Large Magnetic Anisotropy in Mono- and Binuclear cobalt(II) Complexes: The Role of the Distortion of the Coordination Sphere in Validity of the Spin-Hamiltonian Formalism

To get a better insight into understanding the factors affecting the enhancement of the magnetic anisotropy in single molecule (single ion) magnets, two cobalt(II) complexes based on a tridentate ligand 2,6-di(thiazol-2-yl)pyridine substituted at the 4-position with N-methyl-pyrrol-2-yl have been synthesized and studied by X-ray crystallography, AC and DC magnetic data, FIRMS and HFEPR spectra, and theoretical calculations. The change of the counteranion in starting Co(II) salts results in the formation of pentacoordinated mononuclear [Co(mpyr-dtpy)Cl2]·2MeCN (1) complex and binuclear [Co(mpyr-dtpy)2][Co(NCS)4] (2) compound. The observed marked distortion of trigonal bipyramid geometry in 1 and cationic octahedral and anionic tetrahedral units in 2 brings up a question about the validity of the spin-Hamiltonian formalism and the possibility of determining the value and sign of the zero-field splitting D parameter. Both complexes exhibit field-induced slow magnetic relaxation with two or three relaxation channels at BDC = 0.3 T. The high-frequency relaxation time in the reciprocal form τ(HF)-1 = CTn develops according to the Raman relaxation mechanism (for 2, n = 8.8) and the phonon-bottleneck-like mechanism (for 1, n = 2.3). The high-frequency relaxation time at T = 2.0 K and BDC = 0.30 T is τ(HF) = 96 and 47 μs for 1 and 2, respectively.

Binuclear cobalt(II) and two-dimensional manganese(II) coordination compounds self-assembled by mixed bipyridine-tetracarboxylic ligands with single-ion magnet properties

A cobalt(II) complex and manganese(II) coordination polymer, formulated as [Co2(H2btca)(mbpy)4][H2btca]·4H2O (1) and {Mn2(btca)(mbpy)2(H2O)2}n (2) (H4btca = 1,2,4,5-benzenetetracarboxylic acid; mbpy = 4,4'-dimethyl-2,2'-bipyridyl), constructed by mixed bipyridine-tetracarboxylic ligands were synthesized and characterized. Single-crystal structural analyses reveal that compound 1 is a discrete neutral binuclear molecule, while compound 2 is a two-dimensional (2D) coordination polymer. The metal ions in these compounds are well isolated, with an intramolecular Co2+⋯Co2+ distance of 9.170 Å for 1 and Mn2+⋯Mn2+ separation of 10.984 and 11.164 Å for 2 due to the bulk tetracarboxylic linker. This isolation gives rise to a single-ion magnetism origin of the compounds. Magnetic studies reveal a large zero-field splitting parameter D of 82.6 cm-1 for 1, while a very small D of 0.42 cm-1 was observed for 2. Interestingly, dynamic ac magnetic measurements exhibited slow magnetic relaxation under the external dc field of the two compounds, revealing the field-supported single-ion magnet (SIM) of 1 and 2. The detailed theoretical calculations were further applied to understand the electronic structures, magnetic anisotropy, and relaxation dynamics in 1 and 2. Combined with our recently reported compound (Eur. J. Inorg. Chem., 2022, e202200354), the foregoing results provide not only a rare binuclear cobalt(II) SIM and the first 2D manganese(II) SIM coordination polymer but also a bipyridine-tetracarboxylic ligand approach toward novel SIMs.

Tuning chain topologies and magnetic anisotropy in one-dimensional cobalt(II) coordination polymers via distinct dicarboxylates

Based on a terpyridine derivative and two different dicarboxylate ligands, two new cobalt(II) coordination polymers, namely [Co(pytpy)(DClbdc)]n (1) and [Co(pytpy)(ndc)]n (2) (pytpy = 4'-(4-Pyridyl)-2,2':6',2''-terpyridine, H2DClbc = 2,5-Dichloroterephthalic acid, and H2ndc...

Magnetic relaxation in a Co(ii) chain complex: synthesis, structure, and DFT computational coupling constant

We report a homospin non-solvated Co^II chain compound constructed by two similar tridentate ligands and chloride ions, which displays slow magnetic relaxation below 5 K.

Switching of easy-axis to easy-plane anisotropy in cobalt(ii) complexes

In situ microcalorimetry monitored assembly and coligand induced switching of the magnetic anisotropy sign have been observed in a β-diketonate-Co(ii) system.

