Ligand Effect on the Single-Molecule Magnetism of Tetranuclear Co(II) Cubane

A tetracobalt(II) cluster with a two vertex truncated dicubane topology endogenously supported by carboxylate-based (2-pyridyl)methylamine ligands: magneto-structural and DFT studies

A reaction between CoCl2 and L3-(CO2-)2 (2 : 1 stoichiometry) in CH3OH affords a discrete complex [CoII4-{L3-(CO2-)2}2(μ3-OCH3)2(CH3OH)2(H2O)2Cl2] (1) [L3-(CO2-)2 = 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)]. The structure of 1 reveals two terminal mononuclear CoII{L3-(CO2-)2}Cl units connected by a dimeric CoII2(μ3-OCH3)2(CH3OH)2(H2O) unit present in the centre through two methoxo (μ3-OCH3)- and two carboxylate (μ-1,1-OCO-) bridges affording a tetranuclear coordination cluster of Co(II) with a defective dicubane topology. In 1, Co1 (terminal) has distorted octahedral CoIIN2O3Cl and the central Co2 has CoIIO6 coordination. Such coordination arrangements afford the observed topology. Variable-temperature magnetic studies reveal anti-ferromagnetic coupling in 1. Three magnetic exchange interactions (one anti-ferromagnetic and two ferromagnetic: J1 = +3.3 cm-1 (Co⋯Co 3.176 Å; μ-1,1-OCO- and μ3-OCH3 bridges), J2 = -2.5 cm-1 (Co⋯Co 3.228 Å; μ-1-OCO- and μ3-OCH3 bridges) and J3 = +10.6 cm-1 (Co⋯Co 3.084 Å; two μ3-OCH3 bridges)) have been identified, with the inclusion of the orbital reduction parameter (α = Aκ = 1.38), spin-orbit coupling (λ = -158 cm-1) and axial distortion (energy gap Δ = -975 cm-1 between singlet and doublet levels), rationalized by density functional theory (DFT) calculations.

Structure and magnetic properties of an amine-templated one-dimensional cobalt-fluoro-sulfate containing Co4F4 cubane and hydrogen evolution reaction (HER) performance of its derived carbon-wrapped CoSe2 nanorods

Amine-templated 1D cobalt fluoro sulfate of the composition [(CH₃)₂NH₂]₂[Co₄F₄(SO₄)₃(C₃N₂H₄)₄], consisting of Co₄F₄ cubane-type secondary building unit, has been synthesized under solvothermal condition. The magnetic properties of the Co₄F₄ cubane chain exhibited a low-temperature magnetic ordering below 17 K (T_c) attributed to intra-cluster ferromagnetic coupling and did not show spin-glass freezing. The selenylation of the Co₄F₄ cubane chain leads to the formation of sphere-like CoSe₂ in the hydrothermal route (CoSe₂@HT). At the same time, nanorods of CoSe₂ encapsulated with carbon matrix were obtained in a sealed tube method (CoSe₂@ST). Moreover, CoSe₂@ST exhibited a higher hydrogen evolution reaction (HER) activity than CoSe₂@HT in an acidic medium with 177 mV overpotential to achieve the benchmark current density of 10 mA cm^-2. The promising HER performance of derived CoSe₂@ST could be attributed to an increase in the geometrical and specific activity due to the encapsulation of N-doped carbon matrix over the CoSe₂ nanorods that facilitate faster charge transfer at the electrode-electrolyte interface and higher electrochemical conductivity than the derived CoSe₂@HT. This work demonstrates a low-temperature, solvent- and reducing agent-free new synthetic approach for synthesizing framework-derived materials.

