Earth-Abundant Metal Complexes Binding N-Heterocyclic Carbene-Based Ligands for the Photocatalytic Carbon Dioxide Reduction Reaction

The utilization of solar energy for CO2 conversion into value-added chemicals and fuels offers a viable alternative to fulfill the future energy demands instead of relying on fossil fuels. The light-driven reduction of CO2 to formate and CO is a practical approach for generating fuels from a greenhouse gas, as formate is directly usable in fuel cells, while CO could be used to produce diesel fuels via the Fischer-Tropsch process. However, a robust and efficient earth-abundant transition metal-based photocatalyst for CO2 reduction reaction is needed to achieve this goal. N-heterocylic carbene (NHC) ligands have been used widely in organometallic catalyst applications owing to its strong sigma-donor properties that can enhance the stability of the transition metal-based catalysts. The combination of an NHC-based ligand with a pyridyl ring to form a symmetric multidentate ligands can enhance the stability of catalysts. We have synthesized a series of Ni(II) complexes bearing tridentate CNC pincer ligand scaffold composed of two imidazole or benzimidazole-derived NHC rings and a pyridyl ring with various remote substituents (OMe, Me, H) para to the pyridine ring. A monodentate chloride ligand was also incorporated into this metal complex, forming a four-coordinate nickel complex. These nickel complexes were utilized as catalysts for the photocatalytic CO2 reduction reaction, yielding formate as the major product, and CO as the minor product within 72 h, both in the presence (sensitized) and absence (self-sensitized) of an external photosensitizer. The ability of these Ni(II) CNC pincer complexes to reduce CO2 into formate and CO without an external photosensitizer is attributed to the unusual long-lived excited state lifetime of these complexes in a microsecond range, as shown by the transient absorption spectroscopy (TAS) study. The Ni(II) CNC-pincer complex with monodentate Br ligand is also investigated here, yielding a comparable amount of formate and CO within 24 h in the presence of photosensitizer. Meanwhile, in the absence of photosensitizer, a minimal quantity of formate was generated. A tetradentate NCCN ligand scaffold, consisting of two bidentate NHC-pyridinol ligands connected by a propyl linker, is also under investigation with Ni(II) and Co(II) metal centers. Ni(II)-NCCN and Co(II)-NCCN complexes serve as effective catalysts for the photocatalytic reduction of CO2 in the presence of an external photosensitizer, yielding formate as the primary product. Nonetheless, these Ni(II) and Co(II) tetradentate complexes are inactive in the absence of an external photosensitizer. In summary, while tetradentate NCCN ligands are comparable to tridentate CNC pincer ligands for sensitized CO2 reduction to generate formate and CO, the tridentate CNC pincer ligands provide a distinct advantage for self-sensitized catalysis.

Grantor dc:publisher

University of Alabama Libraries

Year dc:date.issued

2025

Author dc:creator

• Manafe, Sonya Yunike

Advisor dc:contributor.advisor

• Papish, Elizabeth T

Contributors dc:contributor

• Shaughnessy, Kevin H
• Pierce, Brad S
• Sun, Wenfang
• Brewster, Timothy

Statement dc:rights

• All rights reserved by the author unless otherwise indicated.

Language dc:language.iso

en_US, English

Dc Identifier Other

1172280

OAI identifier oai:identifier

oai:ir.ua.edu:123456789/17040