Electron Paramagnetic Resonance and Electron Spin Echo Studies of Co2+ Coordination by Nicotinamide Adenine Dinucleotide (NAD+) in Water Solution

Low-Temperature Ferromagnetic Order in a Two-Level Layered Co2+ Material

The magnetic properties of a 2D layered material consisting of high-spin Co2+ complexes, [Co(NH3NH2)2(H2O)2Cl2]Cl2 (CoHyd 2 Cl 4 ), have been extensively characterized using electron paramagnetic resonance, magnetic susceptibility, and low-temperature heat capacity measurements. Electron paramagnetic resonance spectroscopy studies suggest that below 50 K, the J = 3/2 orbital triplet state of Co is gradually depopulated in favor of the J = 1/2 spin state, which is dominant below 20 K. In light of this, the magnetic susceptibility has been fitted with a two-level model, indicating that the interactions in this material are much weaker than previously thought. This two-level model is unable to fit the data at low temperatures and, combined with electron paramagnetic resonance spectroscopy, suggests that ferromagnetic interactions between Co2+ cations in the J = 1/2 state become significant approaching 2 K. Heat capacity measurements suggest the emergence of a long-range ordered state below 246 mK, which neutron diffraction confirms to be ferromagnetic.

A case study using spectroscopy and computational modelling for Co speciation in a deep eutectic solvent

Cobalt has a vital role in the manufacturing of reliable and sustainable clean energy technologies. However, the forecasted supply deficit for cobalt is likely to reach values of 150 kT by 2030. Therefore, it is paramount to consider end-of-life devices as secondary resources for cobalt. Electrorecovery of cobalt from leached solutions has attracted attention due to the sustainability of the recovery process over solvent extraction followed by chemical precipitation. Recently, we reported Co electrorecovery from two different cobalt sources (CoCl2·6H2O and CoSO4·7H2O) using ethylene glycol : choline chloride (EG : ChCl) in a 4.5 : 1 molar ratio, leading to higher purity and easier electrodeposition when sulfate was present as an additive. Here, Co2+ speciation is reported for the two EG : ChCl systems depending on the cobalt source using several spectroscopic techniques (e.g. NMR, EPR, FTIR) in combination with molecular dynamics simulations. Monodentate coordination of SO42- to Co2+, forming the tetrahedral [CoCl3(SO4)]3- was observed as the dominant structure in the system containing CoSO4·7H2O, whereas the system comprising CoCl2·6H2O shows a homoleptic tetrahedral [CoCl4]2- as the dominant structure. This resulted in knowledge being gained regarding Co2+ speciation and the correlation with electrochemistry will contribute to the science required for designing safe electrolytes for efficient electrorecovery.

Successive short- and long-range magnetic ordering in rosiaite-type CoGeTeO6 prepared by ion-exchange reaction

The missing member of the rosiaite family, CoGeTeO₆, was synthesized by mild ion-exchange reactions and characterized by magnetization M and specific heat C_p measurements. It exhibits a successive short- and long-range magnetic ordering at T_short-range ≈ 45 K and T_N = 15 K, respectively. Based on these measurements, the magnetic H-T phase diagram was established, showing two antiferromagnetic phases separated by a spin-flop transition. The reason why the pronounced short-range correlation occurs at a temperature nearly three times higher than T_N was found by evaluating the Co-O⋯O-Co exchange interactions using energy-mapping analysis. Although CoGeTeO₆ has a layered structure, its magnetic structure consists of three-dimensional antiferromagnetic lattices made up of rhombic boxes of Co²⁺ ions. The experimental data obtained at high temperatures agree well with the computational results by treating the Co²⁺ ions of CoGeTeO₆ as S = 3/2 ions, but the heat capacity and magnetization data were obtained at low temperatures by treating the Co²⁺ ion as a J_eff = 1/2 ion.