The dominance of sulfate over two organic ligands in the solvothermal assembly of an undecanuclear cobaltous cluster: crystallography and mass spectrometry

An unexpected dominance of the coordination affinity of the sulfate anion over those of two organic chelating ligands, (1-methyl-1H-benzo[d]imidazol-2-yl)methanol (Hmbm) and pyridin-2-ylmethanamine (pma), was observed during the study of the solvothermal assembly of [Co11(mbm)6(pma)2(SO4)8(H2O)6(CH3OH)6]·5CH3OH (1), from CoSO4·7H2O at 100 °C in methanol. ESI-MS of solutions at different periods of the assembly reveal a hierarchical sequence, [Co1] → [Co2] → [Co3] → [Co4] → [Co5] → [Co9], where the lower nuclearity species are richer in Co2+ and SO42- before the inclusion of more mbm ligands at the last step. Its crystal structure consists of an almost symmetrical wheel of nine cobalt atoms, [Co9(mbm)6(SO4)8(H2O)6] in edge-sharing octahedral coordination arranged in triangles with two peripheral dangling [Co(pma)(CH3OH)3]. The sulfate ions exhibit three different coordination modes, two in μ6-, two in μ3- and four as μ2-bridging. Its thermal, optical and magnetic properties are also reported.

One-Pot Self-Assembly of Dinuclear, Tetranuclear, and H-Bonding-Directed Polynuclear Cobalt(II), Cobalt(III), and Mixed-Valence Co(II)/Co(III) Complexes of Schiff Base Ligands with Incomplete Double Cubane Core

The reaction of 2,6-diformyl-4-methylphenol (DFMF) with 1-amino-2-propanol (AP) and tris(hydroxymethyl)aminomethane (THMAM) was investigated in the presence of Cobalt(II) salts, (X = ClO₄⁻, CH₃CO₂⁻, Cl⁻, NO₃⁻), sodium azide (NaN₃), and triethylamine (TEA). In one pot, the variation in Cobalt(II) salt results in the self-assembly of dinuclear, tetranuclear, and H-bonding-directed polynuclear coordination complexes of Cobalt(III), Cobalt(II), and mixed-valence Co^IICo^III: [Co₂^III(H₂L⁻¹)₂(AP⁻¹)(N₃)](ClO₄)₂ (1), [Co₄(H₂L⁻¹)₂(µ₃-1,1,1-N₃)₂(µ-1,1-N₃)₂Cl₂(CH₃OH)₂]·4CH₃OH (2), [Co₂^IICo₂^III(HL⁻²)₂(µ-CH₃CO₂)₂(µ₃-OH)₂](NO₃)₂·2CH₃CH₂OH (3), and [Co₂^IICo₂^III (H₂L1²⁻)₂(THMAM⁻¹)₂](NO₃)₄ (4). In 1, two cobalt(III) ions are connected via three single atom bridges; two from deprotonated ethanolic oxygen atoms in the side arms of the ligands and one from the1-amino-2-propanol moiety forming a dinuclear unit with a very short (2.5430(11) Å) Co-Co intermetallic separation with a coordination number of 7, a rare feature for cobalt(III). In 2, two cobalt(II) ions in a dinuclear unit are bridged through phenoxide O and μ₃-1,1,1-N₃ azido bridges, and the two dinuclear units are interconnected by two μ-1,1-N₃ and two μ₃-1,1,1-N₃ azido bridges generating tetranuclear cationic [Co₄(H₂L⁻¹)₂(µ₃-1,1,1-N₃)₂(µ-1,1-N₃)₂Cl₂(CH₃OH)₂]²⁺ units with an incomplete double cubane core, which grow into polynuclear 1D-single chains along the a-axis through H-bonding. In 3, HL²⁻ holds mixed-valent Co(II)/Co(III) ions in a dinuclear unit bridged via phenoxide O, μ-1,3-CH₃CO₂⁻, and μ₃-OH⁻ bridges, and the dinuclear units are interconnected through two deprotonated ethanolic O in the side arms of the ligands and two μ₃-OH⁻ bridges generating cationic tetranuclear [Co₂^IICo₂^III(HL⁻²)₂(µ-CH₃CO₂)₂(µ_3-OH)₂]²⁺ units with an incomplete double cubane core. In 4, H₂L1⁻² holds mixed-valent Co(II)/Co(III) ions in dinuclear units which dimerize through two ethanolic O (μ-RO⁻) in the side arms of the ligands and two ethanolic O (μ₃-RO⁻) of THMAM bridges producing centrosymmetric cationic tetranuclear [Co₂^IICo₂^III (H₂L1⁻²)₂(THMAM⁻¹)₂]⁴⁺ units which grow into 2D-sheets along the bc-axis through a network of H-bonding. Bulk magnetization measurements on 2 demonstrate that the magnetic interactions are completely dominated by an overall ferromagnetic coupling occurring between Co(II) ions.

