UC Irvine

The series of projects discussed in this dissertation are linked by the universal theories of electron transfer and proton-coupled electron transfer. Each work has focused on the synthesis and characterization of transition metal complexes containing redox-active ligands.

The ability to understand and subsequently manipulate interligand interactions between redox-active ligands bound to the same metal center is the focus of the work presented in Chapter 2. A series of square-planar metal complexes containing a group 10 metal and the redox-active ligand, 3,5-di-tert-butyl(2,6-diisopropylphenyl)-ortho-iminosemiquinonate (isq●–), were prepared and used to generate the mixed-valence, one-electron reduced species [M(isq●)(ap)]– and one-electron oxidized species [M(isq●)(iq)]+. The degree of ligand-ligand coupling was determined by calculating the electronic coupling parameter, Hab. The mixed-valence anions are strongly delocalized Class III complexes, whereas the mixed-valence cations fall into the Class II regime. The effect of metal ion on ligand-ligand communication follows a less-pronounced, but non-Periodic trend with Hab values following the trend Pt > Ni > Pd.

An understanding of the influence of metal choice and complex charge allowed the work to progress towards the generation of ligand frameworks that are also proton active. Chapter 3 details the synthesis of a family of donor–acceptor Pd(II)/Pt(II) complexes coordinated to a bipyridyl acceptor ligand and 2,4-di-tert-butyl-6-(phenylamino)phenol ([HapH2]) or one of its derivatives, [RapH2] (R = H, CF3, OMe, Me2) as the donor ligand. Protonation of the amidophenolate amine was carried out to generate protonated cationic complexes of the form [M(RapH)(Xbpy)]1+ (R = H, CF3, OMe, Me2; X = H, tBu). Subsequent pKa measurements and reactivity studies with galvinoxyl• confirmed that the [RapH]1– ligand framework could act as a redox, proton, and hydrogen-atom noninnocent ligand. These systems provide a deeper understanding of the factors that influence BDFE values and how they can be modulated to control the thermodynamics of hydrogen-atom transfer.

Lastly, Chapter 4 describes previous work that ties together that investigates excited-state proton transfer in transition-metal complexes. The synthesis and photophysical characterization of a series of low-spin d6 iridium compounds was carried out to probe their viability as inorganic photoacids or photobases for intermolecular excited-state proton-transfer reactions. [IrIII(2-(3-hydroxyphenyl)pyridine)2(N^N)]PF6,) complexes, where N^N = 1,10-phenanthroline (Ir-phen), 2,2’-biquinoline (Ir-bq), or 4,4’-di-tert-butyl-2,2’-bipyridine (Ir-dtb), were investigated as potential photoacids and [Ir(2-(3-methoxyphenyl)pyridine)2(2,2’-biquinoline-4,4’-dicarboxylic acid)]PF6 and [RuII(bipyridine)2(dcbq)] as potential photobases. Spectroscopic characterization showed that functionalizing the ligand scaffold with a protic functional group quenches the intense photoluminescence observed in prototypical iridium-based emitters. This unforeseen result hindered the ability to measure excited state pKa values by fluorescence titrations. Further analyses of the electronic structure and potential quenching mechanisms of the complexes must be done in order to make the structural changes required to promote long-lived excited states with large ΔpKa values.