the uncoordinated water molecules act as an exchange-breaking impurity by allowing only a fraction of the molecule to undergo a spin state change. The paramagnetic centres prevail throughout the chain on desolvation, thereby increasing the number of correlated units in the chain.

Ferromagnetically coupled single-chain magnets exhibiting a magnetic hysteresis of 0.42 Tesla in cyano-bridged FeIII2MII (M = Ni, Fe) coordination polymers

The synthesis, single-crystal structures and magnetic properties of two new double-zigzag-chain cyano-bridged heterobimetallic {[MII(Py-NOH)2][FeIII(Tp*)(CN)3]2}·H2O ([FeIII2MII]) (Py-NOH = 4-pyridinealdoxime, Tp* = tris(3,5-dimethylpyrazol-1-yl)borohydride, M = Ni (1), Fe (2)) compounds are reported. The crystal structures of both compounds were determined by single-crystal X-ray diffraction. Complexes 1 and 2 are isostructural, with the crystal structure comprising neutral double-zigzag (4,2-ribbon-like) bimetallic chains. The FeIII ion is coordinated by three cyanide carbon atoms and three nitrogen atoms of Tp* anions. However, the MII ion is surrounded by four cyanide nitrogen atoms and two nitrogen atoms from two Py-NOH ligands. The crystal structures and magnetic studies demonstrate that both complexes behave as single-chain magnetics (SCMs) with intrachain ferromagnetic coupling. Furthermore, [FeIII2NiII] exhibits an excellent coercive field of 0.42 T at 1.8 K, among cyano-bridged 3d transition-metal-based SCMs reported thus far. Preliminary theoretical calculations provide a deep understanding of the magnetic properties of [FeIII2NiII].

Syntheses, Structures, and Magnetic Properties of Three Cyano-Bridged FeII-MoIII Single-Molecule Magnets

Three new cyano-bridged FeII-MoIII complexes assembled from the [MoIII(CN)7]4- unit, FeII ions, and three pentadentate N3O2 ligands, namely {[Fe2H3(dapab)2][Mo(CN)6]}n·2H2O·3.5MeCN (1), [Fe(H2dapb)(H2O)][Fe(Hdapb)(H2O)][Mo(CN)6]·4H2O·3MeCN (2), and [Fe(H2dapba)(H2O)]2[Mo(CN)7]·6H2O (3) (H2dapab = 2,6-diacetylpyridine bis(2-aminobenzoylhydrazone), H2dapb = 2,6-diacetylpyridine bis(benzoylhydrazone), H2dapba = 2,6-diacetylpyridine bis(4-aminobenzoylhydrazone)), have been synthesized and characterized. Single-crystal structure analyses suggest that complex 1 contains a one-dimensional (1D) chain structure where two FeII ions are bridged by the in situ generated [MoIII(CN)6]3- unit through two trans-cyanide groups into trinuclear Fe2IIMoIII clusters that are further linked by the amino of the ligand into an infinite chain. Complexes 2 and 3 are cyano-bridged Fe2IIMoIII trinuclear clusters with two FeII ions connected by the [MoIII(CN)6]3- and [MoIII(CN)7]4- units, respectively. Direct current magnetic studies confirmed the ferromagnetic interactions between the cyano-bridged FeII and MoIII centers and significant easy-axis magnetic anisotropy for all three complexes. Furthermore, complexes 1-3 exhibit slow magnetic relaxation under a zero dc field, with relaxation barriers of 42.3, 21.6, and 14.4 K, respectively, making them the first examples of cyano-bridged FeII-MoIII single-molecule magnets.

Anthryl-functionalized cyanide-bridged Fe/Co cubes

Two anthryl-functionalized cyanide-bridged [Fe4Co4] cube complexes, [(pzTp)Fe(CN)3Co(TpEtOAn)]4[OTf]4·8MeCN·7Et2O (1) and [NEt4]3[(pzTp)Fe(CN)3Co(TpEtOAn)]4[OTf]7·5MeCN·2Et2O (2) (pzTp- = tetrapyrazolylborate, TpEtOAn = 2,2,2-tris-(pyrazol-1-yl)ethoxy(9-methyl-anthracene)), were synthesized and characterized. The crystallographic study revealed that the [Fe4Co4] cubes are arranged into a linear supramolecular chain through significant anthryl-anthryl π-π stacking interactions in complex 1, whereas a zigzag supramolecular 1D assembly is observed in 2. The magnetic measurements showed that both compounds exhibited incomplete transitions from the paramagnetic {FeIIILS(μ-CN)CoIIHS} state to the diamagnetic {FeIILS(μ-CN)CoIIILS} state at about 200 K. The luminescence measurement of 1 in solution revealed an enhancement of the emission upon dilution or addition of perfluoronaphthalene (PFN) molecules, which could be attributed to the suppression of the aggregation-caused quenching (ACQ) effect, suggesting possible aggregation of the cube units in the solution.

