@article{Dihydrazonopyrrole:3433,
      recid = {3433},
      author = {Jesse, Kate Ashley},
      title = {Leveraging Control of the Secondary Sphere with Fe and Ni  Complexes of a Redox-Active Dihydrazonopyrrole Ligand  Featuring Pendant Protons},
      publisher = {University of Chicago},
      school = {Ph.D.},
      address = {2021-08},
      pages = {243},
      abstract = {Hydrogen transfer chemistry is important due to its wide  variety of applications in industrial processes and  pharmaceutical development. There has been extensive  research into catalyst design for reactions involving  hydrogen transfer reactivity. Homogenous catalysts are  attractive for studies due to the relative ease of their  characterization. Metal-ligand cooperativity can allow  first row transition metals to catalyze multi-electron and  multi-proton processes, and, as such, has become an  important tool in in transition metal catalyzed chemical  synthesis and industrial transformations such as  hydrogenation. For example, the use of redox-active ligands  can allow first row transition metals that often facilitate  1 electron chemistry to facilitate 2 electron chemistry.  Pendant protons on the ligand have been found to facilitate  proton shuttling and engage in hydrogen bonding  interactions that stabilize reactive intermediates. Recent  work combining these two strategies into a single ligand  scaffold has been found to be extremely effective at  facilitating challenging multi-electron and multi-proton  reactivity. In these studies, a 2,5-dihydrazonopyrrole  (DHP) ligand scaffold was utilized in complexes with Ni and  Fe. This ligand scaffold can store a full H2 equivalent in  the ligand scaffold itself in addition to the redox  capabilities of the metal center. 

In Chapter 1, I discuss  a DHP complex with Ni, where an H2 equivalent can be stored  on the ligand periphery without metal-based redox changes  and can be leveraged for catalytic hydrogenations. This  complex is an unusual example where a synthetic system can  mimic biology’s ability to mediate H2 transfer via  secondary coordination sphere-based processes.

DHP ligands  have been isolated in a variety of redox and protonation  states when complexed to Ni, but the redox-state of this  ligand scaffold is less obvious when complexed to metal  centers with more accessible redox couples. In Chapter 2, I  discuss the synthesis of a new series of Fe-DHP complexes  with phenyl groups on the hydrazone arms in two distinct  oxidation states. Detailed characterization supports that  the redox-chemistry in this set is still primarily ligand  based. 

In Nature, enzymes carefully control the movement  of protons and electrons via amino acids in the secondary  sphere of the enzyme active site. This allows for precise  reactivity using kinetically inert oxidants, such as O2.  Harnessing metal-ligand cooperativity to control the  secondary sphere of molecular catalysts mimics the  strategies used in nature. Chapter 3 discusses the DHP  ligand with tert-butyl groups complexed to Fe. This complex  has a hydrogenated ligand which can donate two electrons  and two protons to a substrate. In the presence of O2, this  complex reduces O2 via a high spin Fe(III)-hydroperoxo  intermediate which features a DHP• ligand radical. This  intermediate is characterized by a variety of spectroscopic  and computational techniques. 

In Chapter 4, we discuss a  family of bisneocuproine complexes of Fe2+ and Co2+ have  been investigated for neocuproine redox noninnocence. A  series of redox isomers of M(neocuproine)2n+ (where n = 2,  1, and 0 for Co and 2 and 0 for Fe) were synthesized and  thoroughly characterized. All techniques were consistent  with ligand-based reduction events to generate radical  neocuproine complexes. },
      url = {http://knowledge.uchicago.edu/record/3433},
      doi = {https://doi.org/10.6082/uchicago.3433},
}