Microwave synthesis and crystal structures of two cobalt-4,4′-bipyridine-sulfate frameworks constructed from 1-D coordination polymers linked by hydrogen bonding

Impact of Iron in Three-Dimensional Co-MOF for Electrocatalytic Water Oxidation

The integration of iron (Fe) into a cobalt metal-organic framework (Co-MOF) tunes the electronic structure of the parent MOF as well as enhances their electrocatalytic characteristics. By using pyrazine and hydrofluoric acid, we have synthesized three-dimensional Co-MOF [CoFC₄H₄N₂(SO₄)_0.5], (1), and Fe-MOF [FeFC₄H₄N₂(SO₄)_0.5], (2), through a single-step solvothermal method. Further, a series of bimetallic (having both Co and Fe metal centers) MOFs [Co_1-xFe_xFC₄H₄N₂(SO₄)_0.5] were synthesized with variable concentrations of Fe, and their electrocatalytic performances were analyzed. The optimized amount of Fe significantly impacted the electrocatalytic behavior of the bimetallic MOF toward water oxidation. Particularly, the Co_0.75Fe_0.25-MOF needs only 239 and 257 mV of overpotential to deliver 10 and 50 mA/cm² current density, respectively, in alkaline electrolytic conditions. The Co_0.75Fe_0.25-MOF shows a lower Tafel slope (42 mV/dec.) among other bimetallic MOFs and even the commercial RuO₂, and it has excellent durability (with ∼8 mV increases in overpotential after 18 h of electrolysis) and 97.05% Faradaic efficiency, which further evident its catalytic excellency. These findings explore the intrinsic properties of MOF-based electrocatalysts and prospect the suitability for future water electrolysis.

Poly[diaqua-bis-(μ-4,4'-bipyridine-κ(2) N:N')bis-(ethane-1,2-diol-κO)bis(μ-sulfato-κ(2) O:O')dicobalt(II)]

In the title compound, [Co₂(SO₄)₂(C₁₀H₈N₂)₂(C₂H₆O₂)₂(H₂O)₂]_n, there are two crystallographically independent Co^II ions, each of which lies on a twofold rotation axis and has a slightly distorted octa­hedral environment. One Co^II ion is coordinated by two N atoms from two bridging 4,4′-bipyridine (4,4′-bipy) ligands, two O atoms from two sulfate ions and two O atoms from aqua ligands. The second Co^II ion is similar but with ethane-1,2-diol ligands in place of water mol­ecules. The sulfate anions act as bridging ligands to link two adjacent Co^II ions together, leading to the formation of linear ⋯Co1Co2Co1Co2⋯chains along the a axis. Adjacent chains are further bridged by 4,4′-bipy ligands, which are also located on the twofold rotation axis, resulting in a two-dimensional layered polymer extending parallel to (001). In the crystal, the layers are linked by extensive O—H⋯O hydrogen-bonding inter­actions involving the O atoms of the water mol­ecules and ethane-1,2-diol mol­ecules, resulting in a three-dimensional supra­molecular network.

Solvent-Induced Reversible Crystal-to-Amorphous Transformation Properties of Cobalt(II) 4-Aminomethylpyridine-Sulfate with Chromotropism

The crystalline CoII coordination compound with empirical formula [Co(Hampy)2(H2O)4](SO4)2(H2O)3 (1); ampy = 4-aminomethylpyridine was obtained. The structure contains a mononuclear [Co(Hampy)2(H2O)4]4+ cation unit, two sulfate ions, and three lattice water molecules. The Co2+ cation shows an elongated octahedral geometry comprised of four oxygen atoms from water molecules at equatorial positions and two nitrogen atoms from Hampy ligands which are protonated at NH2. Each mononuclear cation unit is assembled by intermolecular hydrogen bonding and π–π stacking interactions by the coordinated and lattice water molecules, amino group, and sulfate anions to form a 3D supramolecular network. Investigations of the dynamic structural behaviour demonstrate that the title compound exhibits a solvent-induced reversible crystal-to-amorphous transformation with chromotropism when exposed to water and methanol vapour. This indicates that the dehydrated amorphous form, [Co(Hampy)2(SO4)2] 1A, may be utilised as an indicator for humidity and methanol vapour.

A novel one-dimensional Co(II) coordination polymer: catena-poly[[hexaaquacobalt(II)] [[diaquabis(sulfato-κO)cobalt(II)]-μ-4,4'-bipyridine-κ2N:N'] [[triaqua(sulfato-κO)cobalt(II)]-μ-4,4'-bipyridine-κ2N:N']]

The title compound, {[Co(H(2)O)(6)][Co(SO(4))(C(10)H(8)N(2))(H(2)O)(3)][Co(SO(4))(2)(C(10)H(8)N(2))(H(2)O)(2)]}(n), contains three crystallographically unique Co(II) centres, all of which are in six-coordinated environments. One Co(II) centre is coordinated by two bridging 4,4'-bipyridine (4,4'-bipy) ligands, one sulfate ion and three aqua ligands. The second Co(II) centre is surrounded by two N atoms of two 4,4'-bipy ligands and four O atoms, i.e. two O atoms from two monodentate sulfate ions and two from water molecules. The third Co(II) centre forms part of a hexaaquacobalt(II) ion. In the crystal structure, there are two different one-dimensional chains, one being anionic and the other neutral, and adjacent chains are arranged in a cross-like fashion around the mid-point of the 4,4'-bipy ligands. The structure features O-H···O hydrogen-bonding interactions between sulfate anions and water molecules, resulting in a three-dimensional supramolecular network.