All-electron APW+lo calculation of magnetic molecules with the SIRIUS domain-specific package

We report APW+lo (augmented plane wave plus local orbital) density functional theory (DFT) calculations of large molecular systems using the domain specific SIRIUS multi-functional DFT package. The APW and FLAPW (full potential linearized APW) task and data parallelism options and the advanced eigen-system solver provided by SIRIUS can be exploited for performance gains in ground state Kohn-Sham calculations on large systems. This approach is distinct from our prior use of SIRIUS as a library backend to another APW+lo or FLAPW code. We benchmark the code and demonstrate performance on several magnetic molecule and metal organic framework systems. We show that the SIRIUS package in itself is capable of handling systems as large as a several hundred atoms in the unit cell without having to make technical choices that result in the loss of accuracy with respect to that needed for the study of magnetic systems.

Interplay of Anisotropic Exchange Interactions and Single-Ion Anisotropy in Single-Chain Magnets Built from Ru/Os Cyanidometallates(III) and Mn(III) Complex

Two novel 1D heterobimetallic compounds {[Mn^III(SB²⁺)M^III(CN)₆]·4H₂O}_n (SB²⁺ = N,N'-ethylenebis(5-trimethylammoniomethylsalicylideneiminate) based on orbitally degenerate cyanidometallates [Os^III(CN)₆]^3- (1) and [Ru^III(CN)₆]^3- (2) and Mn^III Schiff base complex were synthesized and characterized structurally and magnetically. Their crystal structures consist of electrically neutral, well-isolated chains composed of alternating [M^III(CN)₆]^3- anions and square planar [Mn^III(SB²⁺)]³⁺ cations bridged by cyanide groups. These -ion magnetic anisotropy of Mn^III centers. These results indicate that the presence of compounds exhibit single-chain magnet (SCM) behavior with the energy barriers of Δτ₁/k_B = 73 K, Δτ₂/k_B = 41.5 K (1) and Δτ₁/k_B = 51 K, Δτ₂ = 27 K (2). Blocking temperatures of T_B = 2.8, 2.1 K and magnetic hysteresis with coercive fields (at 1.8 K) of 8000, 1600 Oe were found for 1 and 2, respectively. Theoretical analysis of the magnetic data reveals that their single-chain magnet behavior is a product of a complicated interplay of extremely anisotropic triaxial exchange interactions in M^III(4d/5d)-CN-Mn^III fragments: -J_xS_M^xS_Mn^x-J_yS_M^yS_Mn^y-J_zS_M^zS_Mn^z, with opposite sign of exchange parameters J_x = -22, J_y = +28, J_z = -26 cm^-1 and J_x = -18, J_y = +20, J_z = -18 cm^-1 in 1 and 2, respectively) and single orbitally degenerate [Os^III(CN)₆]^3- and [Ru^III(CN)₆]^3- spin units with unquenched orbital angular momentum in the chain compounds 1 and 2 leads to a peculiar regime of slow magnetic relaxation, which is beyond the scope of the conventional Glaubers's 1D Ising model and anisotropic Heisenberg model.

Magnetic interactions controlled by light in the family of Fe(II)-M(IV) (M = Mo, W, Nb) hybrid organic-inorganic frameworks

Three new hybrid organic-inorganic frameworks employing octacyanidometallates and 4,4'-bypiridine dioxide (4,4'-bpdo) as bridging molecules were prepared and characterized. The three-dimensional coordination frameworks {[Fe^II(μ-4,4'-bpdo)(H₂O)₂]₂[M^IV(CN)₈]·9H₂O}_n (Fe2Mo, Fe2W and Fe2Nb; M = Mo, W and Nb) are composed of cyanido-bridged chains, which are interconnected by the organic linkers. Magnetic measurements for Fe2Nb show a two-step transition to the antiferromagnetic state, which results from the cooperation of antiferromagnetic intra- and inter-chain interactions. Fe2Mo and Fe2W, on the other hand, behave as paramagnets at 2 K because of the diamagnetic character of the corresponding octacyanidometallate(IV) building units. However, after 450 nm light irradiation they show transition to the metastable high spin Mo^IV or W^IV states, respectively, with distinct ferromagnetic intrachain spin interactions, as opposed to the antiferromagnetic ones observed in the Fe2Nb framework.

