Manganese(II), Iron(II), and Mixed-Metal Metal–Organic Frameworks Based on Chains with Mixed Carboxylate and Azide Bridges: Magnetic Coupling and Slow Relaxation

Highly Selective MOF-Based Turn-Off Luminescence Detection of Hg2+ Ions in an Aqueous Medium and Its Dual Functional Catalytic Activity toward Aldol Condensation and β-Enamination Reactions

A new organic ligand, 5-(carboxyformamido)isophthalic acid (5-CFIA), was prepared and employed for the synthesis of two compounds [M₃(C₁₀H₄O₇N₁)₂(8H₂O)]·H₂O (M = Cd, Mn). The compounds have three-dimensionally extended structures. Both the compounds were found to be luminescent at room temperature. The luminescence nature was exploited for the detection of Hg²⁺ ions in an aqueous medium with good selectivity. The interactions between Hg²⁺ ions and the compounds quench the luminescence intensity and act as a turn-off sensor. Both the compounds exhibited low limits for the detection of Hg²⁺ ions and in the range mandated by the WHO. The interactions between Hg²⁺ ions and the compound involve the -NH group, which was probed using Raman and IR spectroscopic techniques. These studies provide important pointers toward the mechanism of this turn-off luminescence behavior. The compounds were explored for base-catalyzed aldol condensation and Lewis acid-promoted β-enaminoester formation reactions. The aldol condensation reaction uses the -NH functionality as a base. The studies indicate that the electron-withdrawing group produces products with higher yields. The β-enaminoester reaction uses the Lewis acid centers, and the studies reveal that the electron-withdrawing groups produce lesser yields of the products. The catalytic nature of the reaction and recyclability of the catalysts were also established. The catalytic reactions employ ethanol (aldol condensation) and no solvent (β-enaminoester), which suggests that the reactions are green and environmentally friendly. The Mn compound was observed to be anti-ferromagnetic.

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.

Systematic Tuning of the Magnetic Properties in Mixed-Metal MOF-74

Recently, mixed-metal metal-organic frameworks (MOFs) have been attracting much attention in various fields. In this study, we have systematically investigated the magnetic properties of Co_xNi_1-x-MOF-74 [Co_2xNi_2(1-x)(dhtp), where H₄dhtp = 2,5-dihydroxyterephthalic acid] with two different kinds of metals (Co and Ni) across the composition range 0 ≤ x ≤ 1. Bimetallic Co_xNi_1-x-MOF-74 (x = 0.752, 0.458, and 0.233) were successfully synthesized and confirmed to have homogeneous metal distributions. Weak ferromagnetic (canted antiferromagnetic) behavior was exhibited, while homometallic Co-MOF-74 and Ni-MOF-74 are antiferromagnetic. We also investigated the effects of C₂H₄ sorption on the magnetic properties and found that C₂H₄-adsorbed Co_0.5Ni_0.5-MOF-74 exhibited a change in the interchain magnetic interaction.

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.

Single-chain magnets assembled in cobalt(ii) metal–organic frameworks

This feature article discusses the advantages, progress and prospects of constructing single-chain magnets in metal–organic frameworks.

Mixed metal CoII1−xZnIIx–organic frameworks based on chains with mixed carboxylate and azide bridges: magnetic coupling and slow relaxation

A series of isomorphous three-dimensional metal–organic frameworks [Co^II_1−xZn^II_x(L)(N₃)]·H₂O (x = 0.26, 0.56 and 0.85) based on bimetallic Co^II_1−xZn^II_x (x = 0.26, 0.56 and 0.85) chains with random metal sites have been synthesized and magnetically characterized. The Co^II_1−xZn^II_x series, which intrinsically feature random anisotropic/diamagnetic sites, shows complex magnetic interactions. By gradually introducing the diamagnetic Zn^II ions into the pure anisotropic Co^II single-chain magnets system, the ferromagnetic interactions between Co^II ions are gradually diluted. Moreover, the slow magnetic relaxation behaviour of the mixed metal Co^II_1−xZn^II_x systems also changes. In this bimetallic series Co^II_1−xZn^II_x, the Co-rich materials exhibit slow relaxation processes that may arise from SCM mechanism, while the Zn^II-rich materials show significantly low slow magnetic relaxation. A general trend is that the activation energy and the blocking temperature decrease with the increase in diamagnetic Zn^II content, emphasizing the importance of anisotropy for slow relaxation of magnetization.

