Partial Synthetic Models of the FeMoco Nitrogenase Cluster with Bridging C-Based Ligands

Citation

Le, Linh Nguyen Vuong (2024) Partial Synthetic Models of the FeMoco Nitrogenase Cluster with Bridging C-Based Ligands. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9xgc-d212. https://resolver.caltech.edu/CaltechTHESIS:06032024-022957246

Abstract

Biological N₂ reduction to NH₃ occurs in microorganisms using the enzyme nitrogenase. This complex system consists of several iron-sulfur clusters, where the active site contains a MFe₇S₉C cluster (M = Mo, V, Fe) known as FeM cofactor (FeMco). The cluster includes an unusual interstitial carbide ligand, which is rare in both inorganic chemistry and biology. In addition, the role of this motif within the enzyme is not well-understood, and studies on synthetic model complexes are limited due to the absence of any previously reported iron-sulfur cluster systems bearing a carbon-based ligand that bridges the Fe atoms. Thus, this thesis focuses on developing strategies to insert a bridging carbon-based ligand into an iron-sulfur cluster platform.

Chapter 1 provides a general introduction and overview of complex biologically relevant iron-sulfur clusters and their corresponding synthetic analogs, with focus on NiFe CO dehydrogenase (CODH), acetyl CoA synthase (ACS), [FeFe] hydrogenase, P-cluster, and M-cluster of nitrogenase.

Chapter 2 discusses the formation of a cluster with a μ₃-carbyne ligand resulting from the ring-opening of a bisaminocyclipropenylidene ligand. Electrochemical studies on this system and related species suggest that a chelating μ₃-carbyne leads to clusters with highly negative reduction potentials compared to μ₃-N or S ligands, suggesting that the interstitial carbide in FeMco may play a role in modulating the redox potential of the cluster to allow for the reduction of difficult substrates like N₂.

Chapter 3 focuses on the binding of CO to the cluster with a μ₃-carbyne fragment, resulting in a high level of CO activation at 1851 cm⁻¹ in the neutral cluster and 1782 cm⁻¹ in the reduced cluster, Computational studies suggest that the bridging carbyne stabilizes the intermediate spin state at the Fe sites, resulting in more electrons in orbitals that can backbond with CO and greater activation. This suggests that the carbide in FeMco might play a role in modulating the electronic structure at the Fe sites to allow for greater activation of substrates.

Chapter 4 highlights the synthesis of a cluster bearing a μ₄-carbide ligand using a previously reported terminal Mo carbide complex, with a bridging CO ligand that resembles the lo-CO form. The S = 1/2 spin state provides an opportunity to study the metal-carbon interaction by pulse EPR spectroscopy.

In Chapter 5, a cluster ligated by an anthracene-bridged bisphenoxide ligand is described. Upon reduction, the anthracene bridge moves closer to one Fe site and interacts with it in an η² manner. This species can catalyze the electrochemical reduction of proton to form H2, possibly through a protonated cluster intermediate. The studies demonstrate the ability of the cluster to catalyze a biologically relevant reaction, and possibility for future studies on protonated species that have only been proposed in reactions of synthetic iron-sulfur clusters.