The rationalized pathway from field-induced slow magnetic relaxation in CoII–WIV chains to single-chain magnetism in isotopological CoII–WV analogues

The change of the oxidation state from WIV to WV in isotopological CoII–[W(CN)8]n− chains leads to the appearence of pronounced single-chain magnet behaviour.

Polyoxometalate-Assisted Assembly of Pearl-Chain-Like Cyanide-Bridged Single-Chain Magnets

The introduction of Keggin-type POMs of [PMo₁₂O₄₀]^3- or [SiW₁₂O₄₀]^4- as counteranions into the Fe^III-M^II cyanometalate system afforded three chain complexes: [(Tp*)Fe(CN)₃Ni(DMF)₄]₂{[(Tp*)Fe(CN)₃Ni(DMF)₃(H₂O)]₂Ni(DMF)₄}[PMo₁₂O₄₀]₂·14DMF (1, Tp*= hydridotris(3,5-dimethylpyrazol-1-yl)borate) and {[(Tp*)Fe(CN)₃M(DMF)₃(H₂O)]₂M(DMF)₄}[SiW₁₂O₄₀]·3DMF (2, M = Ni^II; 3, M = Co^II). Complex 1 contains both discrete cationic [Fe₂Ni₂]²⁺ squares and less-studied {Fe₂Ni₃}_n pearl chains, namely 3,2-chains, while 2 and 3 consist of pure 3,2-chains due to the replacement of [PMo₁₂O₄₀]^3- with [SiW₁₂O₄₀]^4- bearing one more negative charge. Magnetic studies revealed that all of the complexes exhibit single-chain-magnet (SCM) behaviors with the effective thermal barriers of Δ_τ1/k_B = 61.6 K (infinite regime) and Δ_τ2/k_B = 36.5 K (finite regime) for 1, Δ_τ/k_B = 46.9 K for 2 (finite), and Δ_τ/k_B = 30.6 K for 3 (finite). The POM moieties may play a pivotal role for the realization of this promising archetype of favoring SCM property: (1) the highly negatively charged POMs may facilitate the formation of the uncommon highly positive "pearl chain"; (2) the nanosized POMs necessarily led to the good isolation of the chains in the title complexes, and (3) the employment of POMs with different charges may regulate the resultant complexes in both structure and magnetism.

A cyanide-bridged Fe-Co pearl-chain-like single-chain magnet containing 4-coordinate cobalt(II) ions

Treatment of CoCl₂·6H₂O and tris(pyrazolyl-1-yl)borate tricyanoiron(III) anions at an elevated temperature (55 °C) afforded two less-common pearl-chain-like compounds, {[(Tp^R)Fe(CN)₃CoCl₂]₂Co(DMF)₄}·nDMF (1, Tp^R = Tp^4-Me = hydridotris(4-methylpyrazol-1-yl)borate, n = 1 and 2, Tp^R = Tp*^Me = hydridotris(3,4,5-trimethylpyrazol-1-yl)borate, n = 4.5), in which the 4-coordinate Co(II) ions and [(Tp^R)Fe^III(CN)₃]^- units are alternately bridged by cyanide groups into squares, which are further linked with the 6-coordinate Co(II) ions into an infinite chain. Interestingly, the magnetic study revealed that 1 exhibits a typical single-chain magnet behaviour with an effective energy barrier of 28.0 K, while surprisingly no Glauber dynamics was observed for 2 despite their very similar structures. The variations of the local coordination environments of the cobalt ions and the cyanide linkages were evidenced, and they may account for the significant difference in their magnetic properties related to the global magnetic anisotropy and magnetic exchange of the chain.

