Electrochemical carbon dioxide (CO2) reduction powered by renewable electricity offers a path to produce valuable products from CO2 -this earth-scale human waste-and to store intermittent renewable energy in the form of chemical fuels. Recently, single metal atoms (SMAs) immobilized on a conductive substrate have been shown as effective catalysts for the electrochemical CO2 reduction, opening the door to a generation of low-cost and high-performance catalysts for fuel and chemical production. The unique physical and chemical properties of a single-atomic structure, the homogeneity of the active sites, combined with tunable coordination environments, are essential for realizing highly active and selective catalysts. In this review, we focus on the structure-performance relationship in SMA catalysts for CO2 reduction from both theoretical and experimental aspects. We discuss why SMA catalysts exhibit distinct catalytic performance compared to their counterpart nanoparticles. Recent strategies for improving the CO2 reduction selectivity and activity by tuning the nature and coordination environment of SMA active sites are described. Finally, we highlight potential applications of SMA catalysts in practical CO2 reduction 2 conditions, critical challenges, and the path toward efficient electrochemical CO2 reduction catalysis based on SMAs.

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Cobalt(III) complexes with Schiff base ligands derived from hydrazone, (HL 1 = (E)-N 0 -(3,5-dichloro-2-hydroxybenzylidene)-4-hydroxybenzohydrazide, HL 2 = (E)-N 0 -(3,5-dichloro-2-hydroxybenzylidene)-4-hydroxybenzohydrazide (3,5-dibromo-2-hydroxybenzylidene), and HL 3 = (E)-4-hydroxy-N 0 -(2-hydroxy-3-ethoxybenzylidene)benzohydrazide), were synthesized and characterized by elemental analysis, Fourier transform infrared (FT-IR) spectroscopy, UV-Vis spectroscopy, and cyclic voltammetry. X-ray diffraction was used to determine the single crystal structure of the complex (1). Co(III) was formed in a distorted, very regular octahedral coordination in this complex; three pyridine moieties complete this geometry. Schiff base complexes' redox behaviors are represented by irreversible (1), quasi-reversible (2), and quasi-reversible (3) voltammograms. A density functional theory (DFT)/B3LYP method was used to optimize cobalt complexes with a base set of 6-311G. Furthermore, fragments occupying the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were investigated at the same theoretical level. Quantum theory of atoms in molecules (QTAIM) computations were also done to study the coordination bonds and non-covalent interactions in the investigated structures. Hirshfeld surface analysis was used to investigate the nature and types of intermolecular exchanges in the crystal structure of the complex (1). The capacity of cobalt complexes to bind to the major protease SARS-CoV-2 and the molecular targets of human angiotensin-converting enzyme-2 (ACE-2) was investigated using molecular docking. The molecular simulation methods used to assess the probable binding states of cobalt complexes revealed that all three complexes were stabilized in the active envelope of the enzyme by making distinct interactions with critical amino acid residues. Interestingly, compound (2) performed better with both molecular targets and the total energy of the system than the other complexes. K E Y W O R D S4-hydroxybenzohhydrazide, Co(III) complex, electrochemical properties, theoretical study, X-ray crystallography

The trans-[CoIII(acacen)(ta)2]ClO4 (1) and trans-[CoIII((BA)2en)(ta)2]PF6 (2) complexes, where H2acacen = bis(acetylacetone)ethylenediimine, H2(BA)2en = bis(benzoylacetone)ethylenediimine, and ta = thioacetamide, have been synthesized by a solid-state method, and characterized by elemental analyses, IR, UV–vis, and 1H NMR spectroscopy. The crystal and molecular structures of 1 and 2 were determined by X-ray crystallography. Both compounds crystallize in the monoclinic space group P2/n. The ClO4 and PF6 ions are both disordered, ClO4 on a twofold axis in 1 and PF6 on an inversion center in 2. Also bridging N-CH2-CH2-N is disordered in both compounds. The octahedral coordination of Co(III) is slightly distorted in both cases. The thioacetamide ligands are S-bonded and occupy the axial position. The IR, UV–vis, and 1H NMR spectra of the two complexes and their solvatochromic properties are also discussed. The longest wavelength absorption that appears at 517 nm for 1 and at 528 nm for 2 in chloroform is solvent dependent, and is assigned as a metal-mediated ligand-to-ligand charge transfer (LLCT).Key words: solid-state synthesis, thioactamide, Co(III) (Schiff base), crystal structure, solvatochromism, metal-mediated LLCT.