UC Irvine

An attractive feature of tridentate, redox-active [ENE] ligands (E = a heteroatom donor such as O, NR, or S) is their ability to serve as both electron- and proton-reservoirs in coordination complexes. In this dissertation, some of the structural and electronic properties that influence the coupled transfer of protons and electrons from [ENE]-type ligands in transition metal complexes will be explored.

Chapter 2 contains a study of the effect of ancillary donor ligands L on the electron-transfer behavior of the [ONO] tridentate redox-active ligand in complexes {[ONO]Ni(L)}K. ([ONO] = bis(3,5-di-tert-butyl-2-phenoxy)amide). The ancillary ligands L varied in their σ-donor ability, as measured by the pKa of their conjugate acid, and π-acidity, as indicated by their electrochemical ligand parameter (EL). Structural, spectroscopic, and electrochemical characterization techniques were employed in conjunction with computational estimates to investigate subtle electronic changes and their effects on reactivity.

Chapter 3 reports the behavior of a [ON(H)O]Co(DMAP)2 complex with respect to proton-coupled electron transfer (PCET). Acid-base titrations indicate that the complex may qualify as a very weak acid and a potent reductant. Reactions with hydrogen atom transfer (HAT) reagents are reported, placing the ligand-based bond dissociation free energy (BDFE) in a range more typically associated with metal-hydride systems. However, an equilibrium between five- and six-coordinate species was observed upon HAT transfer from the complex, precluding direct spectrophotometric measurement of the BDFE. Experimental evidence and computational investigations indicate that additional equilibria between low- and high-spin species may influence the reactivity of this system.

Chapter 4 contains the attempted protonation of the [ONO] ligand in a complex of tantalum(V). In its monomeric form, {[ONO]TaCl3}{FeCp2*} appears to be an extremely weak base, contrasting with to a previously-reported {Ta}2 ¬dimer, in which one redox-active ligand is protonated and apparently highly basic. Experimental evidence suggesting the existence of both a mer- and fac-{[ONO]TaCl3}− isomer, each with different pKas, is presented in the form of electrochemical, 1H NMR, and spectroscopic measurements.