Synthesis and Structure of Fluorinated (Benzo[d]imidazol-2-yl)methanols: Bench Compounds for Diverse Applications

A simple and general approach to the synthesis (from commercial precursors) of eight out of nine possible (benzo[d]imidazol-2-yl)methanols fluorinated on the benzene ring is reported. Molecular and crystalline structures of most compounds were solved by X-ray diffraction analysis. This made it possible to reveal the influence of the number and arrangement of fluorine atoms in the benzene cycle on the formation of intermolecular hydrogen bonds. It was found that the more fluorine atoms are present in a compound, the higher the dimensionality of the H-bonded structure is. Moreover, the presence of fluorine atoms in the synthesized compounds leads to the emergence of C–F…π interactions affecting crystal packing. The synthesized fluorinated (benzo[d]imidazol-2-yl)methanols may serve as excellent bench compounds for the synthesis of a systematic series of fluorine-containing derivatives to study structure–property correlations in various fields of research from medicine to materials science.

A Chiral and Polar Single-Molecule Magnet: Synthesis, Structure, and Tracking of Its Formation Using Mass Spectrometry

The solvothermal reaction of cobalt(II) sulfate with S, S-1,2- bis(1-methyl-1 H-benzo[ d]imidazol-2-yl)ethane-1,2-diol, (H₂L), neutralized with triethylamine (Et₃N) in a mixture of methanol and water (2:1), resulted in triangular red crystals of [Co^II₇(L)₃(SO₄)₃(OH)₂(H₂O)₉]·4H₂O·3CH₃OH (Co₇). It is formed of chiral and polar clusters crystallizing in the R₃ space group. Co₇ consists of apex-shared asymmetric dicubane units where all of the metals adopt an octahedral coordination and the three ligands wrap diagonally around the unit. One end of the cluster is bonded by six water molecules and the other end by three monodentate sulfates. The head-to-tail packing through extended H-bonds leads to polar chains. The ligand has lost two protons, adopts a cis-conformation, and is coordinated to five metals around the waist of the dicubane. Electrospray ionization mass spectrometry (ESI-MS) of solutions of the reaction as a function of time reveals the possible step-by-step assembly process of the cluster: the initial product [Co^II(HL)(SO₄)]^2- combines with CoSO₄, forming [Co^II₂(HL)(SO₄)₂]^2-, and then, upon addition of Et₃N, dimerizes through a [OH]^- bridge to [Co^II₄(HL)₂(OH)(CH₃OH)₂(SO₄)₃]^- followed by capture of one Co²⁺ and one CoSO₄ to form [Co^II₆(L)₂(OH)(CH₃O)(SO₄)₄]^2- before eventually binding to CoL to form [Co^II₇(L)₃(OH)₂(SO₄)₄]^2-. These results allow us to propose a possible process for the formation of Co₇, which is a good example for chiral multidentate chelating ligand-controlled assembly of clusters. Magnetization measurements as a function of the temperature, field, and ac-frequency reveal ferromagnetic coupled moments and single-molecule magnetism (SMM).

Magnetic relaxation in cobalt(ii)-based single-ion magnets influenced by distortion of the pseudotetrahedral [N2O2] coordination environment

Cobalt(ii) complexes with different dihedral angles between the bidentate ligands show a significant variation in their magnetic relaxation behavior.