Tunable magnetic anisotropy in luminescent cyanido-bridged {Dy2Pt3} molecules incorporating heteroligand PtIV linkers

The interest in the generation of photoluminescence in lanthanide(III) single-molecule magnets (SMMs) is driven by valuable magneto-optical correlations as well as perspectives toward magnetic switching of emission and opto-magnetic devices linking SMMs with optical thermometry. In the pursuit of enhanced magnetic anisotropy and optical features, the key role is played by suitable ligands attached to the 4f metal ion. In this context, cyanido complexes of d-block metal ions, serving as expanded metalloligands, are promising. We report two novel discrete coordination systems serving as emissive SMMs, {[Dy^III(H₂O)₃(tmpo)₃]₂[Pt^IVBr₂(CN)₄]₃}·2H₂O (1) and {[Dy^III(H₂O)(tmpo)₄]₂[Pt^IVBr₂(CN)₄]₃}·2CH₃CN (2) (tmpo = trimethylphosphine oxide), obtained by combining Dy^III complexes with uncommon dibromotetracyanidoplatinate(IV) ions, [Pt^IVBr₂(CN)₄]^2-. They are built of analogous Z-shaped cyanido-bridged {Dy₂Pt₃} molecules but differ in the coordination number of Dy^III (C.N. = 8 in 1, C.N. = 7 in 2) and the number of coordinated tmpo ligands (three in 1, four in 2) which is related to the applied solvents. As a result, both compounds reveal Dy^III-centred slow magnetic relaxation but only 1 shows SMM character at zero dc field, while 2 is a field-induced SMM. The relaxation dynamics in both systems is governed by the Raman relaxation mechanism. These effects were analysed using ac magnetic data and the results of the ab initio calculations with the support of magneto-optical correlations based on low-temperature high-resolution emission spectra. Our findings indicate that heteroligand halogeno-cyanido Pt^IV complexes are promising precursors for emissive SMMs with the further potential of sensitivity to external stimuli that may be related to the lability of the axially positioned halogeno ligands.

New Materials and Effects in Molecular Nanomagnets

Molecular magnets are a relatively new class of purely organic or metallo-organic materials, showing magnetism even without an external magnetic field. This interdisciplinary field between chemistry and physics has been gaining increased interest since the 1990s. While bulk molecular magnets are usually hard to build because of their molecular structures, low-dimensional molecular magnets are often easier to construct, down to dot-like (zero-dimensional) structures, which are investigated by different scanning probe technologies. On these scales, new effects such as superparamagnetic behavior or coherent switching during magnetization reversal can be recognized. Here, we give an overview of the recent advances in molecular nanomagnets, starting with single-molecule magnets (0D), typically based on Mn12, Fe8, or Mn4, going further to single-chain magnets (1D) and finally higher-dimensional molecular nanomagnets. This review does not aim to give a comprehensive overview of all research fields dealing with molecular nanomagnets, but instead aims at pointing out diverse possible materials and effects in order to stimulate new research in this broad field of nanomagnetism.

Unusual structural changes going from Li+ to Cs+ in [W(CN)6(bpy)]2− ion salts: the Na+ case

Two sodium salts with the [W(CN)₆(bpy)]²⁻ ion were synthesized and compared to Li⁺, K⁺, Rb⁺ and Cs⁺ structures. The comparison of molecular packing, inter- and intra-molecular interactions and cation properties is discussed.

X-ray crystal structures of K+ and Rb+ salts of [W(CN)6(bpy)]2− ion. The unusual cation–anion interactions and structure changes going from Li+ to Cs+ salts

The K⁺ and Rb⁺ salts of [W(CN)₆(bpy)]²⁻ ion were compared to the Li⁺ and Cs⁺ ones. The layered structure is independent on cation radii and decreasing strength of the cation–anion interactions on going from Li⁺ to Cs⁺.

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.

Magnetic, Structural and Spectroscopic Properties of Iron(II)-Octacyanoniobate(IV) Crystalline Film Obtained by Ion-Exchange Synthesis

Over recent years, investigations of coordination polymer thin films have been initiated due to their unique properties, which are expected to be strongly enhanced in the thin film form. In this work, a crystalline [Fe^II(H₂O)₂]₂[Nb^IV(CN)₈]∙4H₂O (1) film on a transparent Nafion membrane was obtained, for the first time, via ion-exchange synthesis. The proper film formation and its composition was confirmed with the use of energy dispersive X-ray spectroscopy and infrared spectroscopy, as well as in situ Ultraviolet-Visible (UV-Vis) spectroscopy. The obtained film were also characterized by scanning electron microscopy, X-ray diffraction, and magnetic measurements. The [Fe^II(H₂O)₂]₂[Nb^IV(CN)₈]∙4H₂O film shows a sharp phase transition to a long-range magnetically ordered state at T_c = 40 K. The 1 film is a soft ferromagnet with the coercive field H_c = 1.2 kOe. Compared to the bulk counterpart, a decrease in critical temperature and a significant increase in the coercive field were observed in the films indicating a distinct size effect. The decrease in T_c could also have been related to the possible partial oxidation of Fe^II ions to Fe^III, which could be efficient, due to the large surface of the thin film sample.

