Development of Dinuclear Metal Complexes for Catalytic Transformations in Artificial Photosynthesis

The storage of solar energy in chemical bonds has attracted much interest as a “clean” alternative to petroleum-based fuels. In such systems, a current major challenge is the development of high efficiency catalysts for the oxidation of water to dioxygen and the reduction of carbon dioxide to carbon-based fuels. This dissertation examines novel dinucleating ligands for the formation of dinuclear first-row transition metal complexes and studies their potential for catalytic applications. In Chapter 1, a dinuclear cobalt molecular structural analog of a proposed Co-Pi active site was synthesized from the novel dinucleating ligand 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine (DPFN) as [Co2(μ-OH)2(OH2)2(DPFN)][NO3]4. The structure features a bis(μ-OH) dinuclear cobalt “diamond-shaped” core with a terminal aqua ligand on each cobalt center in a syn configuration analogous to a dinuclear unit of a cobalt oxide surface site. The compound was found to be ineffective for electrocatalytic water oxidation activity. The low catalytic activity of the complex may result from the substitution of terminal aqua ligands for phosphate in KPi electrochemical solutions. Reaction of the complex with potassium phosphate forms the κ2,κ2-phosphate-bridged tetranuclear cobalt complex [{Co2(μ-OH)2(DPFN)}2( κ2, κ2-PO4)][NO3]5. It is suspected that formation of similar phosphate complexes under electrochemical conditions is responsible for the low catalytic activity of [Co2(μ-OH)2(OH2)2(DPFN)][NO3]4. A series of dinuclear and tetranuclear first-row transition metal complexes, [Co2(μ-Cl)2Cl(CH3OH)(DPFN)]2[CoCl4]·8H2O, [Ni2(μ-Cl)2Cl(CH3OH)(DPFN)][Cl]·4H2O, [Cu4(μ-Cl)6(DPFN)2][Cl]2·6H2O, and [Cu2(μ-OH)2(NO3)(OH2)(DPFN)]·2H2O, were synthesized with DPFN and compared to [Co2(DPFN)(μ-OH)2(OH2)2][NO3]4 in Chapter 2. The dinuclear and tetranuclear complexes possess pseudo-octahedral geometries about the metal centers and contain chloro, hydroxo, and aqua bridging ligands forming a “diamond” shape. The metal-metal distance between the two metal centers varies from 2.7826(5) to 3.2410(11) Å. High-spin metal centers are formed with 2+ oxidation state metal centers, in contrast to the low-spin diamagnetic Co(III) complex. The complexes are characterized by electronic spectroscopy, electrochemical and potentiometric titration methods. In Chapter 3, the ligand 2,7-bis(di(2-pyridyl)ethyl)-1,8-naphthyridine (DPEN) promotes the formation of a rare μ-η1:η1 acetonitrile-bridged dicopper(I) complex, [(DPEN)Cu2(μ-NCMe)][PF6]2. The acetonitrile has unusually short Cu—N bond distances for an acetonitrile-bridged complex of 2.004(3) and 1.979(3) Å. Infrared spectroscopy and computational studies indicate that the acetonitrile is involved in a primarily 3-center 2-electron bond that is stabilized by a cuprophilic interaction. The labile acetonitrile of the acetonitrile complex can be substituted with xylyl isocyanide and carbon monoxide. These complexes may lead to the development of catalysts for the reduction of carbon dioxide to fuels. Chapter 4 investigates the interactions of molecular water oxidation complexes with inorganic frameworks for eventual use in photoactive materials. The “ruthenium blue dimer” was incorporated into the mesoporous silica material SBA15. Base treatment of SBA15 was found to greatly increase loading of the complex onto the surface. The identity of the adsorbed species was determined to be the ruthenium dimer cation electrostatically adsorbed onto the surface using diffuse reflectance UV-Vis spectroscopy, diffuse reflectance infrared spectroscopy, Raman scattering. The activity of the adsorbed dimer towards water oxidation was explored using aqueous (NH4)2Ce(NO3)6 as a sacrificial oxidant