X-ray Structure and Magnetic Properties of Heterobimetallic Chains Based on the Use of an Octacyanidodicobalt(III) Complex as Metalloligand

The assembly of [Co2III(μ-2,5-dpp)(CN)8]2− anions and [MII(CH3OH)2(DMSO)2]2+ cations resulted into the formation of two heterobimetallic 1D coordination polymers of formula [MII(CH3OH)2(DMSO)2(μ-NC)2Co2III(μ-2,5-dpp)(CN)6]n·4nCH3OH [M = CoII (1)/FeII (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine. The [Co2III(μ-2,5-dpp)(CN)8]2− metalloligand coordinates the paramagnetic [MII(CH3OH)2(DMSO)2]2+ complex cations, in a bis-monodentate fashion, to give rise to neutral heterobimetallic chains. Cryomagnetic dc (1.9–300 K) and ac (2.0–13 K) magnetic measurements for 1 and 2 show the presence of Co(II)HS (1) and Fe(II)HS (2) ions (HS – high-spin), respectively, with D values of +53.7(5) (1) and −5.1(3) cm−1 (2) and slow magnetic relaxation for 1, this compound being a new example of SIM with transversal magnetic anisotropy. Low-temperature Q-band EPR study of 1 confirms that D value is positive, which reveals the occurrence of a strong asymmetry in the g-tensors and allows a rough estimation of the E/D ratio, whereas 2 is EPR silent. Theoretical calculations by CASSCF/NEVPT2 on 1 and 2 support the results from magnetometry and EPR. The analysis of the ac magnetic measurements of 1 shows that the relaxation of M takes place in the ground state under external magnetic dc fields through dominant Raman and direct spin-phonon processes.

Slow magnetic relaxation in hexacoordinated cobalt(ii) field-induced single-ion magnets

To gain an insight into the factors affecting the enhancement of the energy barrier in SMM/SIM, hexacoordinate pseudohalide Co(ii) complexes based on the tridentate ligand were investigated.

Luminescent sensing of nitroaromatics by crystalline porous materials

Designing strategies for the syntheses of targeted luminescent MOFs, nanoparticle/MOF composites and COFs described and their application in sensing nitroaromatic compounds and explosives discussed.

Magneto-structural diversity of Co(ii) compounds with 1-benzylimidazole induced by linear pseudohalide coligands

The magneto-structural diversity of 1-benzylimidazole-containing cobalt(ii) compounds with linear pseudohalide ions (NCS⁻, NCO⁻, and N₃⁻) is explored.

Field-induced slow magnetic relaxation in two interpenetrated cobalt(ii) metal–organic framework isomers

Two interpenetrated cobalt(ii) metal–organic framework (MOF) isomers were successfully synthesized and magnetically characterized. These compounds are the first example of MOF isomers showing field-induced single-ion magnet behavior.

Temperature-Controlled Assembly/Reassembly of Two Dicarboxylate-Based Three-Dimensional Co(II) Coordination Polymers with an Antiferromagnetic Metallic Layer and a Ferromagnetic Metallic Chain

Two new dicarboxylate-based three-dimensional cobalt coordination polymers, [Co(Me₂mal)(bpe)_0.5(H₂O)]_n (1) and [Co(Me₂mal)(bpe)_0.5]_n (2), were synthesized from dimethylmalonic acid (H₂-Me₂mal) in temperature-controlled solvothermal reactions. Lower temperatures (6080 °C) favored the formation of 1, while higher temperatures (120 °C) favored the production of 2. Compound 1 is comprised of Co(II) corrugated layers linked by syn-anti carboxylate bridges from the Me₂mal^2- ligands and pillared through bis-monodentate bpe groups. Compound 2 is comprised of a three-dimensional network involving one-dimensional Co-carboxylate chains bonded by antisymmetric µ₄-Me₂mal^2- ligands and aligned parallel to the [001] direction. The solvothermal retreatment of crystalline samples of 1 in a DMF/H₂O solvent at 120 °C allowed the structural reassembly, with complete conversion within 2 over 48 h. Magnetic analyses revealed that compound 1 exhibits both spin-orbital coupling and antiferromagnetic interactions through a syn-anti carboxylate (Me₂mal^2-) bridge exchange pathway [Co-Co separation of 5.478 Å] and compound 2 showed a ferromagnetic interaction resulting from the short Co-Co separation (3.150 Å) and the small Co-O-Co bridging angles (98.5° and 95.3°) exchange pathway which was provided by µ₄-Me₂mal^2- bridging ligand.