A series of mixed-metal systems consisting of Co^II_1−xZn^II_x chains with (μ-COO)₂(μ-EO-N₃) bridges were prepared, exhibiting interesting magnetic coupling and slow magnetic relaxation.

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.

An Ising iron(ii) chain exhibits a large finite-size energy barrier and "hard" magnetic behaviour

One-dimensional spin chains featuring strong axial anisotropic magnetism are promising candidates for isolatable and miniatured information storage materials, the so-called single-chain magnets (SCMs). Here we show a mixed azido/carboxylato bridged metamagnetic iron(ii) chain [Fe(N₃)₂(4-mpc)]_n (4-mpc = N-methylpyridinium-4-carboxylate) with a large energy barrier of 150 K, a large remnant magnetization (1.55Nβ) and coercivity (1.7 T at 2 K) for homo-spin SCMs. Heat capacity and Mössbauer spectroscopy studies corroborate the intrinsic nature of SCM behavior regardless of weak interchain magnetic interactions, which lead to the coexistence of metamagnetism but not long-range magnetic ordering. Moreover, detailed magnetic investigations indicate that the system is not only within the "Ising limit" but also in the "finite-size" regime.

Tuning of Charge Transfer Assisted Phase Transition and Slow Magnetic Relaxation Functionalities in {Fe(9-x)Co(x)[W(CN)8]6} (x = 0-9) Molecular Solid Solution

Precisely controlled stoichiometric mixtures of Co(2+) and Fe(2+) metal ions were combined with the [W(V)(CN)8](3-) metalloligand in a methanolic solution to produce a series of trimetallic cyanido-bridged {Fe(9-x)Co(x)[W(CN)8]6(MeOH)24}·12MeOH (x = 0, 1, ..., 8, 9; compounds 0, 1, ..., 8, 9) clusters. All the compounds, 0-9, are isostructural, and consist of pentadecanuclear clusters of a six-capped body-centered cube topology, capped by methanol molecules which are coordinated to 3d metal centers. Thus, they can be considered as a unique type of a cluster-based molecular solid solution in which different Co/Fe metal ratios can be introduced while preserving the coordination skeleton and the overall molecular architecture. Depending on the Co/Fe ratio, 0-9 exhibit an unprecedented tuning of magnetic functionalities which relate to charge transfer assisted phase transition effects and slow magnetic relaxation effects. The iron rich 0-5 phases exhibit thermally induced reversible structural phase transitions in the 180-220 K range with the critical temperatures being linearly dependent on the value of x. The phase transition in 0 is accompanied by (HS)Fe(II) W(V) ↔ (HS)Fe(III) W(IV) charge transfer (CT) and the additional minor contribution of a Fe-based spin crossover (SCO) effect. The Co-containing 1-5 phases reveal two simultaneous electron transfer processes which explore (HS)Fe(II) W(V) ↔ (HS)Fe(III) W(IV) CT and the more complex (HS)Co(II) W(V) ↔ (LS)Co(III) W(IV) charge transfer induced spin transition (CTIST). Detailed structural, spectroscopic, and magnetic studies help explain the specific role of both types of CN(-)-bridged moieties: the Fe-NC-W linkages activate the molecular network toward a phase transition, while the subsequent Co-W CTIST enhances structural changes and enlarges thermal hysteresis of the magnetic susceptibility. On the second side of the 0-9 series, the vanishing phase transition in the cobalt rich 6-9 phases results in the high-spin ground state, and in the occurrence of a slow magnetic relaxation process at low temperatures. The energy barrier of the magnetic relaxation gradually increases with the increasing value of x, reaching up to ΔE/kB = 22.3(3) K for compound 9.

