Developing molecular electrocatalysts for fuel oxidation reactions using earth-abundant metal centers
Creators: name: Katipamula, Sriram
Publisher: name: No Publisher Supplied
Year: 2024

description: Chapter 1. Cyclopentadienyl complexes have been of interest to organometallic chemists sincethe discovery of ferrocene in 1951. Over the past two decades, a number of cyclopentadienyl complexes based on earth-abundant metal centers have been reported as electrocatalysts for solar fuels reactions. In this Chapter, we detail recent progress in this area. We first focus on electrocatalysts for hydrogen evolution and oxidation, before discussing recent developments in carbon dioxide and nitrogen electrocatalysis. Suggestions for future avenues of research in the design of earth-abundant cyclopentadienyl-based electrocatalysts is also presented. Chapter 2. As global energy demands continue to increase, the urgency at which we need toend our reliance on fossil fuel resources has prompted significant interest in the development of molecular electrocatalysts to mediate the electrochemical reactions relevant to renewable energy storage and energy usage. Voltametric methods, especially cyclic voltammetry, and bulk electrolysis are powerful electrochemical techniques that can provide quantitative metrics for benchmarking the activity of such molecular systems under electrocatalytic conditions. In particular, by using controlled-potential electrolysis, the selectivity, efficiency, and stability of an electrocatalyst can be probed. In this review, we provide an introduction to this technique to assist researchers in their study of molecular electrocatalysts in the context of renewable energy. Fundamental aspects of the experimental setup, practical considerations, and best-practice recommendations are discussed. Several case studies, which focus on electrochemical transformations that are pertinent energy-related applications are then presented to highlight this electrochemical method in action for the analysis of molecular electrocatalysts. Chapter 3. We report a family of cobalt complexes based on bidentate phosphine ligands withtwo, one, or zero pendent amine groups in the ligand backbone. The pendent amine complexes are active electrocatalysts for the formate oxidation reaction, generating CO2 with near-quantitative faradaic efficiency at moderate overpotentials (0.45 – 0.57 V in acetonitrile). These homogeneous electrocatalysts are the first cobalt example and third first-row transition metal example for formate oxidation. Thermodynamic measurements reveal these complexes are energetically primed for formate oxidation via hydride transfer to the cobalt center, followed by deprotonation of the resulting cobalt-hydride by formate acting as a base. The complex featuring a single pendent amine arm is the fastest electrocatalyst in this series, with an observed rate constant for formate oxidation of 135 ± 8 h−1 at 25°C, surpassing the activity of the bis-pendent amine analogue. Electrocatalytic turnover is not observed for the complex with no pendent amine groups: decomposition of the complex is evident in the presence of high formate concentrations. Chapter 4. The thermodynamic hydricity of a series of [CpCo(P-P)(H)]+ (P-P = dppe, depe,dcpe or dpype) complexes is determined and their reactivity with formate is investigated. While no electrocatalytic turnover is observed by cyclic voltammetry, NMR studies resulted in formate oxidation and formation of the CoIII-hydride, demonstrating that hydride transfer is accessible. DFT calculations were also employed to understand the effect of ligand identity on hydride donating ability. Our studies showed that significant steric and electronic changes can be made to the ligand framework while still maintaining strong hydride accepting ability at the metal. These results are promising for the further design and optimization of electrocatalysts for formate oxidation. Chapter 5. We report a class of cobalt-phosphine complexes that facilitate dihydrogen splittingat ambient pressure via either a homolytic or heterolytic pathway. Homolytic hydrogen splitting is achieved with CoII complexes, whereas heterolytic hydrogen splitting is accomplished with CoIII complexes in conjunction with weak bases. The strength of the base is critical, as stronger bases lead to faster hydrogen activation. Additionally, the rate of heterolytic hydrogen splitting is enhanced by the presence of pendent amine groups within the ligand architecture.iv Chapter 6. The synthesis and characterization of a half-sandwich cobalt(II) complex supportedby a bidentate, pendent-amine phosphine ligand (PPh2NBn2 = 1,5-diaza-3,7- diphosphacyclooctane) are reported. Oxidation of a cobalt(I)-phosphine precursor with silver(I) salts yielded the paramagnetic complex [CpCo(PPh2NBn2)]+. This species rapidly reacts with oxygen upon air exposure under ambient conditions, resulting in the insertion of oxygen into the cobalt-phosphine bonds. Two cobalt complexes were generated from this reaction: the cobaltocenium cation, bearing two Cp ligands (Cp = cyclopentadienyl); and a cobalt(II)-bis(phoshine oxide) dicationic complex in which two O2P2N2 ligands coordinate in a fac fashion via two oxygen and one nitrogen centers to establish an overall octahedral geometry. Thus, the aerobic oxidation of the phosphine induces the exchange and rearrangement of the ligands at cobalt.