UC Davis

This dissertation describes metal ion effects on electron and proton transfers to the pyridine ring of bis(pyrazolyl)pyridine complexes. Insights from this work aim to inform future ligand-based hydride transfer processes. Chapter 1 presents an account of how non-innocent ligands have expanded aluminum chemistry. This introduces redox-active ligands which are discussed throughout the dissertation. Emphasis is placed on redox-active ligand complexes of aluminum, Al, reported by our lab. This helps set the stage for subsequent discussions of redox-active complexes of dihydropyridinates discussed in later chapters.Chapter 2 discusses hydride transfer (HT), a fundamental step in a wide range of reaction pathways, including those mediated by dihydropyridinates (DHP−s). Coordination of ions directly to the pyridine ring or functional groups stemming therefrom provides a powerful approach for influencing the electronic structure and in turn HT chemistry. Much of the work in this area is inspired by the chemistry of bioinorganic systems including NADH. Coordination of metal ions to pyridines lowers the electron density in the pyridine ring and lowers the reduction potential: lower-energy reactions and enhanced selectivity are two outcomes from these modifications. Herein, we discuss approaches for the preparation of DHP–-metal complexes and selected examples of their reactivity. We suggest further areas in which these metallated DHP−s could be developed and applied in synthesis and catalysis. In Chapter 3, N-alkylation and N-metallation of pyridine are explored to understand how metal-ligand complexes can model NAD+ redox chemistry. Syntheses of substituted dipyrazolylpyridine (pz2P) compounds (pz2P)CH3+ (3.1+) and (pz2P)GaCl2+ (3.2+) are reported, and compared with (pz2P)AlCl2(THF)+ and transition element pz2P complexes from previous reports. Cyclic voltammetry measurements of cationic 3.1+ and 3.2+ show irreversible reduction events ~900 mV anodic those for neutral pz2P complexes of divalent metals. We proposed that N-metallation using Group 13 ions of 3+ charge provides an electrochemical model for N-alkylated pyridyls like NAD+. In Chapter 4, the relationship Ep vs. ΔGH– correlates the applied potential (Ep) needed to drive organohydride formation with the strength of the hydride donor that is formed: in the absence of kinetic effects Ep vs. ΔGH– should be linear but it would be more energy efficient if Ep could be shifted anodically using kinetic effects. Biological hydride transfers (HT) performed by NADH do occur at low potentials and functional modeling of those processes could lead to low-energy HT reactions in electrosynthesis and accurate models for NADH chemistry. Herein we probe the influence of N-alkylation or N-metallation on ΔGH– for dihydropyridinates (DHP–) and on Ep of the DHP– precursors. We synthesized a series of DHP– complexes of the form (pz2HP–)E via hydride transfer from their respective [(pz2P)E]+ forms where E = AlCl2+, GaCl2+, or CH3+. Relative ΔGH– for the (pz2HP–)E series all fall within 1 kcal mol–1, and ΔGH– for (pz2HP)CH3 was approximated as 47.5  2.5 kcal mol–1 in MeCN solution. Plots of Ep vs. ΔGH– including [(pz2P)E]+ suggest kinetic effects shift Ep anodically by ~ 215 mV, and possible origins for the effects are discussed. Appendix A and B provide supplementary information for Chapters 3 and 4 respectively. Appendix C details the synthesis and study of a novel diiminoacridine ligand, 1,1’-(Acridine-4.5-diyl)bis(N-(2,6-dimethylphenyl)methyanimine), I2A. Attempts to isolate Al and Ga complexes of I2A are discussed and solid-state structures of I2A are compared to reported 4,5-substituted acridine ligand complexes, I2A displays a unique twisted geometry. Appendix D outlines the synthesis and characterization of the octahedral complex, [(F5I2P–)2Ga]PF6, and places this work within the context of related complexes reported by our lab.