Multimetallic Models of the Nitrogenase Active Site

Citation

Arnett, Charles Haden (2021) Multimetallic Models of the Nitrogenase Active Site. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fgje-ns05. https://resolver.caltech.edu/CaltechTHESIS:09042020-064530373

Abstract

Motivated by the lack of an atomic-level understanding of the reduction of small molecule substrates by nitrogenase, this dissertation describes the synthesis, characterization and reactivity of well-defined model clusters of the enzyme active site.

Chapter 2 describes a series of site-differentiated, high spin iron clusters which reversibly bind carbon monoxide in redox states Fe^II₄ through Fe^IIFe^III₃. Detailed spectroscopic and thermochemical studies reveal that this remarkable reactivity can be attributed to the ability of remote metal centers to shuttle reducing equivalents to the small molecule binding site.

Chapter 3 further explores the consequences of internal electron transfer events on the thermodynamics of small molecule binding by site-differentiated, tetranuclear iron clusters. To systematically tune the electronic properties of the cluster, a Hammett series was prepared. Counterintuitively, introduction of electron-donating substituents suppresses the first CO binding event but enhances the second. Detailed spectroscopic studies revealed that the origin of this behavior can be traced to the effect of the substituents on the redox reorganization energy associated with internal electron transfer.

Chapter 4 presents the synthesis and characterization of the first open-shell diiron µ-carbyne complex, which also features a biologically relevant Fe(µ-C)(µ-H)Fe core. This electronically unusual species could be activated toward binding of N₂ upon addition of H⁺/e, which initially involves an iron-carbene intermediate.

Chapter 5 describes the synthesis and spectroscopic investigation of the first carbonbridged, bimetallic complexes featuring odd numbers of valence electrons as spectroscopic models of the critical E₄(4H) intermediate of nitrogenase. Detailed pulse EPR studies revealed the effects of electronic localization on the spectroscopic signatures of the µ-hydride motif and provide insight into the electronic distribution in a reduced state of FeMoco.

Chapter 6 describes the synthesis and characterization of terminal iron-carbene complexes, including EPR characterization of open-shell variants.

Appendix A describes unpublished efforts to prepared site-differentiated models of FeMoco featuring carbon- or sulfur-based donors.

Appendix B presents unpublished work towards modelling the cooperative activation and reduction of N₂ by diiron complexes featuring carbon-based bridging ligands.

Thesis Files