Prediction of reduction potentials from calculated electron affinities for metal-salen compounds

Modeling Adsorption of CO2 in Rutile Metallic Oxide Surfaces: Implications in CO2 Catalysis

CO₂ is the most abundant greenhouse gas, and for this reason, it is the main target for finding solutions to climatic change. A strategy of environmental remediation is the transformation of CO₂ to an aggregated value product to generate a carbon-neutral cycle. CO₂ reduction is a great challenge because of the large C=O dissociation energy, ~179 kcal/mol. Heterogeneous photocatalysis is a strategy to address this issue, where the adsorption process is the fundamental step. The focus of this work is the role of adsorption in CO₂ reduction by means of modeling the CO₂ adsorption in rutile metallic oxides (TiO₂, GeO₂, SnO_2, IrO₂ and PbO₂) using Density Functional Theory (DFT) and periodic DFT methods. The comparison of adsorption on different metal oxides forming the same type of crystal structure allowed us to observe the influence of the metal in the adsorption process. In the same way, we performed a comparison of the adsorption capability between two different surface planes, (001) and (110). Two CO₂ configurations were observed, linear and folded: the folded conformations were observed in TiO₂, GeO₂ and SnO₂, while the linear conformations were present in IrO₂ and PbO₂. The largest adsorption efficiency was displayed by the (001) surface planes. The CO₂ linear and folded configurations were related to the interaction of the oxygen on the metallic surface with the adsorbate carbon, and the linear conformations were associated with the physisorption and folded configurations with chemisorption. TiO₂ was the material with the best performance for CO₂ interactions during the adsorption.

How the Ancillary Ligand X Drives the Redox Properties of Biscyclopentadienyl Pentavalent Uranium Cp2U(═N-Ar)X Complexes

Relativistic zero order regular approximation (ZORA) density functional theory computations, coupled with the conductor-like screening model for solvation effects, are used to investigate the redox properties of a series of biscyclopentadienyl pentavalent uranium(V) complexes Cp₂U(═N-Ar)X (Ar = 2,6-Me₂-C₆H₃; X = OTf, C₆F₅, SPh, C═CPh, NPh₂, Ph, Me, OPh, N(TMS)₂, N═CPh₂). Regarding the U^V/U^IV and U^VI/U^V couple systems, a linear correlation (R² ∼ 0.99) is obtained at the ZORA/BP86/TZP level, between the calculated ionization energies and the measured experimental E_1/2 half-wave oxidation potentials (U^VI/U^V) and between the electron affinities and the reduction potentials E_1/2 (U^V/U^IV). The study brings to light the importance of solvation effects that are needed in order to achieve a good agreement between the theory and experiment. Introducing spin-orbit coupling corrections slightly improves this agreement. Both the singly occupied molecular orbital and the lowest unoccupied molecular orbital of the neutral U^V complexes exhibit a majority 5f orbital character. The frontier molecular orbitals show a substantial ancillary ligand X σ and/or π character that drives the redox properties. Moreover, our investigations allow estimating the redox potentials of the X = Ph, X = C₆F₅, and N(TMS)₂ U^V complexes for which no experimental electrochemical data exist.

Connecting Solution-Phase to Single-Molecule Properties of Ni(Salophen)

We present a strong correlation of the Ni(salophen) structure and properties measured in single-molecule vs bulk quantities and in ultra high vacuum vs solution phase. Under a scanning tunneling microscope (STM), Ni(salophen) forms a self-assembled monolayer (SAM) on Au(111) at 23 °C with molecular structure identical to that of the X-ray crystallographic measurement. The HOMO and LUMO levels are determined using elastic tunneling spectroscopy at the single-molecule level with confirmation by monolayer-quantity ultraviolet photoelectron spectroscopy (UPS) and by cyclic voltammetry (CV) measurements. The STM-determined HOMO-LUMO gap of 3.28 eV and (HOMO-1)-HOMO gap of 0.36 eV form a new foundation for the selection of hybrid functionals with a simple basis set to be effective in accurately calculating single-molecule Ni(salophen) frontier MO levels. Our results suggest that microscopy-based experiments on a surface, along with free-molecule gas-phase calculations, can provide useful insights into the physical properties of metal(salen) complexes, especially when such direct measurements are not available in solution.

Reducing properties of 1,2-dipyridyl-1,2-disodioethanes: chemical validation of theoretical and electrochemical predictions

Theoretical calculations and electrochemical analysis were used to set up a relative scale for the reducing strength of the dianions of 1,2-dipyridylethenes, validated by studying their reactivity towards halogenated benzoic and arylacetic acids.

Revealing the thermodynamic driving force for ligand-based reductions in quinoids; conceptual rules for designing redox active and non-innocent ligands

Metal and ligand-based reductions have been modeled in octahedral ruthenium complexes revealing metal-ligand interactions as the profound driving force for the redox-active behaviour of orthoquinoid-type ligands. Through an extensive investigation of redox-active ligands we revealed the most critical factors that facilitate or suppress redox-activity of ligands in metal complexes, from which basic rules for designing non-innocent/redox-active ligands can be put forward. These rules also allow rational redox-leveling, i.e. the moderation of redox potentials of ligand-centred electron transfer processes, potentially leading to catalysts with low overpotential in multielectron activation processes.