Molybdenum(III) Thiocyanate- and Selenocyanate-Based One-Dimensional Heteronuclear Polymers: Coordination Affinity-Controlled Assemblage of Mixed Spin and Mixed Valence Derivatives with Ni(II) and Co(II/III)

One-dimensional coordination polymers constructed with the 4d metallo-ligand [Mo(NCS)₆]^3- associated with Ni^II and Co^II complexes are reported. The first series consists of anionic NCS-bridged [Mo-M^II]^- coordination polymers associated with a discrete paramagnetic complex or a diamagnetic Co^III complex acting as cations. The latter takes advantage of the coordination preference of the S-ligand for the soft 3d ions and has led to hetero-trimetallic [Mo^IIINi^IICo^III] and mixed valence [Mo^IIICo^IICo^III] compounds. A second series concerns neutral chains in which trinuclear [Mo M₂^II] units are bridged by an additional NCS anion. The soft character of the S atom was also the key to a rare example of a compound involving both high-spin and low-spin Co^II centers associated with [Mo(NCS)₆]. A [Mo-Ni] derivative obtained with [Mo(NCSe)₆]^3- has been considered in order to evaluate the effect of Se versus S on the exchange interaction. The spin distribution for selenocyanate metallo-ligand has been assessed by density functional theory calculations. The crystal structures for all compounds have been characterized, and their magnetic behaviors have been investigated. These ferrimagnetic systems are characterized by antiferromagnetic Mo-M^II interactions in the range of -40 to -90 cm^-1 (based on H = -JS_aS_b formalism) operative both with Co^II and with Ni^II, thus demonstrating the potential of the Mo^III-NCS combination for molecular systems in which exchange interactions play an important role.

Two azido-bridged homospin Fe(ii)/Co(ii) coordination polymers featuring single-chain magnet behavior

Two azido-bridged homospin Fe(ii)/Co(ii) coordination polymers [Fe2(Bzp)2(N3)4]n (1) and [Co4(Bzp)4(N3)8·(MeOH)2]n (2) (bzp = 2-benzoylpyridine) are prepared, which consist of one-dimensional neutral chains with pure EO-azido (μ2-1,1-N3) bridges. Magnetically, both 1 and 2 exhibit considerable intrachain ferromagnetic interactions which benefit from the EO-azido bridging mode, leading to typical single-chain magnet (SCM) behavior under both the "infinite-size" and "finite-size" regime and pronounced hysteresis loops. As far as the bridging network is concerned, complex 1 represents not only a rare example of homospin Fe(ii)-based SCMs but also the first Fe(ii) chain compound with pure EO-azido bridges.

Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe2 Complexes

The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged Fe^II₂ complexes [(Me₃TPyA)₂Fe₂(^RL)]ⁿ⁺ (Me₃TPyA = tris(6-methyl-2-pyridylmethyl)amine; n = 2: ^OMeLH₂ = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, ^ClLH₂ = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na₂[^NO₂L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; n = 4: ^SMe₂L = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electron-reduced analogues. Variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between Fe^II centers in the oxidized species, with exchange constants of J = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO₂, SMe₂) cm^-1. In contrast, X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species reveal the presence of strong antiferromagnetic metal-radical coupling, with J = -57(10), -60(7), -58(6), and -65(8) cm^-1 for R = OMe, Cl, NO₂, and SMe₂, respectively. The minimal effects of substituents in the 3- and 6-positions of ^RL^x-• on the magnetic coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of U_eff = 50(1), 41(1), 38(1), and 33(1) cm^-1 for R = OMe, Cl, NO₂, and SMe₂, respectively. Taken together, these results provide the first examination of how bridging ligand substitution influences magnetic coupling in semiquinoid-bridged compounds, and they establish design criteria for the synthesis of semiquinoid-based molecules and materials.

