Design and Synthesis of Transition Metal Complexes for the Structural Investigation of Reactive Intermediates

Abstract

The conversion of abundant yet unreactive molecules to higher-value and more energy dense products by bioinorganic or inorganic materials systems typically proceeds through fleeting high energy intermediates whose physical and electronic structures are therefore difficult to define. Of particular interest are the intermediates produced in the activation of metal–oxygen and metal–halide bonds, processes that are central to the production of solar fuels, and more recently photoredox-mediated organic transformations. This thesis encompasses work to design and synthesize mono- and bimetallic coordination complexes as well as main group compounds that facilitate the study of transient oxo/oxyl and halide radical intermediates relevant to the activation of C–H bonds. Efforts begin with dicobalt, dinickel, and dicopper cryptands for the stabilization and isolation of a late transition metal bridging oxos. In the course of this work, fluorinated versions of the cryptand are developed that allow the formation of intermediates to be tracked, as well as, serendipitously, the binding of various halide anions in the cryptand pocket by 19F NMR. Synthetic routes are developed toward targeting a new family of macrobicyclic cryptand ligands composed of 1,3–azole donors that allow: histidine binding metal sites in bioinorganic systems to be mimicked, and the electronic effects at the metal center to be tuned by varying the heteroatom in the azole ring. Attempts to isolate metal oxos continue with the use of bulky oxygen donor ligands to isolate a rare example of a terminal MnIV oxo. With the challenges associated with isolating reactive metal-oxos revealed, efforts turned toward employing photocrystallography for the observation of reactive intermediates. As is the case for transient metal-oxos, fleeting halogen radical intermediates are implicated in a host of C–H bond activations. A series of isosteric and isoelectronic pyridine diimine (PDI) scaffolds are developed in order to study stereoelectronic effects governing photohalogen elimination from cationic FeIIICl2 centers. A major focus of this work moves away from the isolation of reactive intermediates and instead turns to generating these transient unstable intermediates in a crystalline lattice for their study by photocrystallography. The major results of these studies are twofold: firstly, the structural observation of the first instance of a C–H bond activation in the solid state and secondly, a concise and scalable synthesis of a wide range of electronically and sterically diverse pyridine diimine ligands. From this starting point, the range of compounds that permit the observation of halogen and pseudo-halogen elimination via photocrystallography is expanded. Namely, a number of hypervalent IIII dichloride and difluoride compounds are developed whose photoreactivity can be tracked both in solution and the solid state. Using a combination of traditional techniques and photocrystallography, new modes of reactivity for iodobenzene dichlorides are established. Finally, with photocrystallography skills honed over a number of compounds, we return to the original goal to structurally study late transition metal oxos by designing a system that allows for the generation of a metal¬–oxo in the solid state from a poised precursor. Studies are presented that build on literature examples showing that oxyanions, specifically chlorate and perchlorate can furnish oxyls and oxos, respectively, upon irradiation. The most promising results are realized using copper tris-pyrazole borate with a chlorate anion bound directly to the copper.