Light-Driven Hydrogen Production and CO2 Reduction Using Earth Abundant Polypyridine Metal Complexes: Toward Sustainable Catalysis

The transition to sustainable energy systems demands efficient methods for producing clean fuels and reducing greenhouse gas emissions. A key step toward this goal is the adoption of renewable energy sources, alongside with strategies to lower atmospheric carbon dioxide levels that could mitigate climate change. One promising approach is the conversion of small molecules, such as H2O and CO2, into valuable chemicals through redox reactions. Significant efforts have focused on proton reduction to produce H2 and CO2 reduction into chemicals like carbon monoxide (CO). Molecular catalysts based on first-row transition metal complexes play a crucial role in these reactions due to their high catalytic rates, excellent selectivity, and tunable properties achieved through synthetic modifications. Furthermore, integrating molecular catalysis with solar energy, known as light-driven catalysis, offers a sustainable and energy-efficient alternative to conventional processes. This doctoral thesis explores the application of molecular catalysts for two critical reactions in sustainable energy conversion: the hydrogen evolution reaction (HER) and the carbon dioxide reduction reaction (CO2RR). Transition-metal-based molecular systems are studied, with a focus on how ligand design, electronic properties, and reaction conditions can affect catalytic performance. Combining spectroscopic, electrochemical, and computational techniques, this thesis identifies key structure-function relationships and explores strategies to improve catalytic performances, including electronic modifications and synergistic effects in multi-component systems. After a brief introduction (Chapter 1), in Chapter 2, the impact of proton shuttles and enhanced structural rigidity on the catalytic performance of cobalt(II) TPMA complexes has been investigated. Notably, while a modest improvement in catalytic activity was observed with a rigid cage structure, the integration of ammonium groups as proton transfer relays near the cobalt center resulted in a 4-fold increase in the quantum efficiency of H₂ production and an impressive 22-fold enhancement in the maximum turnover number at low catalyst concentrations. In Chapter 3, three new cobalt-based proton reduction catalysts featuring hexadentate amino-pyridyl ligands, incorporating ortho-substituted pyridine ligands with carboxylate or hydroxyl groups, have been studied to enhance proton transfer and catalytic efficiency. In Chapter 4, the catalytic performance of cobalt and iron complexes featuring the same redox-active ligand and a unique seven-coordinated geometry has been examined toward CO2RR under both electrochemical and photochemical conditions in acetonitrile/water mixtures showing that the cobalt complex favors H₂ formation, while the iron analog achieves nearly quantitative CO production, thus underscoring how metal substitution profoundly influences the reactivity towards solar fuel generation. Chapter 5 focuses on a detailed investigation of the catalytic ability of the iron complex discussed in Chapter 4. The integration of catalytic studies with transient absorption spectroscopy allowed for the identification of the turnover-limiting factors and the rate-determining step in the light-driven catalytic process. Based on these findings, two strategies have been considered aiming at enhancing photochemical performance. 6 In Chapter 6, the catalytic activity of three cobalt(II) complexes based on synthetic variation of the complex presented in Chapter 4 have been explored towards CO₂ reduction, focusing on understanding how substituents with varying electronic properties can impact the activity and selectivity towards CO₂ reduction. Overall, the findings obtained in the present thesis provide critical insights and design principles for the development of next-generation molecular catalysts, advancing sustainable energy technologies and addressing pressing environmental challenges.

La transizione verso sistemi energetici sostenibili richiede metodi efficienti per la produzione di combustibili puliti e la riduzione delle emissioni di gas serra. Un passaggio chiave verso questo obiettivo è l'adozione di fonti di energia rinnovabile, insieme a strategie per ridurre i livelli di anidride carbonica atmosferica, che potrebbero mitigare i cambiamenti climatici. Un approccio promettente è la conversione di piccole molecole, come H₂O e CO₂, in prodotti chimici di valore attraverso reazioni redox. Sforzi significativi si sono concentrati sulla riduzione dei protoni per produrre H₂ e sulla riduzione della CO₂ in prodotti chimici come il monossido di carbonio (CO). I catalizzatori molecolari basati su complessi di metalli di transizione della prima serie svolgono un ruolo cruciale in queste reazioni grazie alle loro elevate velocità catalitiche, eccellente selettività e proprietà regolabili ottenute tramite modificazioni sintetiche. Inoltre, l'integrazione della catalisi molecolare con l'energia solare, nota come catalisi fotoindotta, offre un'alternativa sostenibile ed energeticamente efficiente rispetto ai processi convenzionali. Questa tesi di dottorato esplora l'applicazione di catalizzatori molecolari per due reazioni fondamentali nella conversione sostenibile dell'energia: la reazione di evoluzione dell'idrogeno (HER) e la reazione di riduzione della CO₂ (CO₂RR). Vengono studiati sistemi molecolari a base di metalli di transizione, con particolare attenzione all'influenza della progettazione del legante, delle proprietà elettroniche e delle condizioni di reazione sulle prestazioni catalitiche. Combinando tecniche spettroscopiche, elettrochimiche e computazionali, questa tesi identifica relazioni chiave tra struttura e funzione ed esplora strategie per migliorare le prestazioni catalitiche, tra cui modificazioni elettroniche ed effetti sinergici in sistemi multicomponente.