'Metal Complexes as Ligands' for the Synthesis of Coordination Polymers: A MnIII Monomer as a Building Block for the Preparation of an Unprecedented 1-D {MnIIMnIII}n Linear Chain

A relatively unexplored synthetic route in redox-active Mn(II/III) coordination chemistry has been employed toward the preparation of a new mixed-valence Mn^II/III 1-D linear chain from the reaction of [Mn^III(sacb)₂]⁻ precursor with a Mn^II source, where sacbH₂ is the Schiff base ligand N-salicylidene-2-amino-5-chlorobenzoic acid. The mononuclear (Pr₂NH₂)[Mn^III(sacb)₂] (1) compound was obtained in excellent yields (>85%) from the 1:2:3 reaction of Mn(O₂CMe)₂∙4H₂O, sacbH₂ and Pr₂NH, respectively. In 1, the two doubly deprotonated sacb²⁻ ligands act as O_carboxylate,N_imine,O_phenoxide-tridentate chelates, while the second carboxylate O atom of sacb²⁻ is dangling and H-bonded to the Pr₂NH₂⁺ countercation. Complex 1 was subsequently used as a ‘ligand’ to react stoichiometrically with the ‘metal’ Mn(NO₃)₂∙4H₂O, thus leading to the 1-D coordination polymer {[Mn^IIMn^III(sacb)₂(H₂O)₂(MeOH)₂](NO₃)}_n (2) in good yields (~50%). The removal of Pr₂NH₂⁺ from the vicinity of the [Mn^III(sacb)₂]⁻ metalloligand has rendered possible (vide infra) the coordination of the second O_carboxylate of sacb²⁻ to neighboring {Mn^II(H₂O)₂(MeOH)₂}²⁺ units, and consequently the formation of the 1-D polymer 2. Direct-current (dc) magnetic susceptibility studies revealed the presence of very weak antiferromagnetic exchange interactions between alternating Mn^III and Mn^II atoms with a coupling constant of J = −0.08 cm⁻¹ for g = 2.00. The combined results demonstrate the potential of the ‘metal complexes as ligands’ approach to yield new mixed-valence Mn(II/III) coordination polymers with interesting structural motifs and physicochemical properties.

Structural Disorder in High-Spin {CoII9WV6} (Core)-[Pyridine N-Oxides] (Shell) Architectures

The combinations of Co(II), octacyanidotungstate(V), and monodentate pyridine N-oxide (pyNO) or 4-phenylpyridine N-oxide (4-phpyNO) led to crystallization of novel crystalline phases {Co^II[Co^II₈(pyNO)₁₂(MeOH)₁₂][W^V(CN)₈]₆} (1) and {Co^II[Co^II₈(4-phpyNO)₇(MeOH)₁₇][W^V(CN)₈]₆}·7MeOH·(4-phpyNO)₃ (2). In both architectures, metal-cyanide clusters are coordinated by N-oxide ligands in a simple monodentate manner to give the spherical objects of over 1 nm core diameter and about 2.2 nm (1) and 3 nm (2) of the total diameter, terminated with the aromatic rings. The supramolecular architecture is dominated by dense and rich π-π interaction systems. Both structures are characterized by a significant structural disorder in ligand shell, described with the suitable probability models. For 1, the π-π interactions between the pyNO ligands attached to the same metal centers are suggested for the first time. In 2, 4-phpyNO acts as monodentate ligand and as the crystallization molecule. Magnetic studies indicate the high-spin ground state due to the ferromagnetic interactions Co(II)-W(V) through the cyanido bridges. Due to the high symmetry of the clusters, no signature of slow magnetic relaxation was observed. The characterization is completed by solid-state IR and UV-Vis-NIR spectroscopy. The conditions for the stable M₉M'₆-based crystals formation are synthetically discussed in terms of the type of capping ligands: monodentate, bridging, and chelating. The potential of the related polynuclear forms toward the magnetism-based functional properties is critically indicated.

A cobalt(ii) chain based on pymca generated in situ from the hydrolysis of 2-cyanopyrimidine: spin canting and magnetic relaxation

A one-dimensional (1D) coordination polymer, [{Co2(pymca)2·(H2O)4}SO4·2H2O] n (1) (pymca = 2-carboxypyrimidine), was solvothermally synthesized via the reaction of 2-cyanopyrimidine and Co(SCN)2. A bidentate pymca ligand was formed in situ by the hydrolysis of 2-cyanopyrimidine. Furthermore, in this study, the magnetic properties of complex 1 were investigated in detail. The results indicated that complex 1 showed a single-chain magnet (SCM) behavior below ca. 3 K. The energy barrier (Δτ 1/k B) and preexponential factor (τ 0) of SCM were 31.2 K and 5.4 × 10-9 s, respectively.