Non-covalent ligand-oxide interaction promotes oxygen evolution

Strategies to generate high-valence metal species capable of oxidizing water often employ composition and coordination tuning of oxide-based catalysts, where strong covalent interactions with metal sites are crucial. However, it remains unexplored whether a relatively weak "non-bonding" interaction between ligands and oxides can mediate the electronic states of metal sites in oxides. Here we present an unusual non-covalent phenanthroline-CoO₂ interaction that substantially elevates the population of Co⁴⁺ sites for improved water oxidation. We find that phenanthroline only coordinates with Co²⁺ forming soluble Co(phenanthroline)₂(OH)₂ complex in alkaline electrolytes, which can be deposited as amorphous CoO_xH_y film containing non-bonding phenanthroline upon oxidation of Co²⁺ to Co^3+/4+. This in situ deposited catalyst demonstrates a low overpotential of 216 mV at 10 mA cm^-2 and sustainable activity over 1600 h with Faradaic efficiency above 97%. Density functional theory calculations reveal that the presence of phenanthroline can stabilize CoO₂ through the non-covalent interaction and generate polaron-like electronic states at the Co-Co center.

Ligand-free Guerbet-type reactions in air catalyzed by in situ formed complexes of base metal salt cobaltous chloride

Inexpensive, earth-abundant & environmentally benign CoCl2 efficiently catalyses the β-alkylation of alcohol in unprecedented yields (89%) & turnovers (8900). Mechanistic studies are indicative of in situ generated homogeneous molecular Co catalysts.

Dynamic interactions of CbiN and CbiM trigger activity of a cobalt energy-coupling-factor transporter

Energy-coupling factor (ECF) transporters for uptake of vitamins and transition-metal ions into prokaryotic cells share a common architecture consisting of a substrate-specific integral membrane protein (S), a transmembrane coupling protein (T) and two cytoplasmic ATP-binding-cassette-family ATPases. S components rotate within the membrane to expose their binding pockets alternately to the exterior and the cytoplasm. In contrast to vitamin transporters, metal-specific systems rely on additional proteins with essential but poorly understood functions. CbiN, a membrane protein composed of two transmembrane helices tethered by an extracytoplasmic loop of 37 amino-acid residues represents the auxiliary component that temporarily interacts with the CbiMQO₂ Co²⁺ transporter. CbiN was previously shown to induce significant Co²⁺ transport activity in the absence of CbiQO₂ in cells producing the S component CbiM plus CbiN or a Cbi(MN) fusion. Here we analyzed the mode of interaction between the two protein domains. Any deletion in the CbiN loop abolished transport activity. In silico predicted protein-protein contacts between segments of the CbiN loop and loops in CbiM were confirmed by cysteine-scanning mutagenesis and crosslinking. Likewise, an ordered structure of the CbiN loop was observed by electron paramagnetic resonance analysis after site-directed spin labeling. The N-terminal loop of CbiM containing three of four metal ligands was partially immobilized in wild-type Cbi(MN) but completely immobile in inactive variants with CbiN loop deletions. Decreased dynamics of the inactive form was also detected by solid-state nuclear magnetic resonance of isotope-labeled protein in proteoliposomes. In conclusion, CbiM-CbiN loop-loop interactions facilitate metal insertion into the binding pocket.

Paramagnetic Resonance of Cobalt(II) Trispyrazolylmethanes and Counterion Association

Paramagnetic resonance studies (EPR, ESEEM, ENDOR, and NMR) of a series of cobalt(II) bis-trispyrazolylmethane tetrafluoroborates are presented. The complexes studied include the parent, unsubstituted ligand (Tpm), two pyrazole-substituted derivatives (4Me and 3,5-diMe), and tris(1-pyrazolyl)ethane (Tpe), which includes a methyl group on the apical carbon atom. NMR and ENDOR establish the magnitude of ¹H hyperfine couplings, while ESEEM provides information on the coordinated ¹⁴N. The data show that the pyrazole 3-position is more electron rich in the Tpm analogues, that the geometry about the apical atom influences the magnetic resonance, and that apical atom geometry appears more fixed in Tpm than in Tp. NMR and ENDOR establish that the BF₄^- counterion remains associated in fluid solution. In the case of the Tpm^3,5Me complex, it appears to associate in solution, in the same position it occupies in the X-ray structure.