Synthesis, characterization and coordination chemistry of pincer ligands based on (benzannulated) pyridines: an investigation into their electronic structure and applications
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Date
2020
Authors
Braun, Jason
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Abstract
The idea of ‘non-innocent’ ligands is that the site of chemical or redox activity on a coordination complex may not be solely limited to the central atom. Ligands may participate either by themselves or in cooperation with a central atom to facilitate electron-transfer and/or chemical reactivity in a more active fashion than in a typical coordination complex. Each of the projects explored in this thesis include the common theme of pyridine-based pincer-type ligands, their coordination to late transition metals and main group elements, and how the reactivity and properties of these complexes involves both the ligand and the ligated element. To do so, a comprehensive investigation of the electronic structure of complexes supported by pincer-type, ‘NNN’ ligands is also described.
Diiminepyridine ligands tend to actively participate in the chemistry of their complexes. Taking advantage of these non-innocent ligands has allowed for the stabilization of unusual oxidation states of transition metal complexes as well as the facilitation of chemical transformations with metals unable to do so using spectator ligands. Pseudo-octahedral iron complexes of these ligands exhibit multiple reversible reductions and are shown to be viable candidates as anolytes for redox flow battery applications. In synthesizing these, a synthetic scheme to prepare these ligands with electron withdrawing groups on the flanking aryl groups was developed, opening up the chemical space for these new ligands in the already well-established field of diiminepyridine ligand chemistry. In addition, bulky analogues engender stabilization of phosphorus complexes in the +1 and +3 oxidations states, not seen before with traditional diiminepyridine ligands.
The synthetic route to diarlyamido ligands comprised of flanking quinoline (2,3-benzopyridine) and phenanthridine (3,4-benzoquinoline) donors has also been developed and the coordination of these monoanionic ligands to a variety of late transition metals and main group elements was explored. Several aspects of this ligand can be deemed non-innocent and will be highlighted throughout the course of this thesis. We see an unusual electronic environment of pseudo-octahedral iron complexes of these ligands that exhibit record length charge-transfer excited states and panchromatic absorption. Exchanging the central metal can bring about mixed-valent species upon oxidation evidenced by strong absorption in the near infrared region of the electromagnetic spectrum that significantly depends on metal-ligand orbital overlap.
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Keywords
Iron, Photosensitizer, Redox flow battery, Chemistry