In Situ Ligand Transformation for Two-Step Spin Crossover in FeII[MIV(CN)8]4- (M = Mo, Nb) Cyanido-Bridged Frameworks

We report a unique synthetic route toward the multistep spin crossover (SCO) effect induced by utilizing the partial ligand transformation during the crystallization process, which leads to the incorporation of three different Fe^II complexes into a single coordination framework. The 3-acetoxypyridine (3-OAcpy) molecules were introduced to the self-assembled Fe^II-[M^IV(CN)₈]^4- (M = Mo, Nb) system in the aqueous solution which results in the partial hydrolysis of the ligand into 3-hydroxypyridine (3-OHpy). It gives two novel isostructural three-dimensional {Fe^II₂(3-OAcpy)₅(3-OHpy)₃[M^IV(CN)₈]}· nH₂O (M = Mo, n = 0, FeMo; M = Nb, n = 1, FeNb) coordination frameworks. They exhibit an unprecedented cyanido-bridged skeleton composed of {Fe₃M₂} _n coordination nanotubes bonded by additional Fe complexes. These frameworks contain three types of Fe sites differing in the attached organic ligands, [Fe1(3-OAcpy)₄(μ-NC)₂], [Fe2(3-OHpy)₄(μ-NC)₂], and [Fe3(3-OAcpy)₃(3-OHpy)(μ-NC)₂], which lead to the thermal two-step Fe^II SCO, as proven by X-ray diffraction, magnetic susceptibility, UV-vis-NIR optical absorption, and ⁵⁷Fe Mössbauer spectroscopy studies. The first step of SCO, going from room temperature to the 150-170 K range with transition temperatures of 245(5) and 283(5) K for FeMo and FeNb, respectively, is related to Fe1 sites, while the second step, occurring at the 50-140 K range with transition temperatures of 70(5) and 80(5) K for FeMo and FeNb, respectively, is related to Fe2 sites. The Fe3 site with both 3-OAcpy and 3-OHpy ligands does not undergo the SCO at all. The observed two-step SCO phenomenon is explained by the differences in the ligand field strength of the Fe complexes and the role of their alignment in the coordination framework. The simultaneous application of two related pyridine derivatives is the efficient synthetic route for the multistep Fe^II SCO in the cyanido-bridged framework which is a promising step toward rational design of advanced spin transition molecular switches.

Two three-dimensional [MoIII(CN)7]4−-based magnets showing new topologies and ferrimagnetic ordering below 80 K

Two 3D [Mo^III(CN)₇]⁴⁻-based magnets with new topologies and ferrimagnetic ordering below 80 K have been prepared using different amides as auxiliary ligands.

Wide-Range UV-to-Visible Excitation of Near-Infrared Emission and Slow Magnetic Relaxation in LnIII(4,4'-Azopyridine-1,1'-dioxide)[CoIII(CN)6]3- Layered Frameworks

Trivalent lanthanide ions combined with two molecular linkers, organic 4,4'-azopyridine-1,1'-dioxide (apdo), and inorganic hexacyanidocobaltate(III), gave a series of magnetoluminescent coordination polymers, [{Ln^III(apdo)(H₂O)₄}{Co^III(CN)₆}]·2H₂O (Ln = Nd, 1; Tb, 2; Dy, 3; Er, 4; Tm, 5; Yb, 6). They are hybrid organic-inorganic layered frameworks composed of cyanido-bridged {Ln₂(μ-NC)₄Co₂} squares linked by Ln-apdo-Ln bridges into a coordination network of a mixed 4- and 8-metal ring topology. Lanthanide(III) complexes, [Ln^III(μ-apdo)₂(H₂O)₄(μ-NC)₂]⁺, of a distorted dodecahedral geometry are isolated by diamagnetic [Co^III(CN)₆]^3- and apdo linkers. As a result, 1-6 reveal field-induced slow relaxation of magnetization, with typical temperature-dependent relaxation of a single-ion origin for Nd^III-containing 1, Dy^III-containing 3, and Yb^III-containing 6. The related alternate-current magnetic data were precisely analyzed, indicating the multiple magnetic relaxation pathways, including a direct process, strong quantum tunneling of magnetization, non-negligible Raman processes, and crucial two-phonon Orbach thermal relaxation. The thermal energy barriers of the Orbach process, Δ E/ k_B, are 15.1(9) K with τ₀ = 9.8(9) × 10^-6 s at H_dc = 4500 Oe, 16.1(8) K with τ₀ = 9.0(9) × 10^-5 s at H_dc = 1500 Oe, and 17.3(6) K with τ₀ = 3.2(7) × 10^-6 s at H_dc = 700 Oe, for 1, 3, and 6, respectively, proving the single-molecule magnet (SMM) behavior. Because of the presence of [Co(CN)₆]^3-, 1-6 show strong UV absorption, while the chromophoric apdo leads to the strong absorption in the visible range. As a result, the visible 4f/3d metal-centered emission is quenched, but the near-infrared luminescence from Nd^III and Yb^III is observed in 1 and 6, respectively. It is realized by Co-to-Ln metal-to-metal, and apdo-to-Ln ligand-to-metal energy transfers; thus, broad UV-to-visible excitation can be explored. Compounds 1-6 form a novel family of functional bimetallic assemblies, incorporating NIR-emissive SMMs as presented for NdCo (1) and YbCo (6) derivatives.

