This work explores the effect of geometry and the presence of a site available for carbon dioxide coordination in molecular catalysis of CO₂ reduction for cobalt complexes using electrochemical and spectroelectrochemical studies. The octahedral complexes [Co^II(bztpen)Br]PF₆ and [Co^II(bpy)₃](BF₄)₂, along with the trigonal bipyramidal complex [Co^II(TPA)Cl]Cl, were selected for this study (where bztepen = N-benzyl-N,N′,N′-tris-(pyridine-2-ylmethyl)-ethylenediamine), TPA = tris (2-pyridimethyl)-amine, and bpy = 2′-2′- pyridine). DFT calculations were performed to predict the geometries of the complexes and to propose the sites at which electron transfer occurs. Among the studied compounds, [Co^II(bpy)₃(BF₄)₂] exhibited the highest catalytic rate constant for CO₂ reduction (k = 1.22 × 10² M⁻¹·s⁻¹) compared to [Co^II(bztpen)Br]PF₆ (k = 8.93 × 10¹ M⁻¹·s⁻¹). The trigonal bipyramidal complex [Co^II(TPA)Cl]Cl presented the lowest catalytic rate constant for CO₂ reduction (k = 1.47 × 10¹ M⁻¹·s⁻¹). UV-Vis spectroelectrochemical studies and DFT calculations suggested the formation of [Co^I(bztpen)(CO)]⁺ and [Co^I(tpa)(CO)]⁺ species, which are associated with catalyst deactivation and may account for the lower performance of CO₂ reduction.

In this study, Cu(II) complexes containing the bidentate ligand N,N-dimethylethylendiamine (dmen), i.e., [Cu^II(dmen)₂(CH₃COO)₂], [Cu^II(dmen)₂(NO₃)₂], and [Cu^II(dmen)₂Cl₂], were developed to explore molecular catalysis for the oxygen reduction reaction (ORR). Cyclic voltammetry and UV–vis spectroelectrochemical and electrochemical impedance spectroscopy experiments were performed in the absence and presence of oxygen. The UV–vis spectroscopy results suggested that the aforementioned Cu(II) complexes present an octahedral geometry in the solid state; meanwhile, they show a square pyramidal geometry in an aqueous solution. It is proposed that the chemical species [Cu^I(dmen)₂H₂O]⁺ reacts with O₂, exhibiting an outer-sphere electron transfer mechanism. The same UV–vis spectroelectrochemical response obtained with and without O₂ indicated a direct electron transfer from Cu(II) to Cu(I), with the regeneration of catalyst and the absence of other intermediate species. Among the reported compounds, [Cu(dmen)₂(NO₃)₂] exhibited the highest catalytic rate (TOF = 1.3 × 10⁴ s⁻¹). The impedance spectroscopy results showed that the resistance charge transfer (R_ct) of the redox pair Cu^II|Cu^I decreased in the presence of O₂ from 36.391 kΩ to 5.54 kΩ. For a better understanding of the effect of aliphatic amines on the ORR, a comparison with the complex [Cu(1,10-phen)₂NO₃]NO₃ is also presented.

In this work, the coordination compound sulfate of aqua{tris(2-pyridylmethyl) amine}zinc(II) ([Zn(TPA)(H₂O)]SO₄) is investigated as a catalyst for the molecular reduction of CO₂. The complex was synthesized and characterized by FT–IR, UV–Vis, TGA, and NMR spectroscopy. Cyclic voltammetry reveals irreversible electrochemical behavior, with two cathodic peaks at E_pc = −1.72 V and E_pc = −1.99 V vs. Fc/Fc⁺, respectively. Under a CO₂ atmosphere, a catalytic wave appears at E_pc = −1.87 V vs. Fc/Fc⁺, indicating catalytic activity toward CO₂ reduction. This behavior was further confirmed by Foot-of-the-Wave Analysis (FOWA), which yielded a catalytic rate constant of (k = 1.352 × 10³ M⁻¹ s⁻¹). Bulk electrolysis experiments combined with FT–IR analysis suggest that format is the main product of the CO₂ reduction catalyzed by [Zn(TPA)(H₂O)]SO₄. Electrochemical impedance spectroscopy was used to examine the catalytic process at the electrode–electrolyte interface. In addition, density functional theory (DFT) calculations were conducted to analyze the interaction between the [Zn(TPA)(H₂O)]SO₄ complex and CO₂.