1
|
Montgomery CL, Ertem MZ, Chevalier L, Dempsey JL. Circumventing Kinetic Barriers to Metal Hydride Formation with Metal-Ligand Cooperativity. J Am Chem Soc 2024. [PMID: 39441948 DOI: 10.1021/jacs.4c01716] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2024]
Abstract
We report the two-electron, one-proton mechanism of cobalt hydride formation for the conversion of [CoIIICp(PPh2NBn2)(CH3CN)]2+ to [HCoIIICp(PPh2NBn2)]+. This complex catalytically converts CO2 to formate under CO2 reduction conditions, with hydride formation as a key elementary step. Through a combination of electrochemical measurements, digital simulations, theoretical calculations, and additional mechanistic and thermochemical studies, we outline the explicit role of the PPh2NBn2 ligand in the proton-coupled electron transfer (PCET) reactivity that leads to hydride formation. We reveal three unique PCET mechanisms, and we show that the amine on the PPh2NBn2 ligand serves as a kinetically accessible protonation site en route to the thermodynamically favored cobalt hydride. Cyclic voltammograms recorded with proton sources that span a wide range of pKa values show four distinct regimes where the mechanism changes as a function of acid strength, acid concentration, and timescale between electrochemical steps. Peak shift analysis was used to determine proton transfer rate constants where applicable. This work highlights the astute choices that must be made when designing catalytic systems, including the basicity and kinetic accessibility of protonation sites, acid strength, acid concentration, and timescale between electron transfer steps, to maximize catalyst stability and efficiency.
Collapse
Affiliation(s)
- Charlotte L Montgomery
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
| | - Mehmed Z Ertem
- Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973, United States
| | - Leo Chevalier
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
| | - Jillian L Dempsey
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
| |
Collapse
|
2
|
Barakat M, Elhajj S, Yazji R, Miller AJM, Hasanayn F. Kinetic Isotope Effects and the Mechanism of CO 2 Insertion into the Metal-Hydride Bond of fac-(bpy)Re(CO) 3H. Inorg Chem 2024; 63:12133-12145. [PMID: 38901030 DOI: 10.1021/acs.inorgchem.4c01246] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/22/2024]
Abstract
The 1,2-insertion reaction of CO2 into metal-hydride bonds of d6-octahedral complexes to give κ1-O-metal-formate products is the key step in various CO2 reduction schemes and as a result has attracted extensive mechanistic investigations. For many octahedral catalysts, CO2 insertion follows an associative mechanism in which CO2 interacts directly with the coordinated hydride ligand instead of the more classical dissociative mechanism that opens an empty coordination site to bind the substrate to the metal prior to a hydride migration step. To better understand the associative mechanism, we conducted a systematic quantum chemical investigation on the reaction between CO2 and fac-(bpy)Re(CO)3H (1-Re-H; bpy = 2,2'-bipyridine) starting with the gas phase and then moving to THF and other solvents with increased dielectric constants. Detailed analyses of the potential energy surfaces (PESs) and intrinsic reaction coordinates (IRCs) reveal that the reaction is enabled in all media by an initial stage of making a 3c-2e bond between the carbon of CO2 and the metal-hydride bond that is most consistent with an organometallic bridging hydride Re-H-CO2 species. Once CO2 is bent and anchored to the metal-hydride bond, the reaction proceeds by a rotation motion via a cyclic transition state TS2 that interchanges Re-H-CO2 and Re-O-CHO coordination. The combined stages provide an asynchronous-concerted pathway for CO2 insertion on the Gibbs free energy surface with TS2 as the highest energy point. Consideration of TS2 as a rate-determining TS gives activation barriers, inverse KIEs, substituent effects, and solvent effects that agree with the experimental data available in this system. An important new insight revealed by the analyses of the results is that the initial stage of the reaction is not a hydride transfer step as has been assumed in some studies. In fact, the loose vibration of the TS that can be identified for the first stage of the reaction in solution (TS1) does not involve the Re-H stretching vibrational mode. Accordingly, the imaginary frequency of TS1 is insensitive to deuteration, and therefore, TS1 leads to no significant KIE.