Cyano-Bridged Fe(II)-Cr(III) Single-Chain Magnet Based on Pentagonal Bipyramid Units: On the Added Value of Aligned Axial Anisotropy

A cyano-bridged Fe(II)-Cr(III) single-chain magnet designed to ensure a parallel orientation of the axial anisotropy of the building units is reported. This ferromagnetic chain compound consists of a pentagonal bipyramid Fe(II) complex with Ising-type anisotropy and a dicyanide Cr(III) complex interlinked through their apical positions. It is characterized by an energy gap for the magnetization reversal of Δ_eff/ k_B = 113 K and exhibits magnetic hysteresis with a coercive field of 1400 Oe at 2 K which positions this compound among the very few examples of SCMs with spin reversal barriers above 100 K. The quite remarkable performances of this single-strand SCM are attributed to the alignment of the local anisotropy axes, which is supported by ab initio modeling. A discrete Cr₂Fe complex based on the same building units and behaving as a SMM in zero field is also reported.

Syntheses, crystal structures, and magnetic properties of cyanide-bridged complexestrans-RuII(dppe)2(CN)2(FeIIIX3)2(X = Cl and Br)

Cyanide-bridged complexestrans-Ru^II(dppe)₂(CN)₂(Fe^IIIX₃)₂(X = Cl,2; X = Br,3) and their parenttrans-ClRu^II(dppe)₂(CN) (1) were synthesized and fully characterized. The magnetic properties of2and3have been investigated.

Random mixed-metal Co1−xNix single-chain magnets with simultaneous azide, carboxylate and tetrazolate bridges

A series of MOFs based on random ferromagnetic Co_1−xNi_x chains with simultaneous azide, carboxylate and tetrazolate bridges show metamagnetism and slow magnetic relaxation with complicated composition dependence and synergistic effects on single-chain slow dynamics.

A [Cr2Ni] coordination polymer: slow relaxation of magnetization in quasi-one-dimensional ferromagnetic chains

The first example of a Ni^II–Cr^III chain to exhibit slow relaxation of magnetization is reported.

Enforcing Ising-like magnetic anisotropy via trigonal distortion in the design of a W(v)-Co(ii) cyanide single-chain magnet

A new octacyanotungstate(v) singe chain magnet with an effective energy barrier of 39.7(3) cm^–1 is achieved by enforcing Ising-like magnetic anisotropy via introduction of trigonal distortion with a fac-tridentate capping ligand.

A new octacyanotungstate(v) based single chain magnet {[(Tpm)Co(DMF)W(CN)₈]₂[Co(DMF)₄]·2DMF}_n (1, Tpm = 1,1,1-trispyrazoylmethane), with an effective barrier of 39.7(3) cm^–1 is reported. The Ising-like magnetic anisotropy of the chain originates from the nearly parallel local orientations of the Co(ii) ions with the source of the uniaxial magnetic anisotropy being a trigonal distortion of the octahedral environment with the fac-tridentate capping Tpm ligand.

Modulation of the FeII spin crossover effect in the pentadecanuclear {Fe9[M(CN)8]6} (M = Re, W) clusters by facial coordination of tridentate polyamine ligands

N,N,N-tridentate ligands coordinated to {Fe₉[M(CN)₈]₆} (M = Re, W) induce the spin crossover on the external Fe sites of the cluster.

Two-dimensional frameworks formed by pentagonal bipyramidal cobalt(ii) ions and hexacyanometallates: antiferromagnetic ordering, metamagnetism and slow magnetic relaxation

Two-dimensional frameworks constructed by pentagonal bipyramidal Co^II and [M(CN)₆]³⁻ units have been synthesized and characterized structurally and magnetically.