Tetranuclear CoII4O4 Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation

The reaction of Co(OAc)2·6H2O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co4(L)2(μ-η1:η1-OAc)2(η2-OAc)2]· 1.5 CH3OH· 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L]2- and four acetate ligands. Two distinct types of CoII centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm-1), while the magnetic anisotropy is the easy-plane type (D1 = 8.8, D2 = 0.76 cm-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec-1, required to attain a current density of 10 mA cm-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (Rct) of 2.927 Ω, which is comparatively lower than standard Co3O4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.

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.

Slow magnetic relaxation in dinuclear Co(III)-Co(II) complexes containing a five-coordinated Co(II) centre with easy-axis anisotropy

Two air-stable Co(III)-Co(II) mixed-valence complexes of molecular formulas [Co^IICo^III(L)(DMAP)₃(CH₃COO)]·H₂O·CH₃OH (1) and [Co^IICo^III(L)(4-Pyrrol)₃ (CH₃COO)]·0.5CH₂Cl₂ (2) (H₄L = 1,3-bis-(5-methyl pyrazole-3-carboxamide) propane; DMAP = 4-dimethylaminopyridine; and 4-Pyrrol = 4-pyrrolidinopyridine) were synthesized and characterized by single-crystal X-ray crystallography, high-field electron paramagnetic resonance (HFEPR) spectroscopy, and magnetic measurements. Both complexes possess one five-coordinated paramagnetic Co(II) ion and one six-coordinated Co(III) ion with octahedral geometry. Direct-current magnetic susceptibility and magnetization measurements show the easy-axis magnetic anisotropy that is also confirmed by low-temperature HFEPR measurements and theoretical calculations. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal their field-assisted slow magnetic relaxation, which is a characteristic behavior of single-molecule magnets (SMMs), caused by the individual Co(II) ion. The effective energy barrier of complex 1 (49.2 cm^-1) is significantly higher than those of the other dinuclear Co(III)-Co(II) SMMs. This work hence presents the first instance of the dinuclear Co(III)-Co(II) single-molecule magnets with a five-coordinated environment around the Co(II) ion.

Photo-sensitive hybrids constructed from diphenyliodonium and metal-thiocyanates: photo-induced structure and property transformations

New hybrids constructed from diphenyliodonium and metal thiocyanates have been prepared, and their photo-induced structural transformations and corresponding color/fluorescence/magnetism variations have been investigated.

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.

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 CoIICoIII: [Co2III(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), [Co2IICo2III(HL-2)2(µ-CH3CO2)2(µ3-OH)2](NO3)2·2CH3CH2OH (3), and [Co2IICo2III (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 [Co2IICo2III(HL-2)2(µ-CH3CO2)2(µ3-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 [Co2IICo2III (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.

Preparation, crystal structures, properties, and time-dependent density functional theory of two cobalt complexes with 3-hydroxy-2-methyl-quinoline-4-carboxylate

One organic compound [HMCA]2 (1) and two novel cobalt complexes [Co(MCA)(bipy)(H2O)]·(H2O) (2), [Co2(MCA)2(Phen)3]·3(H2O) (3) are synthesized by a solvothermal approach and are structurally determined by single-crystal X-ray diffraction. Compound 1 exhibits a two-dimensional structure by hydrogen bond and π. . .π stacking interaction. The complexes exhibit a three-dimensional and one-dimensional metal-organic framework. Solid-state photoluminescence spectrums reveal that they show blue emission bands at 449 nm, and theoretical calculation results of time-dependent density functional theory show that all belong to ligand-to-ligand charge transfer. Solid-state diffuse reflectance spectrums reveal the presence of a narrow optical band gap of 1.72 and 1.60 eV in the complexes 2 and 3, respectively.