Collapse
Affiliation(s)
- Mariam Barakat
- Department of Chemistry, American University of Beirut, Beirut 1107 2020, Lebanon
| | - Sarah Elhajj
- Department of Chemistry, American University of Beirut, Beirut 1107 2020, Lebanon
| | - Riyad Yazji
- Department of Chemistry, American University of Beirut, Beirut 1107 2020, Lebanon
| | - Alexander J M Miller
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
| | - Faraj Hasanayn
- Department of Chemistry, American University of Beirut, Beirut 1107 2020, Lebanon
| |
Collapse
|
3
|
Peng X, Zhang M, Qin H, Han J, Xu Y, Li W, Zhang XP, Zhang W, Apfel UP, Cao R. Switching Electrocatalytic Hydrogen Evolution Pathways through Electronic Tuning of Copper Porphyrins. Angew Chem Int Ed Engl 2024; 63:e202401074. [PMID: 38311965 DOI: 10.1002/anie.202401074] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2024] [Revised: 02/02/2024] [Accepted: 02/03/2024] [Indexed: 02/06/2024]
Abstract
The electronic structure of metal complexes plays key roles in determining their catalytic features. However, controlling electronic structures to regulate reaction mechanisms is of fundamental interest but has been rarely presented. Herein, we report electronic tuning of Cu porphyrins to switch pathways of the hydrogen evolution reaction (HER). Through controllable and regioselective β-oxidation of Cu porphyrin 1, we synthesized analogues 2-4 with one or two β-lactone groups in either a cis or trans configuration. Complexes 1-4 have the same Cu-N4 core site but different electronic structures. Although β-oxidation led to large anodic shifts of reductions, 1-4 displayed similar HER activities in terms of close overpotentials. With electrochemical, chemical and theoretical results, we show that the catalytically active species switches from a CuI species for 1 to a Cu0 species for 4. This work is thus significant to present mechanism-controllable HER via electronic tuning of catalysts.
Collapse
Affiliation(s)
- Xinyang Peng
- Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China
| | - Mengchun Zhang
- Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China
| | - Haonan Qin
- Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China
| | - Jinxiu Han
- Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China
| | - Yuhan Xu
- Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China
| | - Wenzi Li
- Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China
| | - Xue-Peng Zhang
- Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China
| | - Wei Zhang
- Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China
| | - Ulf-Peter Apfel
- Ruhr-Universität Bochum, Fakultät für Chemie und Biochemie, Anorganische Chemie I, Universitätsstrasse 150, 44801, Bochum, Germany
- Fraunhofer UMSICHT, Osterfelder Strasse 3, 46047, Oberhausen, Germany
| | - Rui Cao
- Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China
| |
Collapse
|
4
|
Amtawong J, Montgomery CL, Bein GP, Raithel AL, Hamann TW, Chen CH, Dempsey JL. Mechanism-Guided Kinetic Analysis of Electrocatalytic Proton Reduction Mediated by a Cobalt Catalyst Bearing a Pendant Basic Site. J Am Chem Soc 2024; 146:3742-3754. [PMID: 38316637 DOI: 10.1021/jacs.3c10408] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2024]
Abstract
Cobalt polypyridyl complexes stand out as efficient catalysts for electrochemical proton reduction, but investigations into their operating mechanisms, with broad-reaching implications in catalyst design, have been limited. Herein, we investigate the catalytic activity of a cobalt(II) polypyridyl complex bearing a pendant pyridyl base with a series of organic acids spanning 20 pKa units in acetonitrile. Structural analysis, as well as electrochemical studies, reveals that the Co(III) hydride intermediate is formed through reduction of the Co(II) catalyst followed by direct metal protonation in the initial EC step despite the presence of the pendant base, which is commonly thought of as a more kinetically accessible protonation site. Protonation of the pendant base occurs after the Co(III) hydride intermediate is further reduced in the overall ECEC pathway. Additionally, when the acid used is sufficiently strong, the Co(II) catalyst can be protonated, and the Co(III) hydride can react directly with acid to release H2. With thorough mechanistic understanding, the appropriate electroanalytical methods were identified to extract rate constants for the elementary steps over a range of conditions. Thermodynamic square schemes relating catalytic intermediates proposed in the three electrocatalytic HER mechanisms were constructed. These findings reveal a full description of the HER electrocatalysis mediated by this molecular system and provide insights into strategies to improve synthetic fuel-forming catalysts operative through metal hydride intermediates.
Collapse
Affiliation(s)
- Jaruwan Amtawong
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
| | - Charlotte L Montgomery
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
| | - Gabriella P Bein
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
| | - Austin L Raithel
- Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States
| | - Thomas W Hamann
- Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States
| | - Chun-Hsing Chen
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
| | - Jillian L Dempsey
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
| |
Collapse
|