Correlating Physical and Electronic Structure in Trinuclear Iron Clusters

Abstract

The study of biological nitrogen fixation carried out by the nitrogenase enzyme is of scientist interest in order to propose alternatives to the high-energy Haber Bosch process performed industrially. Nitrogenase contains a polynuclear active site, FeMoco, which stores and delivers multiple electron equivalents and allows for variable modes of substrate binding across multiple metal centers. While the resting state of FeMoco has been characterized, the understanding of key intermediates responsible for substrate binding and reduction remain elusive. Therefore, scientists have turned to synthetic polynuclear models to offer insight into the fundamental mechanistic questions that remain. This thesis utilizes a polynucleating ligand, much like the protein scaffolds that anchor polynuclear cofactors, to template the synthesis of open-shell trinuclear clusters. The highly tunable nature of these clusters permits interrogation of the electronic structure to offer insight into these fundamental questions: (1) What are the dominant pathways by which charge is delocalized? (2) What is the effect of ligand charge on the electronic structure of open-shell polynuclear clusters? (3) How do changes in oxidation state affect the aggregate electronic structure? We first studied the effect of local coordination environment on multiwavelength anomalous diffraction (MAD) data using a series of trinuclear clusters, to conclude that appropriate control compounds must be employed. We then studied a series of mixed-valent [Fe3] chalcogenide clusters featuring increasing bridging ligand size to show that the dominant pathway by which electron delocalization occurs is through direct metal orbital overlap despite the M–M distances being too long to be considered a formal bond. We then prepared a series of isoelectronic clusters bearing bridging ligands with increasing charge to reveal that an unprecedented cathodic shift of 1 V per charge is observed. Finally, we show that a large increase in spin density at the metal centers is observed upon one-electron reduction of the cluster.