Chapter 1. Introduction to Phosphinimide Ligands and the Development of a Generable Methodology for Their SynthesisHigh-valent late transition metals are invoked in myriad synthetic and biological processes but are often highly unstable, precluding their full characterization. Phosphinimides are strongly π-basic ligands that have been primarily utilized to stabilize electron-deficient early metals and lanthanides. The oxidative robustness and high level of tunability of these platforms should, in principle, allow access to high-valent late first row transition metals. A general and scalable electrophilic amination methodology giving access to rationally designed multi-dentate phosphinimide frameworks was developed.
Chapter 2. 1,2-Addition at a Phosphinimide Supported Zirconium Complex Highlights Isolobality to Imido Ligands
Early transition metal imido complexes have been shown to activate X-H type bonds across a metal ligand multiple bonds. Phosphinimide (PN) moieties are isolobal to imidos and have been shown to have metal-ligand π-bonding character. To highlight the multiple bonding character of the PN-Metal bond, a trisphosphinimide zirconium platform was synthesized. Reaction of the trisphosphinimide zirconium arene complex with phenylacetylene and aniline resulted in the formation of the corresponding bisphosphinimide, phosphinimine zirconium acetylide and anilide, respectively. Addition of acetophenone and nitromethane resulted in the oxygen bound products upon activation of the C-H bond across the PN-Zr bond. This reactivity showcases the ability of the isolobal analogy to predict the reactivity of ligand fragments that are analogous to each other.
Chapter 3. Multi-Electron Processes Mediated by a Phosphinimide Supported Chromium(II) ComplexThe activation of small molecules via multi-electron transfer processes has drawn vast interest in the organometallic field due to its diverse application in performing difficult transformations by enzymatic systems. To that end, an electron rich and multi-dentate phosphinimide platform was designed to stabilize a low-coordinate, reducing chromium(II) complex capable of performing a series of net 4-electron processes, including the reduction of dioxygen, nitrosoarenes, and aryl azides to the corresponding chromium (VI) dioxo, imido/oxo, and bisimido complexes, respectively. Additionally, the chromium(VI) imido/oxo complexes are capable of performing oxo/imido heterometathesis to an additional equivalent of nitrosobenzene to form the corresponding azobenzene and chromium(VI) dioxo. The structural and spectroscopic characterization of these compounds highlights the flexibility of phosphinimides to support both low- and high- valent transition metal complexes.
Chapter 4. Structure and Reactivity of a High-Spin, Nonheme Iron(III)- Superoxo Complex Supported by Phosphinimide Ligands
Nonheme iron oxygenases utilize dioxygen to accomplish challenging chemical oxidations. A further understanding of the Fe-O2 intermediates implicated in these processes is challenged by their highly transient nature. To that end, we have developed a ligand platform featuring phosphinimide donors intended to stabilize oxidized, high-spin iron complexes. O2 exposure of single crystals of a three-coordinate Fe(II) complex of this framework allowed for in crystallo trapping of a terminally bound Fe-O2 complex suitable for XRD characterization. Spectroscopic and computational studies of this species support a high-spin Fe(III) center antiferromagnetically coupled to a superoxide ligand, similar to that proposed for numerous nonheme iron oxygenases. In addition to the apparent stability of this synthetic Fe-O2 complex, its ability to engage in a range of stoichiometric and catalytic oxidation processes demonstrates that this iron-phosphinimide system is primed for development in modelling oxidizing bioinorganic intermediates and green oxidation chemistry.
