The work described herein focuses on the ability of redox-active ligands to enable multi-electron reactivity at transition metal centers. A parallel theme is the effect of ancillary ligands on controlling and modulating the electronic structure of the redox-active ligand and metal center in addition to ancillary ligand effects as they relate to controlling the primary coordination sphere of the metal.
Chapter 1 provides a basic introduction to the field of redox-active ligands, describing general features of this ligand class and providing a sample of specific case-studies which detail how redox-active ligands have been utilized in enabling redox-reactions at a transition metal center.
Chapter 2 describes the effects of ancillary ligands on a redox-active ligand containing complex of the general formula (ONOsq)TaX3 (where (ONOcat)H3 = bis(3,5-di-tert-butyl- 2-phenol)amine). The effect of the ancillary ligand X on the electronic structure of the (ONOsq)2− ligand is probed by cyclic voltammetry and electron paramagnetic resonance spectroscopy, both of which indicate a strong correlation between the (ONOsq)2− electronic structure and the donor strength of ligand X.
Chapter 3 details the synthesis and characterization of tantalum-oxo complexes of the ONO ligand platform. Oxo complexes of the formula [(ONOq)Ta(OR)2(μ–O)]2 were prepared with two alkoxide –R groups, tert-butoxide and trifluoroethoxide, which were also compared in Chapter 2. Both complexes were found to affect C–H activation of 9,10-dihydroanthracene and also oxygen-atom transfer to trimethylphosphine, redox processes enabled at a d0 metal center by the redox-active ligand. A kinetic comparison between the two complexes is also presented.
Chapter 4 presents the synthesis and characterization of chromium complexes of the ONO ligand. Complexes bearing the three stable redox forms of the ONO ligand have been prepared, with the chromium center showing a strong proclivity for the Cr(III) oxidation state. Dimeric ONO complexes of chromium are also described and the affects of neutral ancillary ligands on structural morphology are presented.
Chapter 5 reports the synthesis of ONO and DOPO (where DOPO = 2,4,6,8-tetra-tert-butyl-1,9-dioxophenoxazinate) complexes of chromium which bear weakly coordinating trifluoromethanesulfonate (triflate) groups. The complexes are used to pursue chromium-oxo complexes by deprotonation of coordinated water in an attempt to achieve sulfide oxidation. Evidence for water coordination and deprotonation via electronic absorption spectroscopy is presented.
Chapter 6 describes the synthesis of heteroleptic ONO complexes of titanium, vanadium, manganese, and nickel. The titanium, manganese, and nickel complexes represent the final members of the first-row transition metal series of heteroleptic ONO complexes. Fundamental studies establishing the metal oxidation state and ONO ligand form are achieved by electronic absorption spectroscopy, X-ray diffraction analysis, and EPR spectroscopy.
