Browsing by Author "Siek, Sopheavy"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Catalytic small molecule reduction using late transition metal complexes of carbon and nitrogen donor chelates(University of Alabama Libraries, 2017) Siek, Sopheavy; Papish, Elizabeth T.; University of Alabama TuscaloosaHydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible that energy can be released on demand. A new bidentate chelating ligand was designed and synthesized for this project, using an N-heterocyclic carbene ring bound directly to a pyridinol ring (NHC-pyOR). This new ligand was used to make iridium complexes that were studied as catalysts for the hydrogenation of CO2 and dehydrogenation of formic acid. For comparison, analogous bipy derived iridium and ruthenium complexes were also tested. In general, the NHC-pyOR complexes demonstrated modest activity, where hydroxyl-pyridines found in the bipy derived systems are more active for CO2 hydrogenation under basic conditions. However, the trends were quite different for formic acid dehydrogenation reaction which will be discussed in Chapter 2. Other ruthenium (II) and iridium (III) complexes of the NHC-pyOR ligand with difference counter anions from above complexes were also synthesized. The ruthenium complexes were tested for their ability to accelerate CO2 (de)hydrogenation, but our studies show that these complexes all undergo transformations in solution and thus they are not true catalysts, but rather pre-catalysts. The use of new tridentate pincer ligands derived from NHC and pyridinol is also described. A new ligand containing (NHC-pyOR-NHC) rings binding to a metal with the pyridinol derivative were synthesized. A series of metal complexes of the type LnM were synthesized (n = 1 and 2; M = Fe2+, Co3+, and Ru2+). Preliminary results of photocatalytic reduction of CO2 to CO show that ruthenium complexes are the most active catalysts followed by cobalt and iron, respectively. The activation of carbon dioxide and nitrite utilizing bio-inspired and proton responsive catalysts were also studied with tris(triazolyl)hydroborate (Ttz) complexes of zinc(II) and copper(II). For the biomimetic zinc complexes for CO2 activation, the synthetic result was found to be greatly depend on the steric bulk of Ttz ligand which will be discussed in detail in Chapter 6. Moreover, the electrochemical reduction of Ttz-Cu(II) complexes in the presence and absence of a proton source shows processes that are relevant to enzymatic nitrite reduction which also will be studied in Chapter 7.Item Iridium and Ruthenium Complexes of N-Heterocyclic Carbene- and Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation: The Role of the Alkali Metal(American Chemical Society, 2017) Siek, Sopheavy; Burks, Dalton B.; Gerlach, Deidra L.; Liang, Guangchao; Tesh, Jamie M.; Thompson, Courtney R.; Qu, Fengrui; Shankwitz, Jennifer E.; Vasquez, Robert M.; Chambers, Nicole; Szulczewski, Gregory J.; Grotjahn, Douglas B.; Webster, Charles Edwin; Papish, Elizabeth T.; University of Alabama Tuscaloosa; Mississippi State University; San Diego State UniversityHydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO, hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6 '-dihydroxybipyridine (6,6 '-dhbp)) for both their proton-responsive features and for metal-ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir(NHC-py(OR))Cl]OTf complexes where R = Bu-t (1), H (2), or Me (3). For comparison, we tested analogous bipyderived iridium complexes as catalysts, specifically [Cp*Ir(6,6 '-dxbp)Cl]OTf, where x = hydroxy (4(Ir)) or methoxy (5(Ir)); 4(Ir) was reported previously, but 5(Ir) is new. The analogous ruthenium complexes were also tested using [(eta(6)-cymene)Ru(6,6 '-dxbp)CljOTf, where x = hydroxy (4(Ru)) or methoxy (5(Ru)); 4(Ru) and 5(Ru) were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5(Ir), and for two [Ag(NHC-py(OR))(2)]OTf complexes 6 (R = Bu-t) and 7 (R = Me). The aqueous catalytic studies of both CO2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1-5. In general, NHC-py(OR) complexes 1-3 were modest precatalysts for both reactions. NHC complexes 1-3 all underwent transformations under basic CO2 hydrogenation conditions, and for 3, we trapped a product of its transformation, 3(sp), which we, characterized crystallographically. For CO2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4(Ir)) is 5-8 times more active than x = methoxy (5(Ir)). Notably, ruthenium complex 4(Ru) showed 95% of the activity of 4(Ir). For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4(Ir) >> 4(Ru) and 4(Ir) approximate to 5(Ir). Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6 '-dhbp are deprotonated and alkali metals can bind and help to activate CO2. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO2, hydrogenation and formic acid dehydrogenation.Item Ruthenium (II) and iridium (III) complexes of N-heterocyclic carbene and pyridinol derived bidentate chelates: Synthesis, characterization, and reactivity(Elsevier, 2017) Gerlach, Deidra L.; Siek, Sopheavy; Burks, Dalton B.; Tesh, Jamie M.; Thompson, Courtney R.; Vasquez, Robert M.; White, Nicholas J.; Zeller, Matthias; Grotjahn, Douglas B.; Papish, Elizabeth T.; University of Alabama Tuscaloosa; San Diego State University; Purdue University; Purdue University West Lafayette CampusWe report the synthesis and characterization of new ruthenium(II) and iridium(III) complexes of a new bidentate chelate, NHCR'-py(OR) (OR = OMe, OtBu, OH and R' = Me, Et). Synthesis and characterization studies were done on the following compounds: four ligand precursors (1-4); two silver complexes of these NHCR'-py(OR) ligands (5-7); six ruthenium complexes of the type [eta(6)-(p-cymene) Ru(NHCR'-py(OR))Cl]X with R' = Me, Et and R = Me, tBu, H and X = OTf, PF6 and PO2F2 (8-13); and two iridium complexes, [Cp*Ir (NHCMe-py(OtBu)) Cl]PF6(14) and [Cp*Ir(NHCMe-py(OH))Cl]PO2F2(15). The complexes are air stable and were isolated in moderate yield. However, for the PF6 salts, hydrolysis of the PF6 counter anion to PO2F2 during t-butyl ether deprotection was observed. Most of the complexes were characterized by H-1 and C-13 NMR, MS, IR, and X-ray diffraction. The ruthenium complexes [eta(6)-(p-cymene) Ru(NHCMe-py(OR))Cl]OTf(R = Me (8) and tBu (9)) were tested for their ability to accelerate CO2 hydrogenation and formic acid dehydrogenation. However, our studies show that the complexes transform during the reaction and these complexes are best thought of as pre-catalysts. (C) 2017 Elsevier B.V. All rights reserved.