Source: Dissertation Abstracts International, Volume: 79-12(E), Section: B.

Advisers: Robert Crabtree; Gary Brudvig.

This thesis describes work on several projects, all related to the utilization of pyridine alkoxide ligands for stabilizing transition metals in high oxidation states. The main focus is on the ligand 2-(2-pyridinyl)-2-propanolate (pyalk), a bidentate chelator with one pyridine and one tertiary alkoxide group; several analogues are also discussed. This ligand exhibits several properties which allow it to give rise to unique complexes. Its combination of a strongly donor alkoxide group and an oxidation resistant structure allow its transition metal complexes to attain high oxidation states under oxidizing conditions without degradation. At the same time, the pyridine group provides coordinative stability against alkoxides' tendencies to dissociate on protonation, and the high disparity between donor strengths of the two groups leads to interesting ligand field perturbation effects that significantly affect the metal's electronic properties.

Driven by interest in finding a synthetic route to oxo-bridged iridium pyalk dimers to study as analogues to a highly active but structurally ambiguous water oxidation catalyst, a series of Ir(pyalk)1-3Cl4-0 complexes were prepared (Chapters 2 and 3). These species showed reversible Ir(III)/Ir(IV) redox, with remarkably stable Ir(IV) states. All possible diastereomers were isolated and found to possess drastically different redox properties, thanks to what our group has dubbed the anisotropic field oxidation enhancement (AFOE) effect, which is especially strong with pyalk. Furthermore, the complexes were found to be highly substitutionally inert, to the point that all isomers are kinetically stable.

The Ir(pyalk)2Cl2 complexes could then be reacted with Ag2O to give a novel class of stable Ir(IV,IV) micro-oxo dimers of the type (Ir(pyalk)2Cl)2O (Chapter 4). Like the monomers, these were redox-active, with an accessible Ir(III,III) state, but thanks to the presence of an additional donor oxo ligand, oxidation to a mixed-valence Ir(IV,V) state is possible under fairly mild conditions (0.99 V vs NHE). Further oxidation at 1.9 V vs NHE in non-aqueous environment yielded a reactive Ir(V,V) state without degradation of the ligands. This was crystallized thanks to a specialized bulk electrolysis and crystallization protocol for preparing high-valent species, developed by the author and his coworkers.

In the course of research with pyalk, a series of dimeric and trimeric analogues of this ligand were found to form as side products during the synthesis (Chapter 5). These share the interesting properties of pyalk itself but provide tetradentate or hexadentate chelation, which was utilized for preparing a wide variety of stable polynuclear first-row transition metal clusters, as well as a stable mononuclear Ir(V) complex.

The work presented in this thesis contributes to the field of high oxidation-state coordination chemistry. Specifically, it significantly expands the understanding of iridium(IV) and iridium(V) coordination complexes, demonstrates the effectiveness of pyridine-alkoxide ligands as well as the importance of diastereomerism, and introduces a synthetic protocol for targeted Ir micro-oxo dimer preparation as well as an electrochemical method for crystallizing highly reactive oxidized species.