Ligand-Structure Effects on N-Heterocyclic Carbene Rhenium Photo- and Electrocatalysts of CO2 Reduction

Lauren Kearney; Michael P Brandon; Andrew Coleman; Ann M Chippindale; František Hartl; Ralte Lalrempuia; Martin Pižl; Mary T Pryce

doi:10.3390/molecules28104149

Ligand-Structure Effects on N-Heterocyclic Carbene Rhenium Photo- and Electrocatalysts of CO₂ Reduction

Molecules. 2023 May 17;28(10):4149. doi: 10.3390/molecules28104149.

Authors

Lauren Kearney¹, Michael P Brandon¹, Andrew Coleman¹, Ann M Chippindale², František Hartl², Ralte Lalrempuia^{1

3}, Martin Pižl⁴, Mary T Pryce¹

Affiliations

¹ School of Chemical Sciences, Dublin City University, D09 K20V Dublin, Ireland.
² Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6DX, UK.
³ Department of Chemistry, School of Physical Sciences, Mizoram University, Aizawl 796004, India.
⁴ Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 16628 Prague, Czech Republic.

Abstract

Three novel rhenium N-heterocyclic carbene complexes, [Re]-NHC-1-3 ([Re] = fac-Re(CO)₃Br), were synthesized and characterized using a range of spectroscopic techniques. Photophysical, electrochemical and spectroelectrochemical studies were carried out to probe the properties of these organometallic compounds. Re-NHC-1 and Re-NHC-2 bear a phenanthrene backbone on an imidazole (NHC) ring, coordinating to Re by both the carbene C and a pyridyl group attached to one of the imidazole nitrogen atoms. Re-NHC-2 differs from Re-NHC-1 by replacing N-H with an N-benzyl group as the second substituent on imidazole. The replacement of the phenanthrene backbone in Re-NHC-2 with the larger pyrene gives Re-NHC-3. The two-electron electrochemical reductions of Re-NHC-2 and Re-NHC-3 result in the formation of the five-coordinate anions that are capable of electrocatalytic CO₂ reduction. These catalysts are formed first at the initial cathodic wave R1, and then, ultimately, via the reduction of Re-Re bound dimer intermediates at the second cathodic wave R2. All three Re-NHC-1-3 complexes are active photocatalysts for the transformation of CO₂ to CO, with the most photostable complex, Re-NHC-3, being the most effective for this conversion. Re-NHC-1 and Re-NHC-2 afforded modest CO turnover numbers (TONs), following irradiation at 355 nm, but were inactive at the longer irradiation wavelength of 470 nm. In contrast, Re-NHC-3, when photoexcited at 470 nm, yielded the highest TON in this study, but remained inactive at 355 nm. The luminescence spectrum of Re-NHC-3 is red-shifted compared to those of Re-NHC-1 and Re-NHC-2, and previously reported similar [Re]-NHC complexes. This observation, together with TD-DFT calculations, suggests that the nature of the lowest-energy optical excitation for Re-NHC-3 has π→π*(NHC-pyrene) and d_π(Re)→π*(pyridine) (IL/MLCT) character. The stability and superior photocatalytic performance of Re-NHC-3 are attributed to the extended conjugation of the π-electron system, leading to the beneficial modulation of the strongly electron-donating tendency of the NHC group.

Keywords: CO2 reduction; N−heterocyclic carbene; electrocatalysis; photocatalysis; rhenium.

Grants and funding

This research in Dublin was funded by the Sustainable Energy Authority of Ireland under the SEAI National Energy Research, Development and Demonstration Funding Programme 2018, Grant number 18/RDD/282, and the Research, Development and Demonstration Funding Programme 2019, Grant number 19/RDD/566. The spectro−electrochemical research in Reading was directly funded by Spectroelectroelectrochemistry Reading (startup), headed by the F.H. Chemistry Analytical Facilities (CAF) in Reading, which is acknowledged for XRD instrumentation (A.M.C.), and UCT Prague for the access to a computational cluster equipped with quantum chemical packages—Gaussian 16, Orca (M.P.). R.L. would like to thank the European Commission, Marie Sklodowska−Curie Fellowship (No. 799778).