Structural characterization and magnetic property studies of a mixed-valence {CoIIICo} complex with a μ4-oxo tetrahedral {Co} motif

We have synthesized and structurally characterized a new mixed valence pentanuclear Co complex, bearing a rare μ4-O-tetrahedral CoII4 unit, by employing a pyridine-like Schiff base ligand. We have performed DC magnetic susceptibility and magnetization measurements over polycrystalline samples and chemical quantum computations in order to understand the exchange interaction pattern within Co(ii) sites and ground state magnetic anisotropy. This new complex shows an overall antiferromagnetic exchange interaction whose strength strongly depends on the local symmetry of Co(ii) sites. Also, local ion magnetic anisotropy reveals a strongly axial behaviour with the lowest Kramers doublet (KD) at each Co(ii) ion sufficiently isolated from excited states at low temperatures. Two Co(ii) sites show tetrahedral symmetry and the spin only formalism including axial zero-field splitting (ZFS) term properly described them; on the other hand, the other two Co(ii) sites have distorted octahedral and square base pyramidal coordination spheres, and a strong orbital contribution leads to a failure of the spin only formalism. A model of four Seff = 1/2 exchange interacting sites is necessary in order to account for low temperature magnetization behaviour. In view of the strongly anisotropic KD states, the exchange interactions are forced to be modelled as anisotropic ones. Overall, experimental data and quantum chemical computations are in good agreement, supporting the proposed model for magnetic behaviour.

Solvent-Induced Structural Diversity and Magnetic Research of Two Cobalt(II) Complexes

The solvent-induced topological and structural diversities of two Co(II) complexes, namely, [Co(L)₂(SCN)₂] (Co1) and [Co₂(L)₂(SCN)(OAc)₃] (Co2) (L = 8-methoxyquinoline), were comparatively analyzed. Certain proportions of L, Co(OAc)₂·4H₂O, and NaSCN were mixed and dissolved in CH₃OH at 60 °C to obtain complex Co1. Complex Co2, an asymmetric dinuclear compound, was obtained by simply replacing CH₃OH with CH₃CN as the solvent. The Co(II) ion in complex Co1 was coordinated by the N₄O₂ mode provided by two L ligands and two SCN^- anions. The two Co(II) ions in Co2 were in the N₂O₄ and NO₅ coordination environment and were linked by two μ₂-OAc^- bridges and one rare μ₃-OAc^- bridge. Weak interaction analysis revealed that complexes Co1 and Co2 exhibited 6-connected shp and 14-connected fcu nets, respectively. Magnetic studies showed that Co1 demonstrated single-ion magnet behavior under 2000 Oe. These behaviors are indicative of clearly field-induced single-ion magnetic behavior with U _eff = 34.7(2) K and τ₀ = 2.7(2) × 10^-7 s under 2000 Oe dc field, respectively. By contrast, Co2 lacked frequency dependence under zero-field conditions. Electrospray ionization mass spectrometry indicated that two complexes were stable in N,N-dimethylformamide.

Impact of the Schiff base ligand substituents on the solid state and solution properties of eleven iron(iii) complexes

A series of novel iron(iii)-Schiff base complexes have been prepared and structurally characterised. Their stereochemical, magnetic and redox properties were correlated with the molecular design of the corresponding N₃O₂-pentadentate Schiff base ligand.

Variation of nuclearity in NiII complexes of a Schiff base ligand: crystal structures and magnetic studies

Four Ni^II complexes have been synthesized by changing the reaction conditions and stoichiometry of the reactants. The magnetic coupling in the NiII4 complex is ferromagnetic whereas that in the NiII6 complex is antiferromagnetic.

Supramolecular Polymer and Sheet and a Double Cubane Structure in Platinum(IV) Iodide Chemistry: Solution of a Longstanding Puzzle

The complexes [PtMe₂(L)], L = 2-C₅H₄NCH₂NH-x-C₆H₄OH (x = 2, 3, or 4), react with iodine to form [PtI₂Me₂(L)], by trans oxidative addition, when x = 3 or 4, and they are shown to have polymeric or sheet structures formed through NH···I hydrogen bonding. However, ligand dissociation occurs when x = 2 to give [(PtI₂Me₂) _n ] and, with methyl group transfer, the complex [(PtIMe₃·PtI₂Me₂)₂]. This tetraplatinum cluster complex is shown to have a double cubane structure, thus solving a longstanding puzzle.

Anion coordination directed synthesis patterns for [Ni4] aggregates: structural changes for thiocyanate coordination and ligand arm hydrolysis

Competitive bridging modes of HO⁻ and NCS⁻ were explored for four Ni^II-based coordination aggregates in cubane and dicubane topology.