Catalytic small molecule reduction using late transition metal complexes of carbon and nitrogen donor chelates

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Hydrogenation 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.

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