Computational Study of Photocatalytic CO₂ Reduction by a Ni(II) Complex Bearing an S₂N₂-Type Ligand

Abstract

Photocatalytic CO2 reduction to CO by a Ni­(II) complex with an S2N2-type tetradentate ligand exhibits high efficiency and selectivity. Here, a density functional theory (DFT) study of the reaction mechanism is presented. Our calculations support that the four-coordinated Ni0 species formed through a photoinduced process participates in the actual catalytic reaction, which reduces CO2 to CO and then regenerates the NiII species. A CO2 adduct with the η2 CO binding mode is the precursor for the formation of CO, and once its first protonation is completed, further C–O bond cleavage is most likely to occur directly, yielding the CO molecule. Product selectivity calculations suggest that the Ni hydride complex is a crucial intermediate for the generation of H2 and formic acid, while the former is more favorable than the latter. The high CO selectivity is mainly attributed to the fact that a much lower concentration of protons in comparison to that of CO2 restricts the formation of the Ni hydride intermediate. In addition, we found that sulfur coordination could promote the formation of a CO2 adduct.

Mechanistic Insights of Photocatalytic CO2 Reduction: Experimental versus Computational Studies

“…These ligands are also found in combination with carbonyl or halogen ligands [78] . It is also common to find complexes with a coordination position occupied by a labile solvent molecule that can be readily exchanged by the carbon dioxide substrate [63,76,79,80] …”

“…Recently Chen's group published a study [80] of the CO 2 reduction catalysed by a Ni complex containing an uncommon tetradentate ligand of type S 2 N 2 , the [Ni( pmet )X 2 ] 2+ ( pmet =bis(2‐pyridylmethyl)‐1,2‐ethanethiol, X=acetonitrile, 13 , Scheme 7). The experimental study [66] was performed using a photocatalytic system formed by the Ni(II) complex, PS2 as photosensitiser and BIH as an electron donor.…”

“…[78] It is also common to find complexes with a coordination position occupied by a labile solvent molecule that can be readily exchanged by the carbon dioxide substrate. [63,76,79,80] Another component of the catalytic systems for CO 2 reduction that have a great influence on the effectiveness of the reaction is the photosensitiser. Its absorption spectra, lifetime of excited states, photostability and redox properties directly affect the activity and selectivity of the process.…”

“…For the particular case of the mechanisms of catalytic photoreduction reactions from CO 2 to CO using molecular transition metal-based catalysts, the first studies are even more recent (2015) and globally they are not very abundant. All of them use methods based on the density functional theory (DFT) and the most used functionals are the B3LYP, [62,80,96,98] in most cases [46,48,80,92] with the D3 correction for the dispersion proposed by Grimme, [99] and the M06 in some of its variants (M06L, M06-2X, M06-D3). [47,56,62] In general the geometries are optimised in the gas phase with a more modest basis set (they use usually pseudopotential for metals and 6-31G(d) for the rest of the atoms), and the energies are refined including the effect of the solvent with the Polarisable Continuum Model (PCM) or Solvent Model Density (SMD) using somewhat larger bases (the pseudopotential for the metal is maintained and the bases of the rest of the atoms are extended in many cases to 6-311 + G(d)).…”

Photocatalytic Carbon Dioxide Reduction Using Nickel Complexes as Catalysts

“…The mechanism has been discussed by Chen and coworkers using DFT calculations to gain mechanistic insights into the CO formation using 14. [28] Addition of a Lewis acid to the reaction solution for electrochemical CO 2 reduction has been reported to exhibit positive impact on the reaction. [29][30][31][32][33] For example, it has been demonstrated that the presence of Mg 2 + ions enhances the electrochemical CO 2 reduction using a Mn complex as a catalyst in terms of the higher rate of catalysis and lower overpotential.…”