UC Merced

Chemical targets produced by transformations of organic molecules are valuable, serving as building blocks for the agrochemical, pharmaceutical, and polymer industries. These transformations may be carried out catalytically or stoichiometrically, but catalysts are used in the synthesis of 80-90 percent of all industrially manufactured products, demonstrating the indispensable role catalysts play in industry. Several organic transformations ranging from cross-coupling to hydrofunctionalization to oxidation or reduction reactions are typically catalyzed by precious metals like iridium (Ir), palladium (Pd), platinum (Pt), rhodium (Rh) etc. These ‘noble’ metals have revolutionized synthetic organic chemistry,allowing chemists to build novel molecules and functional materials with high selectivity, atom economy, and versatility by using mild reaction conditions facilitated by these catalysts. Density Functional Theory (DFT) is a computational method used to study the electronic structures of molecules and materials. These methods can inform and guide catalysis research by providing valuable insights into the molecular-level mechanisms of catalytic reactions. DFT can predict the electronic and structural properties of molecules, which can help to identify the key steps and intermediates involved in catalytic reactions. DFT can also be used to design and optimize catalysts for specific reactions, comprehend the role of solvents and other environmental factors in catalytic reactions, and predict the behavior of catalytic systems under different conditions. Overall, DFT has proven itself to be a critical tool in catalysis research, assisting in the optimization of reaction conditions and improvement of reaction efficiency. The first chapter of this dissertation describes a recyclable catalytic system which has applications in both Suzuki-Miyaura (SM) and Negishi coupling reactions. We investigated the nature of active species catalyzing the reaction – a controversial topic in light of recent research which has suggested ‘metal–free’ coupling chemistry and in the fundamental nature of the catalyst, whether the system is heterogeneous or homogeneous. We utilized the commercially available cyclohexyldiphenyl phosphine oxide ligand and Pd(OAc)2 (palladium acetate) to catalyze coupling reactions and concluded the catalytic system to be ‘pseudo homogeneous’. All the substrates studied afforded good to excellent reaction yields and the catalyst system could be reused in up to ten cycles. In the second chapter of this dissertation, the synthesis, characterization, and ligand exchange studies of iridium-based pincer complexes is reported. The iridium complex (tBuPOCOP)Ir(PPh3) (tBuPOCOP = 2,6-bis(di-tert-butylphosphonito)benzene) acts as a convenient source of latent Ir(I), a 14e- species [(tBuPOCOP)Ir]; which is susceptible to ligand exchange chemistry. The reactions with acetonitrile and pyridine afford the corresponding (tBuPOCOP)Ir(NCMe) and (tBuPOCOP)Ir(Py) complexes, respectively. NMR, UV-vis spectroscopy, and density functional theory (DFT) calculations were used to evaluate the key equilibria and determine the kinetic and thermodynamic parameters of the ligand exchange process between (tBuPOCOP)Ir(PPh3) and L (L = MeCN or pyridine). These studies provided experimental and computational support of the proposed pathway of phosphine displacement, i.e., it occurs either via an associative or a dissociative pathway. In the third chapter of this dissertation, the electrochemical reduction of captured carbon dioxide using an iridium pincer complex was studied. Recently, the reactive capture of carbon dioxide, (i.e., capturing CO2 and reducing it directly) has garnered a lot of interest. Amines have been used most extensively for carbon dioxide capture. Amines react with dilute CO2 in a 2:1 ratio to form the corresponding ammonium carbamate. We utilized commercially available ammonium carbamate with the highly selective and robust CO2 to formate reduction catalyst [Ir(POCOP)]. When ammonium carbamate was used as the substrate instead of CO2, only hydrogen was produced. Equivalent electrolysis with ammonium hexafluorophosphate also resulted in only hydrogen. These results indicate that the use of amine-captured CO2, which generates an equivalent of ammonium, modifies the H+ activity in solution, which can lead to hydrogen production for catalysts that have high selectivity when CO2 is the substrate. The fourth chapter of this dissertation discusses the utilization of DFT calculations to study the chemistry of elements from the pnictogen family, i.e., arsenic tautomerization chemistry and supramolecular assembly of antimony and bismuth. Unlike arsenic’s analog phosphorus, arsenic doesn’t have extensive synthetic literature to rely upon. To study arsenic in-depth and help biologists by providing them with CRM’s we decided to synthesize As-HCs/ As-sugars using a proposed synthetic strategy with a SAO key intermediate. Because of the scarce literature precedent, we decided to probe the route computationally, deducing the stability of the AsIII or AsV states.