Novel manganese(ii) and cobalt(ii) 2D polymers containing alternating chains with mixed azide and carboxylate bridges: crystal structure and magnetic properties

Two new Mn(ii) and Co(ii) complexes based on AABAA alternating chains with mixed azide and carboxylate bridges were synthesized, structurally and magnetically studied.

A unusual two-dimensional azido-Cu(ii) network with benzoate derivative as a co-ligand exhibiting ferromagnetic order and slow magnetic relaxation

A 2D layer-like Cu(ii) compound with multiple-bridges of azido, carboxylate and hydroxyl ligands, featuring strong ferromagnetic coupling and slow magnetic relaxation.

Structural and magnetic modulations of copper(ii) azido complexes: unexpected in situ reactions of mono-N-donor pyridine-based co-ligands

Hydrothermal reactions of azide anions, Cu(ii) ions, and fourmono-N-donor pyridine ligands lead to four new copper(ii) azides with different structures and magnetic properties, in which three co-ligands were generatedin situby decomposition, substitution, and oxidation reactions.

Composition dependence of magnetic relaxation for CoIIxNiII1−x chain-based compounds with mixed double azide–tetrazolate bridges

A series of isomorphous random bimetallic compounds based on CoIIxNiII1−x (0 ≤ x ≤ 1) chains with simultaneous azide–tetrazolate bridges exhibit composition-dependent magnetic relaxation were investigated.

Spin canting, metamagnetism, and single-chain magnetic behaviour in a cyano-bridged homospin iron(II) compound

Spin canting, antiferromagnetic ordering, metamagnetism and single-chain magnetism were verified in a cyano-bridged Fe(II) compound synthesized from the pentagonal bipyramidal Fe(II) starting material in the presence of excessive BF4(-) anions.

Luminescent MOF material based on cadmium(ii) and mixed ligands: application for sensing volatile organic solvent molecules

A luminescent MOF was constructed by employing a mixed ligands strategy and may be used as a potential luminescent probe for the detection of acetone.

Four tetrazolate-based 3D frameworks with diverse subunits directed by inorganic anions and azido coligand: hydro/solvothermal syntheses, crystal structures, and magnetic properties

Four magnetic frameworks with hourglass-shaped {Cu₅(μ₃-OH)₂}⁸⁺ cluster, linear {Cu₃(μ-Cl)₂}⁴⁺ core, linear {Cu(μ_1,1-N₃)}⁺ chain and trigonal-prismatic {Cu₈(μ₃-tz)₆}¹⁰⁺ motifs were reported.

Structural and magnetic tuning from a field-induced single-ion magnet to a single-chain magnet by anions

By choosing different anions, a mononuclear field-induced single-ion magnet and a one-dimensional single-chain magnet were selectively prepared and characterized.

12-Metal 36-membered ring based WV–CoIIlayers showing spin-glass behavior

Two cyano-bridged bimetallic W^V–Co^IIlayers were synthesized, which contain 12 metal 36 membered rings and present spin-glass behavior.

Self-assembly of two high-nuclearity manganese calixarene-phosphonate clusters: diamond-like Mn16 and drum-like Mn14

Variation of the phosphonic acid “converts” a tetradecanuclear drum-like MnII14 cluster (1) into a hexadecanuclear diamond-like MnII16 cluster (2).

Three sra topological lanthanide–organic frameworks built from 2,2′-dimethoxy-4,4′-biphenyldicarboxylic acid

The first neutral 3D sra-LOF built from a 1D inorganic rod-shaped chain [Ln(CO₂)₂(HCO₂)]_n (Ln = Eu, Gd, Dy) was synthesized.

Two novel cadmium(ii) carboxyphosphonates with 3D framework structure: synthesis, crystal structures, luminescence and molecular recognition properties

Two novel cadmium(ii) carboxyphosphonates with 3D framework structure have been synthesized under hydrothermal conditions. The luminescence properties and molecular recognition properties of the compounds have been investigated.