Micromesoporous Nitrogen-Doped Carbon Materials Derived from Direct Carbonization of Metal-Organic Complexes for Efficient CO2 Adsorption and Separation

Metal-organic complexes (MOCs) are considered as excellent precursors to prepare carbon materials, due to the fact that heteroatoms and functional groups can be naturally reserved in the resulting carbon materials through the carbonization. Herein, micromesoporous nitrogen-doped carbons MPNC-1 and MPNC-2 are successfully obtained by direct carbonization (800 °C, KOH activation) of metal-organic complexes DQA-1 and DQA-2. MPNC-1 and MPNC-2 exhibit high BET surface area (2368.9 and 2327.6 m² g^-1), pore volume (1.95 and 1.89 cm³ g^-1), and N contents (17.2% and 12.3%). At 25 °C and 1 bar, MPNC-1 and MPNC-2 show high CO₂ adsorption of 7.53 and 6.58 mmol g^-1, the estimated CO₂/N₂ selectivity are 20.5 and 22.6, indicating excellent promise for practical CO₂ adsorption and separation applications. Theoretical calculation indicates carbon surfaces with pyridinic-N, pyrrolic-N, and graphitic-N coexistence could strongly change the local electronic distribution and electrostatic surface potential, enhancing the CO₂ adsorption with adsorption energy of -58.96 kJ mol g^-1. Theoretical calculation also highlights that CO₂ adsorption mechanism is electrostatic interaction with a large green isosurface between CO₂ molecules and the carbon surface.

Synthesis of a Zn(ii)-based 1D zigzag coordination polymer for the fabrication of optoelectronic devices with remarkably high photosensitivity

This study presents the fabrication of a novel Zn(ii)-based 1D zigzag coordination polymer, which shows remarkably high photosensitivity and has the potential to be used in photoswitching devices and solar cells.

Modulation of the magnetic anisotropy of octahedral cobalt(ii) single-ion magnets by fine-tuning the axial coordination microenvironment

The alteration of the axial N-donor ligands leads to two octahedral Co(ii) SIMs with varying easy-plane magnetic anisotropies and dynamic magnetic behaviors.

Two zinc(ii)-based coordination polymers with flexible dicarboxylate and pyridine mixed ligands: effect of π⋯π interactions on electrical activity

We synthesized two Zn(ii)-based 1D coordination polymers with paddle-wheel units using flexible 1,4-cyclohexanedicarboxylate linkers; these compounds exhibited electrical conductivity and Schottky barrier diode behavior.

Luminescence sensing, electrochemical, and magenetic properties of 2D coordination polymers based on the mixed ligands p-terphenyl-2,2′′,5′′,5′′′-tetracarboxylate acid and 1,10-phenanthroline

Four 2D CPs were constructed and provided platforms to investigate the electrochemical behavior, luminescence sensing of Cr(vi) and pesticides, and magnetic properties.

Multifunctional Properties of a 1D Helical Co(II) Coordination Polymer: Toward Single-Ion Magnetic Behavior and Efficient Dye Degradation

This contribution deals with the synthesis and utilization of a new pyrazole-based unsymmetrical ligand, 3-(3-carboxyphenyl)-1H-pyrazole-5-carboxylic acid (H₂CPCA), for generating multifunctional materials. The reaction with the Co(II) salt in the presence of a co-ligand 2,9-dimethyl phenanthroline (dmphen) results in the formation of the helical compound {[Co₂(dmphen)₂(CPCA)₂]DMF} _n (1). However, two isostructural monomeric complexes are formed {[M(HCPCA)₂(H₂O)₂], M = Co(II), (2) and Mn(II) (3)} when reactions were carried out in the absence of dmphen. Compound 1 shows some highly encouraging single-ion magnetic (SIM) properties. Detailed magnetic studies unveil slow relaxation of magnetization of compound 1, driven by the higher magnetic anisotropy of the cobalt ion, with the energy barrier of ∼9.2 K and relaxation time of 9.1 × 10^-5 s, suggesting a SIM behavior. Moreover, UV-vis and fluorescence studies confirm the selective dye degradation of compound 1 with methylene blue both in the presence and absence of H₂O₂, with the remarkable degradation efficiency of ∼98 and ∼82%, respectively.